Patent Grant
7141407
This application claims the benefit of priority from Canadian Patent Application No. 2,421,373, filed Mar. 7, 2003, the entire contents of which are incorporated herein by reference.
The invention relates to biocontrol agents for suppressing weed growth. More specifically the present invention relates to bacterial biocontrol agents for suppression of weed growth.
Chickweed (Stellaria media) is one of the most important weeds in Canada. It is a vigorous competitor for space and nutrients; an aesthetic problem in agricultural crops, market gardens, and urban centers; and listed as a noxious weed in the Noxious Weeds Act of Alberta and Manitoba (Mann et al., 1950; Terkington et al., 1980).
Chickweed is distributed across all Canadian provinces. It is found from Vancouver Island to Newfoundland, and from the United States border to the Mackenzie delta at 69 N (Terkington et al., 1980). Previous data showed that chickweed is more common in B.C. and eastern Canada than in the prairies, however, its populations in the prairie provinces of Canada have been dramatically increasing over the past 10 years. Overall, the relative abundance of chickweed over these years rose by 4, 6, and 17 rankings in Alberta, Saskatchewan and Manitoba, respectively (Thomas et al., 1998). Presently, chickweed is the most abundant weed species in Alberta, followed by wild oats (Thomas et al., 1997).
The main control method is the use of chemical herbicides that belong mainly to Groups 2 and 4. However, herbicide resistance in chickweed to Group-2 herbicides in Canada and Group-4 herbicides in Europe has been reported (Devine et al., 1991; Hall and Devine, 1990; Lutman & Heath, 1990; O'Donovan et al., 1994). Herbicide resistance development in chickweed substantially limits herbicide choices for its control in most crops and other non-agricultural lands such as home gardens and lawns. This situation provides an opportunity to develop “biological herbicide” or “bioherbicide” or “microbial herbicides” for chickweed control.
The invention relates to biocontrol agents for suppressing weed growth. More specifically, the present invention relates to bacterial biocontrol agents for suppression of weed growth.
The present invention relates to the development of effective and environmentally safe bioherbicides for biological control of chickweed. More particularly, the present invention
The present invention provides the use of a strain of biocontrol agent Burkholderia andropogonis, such as Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234 for controlling or suppressing the growth of a weed. Preferably, the weed belongs to the order Caryophyllales, more preferably the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
Also provided by the present invention is a biocontrol composition for controlling the growth of a weed, comprising a strain of biocontrol agent Burkholderia andropogonis, such as Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234, and a suitable medium. The acceptable medium may comprise a liquid culture medium, a solid culture medium or a combination thereof. Preferably the acceptable medium is a liquid culture medium. Preferably, the weed belongs to the order Caryophyllales, more preferably the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
The present invention further provides a biocontrol composition comprising biocontrol agent Burkholderia andropogonis (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234, and a suitable medium. The present invention also provides the use of the just described composition for controlling or suppressing the growth of a weed. The present invention also provides a method for suppressing weed growth, comprising applying the just described composition to a weed. for controlling or suppressing the growth of a weed, such as a weed belonging to the order Caryophyllales. Preferably, the weed belongs to the order Caryophyllales, more preferably the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
Also according to the present invention there is provided a method for suppressing weed growth, comprising applying a strain of biocontrol agent Burkholderia andropogonis, such as Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234, to a weed. Preferably, the weed belongs to the order Caryophyllales, more preferably, the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
The present invention also provides a method of suppressing weeds during crop growth comprising:
Preferably, the weed belongs to the order Caryophyllales, more preferably, the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
The biocontrol agent or biocontrol composition may be applied to weeds by any method known in the art, but is preferably applied by spraying, for example, but not limited to airbrush spraying or broadcast spraying. Broadcast application may be effected using a nozzle, which enhances the reduction of the size of the droplets, which are emitted during application of the biocontrol agent or composition as defined above.
In particular, the present invention relates to the use of the bacterial isolate, CW00B006C, for causing severe disease on chickweed. Typical disease symptoms include chlorosis, resulting in slower chickweed growth and death. Based on phenotypic properties, fatty acid composition, and 16s rDNA sequences, the bacterial strain CW00B006C has been identified as Burkholderia andropogonis (Smith 1911) Gillis et al. 1995, comb. nov, and has been deposited under ATCC Accession No. PTA-4234. This bacterium is easily cultured in submerged conditions and its cell production has been characterized using different liquid culture media and various medium pH. Host specificity tests against 36 plant species in 30 genera and 8 families demonstrated that the use of B. andropogonis as a bioherbicide for control of chickweed would not cause major concerns to crops and native flora in Canada. Various factors impact the herbicidal performance of this bacterium against chickweed. Under greenhouse conditions, increasing the bacterial cell concentration, repeat application, addition of the surfactant Silwet L-77® at 0.1%–0.2%, or application of bacterial inoculum to chickweed seedlings at an older growth stage, significantly increases disease severity on chickweed seedlings. Under field conditions, about 65%–80% disease severity is consistently observed with application of B. andropogonis at 109–1010 CFU/ml plus 0.15% Silwet 77®. A maximum percent dry weight reduction of 79.9% under field conditions was obtained. Moreover, B. andropogonis causes equivalent disease severity on Group 2 herbicide resistant chickweed seedlings, providing a new approach for mitigating herbicide resistance development in chickweed. Cell-free culture filtrates cause symptoms similar to those caused by the bacterial cells, indicating the potential of using fermentation broth alone, or in combination with bacterial cells, for chickweed control.
This summary does not necessarily describe all necessary features of the invention but that the invention may also reside in a sub-combination of the described features.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
The invention relates to biocontrol agents for suppressing weed growth. More specifically, the present invention relates to bacterial biocontrol agents for suppression of weed growth.
The following description is of a preferred embodiment by way of example only and without limitation to the combination of features necessary for carrying the invention into effect.
By the term “biocontrol agent” is meant a microorganism which suppresses the growth of, or kills, a target pest, for example, but not limited to a plant or a weed. More specifically, the biocontrol agents of the present invention may be used to suppress the growth of one, or more than one target pest. Without wishing to be bound by theory, the biocontrol agent suppresses the growth of a target pest, for example, a plant or weed (i.e. exhibits weed suppressive activity), by interfering with the normal growth and development of the target plant or weed. For example, but not wishing to be limiting, the biocontrol agent may inhibit root growth, shoot growth, reduce biomass, inhibit seed production, reduce competitiveness of the target plant or weed for a crop's water and nutrients, or a combination thereof.
As someone of skill in the art will understand, in order for the biocontrol agent of the present invention to be grown, cultured or used in accordance with the embodiments of the present invention, it is preferable that the biocontrol agent be grown in a suitable medium to produce a biocontrol composition or formulation. By the term “suitable medium” or “acceptable medium” it is meant any liquid, semi-liquid or solid substrate which allows a biocontrol agent such as Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234, to grow, or to remain viable, or both grow and remain viable. The present invention contemplates a biocontrol composition comprising a bacterial biocontrol agent, such as a strain of Burkholderia andropogonis, for example, biocontrol agent Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234. Preferably, the composition permits an effective amount of the biocontrol agent to remain viable prior to, and after, being applied to a crop.
ATCC Accession No. PTA-4234 was deposited on Apr. 17, 2002. The ATCC is the American Type Culture Collection, P.O. Box 1549, Manassas, Va., 20108, USA.
More preferably, the composition permits the biocontrol agent to remain viable for a period between about 1 day to about 1 month following application of the biocontrol composition of the present invention onto a plant, or soil.
The biocontrol agent or biocontrol composition of the present invention may be applied to plants, soil or both plants and soil. Preferably, the biocontrol agent or composition is applied to plant foliage, for example the foliage of the target weed. Alternatively, the biocontrol agent or composition may be applied directly to soil, either before, during or after seeding a crop. The biocontrol agent may be applied by any method known in the art, for example, but not limited to spraying, pouring, dipping or the like. Preferably, the biocontrol composition of the present invention is applied by spraying.
Therefore, the present invention provides for the use of fungal biocontrol agent Burkholderia andropogonis, (Smith 1911) Gillis et al. 1995, comb. nov., deposited under ATCC Accession No. PTA-4234, grown and formulated in a suitable composition for the suppression of weeds. Preferably, the weed belongs to the order Caryophyllales, more preferably, the weed is a member of the family Caryophyllaceae, even more preferably the weed is a member of the subfamily Alsinoideae, most preferably the weed is a member of the genus Stellaria.
However, as someone of skill in the art will understand, the amount of the biocontrol composition required for suppression of a weed may be dependent on the medium in which the biocontrol agent is formulated and the method by which it is formulated. For example, but not wishing to be limiting, a formulation and medium which permits a greater percentage of the biocontrol agent to remain viable may require less biocontrol composition to suppress weed growth than does another formulation and medium in which the biocontrol agent is less viable. Further, the amount of a biocontrol composition required for suppression of a weed may be influenced by environmental factors such as but not limited to temperature, humidity, soil pH, and soil type.
Naturally occurring fungi and bacteria were isolated from various locations across Alberta and from Saskatoon. A total of 52 fungal and 25 bacterial isolates with some pathogenicity to chickweed were found after the demonstration of Koch's postulates. Of these isolates, the bacterial isolate, CW00B006C, deposited under ATCC Accession No. PTA-4234, typically causes disease symptom on chickweed of chlorosis and necrosis, resulting in slower plant growth and death. CW00B006C was selected as the bioherbicide candidate for further study. The use of this bacterium as a bioherbicide does not require dew or free moisture to cause the disease.
Based on phenotypic properties, fatty acid composition, and 16s rDNA sequences, the bacterial strain CW00B006C has been identified as Burkholderia andropogonis (Smith 1911) Gillis et al. 1995, comb. nov., and deposited under ATCC Accession No. PTA-4234 on Apr. 17, 2002.
This bacterium is easily cultured in submerged conditions. Cell production was significantly affected by the liquid culture media. No lag phase was observed in nutrient glucose broth (NGB), tryptic soy dextrose (TSD), and King's medium B (KB) and about a 2–4 h lag phase was observed in nutrient broth yeast extract (NBY). Exponential growth ended around 22–24 h in all four media tested. Death phase was observed at 42 h in KB and 56 h in NGB and TSD. Growth rate was not significantly different among four media tested, but initial cell production in NBY, NGB, and TSD were similar and significantly greater than that in KB. Estimated generation time (g) was 2.9 h in KB, 2.7 h in NBY, 2.9 h in NGB, and 3.0 in TSD. Medium pH did not affect cell production of B. andropogonis. Addition of chickweed extract reduced cell production by 50% when culture time was more than 48 h.
Thirty-six plant species in 30 genera and 8 families, selected by using the centrifugal phylogenetic method, were screened against the bacterial strain Burkholderia andropogonis (CW00B006C), a bioherbicide candidate collected in Alberta, Canada from diseased chickweed. Trials were performed under greenhouse conditions. Plants were inoculated with a bacterial suspension at 109–1010 CFU/ml plus a surfactant, Silwet L-77®. This bacterial strain of B. andropogonis caused disease of host plants distributed in the Caryophyllaceae, Poaceae, and Fabaceae families. The number of species in the Caryophyllaceae family that this bacterial strain was able to infect is relatively wide including chickweed and six other weeds common in western Canada, expanding B. andropogonis weed control spectrum as a bioherbicide. In the Poaceae family, corn is susceptible to this strain of bacterium. In the Fabaceae family, hosts of our B. andropogonis strain were restricted to Tribe Vicieae containing common vetch and chickpea. The strain of B. andropogonis of the present invention is not pathogenic to the majority of major economically important crops cultivated in western Canada. On the basis of host specificity, the use of B. andropogonis as a bioherbicide for control of common chickweed will not cause major concerns to crops and native flora in Canada.
Efficacy of chickweed control with Burkholderia andropogonis (bacterial isolate CW00B006C) was assessed under both greenhouse and field conditions. Various factors impacted herbicidal performance of this bacterium against chickweed. Under greenhouse conditions, increasing bacterial cell concentration, repeat application, addition of surfactant Silwet L-77® at 0.1%–0.2%, or application of bacterial inoculum to chickweed seedlings at an older growth stage significantly increased disease severity on chickweed seedlings. However, spray pH did not affect the bacterial performance as a bioherbicide against chickweed. Under field conditions, about 65%–80% disease severity was constantly observed with application of B. andropogonis at 109–1010 CFU/ml plus 0.15% Silwet 77®. A maximum percent dry weight reduction of 79.9% was obtained. Moreover, B. andropogonis caused equivalent disease severity on Group 2 herbicide resistant chickweed seedlings, providing a new approach for mitigating herbicide resistance development in chickweed.
Cell-free culture filtrates of Burkholderia andropogonis caused symptoms similar to those caused by the bacterial cells, indicating the potential of using fermentation broth alone, or combination with bacterial cells for chickweed control.
The above description is not intended to limit the claimed invention in any manner, furthermore, the discussed combination of features might not be absolutely necessary for the inventive solution.
The present invention will be further illustrated in the following examples. However, it is to be understood that these examples are for illustrative purposes only, and should not be used to limit the scope of the present invention in any manner.
Collection and Screening of Bioherbicide Candidates for Control of Chickweed
The use of living microorganisms, mainly plant pathogens, for control of weeds has received increasing interest in the last three decades, especially the bioherbicide approach (Charudattan, 2001). The bioherbicide approach. involves three major phases or stages: 1) discovery, 2) development, and 3) deployment (Templeton, 1982). Major activities in the discovery phase involve the collection of diseased plant material, the isolation of causal organisms, and the demonstration of Koch's postulates (Watson, 1993). To date, there has been no research to extensively search for chickweed diseases with an aim to develop a bioherbicide.
Unlike the classical biological control approach, the bioherbicide approach usually relies on the use of endemic pathogens (Watson, 1993). In Canada, three fungi, Septoria stellariae Ro. & Desm., Melampsorella caryophyllacearum Schroet., and Puccinia arenariae (Schum.) Wint., were associated with chickweed (Conners, 1967; Tom, 1964). But it is unknown whether these fungi can be isolated and used as bioherbicide candidates. In addition, the existence of other fungi and bacteria that infect chickweed and possess potential as bioherbicides remains unknown.
In this example, diseased chickweed plants are collected, causal microorganisms are isolated, and pathogenic fungi- and bacteria are identified with bioherbicide potential.
Collection and Isolation of Fungi and Bacteria
Collection of Diseased Chickweed.
Several field trips were made from May to September in 1999 and 2000 to collect diseased chickweed plant materials from agricultural crops and market gardens in the districts of Vegreville, Tofield, Busby, Ryley, Ranfurly, Fort Assiniboine, Lamont, Spruce Grove, Leduc, Red Deer, Calgary, Edmonton, and Saskatoon. Diseased plant parts of chickweed were collected, cut to appropriate size, and transported at 4° C.
Isolation of Causal Organisms.
Isolation was carried out in the lab within 24 h of field collection in order to keep the samples as fresh as possible. Diseased plant material was sorted based on whether fungal or bacterial disease was most likely to have caused disease symptoms. To isolate fungi, leaf, stem or root pieces with lesions were surface sterilized with 0.5% sodium hypochlorite solution and incubated on fresh potato dextrose agar (PDA; Difco, Detroit, Mich.). Fungi that grew from the lesions were isolated. For all isolated fungi, single cell colonies were obtained by using standard single spore or hyphal tip techniques to ensure species purity (Tuite, 1969). To isolate bacteria, pieces of leaves or stems with lesions were washed with running tap water, placed into a drop of sterile distilled water, and macerated. A loopfull of the macerated solution was streaked across a plate of nutrient agar, nutrient glucose agar, and nutrient broth yeast extract agar (Schaad, 1988). Single, morphologically distinct bacterial colonies were then selected from culture plates after 4–7 days and streaked on a fresh plate of culture media to obtain pure cultures.
Maintenance of Microorganisms.
A 15% glycerol solution was prepared as follows: Fifteen ml of glycerol was added to 85 ml of distilled water, the resulting mixture was then autoclaved and cooled to room temperature. Then, 1.8 ml of 15% glycerol solution was dispensed into 2 ml cryovials. A pure culture of fungi or bacteria was scraped with a scalpel blade and placed in each of 3 cryovials of glycerol solution. Cryovials of samples were placed in a Mr. Frosty (Nalgene Cryo Freezing container, Cat No 5100-0001) in a freezer at −80° C. for a minimum time of 85 minutes to freeze at a rate of 1° C. per minute to −80° C. Frozen samples were then placed in racks for storage at −80° C. as stock cultures.
Screening of Fungi and Bacteria
Inoculum Preparation
For fungal pathogens, a cryovial of stock culture was warmed to room temperature in a 36° C. water bath and the contents were dispensed in 200 μl aliquots on the surface of potato dextrose agar (PDA) plates, and spread with a sterile glass rod. The PDA plate was incubated for 5 days at 22° C. under 12 h light provided by two, 20 W, 60 cm long cool white fluorescent tubes placed 30 cm above the PDA plates. Single-spore colonies were then made for all fungal isolates using standard single-spore-technique and cultures were incubated under conditions as described above. Sufficient spores for experiments were produced by transferring spores from a single-cell colony onto several PDA plates (spread plates). Plates were incubated as above for 1–2 wks. Spores were harvested by gently scraping the surface of the agar and transferring into sterilized distilled water. Spore concentrations were determined using a haemocytometer.
For bacterial pathogens, a cryovial of stock culture was warmed to room temperature in a 36° C. water bath. A 50 μl aliquot of suspension from the vial was added to each 18×150 mm glass test tube containing 3 ml nutrient glucose broth (NGB) at a pH of 6.8. The test tubes were incubated on an orbit shaker at 200 rpm for 24–72 h under ambient laboratory conditions (24° C.±3). Contents of tubes were used as ‘seed inoculum’. The bacterial culture for inoculations was produced by placing 75 ml of NGB (pH 6.8) in each 250-ml Erlenmeyer flasks, autoclaving, cooling, inoculating with 1 ml per flask of the ‘seed inoculum’, and incubating flasks on an orbit shaker at 200 rpm for 24–72 h under ambient laboratory conditions (24° C.±3). About 30 ml of cell suspension from each flask was then placed in a 50 ml centrifuge tube and centrifuged 10 minutes at 3700 rcf (Sorvall RC-5B refrigerated superspeed centrifuge). Supernatant was poured from each tube and the bacterial pellet was resuspended in 20 ml of 0.01 M, pH 7 phosphate buffer. Viable bacterial cell production was determined using the dilution plate count method. Serial dilutions (10×) were performed in dilution tubes containing 9 ml of sterile 0.01 M phosphate buffer, pH 7, and 100 μl of three appropriate dilutions were spread plated on nutrient glucose agar (NGA). Plates were incubated under ambient laboratory conditions for about 4 days. Colony forming units (CFU) per ml were determined by counting colonies on plates with 10–200 CFU per plate.
Plant Production
Fungal or bacterial virulence against chickweed was assessed using a single batch of chickweed seeds for all experiments. Seeds were sown in 10-cm-diameter pots filled with pre-prepared soil mix. The soil mix consisted of 140 L soil (loam, pH 6.7, 42% sand, 40% silt, and 18% clay), 107 L sand, 160 L Sunshine Growing Mix (SunGro Horticulture, Bellevue, Wash.), 62 L Fibrous Blond Shagnum Peat Moss (Premier Pro Moss, Riviere-du-Loup, Quebec), 113 L vermiculite (Therm-O-Rock, Chandler, Ariz.), 230 g dolomite lime, and 150 g Super Phosphate (0-45-0) per batch of soil. Seeded pots were placed in a greenhouse with 23/20±4° C. day/night temperature, a 16 h photoperiod, an average light intensity of 300 μEm−1, and an average relative humidity of 45–50%. After germination, seedlings were thinned to five plants per pot.
Inoculation Procedure
For fungal pathogens, chickweed seedlings at the 4- to 6-leaf stage were inoculated with 5×105 to 5×106 spores/ml to run-off with 0.05% gelatin as a wetting agent, using an airbrush at 100 kPa. Unless otherwise indicated, after spraying, pots were placed in a dark dew chamber with 100% relative humidity at 22° C. for 48h. Subsequently, pots were transferred back to the greenhouse with conditions as mentioned above. Control treatments were sprayed with distilled water containing only the wetting agent.
For bacterial pathogens, chickweed seedlings at the 4- to 6-leaf stage were inoculated with 107–1010 CFU/ml of bacterial cells suspended in 0.01 M, pH 7 phosphate buffer plus 0.1% Silwet L-77® (wetting agent) using a airbrush at 100 kPa until all leaf surfaces were evenly wet. Immediately after spraying, pots were returned to the greenhouse. Control treatments were sprayed with 0.01 M phosphate buffer containing only the wetting agent.
Pathogenicity and Virulence to Chickweed
Fungal or bacterial virulence on chickweed were assessed 7 and 14 days after inoculation using a 0 to 3 scale (0—no symptom, 1—light infection, 2—moderate infection, and 3—severe infection to death). For each isolate tested, there were three replications arranged in a completely randomized design. The experiments were repeated at least once.
Disease Symptoms of CW00B006C in Chickweed
To characterize the typical symptoms associated with chickweed disease caused by isolate CW00B006C, chickweed plants inoculated with the bacterium in various greenhouse and field experiments were observed from the onset of symptoms for a three week period after spraying. Symptoms from the various experiments were recorded, compared, and summarized.
Collection and Isolation of Fungi and Bacteria
From 13 different locations, a total of 153 fungal isolates and 448 bacterial strains were isolated from chickweed and stored at −80° C. (Table 1). The findings in this study demonstrated that endemic fungi and bacteria associated with chickweed are much more common than previously reported (Conners, 1967; Toms, 1964). These naturally occurring fungi and bacteria provided a sound base for searching for a bioherbicide candidate.
Screening of Fungi and Bacteria.
A total of 52 fungal and 25 bacterial isolates with some pathogenicity to chickweed were found (Table 2 and Table 3). Of these isolates, only three fungi and one bacterium are highly pathogenic to chickweed and thus deserve consideration as bioherbicide candidates.
Fungal isolate CW98-235 was the most virulent of the fungal isolates (data not shown). All three fungi, however, required a prolonged dew period (48 h) to cause infection, while the bacterium required no free moisture and had excellent virulence against chickweed. Thus, the bacterial isolate, CW00B006C, was selected as the bioherbicide candidate for further study.
Disease Symptoms of CW00B006C in Chickweed
Isolate CW00B006C infection of chickweed causes chlorosis on treated leaves and stems, sometimes with necrotic lesions, within 5–7 days after bacterial treatment (
Bacteria have excellent potential as weed biocontrol agents because they can be delivered into the xylem via stomata and other natural plant openings with the aid of a new type of surfactant—nonionic organosilicone surfactants (e.g. Silwet 77°) (Zidack et al., 1992; Johnson et al., 1996). As a result, the bacteria do not require a prolonged dew period or free moisture for infection and colonization that are rarely met under field conditions, especially in western Canada. This is a significant advantage over the use of a fungus as a bioherbicide. Therefore, the bacterial isolate CW00B006C possesses great potential as a bioherbicide.
Characterization and Identification of Bacterium CW00B006C
The native bacterium, strain CW00B006C, caused severe disease on chickweed. In order to further evaluate and develop this bacterium as a bioherbicide, its identification was conducted to provide information on its pathogenicity, culture conditions, and phylogenetic relations. Various techniques are available for bacterial identification including phenotypic and genotypic analyses (Bagsic-Opulencia et al., 2001; Viallard et al., 1998; Whitford et al., 2001). The bacterium was identified using those available technologies including physiological and biochemical characterization, fatty acid composition, and 16s rDNA sequence. In addition, the bacterial growth and disease symptoms were also characterized.
Phenotypic Analyses
The bacterial isolate CW00B006C was characterized and identified based on phenotypic properties including physiological and biochemical characterization and fatty acid composition.
Physiological and Biochemical Characterization
Characterization included gram staining, motility, carbon substrate assimilation, oxidase and other physiological activities using previously described methods (Coeyne et al. 1999; Viallard et al. 1998; Hu et at. 1991). Carbon substrate assimilation tests were performed using auxanographic API 50CH strips (bioMerieux) as recommended by the manufacturer.
Fatty Acid Composition (MIDI-FAME)
The bacterial isolate CW00B006C was grown overnight at 27° C. on trypticase soy agar and harvested for extraction of total cellular fatty acids. Total cellular fatty acids were extracted and methylated for fatty acid methyl ester (FAME) analysis by gas chromatography (Paisley 1996; Vandamme et al. 1992) and the profiles compared with a computerized database using Sherlock microbial identification system software (MIDI, Newark, Del.).
Genotypic Analyses (16S rDNA Sequence)
Genomic DNA was prepared from bacterial isolate CW00B006C as described by Whitford et al. (1998). 16S ribosomal rRNA genes (rDNA) were amplified using primers FP1 [5′AGA GTT YGA TYC TGG CT 3′ (SEQ ID NO:1)] and R1492 [5′-TAC GGY TAC CTT GTT ACG ACT-3′ (SEQ ID NO:2)] based on primers described by Lane (1991). Primers and PCR reaction conditions have been previously described (Whitford et al., 2001). The 16S rDNA PCR products were purified using a QlAquick PCR purification kit (QIAGEN) and the recovered PCR products were quantified using a DyNAQuant 200 Fluorometer (Hoefer). The PCR products were directly sequenced using a Thermo Sequenase cycle sequencing kit (Amersham Pharmacia Biotech) using IRD800-labeled M13 forward and reverse primers (LI-COR Inc., Lincoln, Nebr.), plus the IRD800-labeled 16S rDNA specific primers FP1, EUB338f [5′-ACT CCT ACG GCA GGC AG-3′ (SEQ ID NO:3)], 519r [5′-GWA TTA CCG CGG CKG CTG-3′ (SEQ ID NO:4)], 926f [5′-AAA CTY AAA KGA ATT GAC GG-3′ (SEQ ID NO:5)], 1100r [5′-AGG GTT GCG CTC GTT G-3′ (SEQ ID NO:6)], and 1492r. Sequence products were analyzed on a LI-COR model 4000L sequencer. Sequence fragments were assembled and edited using Sequencher version 3.1 (Gene Codes Corp., Ann Arbor, Mich.). The resulting sequence was analyzed using the Similarity Matrix Program at the Ribosomal Database Project II (RDPII; rdp.cme.msu.edu/html/).
Phenotypic Analyses
Physiological and Biochemical Characterization
Isolate CW00B006C is a motile, gram negative rod, 0.5×1.5–2.5 μm, that grows only aerobically without production of any soluble or fluorescent pigments. The isolate has urease activity, utilizes citrate and produces poly-13-hydroxybutyrate. The isolate does not produce indole or have oxidase, arginine and lysine dihydrolase, gelatin liquification, lecithinase, (3-galactosidase and I3-glucouronidase activities. The isolate grows well at 30° C. but not at 37° C. or higher. Tests based on the assimilation of 49 carbon sources exhibited that isolate CW00B006C utilizes 20 carbons as a sole carbon source (Table 4). Isolate CW00B006C matches closely to Burkholderia andropogonis (Smith 1911) Gillis et al. 1995, comb. Nov., differing in its utilization of D-xylose but not gluconate (Hu et al. 1991; Gillis et al., 1995; Viallard et al. 1998).
Burkholderia andropogonis
Burkholderia
andropogonis
Fatty Acid Composition
The main fatty acids (>1%) in the cells of strain CW00B006C are 14:0 (6.1%), 16:0 (14.2%), 18:16)7c (22.4%), 16:1 2-OH (4.6%), 16:0 2-OH (5.7%), 16:0 3-OH (4.5%), 18:1 2-OH (2.2%), 17:0 cyclo (5.2%), 19:0 cyclo w8c (7.1%), summed feature 2 (14:0 3-OH; 4.5%) and summed feature 3 (16:1w7c; 21.5%). These results match published values for B. andropogonis (Coeyne et al., 2001 a; Stead, 1992). In particular, the presence of 14:0 3-OH, 16:1 2-OH, 16:0 2-OH and 16:0 3-OH and no other hydroxy fatty acids are indicative of Burkholderia sp. (Stead, 1992). Further, the predominance of 16:0, 18:1ω7c and summed feature 3 are indicative of B. andropogonis (Coeyne et al., 2001a; Coeyne et al., 2001b; Coeyne et al., 2001c; Vandamme et al., 1997; Vandamme et al., 2000; Zhang et al.,2000).
Genotypic Analyses (16S rDNA Sequencing)
A 1408 base pair sequencing product was produced from amplification of the rDNA from isolate CW00B006C. The 16S rDNA sequence of isolate CW00B006C is very similar to that of Burkholderia andropogonis ATCC 19311 (formerly Pseudomonas woodsii; similarity value 98.6%) and Burkholderia andropogonis ATCC 23061T. Therefore, genotypic analyses confirmed the identification of isolate CW00B006C as B. andropogonis (Smith 1911) Gillis et al. 1995, comb. nov.
Chapter 4 Cell Production of Bacterium CW00B006C
The development of low-cost methods for mass production of a bioherbicide is an important step for the commercialization of a bioherbicide (Stowell et al., 1989; Boyette et al., 1991). Submerged liquid culture spore production is the preferred technique for mass production of biocontrol agents because the technology is readily available and the scale-up process from the research phase to the development phase is relatively easy (Churchill, 1982; Stowell et al., 1989; Jackson, 1997). So far, no information has been reported on submerged liquid culture cell production for Burkholderia andropogonis. In this example, the growth curve of B. andropogonis was characterized in different liquid culture media, the effect of medium pH on cell production of B. andropogonis was determined, and (3) the effect of addition of chickweed extracts on cell production of B. andropogonis was evaluated.
For all experiments, stock cultures of CW00B006C were stored in 15% glycerol at −80° C. A cryovial of stock culture was warmed to room temperature in a 36° C. water bath. A 50 μl aliquot of suspension from the vial was added to 18×150 mm glass test tube containing 3 ml of appropriate broth. Test tubes were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes of the same medium were combined to produce ‘seed inoculum’. Replicate 500 ml flasks of each medium were inoculated with 1 ml of the appropriate ‘seed inoculum’. Flasks were placed on an orbit shaker at 200 rpm under ambient laboratory conditions (24° C.±3). Viable bacterial cell production was determined using the dilution plate count method. Serial dilutions (10×) were performed in dilution tubes containing 9 ml of sterile 0.01 M phosphate buffer, pH 7, and 100 μl of three appropriate dilutions were spread plated on nutrient glucose agar (NGA). Plates were incubated under ambient laboratory conditions for about 4 days. Colony forming units (CFU) per ml was determined by counting colonies on plates with 10–600 CFU per plate.
Growth Curve of the Bacterium in Different Liquid Culture Media
The growth of the bacterium in four different liquid culture media was assessed over a 44 h period. Liquid culture medium tested included nutrient glucose broth (NGB; 8 g nutrient broth (Difco), 2.5 g glucose, 1 L distilled water), nutrient broth yeast extract (NBY; 8 g nutrient broth (Difco), 2 g yeast extract (Difco), 2 g K2HPO4 0.5 g KH2PO4, 2.5 g glucose, 1 L distilled water), King's medium B (KB; 20 g proteose peptone #3 (Difco), 1.5 g. K2HPO4, 1.5 g MgSO4.7H2O, 15 ml glycerol, 1 L distilled water), and tryptic soy dextrose (TSD; 17 g Bacto tryptone (Difco), 3 g Bacto Soytone (Difco), 5 g NaCl, 2.5 g K2HPO4, 2.5 g glucose, 1 L distilled water). Two replicate 500 ml flasks of each medium were inoculated with 1 ml of the appropriate ‘seed inoculum’. A 0.5 ml sample from each of the two replicate flasks was taken at 0, 2, 4, 6, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 36, 38, 40, 42 and 44 h after inoculation. The 2–0.5 ml samples from each medium were combined and viable bacterial cell concentration was determined using the dilution plate count method.
Since the death phase were not observed within 44 h culture time, an extension of culture time to 64 h was used to culture CW00B006C in NGB and TSD. Two trials were conducted. For each trial, 3 replicate 500 ml flasks of each medium were inoculated with 1 ml of appropriate ‘seed inoculum’. A 1 ml sample from each replicate flask was taken every 4 hours from 0 to 44 hrs in trial 1 and from 0 to 48 hours in trial 2, and at 56 and 64 hours for both trials. Each sample was separately analyzed using the dilution plate count method to determine viable bacterial cell concentration. Data from the two trials of each experiment were pooled because the variances of trials were homogeneous according to Bartlett's test (Gomez & Gomez, 1984).
Effect of Medium pH on Bacterial Cell Production
A cryovial of stock culture was warmed to room temperature in a 36° C. water bath. A 50 μl aliquot of suspension from the vial was added to each 15 mm glass test tube containing 3 ml nutrient glucose broth (NGB) at a pH of 6.8. The test tubes were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes were used as ‘seed inoculum’. The pH of NGB flasks was adjusted using 1 N HC 1 or NaOH to achieve pHs from 6 to 8 at increments of 0.2 prior to sterilization. Two replicate flasks of each treatment were inoculated with the ‘seed inoculum’ and incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Medium pH of each flask after 24 h culture was measured and compared to the original pH. Bacterial cell production was then determined.
Effect of Growth Medium Modification with Chickweed Extracts.
For all experiments, nutrient glucose broth (NGB) at a pH of 6.8 was used as a control treatment to compare the cell production in chickweed extract medium (CWE). To prepare chickweed extract medium, 3–5 wk old chickweed foliage was harvested from the greenhouse and stored at −20° C. until use. Using liquid nitrogen and a pestle and mortar, tissue was crushed to a powder, and combined with an equal amount of distilled water (w/v) to form thick slurry. The slurry was filtered through 2-ply cheesecloth, filtrate was centrifuged at 18500 rcf (Eppendorf 5810R centrifuge) for 15 minutes, supernatant was filter sterilized using a 0.22 μm bottle-top vacuum filter, and resulting sterile CWE medium was stored at 4° C. until use. Stock cultures of isolate CW00B006C were stored in 15% glycerol at −80° C. For each experiment, a cryovial of stock culture was warmed to room temperature in a 36° C. water bath. A 50 μl aliquot of suspension from the vial was added to each 18×150 mm glass test tube containing 3 ml of appropriate medium. The test tubes were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes were used as ‘seed inoculum’. The bacterial culture for inoculations was produced using 75 ml of sterile NGB or CWE in 250-ml Erlenmeyer flasks inoculated with 0.5 ml per flask of the appropriate ‘seed inoculum’. Unless otherwise indicated, flasks were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). When stated, viable bacterial cell production was determined using the dilution plate count method. Serial dilutions (10×) were performed in dilution tubes containing 9 ml of sterile 0.01 M phosphate buffer, pH 7, and 100 μl of three appropriate dilutions were spread plated on nutrient glucose agar (NGA). Plates were incubated under ambient laboratory conditions for about 4 days. Colony forming units (CFU) per ml was determined by counting colonies on plates with 10–200 CFU per plate.
Growth Curve of the Bacterium in Four Different Liquid Culture Media
Isolate CW00B006C grew well on all four media tested (
Since the death phase were not observed within 44 h culture time, an extension of culture time to 64 h was used to culture CW00B006C in NGB and TSD. Results demonstrated that the growth curve of CW00B006C in both media follows a similar pattern. The death phase began at 56 h (
Effect of Medium pH on Bacterial Cell Production Medium pH did not significantly affect the growth curve of isolate CW00B006C, or the bacterial cell production. A slight decrease in cell production was observed with an increase in pH. The highest number of bacterial cells, 9.2×1010 CFU/ml, was produced at a pH of 6 while the lowest number of bacterial cells, 2.9×1010CFU/ml, was produced at a pH of 8.
Effect of Growth Medium Modification with Chickweed Extracts
Since the addition of chickweed extract in culture medium enhanced the efficacy of B. andropogonis for the control of chickweed, the bacterial cell production was assessed in the culture medium modified with the chickweed extract (
Host Range
Host specificity of a biocontrol agent is an important factor to consider in a biological weed control program (Watson, 1985). Before using a biological control agent in the field, its host specificity to non-target economic and wild plants must be characterized (Wapshere, 1974).
Various plant species were reported as hosts of B. andropogonis. However, cross inoculation studies demonstrated variations in the host reactions of isolates from a particular host genus or species, indicating a degree of host specialization associated with B. andropogonis strains (Moffett et al., 1986). According to Moffett et al. (1986), common hosts of B. andropogonis include sorghum (Sorghum bicolor) and corn (Zea mays) of the family Poaceae, common vetch (Vicia sativa) of the family Fabaceae, and carnation (Dianthus caryophyllus) and baby's breath (Gypsophila paniculata) of the family Caryophyllaceae. Other possible hosts, depending upon the strain, included sudan grass (Sorghum sudanens) of the family Poaceae, chick pea (Cicer arietinum), velvet bean (Mucuna deeringiana), white clover (Trifolium repens), and red clover (Trifolium pratense) of the family Fabaceae, blueberry (Vaccinium sp.) of the family Rubiaceae, and tulip (Tulip sylvestris) of the family Liliaceae (Burkholder, 1957; Allen et al., 1970; Goto and Starr, 1971; Hayward, 1972; Nishiyama et al., 1979; Caruso, 1984).
Burkholderia andropogonis has never been reported as a crop pathogen in Canada (Howard et al., 1994; Martens et al., 1994). In this example, the host range of this strain of B. andropogonis was determined based on the phylogenetic method and the potential risks involved in its use was estimated as a biological control agent of common chickweed in western Canada.
The Organisms
The Target Weeds: Common Chickweed
Common chickweed is native to Europe (Hohm et al., 1977; Turkinton et al., 1980). It is believed to be one of the 12 most successful colonizing species among the noncultivated plants and is reported to be a weed in more than 20 crops in 50 countries (Holm et al., 1977). It is an annual or winter annual, spreading by seed and the stems rooting at the nodes. Stems are slender, round, branching, usually prostrate and spreading with lines of fine hairs and swollen nodes at the leaf base. Leaves are in pairs on opposite sides of the stem at each node, the lower and middle leaves are stalked and the stalk often has a fringe of hairs along each edge, upper leaves are stalkless and are oval with pointed tips. Flowers are located in the axils of the leaves, about 6 mm across with five sepals and five white petals, flowering throughout the growing season and maturing seed rapidly after flowering. Seeds are reddish brown to tan colored, about 1 mm across, and covered with regular rows of small bumps. Common chickweed grows best on moist, heavy, nitrogen rich soils at optimum, constant temperature of 12–20° C. (Turkinton et al., 1980). In Canada, common chickweed can germinate throughout the growing season but has two main flushes: one is early spring while the other is in late fall.
Common chickweed belongs to the genus Stellaria (Table 6), subfamily Alisnoideae, family Caryophyllaceae, order Caryophyllales (Cronquist 1981). The family Caryophyllaceae consists of three subfamilies including Alsinoideae, Paronychioideae, and Silenoideae containing 13, 12, and 14 genera, respectively (Cronquist, 1981; Jalas and Suominen, 1987). The most familiar species in the subfamily Alsinoideae include the weeds such as common chickweed, mouse-eared chickweed (Cerastium vulgatum L.), and Knawel weed (Scleranthus annus L.). Subfamily Paronychioideae contains the weed Corn spurry (Spergula arvensis L). Subfamily Silenoideae contains a number of garden ornamentals, such as carnation (Dianthus caryophyllus L.) and baby's breath (Gypsophila sp), as well as weeds such as Gypsophila paniculata L., white cockle (Lychynis alba Mill.), cow cockle (Saponaria vaccaria L.), bladder Campion (Silene cucubalis Wibel), and night-flowering catchfly (Silene noctiflora L.).
The Order Caryophyllales has 11 families with major economic importance as both garden ornamentals and notable weeds (Bateman, 1985; Jalas and Suominen, 1987a; 1987b). Considering the phenogenetic relationship to common chickweed, important plants in families other than Cayophyllaceae family may include weeds, such as Chenopodium spp., Amaranthus spp., common purslane (Portulaca oleracea L.), and Russian thistle (Salsola kali L. var. tenuifolia Tausch), as well as vegetables, such as beet (Beta vulgaris L.) and spinach (Spinacea oleracea L.). Plants native to Canada in all families of the order Caryophyllales grow exclusively in non-cultivated areas (Scoggan, 1978). Few are common in agricultural fields, home gardens, or golf courses (Table 7).
Bougainvillea and
Mirabilis cultivated as
aThe phylogenetic relationship and other data was based on Cronqiust (1981) and Bateman (1985)
The Biocontrol Agent: Burkholderia Andropogonis
Burkholderia andropogonis (Smith 1911) Gillis et al. 1995, comb. nov. belongs to the family Burkholderiaceae, order Burkholderiales, class Betaproteobacteria, Phylum Proteobacteria (Boone et al., 2001). Several synonyms exist as Pseudomonas andropogonis (Smith 1911) Stapp 1928, Bacterium andropogon (sic) Smith 1911, Aplanobacter stizolobii Wolf 1920, Pseudomonas stizolobii (Wolf 1920) Stapp 1935 (Gillis, 1995; Viallard et al., 1998). B. andropogonis appears in Group A (isolated from diseased plants and nonfluorescent), Section V of Pseudomonas in Bergey's Manual (Palleroni, 1984). B. andropogonis has been reported as pathogenic to sorghum, corn, clover, velvet bean, and carnation. Eight other bacterial species in the same group (Group A) were reported as pathogenic to the bird's-nest fern (Asplenium nidus), pawpaw, cultivated mushroom, almond tree (Prunus dulcis), oats (Avena sativa), foxtail (Chaetochloa lutescens), Ciccus japonica, Cattleya sp., Phalaenopsis sp., tomato, rice, and sugarcane.
Inoculum Preparation
Stock cultures of B. andropogonis were stored in 15% glycerol at −80° C. For each experiment, a cryovial of stock culture was warmed to room temperature in a 36'C. water bath. A 50 gl aliquot of suspension from the vial was added to each 18×150 mm glass test tube containing 3 ml nutrient glucose broth (NGB) at a pH of 6.8. The test tubes were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes were used as ‘seed inoculum’. Unless otherwise stated, the bacterial culture for inoculations was produced by placing 75 ml of NGB (pH 6.8) in each 250-ml Erlenmeyer flasks, autoclaving, cooling, inoculating with 1 ml per flask of the ‘seed inoculum’, and incubating flasks on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). When stated, viable bacterial cell production was determined using the dilution plate count method. Serial dilutions (10×) were performed in dilution tubes containing 9 ml of sterile 0.01 M phosphate buffer, pH 7, and 100 μl of three appropriate dilutions were spread plated on nutrient glucose agar (NGA). Plates were incubated under ambient laboratory conditions for about 4 days. Colony forming units (CFU) per ml was determined by counting colonies on plates with 10–200 CFU per plate.
Plant Production
Using the modified centrifugal phylogenetic and varietal strategy (Wapshere, 1974), 36 plant species in 30 genera and 8 families were selected for the host range trial (Table 6). Test plants were grown from seed or propagated vegetatively depending on the species being tested and availability of material. Plants were planted in 10-cm diameter peat pots containing steam-pasteurized soil mix. The soil mix consisted of 140 L soil (loam, pH 6.7, 42% sand, 40% silt, and 18% clay), 107 L sand, 160 L Sunshine Growing Mix (SunGro Horticulture, Bellevue, Wash.), 62 L Fibrous Blond Shagnum Peat Moss (Premier Pro Moss, Riviere du-Loup, Quebec), 113 L vermiculite (Therm-O-Rock, Chandler, Ariz.), 230 g dolomite lime, and 150 g Super Phosphite (0-45-0) per batch of soil. Seeded pots were placed in a greenhouse with 23/20±4° C. day/night temperature, a 16 h photoperiod, an average light intensity of 300 iEni2s″1, and an average relative humidity of 45–50%. After germination, seedlings were thinned to five plants per pot.
Treatments
For each species, two sets of test plants (each set containing 15 plants) were prepared. One set was inoculated with B. andropogonis and the other set served as uninoculated controls. Seedlings of selected plant species at the 2- to 3-leaf stage were arbitrarily selected. For the inoculated treatment, seedlings were sprayed with the bacterial culture suspension at 109–1010 CFU/ml bacterial cells plus 0.15% Silwet L-77®, using an H-set airbrush (Paasche Airbrush Company, Harwood Heights, Ill.) at a pressure of 100 kPa. The application volume of the culture suspension was 5 ml/pot. For uninoculated control, seedlings were sprayed with 0.15% Silwet L-77®. Immediately after spraying, pots were returned to the greenhouse and placed in a randomized block design for the remainder of the experiment.
Measurements
Disease severity was visually assessed daily until the 14th day after inoculation by using the 0–11 rating scale by Horsfall and Barrett (1945). Chickweed seedlings at the 4–6 leaf growth stage were also inoculated with the same inoculum and served as a control. The experiment was conducted as a factorial experiment with species tested and inoculation levels (0 and 109–1010 CFU ml−1) as factors. Each treatment was replicated three times.
Burkholderia andropogonis has never been reported as a pathogenic bacterium causing crop diseases in Canada (Howard et al., 1994; Martens et al., 1994). To our knowledge, this is first description of B. andropogonis as a plant pathogen in Canada (Howard et al., 1994; Martens et al., 1994). Host specificity test for 36 plant species in 30 genera and 8 families demonstrated that the CW00B006C strain of B. andropogonis is not pathogenic to the majority of major economically important crops cultivated in western Canada. Table 8 shows the plant species with disease ratings greater than 0.
Burkholderia andropogonis
aAll other species remained uninfected, severity ratings were 0.
bSeedlings of plants at the 2- to 3-leaf stage were inoculated with the bacterial culture suspension at 109–1010 CFU/ml bacterial cells plus 0.15% Silwet L-77 ®. The application volume of the culture suspension was 5 ml/pot. Chickweed seedlings at the 4- to 6-leavestage were also inoculated with the same inoculum and served as a control. Disease severity was visually assessed daily until the 14th day after inoculation by using the 0–11 rating scale by Horsfall and Barrett (1945).
Findings in the study demonstrated that hosts of our B. andropogonis strain isolated from common chickweed were distributed in the families Caryophyllaceae, Poaceae, and Fabaceae. No infection on clover confirmed that the bacterial isolate of the present invention possesses host specialization, supporting the conclusions reported by Moffett et al. (1996). Hosts in the Caryophyllaceae family of our B. andropogonis strain are relatively wide (Table 7). All plant species in the Silenoideae subfamily was diseased by the inoculation of B. andropogonis. In the Alsinoideae subfamily, all seedlings of common chickweed serving as positive controls were severely diseased. However, the other plant species in the same subfamily, corn spurry (Spergula arvensis), was not affected by the inoculation of B. andropogonis. In the Paronychioideae subfamily, the selected plant species, Knawel weed (Scleranthus annus) was not affected by the inoculation of B. andropogonis. Results demonstrated that this bacterium also causes disease at various levels on six other weed species common to western Canada in this family, expanding B. andropogonis' weed control spectrum as a bioherbicide.
In the Poaceae family, three plant species including corn (Zea Mays), sorghum (Sorghum bicolor), and sudan grass (Sorghum sudanens (Piper) Stapf.) were reported as hosts of B. andropogonis in the United States (Smith & Hedge, 1905; Ullstrup, 1960). Our results demonstrated that although a water-soaked lesion was observed on two of nine corn seedlings treated with B. andropogonis, disease ratings on bacterial treated corn seedlings were similar to that on untreated corn seedlings two weeks after inoculation. Three weeks after inoculation, no visible difference was observed between treated and untreated corn seedlings. Ullstrup (1960) reported that B. andropogonis caused bacterial stripe of a few very susceptible inbred corn lines and their progenies but not on commercial field corn in the United States. Our results confirmed that our selected commercial corn variety was not significantly susceptible to B. andropogonis to allow development of the typical bacterial stripe symptoms.
In the Fabaceae family, hosts of our B. andropogonis strain were restricted to Tribe Vicieae among 50 tribes (Hutchinson, 1967). Susceptible species in this Tribe include common vetch and chickpea. However, another economically important crop in this Tribe, lentil (Lens culinaris), was unaffected by this bacterium. Other major crops in this family were also unaffected by this bacterium.
Although B. andropogonis has a relatively wide plant host range, wounds plus infiltration are usually pre-requirements for the infections (Moffett et al., 1986). The absence of these prerequisites in the natural Canadian environment may explain why B. andropogonis has never been reported as a plant pathogen. Bioherbicide preparation of B. andropogonis contains a special surfactant, Silwet L-77®, that allows infiltration of bacterial cells into plant tissues without the presence of wounds (Zidack et al., 1992). Therefore, application of B. andropogonis as a bioherbicide will cause disease only on directly sprayed target weed species but not on other susceptible plant species under natural conditions. Since plants native to Canada in all families of the order Caryophyllales grow exclusively in non-cultivated areas (Scoggan, 1978) and few are common in agricultural field, home gardens, or golf courses (Table 7). Therefore, application of B. andropogonis as a bioherbicide will not cause disease on native plant species under natural conditions.
In conclusion, the use of B. andropogonis as a bioherbicide for control of common chickweed will not cause major concerns to crops and native flora in Canada.
Efficacy
After discovery, the key considerations in the decision to develop bioherbicides are efficacy and safety (Watson and Wymore, 1990). Weed control of the candidate agent should be assessed by the speed, amount, and ease of control. However, efficacy should not be determined by weed mortality alone. It has been reported that crop yield or economic threshold may be the most suitable index of bioherbicides because the non-complete eradication of weed populations can cause the significant increases in crop yields. In this example, the effectiveness of B. andropognis for the control of chickweed is quantified.
Efficacy under Greenhouse Conditions
Inoculum Preparation.
For all experiments, nutrient glucose broth (NGB) at a pH of 6.8 and/or chickweed extract medium (CWE) was used. To prepare chickweed extract medium, 3–5 wk old chickweed foliage was harvested from the greenhouse and stored at −20° C. until use. Using liquid nitrogen and a pestle and mortar, tissue was crushed to a powder, and combined with an equal amount of distilled water (w/v) to form thick slurry.
The slurry was filtered through 2-ply cheesecloth, filtrate was centrifuged at 18500 rcf (Eppendorf 581OR centrifuge) for 15 minutes, supernatant was filter sterilized using a 0.22 pm bottle-top vacuum filter, and resulting sterile CWE medium was stored at 4° C. until use. Stock cultures of isolate CW00B006C were stored in 15% glycerol at −80° C. For each experiment, a cryovial of stock culture was warmed to room temperature in a 36° C. water bath. A 50 gl aliquot of suspension from the vial was added to each 18×150 mm glass test tube containing 3 ml of appropriate medium. The test tubes of were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes were used as ‘seed inoculum’. The bacterial culture for inoculations was produced using 75 ml of sterile NGB or CWE in 250-ml Erlenmeyer flasks inoculated with 0.5 ml per flask of the appropriate ‘seed inoculum’. Unless otherwise indicated, flasks were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). When stated, viable bacterial cell production was determined using the dilution plate count method. Serial dilutions (10×) were performed in dilution tubes containing 9 ml of sterile 0.01 M phosphate buffer, pH 7, and 100 μl of three appropriate dilutions were spread plated on nutrient glucose agar (NGA). Plates were incubated under ambient laboratory conditions for about 4 days. Colony forming units (CFU) per ml was determined by counting colonies on plates with 10–200 CFU per plate.
Plant Production.
Chickweed seeds (7–10 seeds per pot) were sown in 10-cm diameter peat pots containing Sunshine Growing Mix (#1 SunGro Horticulture Canada Ltd., Seba Beach, Alberta, Canada). Seeded pots were placed in a greenhouse with 23/20±4° C. day/night temperature, a 16 h photoperiod, an average light intensity of 300 μEm−2s−1, and an average relative humidity of 45–50%. After germination, seedlings were thinned to five plants per pot.
Inoculation.
Unless otherwise stated, three replicate pots containing chickweed seedlings at the 4–6-leaf stage were inoculated with culture suspension using an H-set airbrush (Paasche Airbrush Company, Harwood Heights, Ill.) at a pressure of 100 kPa. Unless otherwise stated, the culture suspension consisted of 109–1010 CFU/ml bacterial. The application volume of the culture suspension was 5 ml/pot. Immediately after spraying, pots were returned to the greenhouse and placed in a randomized block design for the remainder of the experiment. Disease severity was assessed using the 0–11 rating scale by Horsfall and Barrett (1945) one, two or three wks after spraying.
Effect of Bacterial Concentration on Disease Severity.
Culture was produced using NGB as described. Culture contents of flasks were centrifuged at 2440 rcf for 10 min to form a bacterial cell pellet (Sorvall RC-5B refrigerated superspeed centrifuge). The supernatant was decanted and the bacterial cell pellet from 60 ml of culture was resuspended in 30 ml of 0.01 M, pH 7, sterile phosphate buffer amended with 0.2% Silwet L-77®. The resulting spray solution was assumed to have a concentration of 1010 cells per ml and subsequent serial dilutions of the suspension with buffer and 0.2% Silwet L-77® were made to achieve spray inoculum with cell concentrations of about 109, 108 107, 106 and 105 cells per ml. The viable bacterial concentration of each spray solution was then determined. The actual viable bacterial concentrations of each spray solution were very close to desired concentrations at 0.98×1010, 1.8×109, 1.0×108, 1.8×107, 1.6×106, and 1.6×105 CFU/ml. Chickweed seedlings were inoculated with each spray solution. Control plants were inoculated with sterile 0.01 M phosphate buffer plus 0.2% Silwet L-77®. Efficacy of each solution was determined as described.
Effect of Spray Solution pH on Disease Severity.
Bacterial inoculum was produced using NGB as described. Culture contents of each flask were centrifuged at 2440 rcf for 10 min to form a bacterial cell pellet (Eppendorf 5810R centrifuge). Supernatant was decanted and the bacterial cell pellet from 40 ml of culture was resuspended in 20 ml of 0.1 M sterile phosphate buffer containing 0.2% Silwet L-77® with pHs from 6 to 8 at increments of 0.2. Chickweed seedlings were inoculated with each spray solution having different pH. Control plants were inoculated with sterile 0.1 M phosphate buffer of the appropriate pH plus 0.2% Silwet L-77®. Efficacy of each solution was determined as described.
Effect of Surfactant Type and Concentration on Disease Severity.
Inoculum was produced using NGB as described. Five organosilicone-based surfactants (Loveland Industries, Inc., Greeley, Colo.) were selected including Silwet L-77® (silicone-polyether copolymer 100%), Freeway (silicone-polyether copolymer and alcohol ethoxylates 100%), Phase (methylated esters of fatty acids and organosilicone surfactant fluids 100%), Tactic (synthetic latex plus organosilicone surfactant 62.5% and inerts 37.5%), and motion (polymethylsiloxane-copolymer and non-ionic surfactant 100%). Concentrations of 0.1, 0.2, and 0.3% were included for Silwet L-77® and Intac while surfactant concentrations of 0.1, 0.2, 0.3, and 0.5% were included for remaining three surfactants. Surfactants were added to bacterial inoculum just prior to spraying. Efficacy of different surfactants was determined as described with a treatment consisting of NGB and surfactant serving as a control for each bacterial treatment.
Effect of Repeat Application on Disease Severity.
Inoculum was produced using NGB as described. Replicate pots containing chickweed seedlings at the 4–6-leaf stage were initially inoculated with bacterial solution produced as described and amended with 0.2% Silwet L-77® just prior to spraying. Immediately after spraying, pots were returned to the greenhouse and placed in a randomized block design. Inoculated common chickweed seedlings were then resprayed with bacterial inoculum, produced as described and amended with 0.2% Silwet L-77 just prior to spraying, 2, 6, and 9 days after initial application. One replicate set of pots with no repeat spray application was maintained. Control treatments with NGB and surfactant were included for all bacterial treatments. Disease severity was assessed beginning 1 wk after the initial bacterial application.
Effect of Bacterial Growth Medium and Inoculum Growth Stage on Efficacy.
Inoculum was produced using both CWE and NGB as described with incubation of bacterial inoculum in each medium for 24, 48 or 72 hours. Inoculation of flasks was staggered at 24 h intervals such that all inoculum was ready at the same time. Viable bacterial cell concentration of each treatment was determined as described prior to amendment with 0.15% Silwet L-77® for spray application. Control treatments consisting of sterile medium and wetting agent were included for both NGB and CWE. Disease severity was assessed as described.
Effect of Bacterial Growth Stage and Chickweed Growth Stage on Disease Severity.
Inoculum was produced using CWE as described. Two trials to study the impact of chickweed growth stage on disease severity were conducted. For trial 1, flasks of bacterial inoculum were incubated 24 hours as described. For trial 2, flasks of bacterial inoculum were incubated either 24 or 48 hrs prior to determination of viable bacterial cell concentration as described and amendment with 0.15% Silwet L-77® for spraying. Replicate pots of chickweed seeded and grown under greenhouse conditions as described for one (cotyledon growth stage), two (2–4 leaf stage), three (6–8 leaf stage) or four weeks were inoculated with bacterial or control solution. Spray solution for control treatments consisted of tap water and 0.15% Silwet L-77 for trial 1 and uninoculated CWE and 0.15% Silwet L-77 for trial 2. Disease severity for both trials was assessed as described.
Infectivity and Disease Severity on Group 2 Herbicide Resistant Chickweed.
Inoculum was produced using NGB as described. Group 2 herbicide resistant chickweed seeds were obtained from the ARC Weeds Research Laboratory. Previous tests indicate that the average survival rate of the herbicide resistant seed set with 1× and 2× Ally (metsulfuron methyl) application was 73.5 and 35.2%, respectively (O'Donovan, et al., 1994). The chickweed seed set used for all other experiments was included as a herbicide susceptible positive control. Both herbicide resistant and susceptible seedlings were produced as described and seedlings of both seed sets at the 6–8 leaf stage were inoculated with the bacterial inoculum using the previously described method. Differences in infectivity and disease severity between the herbicide resistant and the herbicide susceptible populations were compared as described.
Efficacy under Field Conditions
Experiments Conducted in Outside Pots.
Efficacy of B. andropogonis for the control of chickweed under natural conditions was first evaluated in large pots set outside. Two experiments were conducted, one in 2001 (July 24–September 14) and the second in 2002 (May 30–July 31). Results from the two experiments were not pooled because the variances were not homogeneous according to Bartlett's test (Gomez & Gomez, 1984). However, a similar trend was observed and the results from 2001 are presented.
For both experiments, chickweed plants were produced in 53-cm-diameter fiber pots filled with soil collected from a field at the ARC, Vegreville experimental site (loam, pH 6.7, 42% sand, 40% silt, and 18% clay) and placed outside on the ground where building walls would provide some shade. Chickweed seeds were sown in the pots 3–4 weeks prior to spraying by sprinkling seeds evenly over the soil surface, then lightly sprinkling soil over the seeds. Pots were watered as necessary using an outside tap throughout the experiment. Bacterial inoculum was prepared as described using NGB and 24 h incubation of flasks for experiment 1 and CWE and 48 h of incubation for experiment 2. Inoculum at a cell concentration of 3.0×109 CFU/ml for experiment 1 and 5.0×109 CFU/ml for experiment 2, with a surfactant concentration of 0.15% Silwet L-778 for both experiments was used. A control treatment with NGB and wetting agent was included for experiment 1, while a control with tap water and wetting agent was included for experiment 2. Chickweed plants were sprayed at approximately the 6-leaf stage. For experiment 1, pots with chickweed seedlings at the 6-leaf stage were brought inside and placed, 3 replicate pots at a time, in the spray chamber. Each pot was sprayed with 100 ml of appropriate solution using an airbrush at 100 kPa. After spraying, pots were immediately placed back outside. For experiment 2, a 2L Spray-Doc compressed air sprayer (Gilmour Manufacturing Co., Somerset, Pa., USA) was used to spray about 115 ml per pot without moving them from their outside location. Plants were assessed for symptoms using the 0–11 rating scale by Horsfall and Barrett (1945) when they became most apparent, about 2 weeks after spraying. Daily weather conditions during the experiments were recorded including maximum and minimum temperatures (° C.), precipitation (mm), sunlight period (h); and average radiation (watts/m2). Dry weight per pot was determined 4 weeks after spraying by cutting aerial parts at the soil level, drying in a paper bag for 48 h at 70° C., and weighing. The dry weight (DW) data were expressed as percent reduction in biomass compared with biomass of controls and calculated using the formula: Dry weight reduction (%)=(DW in control−DW in inoculated treatment)/DW in control×100. The experiments were arranged in a completely randomized design. Data was analyzed by SAS ANOVA.
Experiments Conducted in Field Plots.
Efficacy of CW00B006C for the control of chickweed at different growth stages under natural conditions was further evaluated in field plots. Two field trials were conducted at the ARC Vegreville experimental site in 2002: trial one was conducted from May 27–July 29 and trial 2 from June 24–August 30. Results from the two trials were not pooled because the variances were not homogeneous according to Bartlett's test (Gomez & Gomez, 1984). However, a similar trend was observed between trials and the results from trial 1 are presented.
A mini-plot (0.5 m×1.0 m) design with a 0.5 m boarder of bare earth surrounding each split plot was used (
Efficacy under Greenhouse Conditions
Effect of Bacterial Concentration on Disease Severity.
Disease was observed on chickweed seedlings with all bacterial concentration treatments two weeks after spraying. Disease severity significantly increased when the bacterial concentration was increased (
Effect of Spray Solution pH.
No damage was observed on control plants. Spray solution pH did not affect weed control efficacy in the pH range of 6–8.
Effect of Surfactant Type and Concentration on Disease Severity.
Effectiveness of chickweed control with B. andropogonis was affected by surfactant type (P>0.001), concentration (P>0.003), and their interaction (P>0.001). Of the five surfactants studied, Silwet L-77® significantly enhanced the bacterial efficacy for the control of chickweed while the remaining surfactants did not show any efficacy enhancement (
Effect of Repeat Application on Disease Severity.
Repeat application of the bacterial inoculum enhanced disease severity on chickweed seedlings caused by B. andropogonis, but this enhancement varied with the time interval of the second application (
Effect of Bacterial Growth Medium and Inoculum Growth Stage on Disease Severity.
Disease severity was significantly affected by the addition of chickweed extract to the culture medium (P>0.0071) and the inoculum growth stage (P>0.0176) (
Effect of Inoculum Growth Stage and Chickweed Growth Stage on Disease Severity.
Chickweed seedlings at different growth stages responded differently to the fungus B. andropogonis. Disease severity was increasing with older chickweed growth stage (
Inoculum growth stage significantly affected disease severity in chickweed seedlings caused by B. andropogonis (
Infectivity and Disease Severity on Group 2 Herbicide Resistant Chickweed.
Herbicide resistant seedlings were also susceptible to the bacterial treatment. There was no difference in disease severity between the herbicide resistant and the herbicide susceptible seedlings when treated with the same bacterial inoculum. Therefore, the use of B. andropogonis provides a new approach to combat Group 2 herbicide resistance in chickweed and may mitigate the development of herbicide resistance.
Efficacy under Field Conditions.
Experiments Conducted in Outside Pots.
Severe disease was observed on chickweed seedlings 1 wk after bacterial treatment. Two wks after inoculation, disease severity was rated as 75% (
Experiments Conducted in Field Plots.
Severe disease was observed on chickweed seedlings 1 wk after bacterial treatment. Disease incidence reached 100% (
Production of two amino acids, ‘rhizobitoxine and hydroxyreonine, has been considered as a general feature of Burkholderia andropogonis (Mitchell and Frey, 1988). However, potential for the use of these two amino acids to control weeds have not been extensively evaluated. Recently, several studies have demonstrated that amino acids and/or peptides can be used as biologically based herbicides (Duke et al., 2002). For example, five dipeptides from corn gluten hydrolysate can control both grasses and broadleaf weeds, with alanylalanine being the most active (Liu and Christians, 1994). In the following example it was determined whether the natural products produced by Burkholderia andropogonis have any herbicidal activity against chickweed.
Activity of Fermentation Broth of Burkholderia andropogonis in Chickweed Control
Materials and Methods
A cryovial of stock culture was warmed to room temperature in a 36° C. water bath. Streak plates were then made on King's medium B agar Petri plates (KB; 20 g proteose peptone #3 (Difco), 1.5 g. K2HPO4, 1.5 g MgSO4. 7H2O, 15 ml glycerol, 15 g Bacto Agar (Difco), 1 L distilled water) using the stock culture. After 24 h incubation under ambient laboratory conditions on KB agar, a loopfull of cells was transferred to each 18×150 mm glass test tube containing 3 ml yeast-glucose broth (YGB; 5 g yeast extract (Difco), 5 g glucose, 0.3 g MgSO4, 3.9 g K2HPO4, 3.8 g KH2PO4, 1 L distilled water) at a pH of 6.8. The test tubes were incubated on an orbit shaker at 200 rpm for 24 h under ambient laboratory conditions (24° C.±3). Contents of tubes were combined as ‘seed inoculum’ and 0.5 ml of the ‘seed inoculum’ was used to inoculate each 500 ml flask containing 150 ml of filter sterilized, pH 6.8, Hoitink & Sinden chemically defined medium (HS; 10 g glucose, 3.6 g K2HPO4, 4.1 KH2PO4, 0.2 g MgSO4, 1 g NH4C1, 1 mg biotin, 1 L distilled water). Flasks were incubated on an orbit shaker at 200 rpm for 6 d under ambient laboratory conditions (24° C.±3). Contents of flasks were combined and viable bacterial cell production was determined using the dilution plate count method as previously described. Culture was centrifuged 10 minutes at 2440 rcf (Eppendorf 5810R centrifuge), supernatant was poured from each tube and passed through a 0.22 gm bottle-top vacuum filter to remove all bacterial cells.
Heat Stability of the Cell-Free Culture Filtrates
A sample of cell-free filtrate was autoclaved 30 minutes at 121° C. to determine the effect of heat on the bacterial phytotoxins.
Effect of the Cell-Free Culture Filtrates on Chickweed Root Growth
A sample of both the autoclaved and unautoclaved cell-free filtrate was used for a laboratory assay designed to study the effects of the bacterial phytotoxins on chickweed seed germination and seedling root growth. A Whatman #1 filter paper was placed in replicate glass Petri Plates. Solutions of autoclaved and unautoclaved cell-free filtrates at 100, 50, 25, 12.5 and 6.25% of the initial concentration were prepared using sterile distilled water. A control of sterile, distilled water was also prepared. A 5 ml aliquot of the appropriate treatment was pipetted into each of three replicate Petri plates. Using forceps, four chickweed seeds, as used for all other experiments, were placed in each plate. Plates were placed on the laboratory bench under ambient laboratory conditions (21±2° C.) for incubation and arranged in a randomized block design. Seed germination, seedling root length (mm), and seedling appearance were assessed after 1 wk.
Diseases Caused by the Cell-Free Culture Filtrates
Remaining autoclaved and unautoclaved cell-free filtrate was then shell frozen in Fast-Freeze Flasks (Labconco), freeze-dried, and used to inoculate chickweed seedlings in the greenhouse. Once dry, samples were stored at −80° C. until use. Freeze-dried samples were resuspended in distilled water at a concentration of 10 times that of the original by gentle shaking. For the concentrated, unautoclaved cell-free filtrate, one 15 ml sample was amended to contain 0.1% Silwet L-77®, a second to contain 0.2% Silwet L-77®, a third was amended to contain 0.5% Tween 80 as a wetting agent and a fourth sample was left unamended. The autoclaved sample was amended to contain 0.2% Silwet L-77®. Control treatments with freeze-dried HS medium and wetting agent were also prepared. Three replicate pots containing chickweed seedlings at the 6–8-leaf stage were inoculated for each treatment using an H-set airbrush (Paasche Airbrush Company, Harwood Heights, Ill.) at a pressure of 100 kPa. Plants were grown as previously described. Immediately after spraying, pots were returned to the greenhouse and placed in a randomized block design for the remainder of the experiment. Plants were monitored for any symptoms for 3 wks following spraying
Cell free culture filtrates from HS medium did not affect chickweed seed germination. However, they caused completely chlorotic chickweed seedlings and significantly inhibited root growth (
In the greenhouse studies, approximately 4–6 days after spraying, slight to moderate chlorosis was visible on new leaves of chickweed sprayed with all autoclaved and unautoclaved cell-free filtrates amended with Tween-80 and Silwet L-77®. There was no difference in disease severity on chickweed seedlings between heat treatments, confirming the natural products produced by CW00B006C was heat stable. New leaves on plants showing symptoms were pale yellow or white. No symptoms were seen on chickweed sprayed with cell-free filtrate without wetting agent or on control treatments. Thus, bacterial natural products can be used as a biologically based herbicide for the control of chickweed.
All citations are herein incorporated by reference.
The present invention has been described with regard to preferred embodiments. However, it will be obvious to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention described herein
| Number | Date | Country | Kind |
|---|---|---|---|
| 2421373 | Mar 2003 | CA | national |
| Number | Date | Country | |
|---|---|---|---|
| 20040254075 A1 | Dec 2004 | US |
