Progress 08/01/02 to 07/31/05
Outputs
Growth chamber and field studies evaluated survival and dissemination of Xanthomonas axonopodis pv. allii in association with weed, alternate host, and volunteer onion plants, irrigation water, and crop debris. Epiphytic X. axonopodis pv. allii was recovered from the foliage of nine asymptomatic weed species and Medicago sativa, but the bacterium was not recovered from plants in locations where an epidemic of Xanthomonas leaf blight did not occur the prior year. The bacterium also was isolated from volunteer onion with characteristic Xanthomonas leaf blight symptoms. A rifampicin mutant of X. axonopodis pv. allii was recovered consistently from the irrigation tail water of onion fields inoculated with the bacterium, as well as from infested onion leaves 9 months after they were placed on the soil surface or buried to a depth of 25 cm. Cultural practices that avoid or eliminate X. axonopodis pv. allii inoculum sources should reduce Xanthomonas leaf blight losses to
onion. Frequent applications of copper-based bactericides amended with an ethylenebisdithiocarbamate fungicide (e.g., maneb or mancozeb, class B2 carcinogens) provide some disease suppression. Applications of acibenzolar-S-methyl reduced in planta and epiphytic populations of X. axonopodis pv. allii as effectively as applications of copper hydroxide-mancozeb in growth chamber studies. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced severity of Xanthomonas leaf blight as or more effectively than 9 to 12 weekly applications of copper hydroxide or copper hydroxide-mancozeb. Application of a commercial formulation of both Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 reduced severity of Xanthomonas leaf blight in field experiments. X. axonopodis pv. allii is phenotypically and genetically diverse and its relationship to other X. axonopodis pathovars within DNA homology group 9.2 is unknown. In growth chamber experiments, disease
symptoms were produced on onion only by inoculation with X. axonopodis pv. allii. X. axonopodis pv. allii multiplication and persistence in Duncan grapefruit were equal to those of an aggressive strain of X. axonopodis pv. citrumelo, but populations of X. axonopodis pvs. alfalfae, betlicola, citrumelo, phaseoli, and vesicatoria were 1.3 to 4.0 log units less than X. axonopodis pv. allii in onion. Genomic fingerprinting by repetitive sequence-based polymerase chain reaction demonstrated that X. axonopodis pvs. allii, alfalfae, and citrumelo are distinct from other Xanthomonas species and X. axonopodis pathovars, but these pathovars were indistinguishable from each other. Three genotype groups were apparent among DNA homology group 9.2 strains, and generally correspond to the aggressiveness and genotype groups previously described for X. axonopodis pv. citrumelo. X. axonopodis pvs. allii, alfalfae, and citrumelo appear to have recently diverged from a common ancestral strain.
Impacts
Xanthomonas leaf blight, caused by the bacterium X. axonopodis pv. allii, is an emerging disease of onion in the western United States and worldwide, but few management strategies have been developed because little is known about disease epidemiology and pathogen survival. We documented that bacterial pathogens can survive epiphytically on a variety of plant species and impact future disease epidemics on susceptible hosts. Cultural practices such as crop rotation (time between susceptible crops, crop components and their position in the rotation cycle), planting clean seed and transplants, sanitation of previous crop debris, and management of other plant sources such as weeds that could harbor populations of epiphytic bacteria are all important tools that need to be included in integrated pest management strategies. Disease forecasting can be a valuable tool to monitor crop and pathogen development in relation to integrated pest management strategies which incorporate
timely applications of conventional pesticides such as copper-based bactericides. Recent work with promising biopesticides (systemic acquired resistance inducer and a viral pathogen of the bacterium) could provide comparable disease control with reduced applications (2 - 4 fewer sprays per season) and pesticide exposure (25 to 75 percent reduction in conventional bactericides) to the environment. The economic impacts of these alternative treatments have not been determined yet, but we anticipate that final grower costs will not increase.
Publications
- Gent, D. H., Lang, J. M., and Schwartz, H. F. 2005. Epiphytic survival of Xanthomonas axonopodis pv. allii and X. axonopodis pv. phaseoli on leguminous hosts and onion. Plant Disease 89:558-564.
- Gent, D. H., Schwartz, H. F., Ishimaru, C. A., Louws, F. J., Cramer, R. A., and Lawrence, C. B. 2004. Polyphasic characterization of Xanthomonas strains from onion. Phytopathology 94:184-195.
- Schwartz, H. F., and Gent, D. H. 2005. Xanthomonas Leaf Blight of Onion. CSU Coop. Ext. Fact Sheet 2.951. 3 pp.
- Schwartz, H. F., Otto, K. L., and Gent, D. H. 2003. Relation of temperature and rainfall to development of Xanthomonas and Pantoea leaf blights of onion in Colorado. Plant Disease 87:11-14
- Gent, D. H., and Schwartz, H. F. 2005. Management of Xanthomonas leaf blight of onion with a plant activator, biological control agents, and copper bactericides. Plant Disease 89:631-639.
- Gent, D. H., Al-Saadi, Gabriel, D. W., Louis, F. J., Ishimaru, C. A., and Schwartz, H. F. 2005. Pathogenic and genetic relatedness among Xanthomonas axonopodis pv. allii and other pathovars of X. axonopodis. Phytopathology 95:918-925..
- Gent, D. H., Lang, J. M., Bartolo, M. E., and Schwartz, H. F. 2005. Inoculum sources and survival of Xanthomonas axonopodis pv. allii in Colorado. Plant Dis. 89:507-514.
|
Progress 01/01/04 to 12/31/04
Outputs
Xanthomomas leaf blight, caused by the bacterium Xanthomonas axonopodis pv. allii, is an emerging disease of onion in the western U. S. and worldwide, but few management strategies have been developed because little is known about disease epidemiology and pathogen survival. Therefore, we sought to identify and quantify primary inoculum sources of the pathogen in Colorado. Growth chamber and field studies evaluated survival and dissemination of X. axonopodis pv. allii in association with weed, alternate host, and volunteer onion plants, irrigation water, and crop debris. Epiphytic X. axonopodis pv. allii was recovered from the foliage of nine asymptomatic weed species and Medicago sativa, but the bacterium was not recovered from plants in locations where an epidemic of Xanthomonas leaf blight did not occur the prior year. The bacterium also was isolated from volunteer onion with characteristic Xanthomonas leaf blight symptoms. A rifampicin mutant of X. axonopodis pv.
allii strain O177 was recovered consistently from the irrigation tail water of onion fields inoculated with the bacterium; populations as large as 30200 CFU/ml were recovered. X. axonopodis pv. allii was recovered from infested onion leaves nine months after they were placed on the soil surface or buried to a depth of 25 cm, but culturable populations of the pathogen decreased ten thousand to a million more in buried leaves. Cultural practices that avoid or eliminate X. axonopodis pv. allii inoculum sources should reduce Xanthomonas leaf blight losses to onion. Onion production systems that practice strict sanitation of weed and volunteer onion plants, follow a 2-year or longer rotation to nonhosts such as small grains, avoid reuse of irrigation tail water, and promote rapid breakdown of crop debris by deep tillage should reduce X. axonopodis pv. allii survival and minimize reliance upon copper bactericides for disease management. Acibenzolar-S-methyl reduced in planta and epiphytic
populations of X. axonopodis pv. allii equal to a copper hydroxide-mancozeb in growth chamber studies. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced Xanthomonas leaf blight severity equal or superior to conventional bactericides. Acibenzolar-S-methyl treatment did not improve bulb yield or grade as compared to conventional bactericide treatments, but yield was reduced 22 to 27% when 10 weekly applications were made in the absence of disease. Application of a commercial formulation of Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 reduced Xanthomonas leaf blight severity in field experiments. Copper hydroxide applications initiated one to two weeks before bulb initiation were as effective as those initiated three to four weeks before initiation, irrespective of maneb rate. Integration of acibenzolar-S-methyl and biological control agents with well-timed copper hydroxide applications can replace class B2 carcinogen use on onion
for Xanthomonas leaf blight management.
Impacts
Preliminary studies are documenting that bacterial pathogens can survive epiphytically on a variety of plant species and impact future disease epidemics on susceptible hosts. Cultural practices such as crop rotation (time between susceptible crops, crop components and their position in the rotation cycle), planting clean seed and transplants, sanitation of previous crop debris, and management of other plant sources such as weeds that could harbor populations of epiphytic bacteria are all important management tools that need to be included in integrated pest management strategies. Previous work has shown that disease forecasting can be a valuable tool to monitor crop and pathogen development in relation to integrated pest management strategies which incorporate timely applications of conventional pesticides such as copper-based bactericides. Recent work with promising biopesticides (systemic acquired resistance inducer and a viral pathogen of the bacterium) could
provide comparable disease control with reduced applications (2 - 4 fewer sprays per season) and pesticide exposure (25 to 75 percent reduction in conventional bactericides) to the environment. The economic impacts of these alternative treatments have not been determined yet, but we anticipate that final grower costs will not increase.
Publications
- Gent, D. H., Schwartz, H. F., Ishimaru, C. A., Louws, F. J., Cramer, R. A., and Lawrence, C. B. 2004. Polyphasic characterization of Xanthomonas strains from onion. Phytopathology 94:184-195.
- Gent, D. H., and Schwartz, H. F. 2004. Managing Xanthomonas leaf blight of onion with copper bactericides and acibenzolar. Phytopathol. 94:S34.
- Gent, D. H., Schwartz, H. F., and Ishimaru, C. A. 2004. Inoculum sources of Xanthomonas axonopodis pv. allii in Colorado. Phytopathol. 94:S33.
|
Progress 01/01/03 to 12/31/03
Outputs
Xanthomonas axonopodis pv. allii (Xaa) population dynamics on leaves of various pulse crops and weeds were monitored in growth chamber assays. Epiphytic populations of the spontaneous rifampcin mutant Xaa strain RO177 were recovered and enumerated by leaf rinsing and subsequent spiral plating onto rifampcin-amended media. Populations increased on all pulse crops evaluated and several weeds when maintained at 26 degrees Celsius and 100 percent relative humidity, but disease symptoms were never observed. In other studies, population dynamics of strain RO177 in pulse crops were monitored in planta. Strain RO177 was pressure infiltrated into leaves, and leaf discs were removed and homogenized before plating onto rifampcin amended nutrient agar. Populations of RO177 increased in planta in all pulse crops evaluated when infiltrated into leaves, but disease symptoms were absent. Field studies are currently evaluating survival of Xaa on pulse crops and weeds, and implications
for Xanthomonas leaf blight management. Xaa did survive on dry bean and lentil at 100 to 1000 cfu/ml. Epiphytic populations of Xap were recovered from onion leaves in the field as well. Disease symptoms caused by Xap on dry bean were more severe when onions preceded dry beans in the crop rotation. Field studies showed that high nitrogen fertilization increased bacterial pathogen activity and disease development. Pesticide studies with promising materials such as Acibenzolar (systemic acquired resistance inducer) and bacteriophage (bacterial virus biopesticide) alone or tank-mixed with ManKocide (cupric hydroxide) provided disease suppression or control equal to or superior to conventional programs with copper-based bactericides.
Impacts
Preliminary studies are documenting that bacterial pathogens can survive epiphytically on a variety of plant species and impact future disease epidemics on susceptible hosts. Cultural practices such as crop rotation (time between susceptible crops, crop components and their position in the rotation cycle), planting clean seed and transplants, sanitation of previous crop debris, and management of other plant sources such as weeds that could harbor populations of epiphytic bacteria are all important management tools that need to be included in integrated pest management strategies. Previous work has shown that disease forecasting can be a valuable tool to monitor crop and pathogen development in relation to integrated pest management strategies which incorporate timely applications of conventional pesticides such as copper-based bactericides. Recent work with promising biopesticides (systemic acquired resistance inducer and a viral pathogen of the bacterium) could
provide comparable disease control with reduced applications (2 - 4 fewer sprays per season) and pesticide exposure (25 to 75 percent reduction in conventional bactericides) to the environment. The economic impacts of these alternative treatments have not been determined yet, but we anticipate that final grower costs will not increase.
Publications
- Gent, D. H., Schwartz, H. F., and Ishimaru, C.A. 2003. Population dynamics of Xanthomonas campestris pv. allii on leaves of pulse crops and weeds. APS Annual Meeting, Poster Presentation, Charlotte, NC Phytopathology Vol. 93:S29.
|
Progress 01/01/02 to 12/31/02
Outputs
Xanthomonas leaf blight of onion is a yield limiting disease in Colorado, and is a potential threat to onion, pulse and other seed crops produced in western United States. The overall objective of this study is to develop multi-faceted management strategies that will reduce grower reliance upon at-risk pesticides, while simultaneously improving vegetable production profitability and sustainability. We will investigate cultural practices, biological agents and pesticides, and disease forecasting to enhance onion and dry bean IPM in the western U.S. The relationships between regional weather events and the initial appearance and subsequent severity of Xanthomonas leaf blight in southern Colorado over the past 6 years were modeled using multiple regression equations. Systemic acquired resistance type of products and biocontrol agents did not significantly improve yields or reduce storage rot, while Actigard did reduce yield in the absence of disease. Nitrogen fertility
did not affect epiphytic development of the pathogen in field trials in 2002. Strains of Xanthomonas campestris originally isolated from onion over several years from diverse geographic regions were quantitatively and qualitatively characterized by fatty acid methyl ester profiles, metabolic utilization patterns (Biolog), host range, bactericide resistance, rep-PCR fingerprinting, rDNA spacer sequences, and PCR reaction with various primer pairs. Fatty acid and metabolic profiles revealed close relatedness between onion strains of X. campestris and X. campestris pv. citrumello and dieffenbachiae, respectively. Considerable phenotypic, pathogenic, and genetic diversity exists within strains of Xanthomonas campestris causing Xanthomonas leaf blight of onion, and this diversity is largely explained by the geographic origin of the strains. Rep-PCR genomic fingerprinting revealed that onion Xanthomonas strains form a distinct fingerprint that was highly similar to the described pathovar X.
campestris pv. allii. Based upon pathogenicity to onion, carbon substrate utilization, fatty acid profiles, rDNA genetic diversity, and genomic fingerprints, we conclude the strains examined in this study are pathovar X. campestris pv. allii. The conserved rep-PCR genomic fingerprints within geographic locations suggest that the population structure is largely clonal within production areas. The tight phylogenetic clusters formed between production regions suggests X. campestris pv allii was introduced into these production regions in one or a few events, perhaps on contaminated seed. The climates, cultivars, and production practices between the onion production regions considered in this study are significant and may have exerted a strong selection pressure for fitness and adapted genotypes for each environment. The relatedness of the strains was revealed by the identical rDNA RFLP profiles and high sequence similarity of the non-conserved IGS region.
Impacts
Daily average high temperatures in July during the transition from plant vegetative to reproductive development and precipitation in August explained over 99.7% of the variability in disease appearance, and closely followed crop development. This disease forecast model could be used by growers on a regional basis to improve timing of the first application of a copper-based bactericide and eliminate unnecessary early-season sprays (1 - 2 fewer sprays per field). The critical period for bactericide applications occurred when host susceptibility near bulbing, favorable environmental conditions, and increasing pathogen populations overlapped. Identification of the pathogen from onion as Xanthomonas campestris pv. allii provides a critical step in elucidating the biology, and more effectively directing future research efforts with this bacterium. Based upon these preliminary tests, onion seed produced in the Pacific Northwest region does not appear to be a source of the
pathogen. However, the pathogen can survive in planta within other pulse crops such as dry bean, soybean, lentil, chickpea and alfalfa, and thus could be a potential source of contamination.
Publications
- Gent, D. H., and Schwartz, H. F. 2002. Polyphasic characterization of Xanthomonas campestris from onion. 2002 National Allium Research Conference Poster Abstracts - Page 48, Pasco, WA.
- Gent, D. H. and Schwartz, H. F. 2002. Epidemiology of Xanthomonas leaf blight in relation to the critical period for bactericide applications. 2002 National Allium Research Conference Poster Abstracts - Page 49, Pasco, WA.
- Schwartz, H. F., Otto, K. L., and Gent, D. H. 2003. Relation of temperature and rainfall to development of Xanthomonas and Pantoea leaf blights of onion in Colorado. Plant Disease 87:11-14.
|
|