Progress 08/01/03 to 10/30/05
Outputs
Xanthomonas leaf blight, caused by the bacterium Xanthomonas 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. The effects of rates of nitrogen fertilization and levels of seed contamination by X. axonopodis pv. allii on epiphytic populations of the pathogen and subsequent disease development were determined in field studies over two years. Epiphytic populations of a rifampicin-resistant mutant were quantified from bulked, asymptomatic leaf tissue using a leaf wash assay. There was no significant relationship between nitrogen fertilization treatment and epiphytic population development, but excessive nitrogen (> 200 kg/ha)fertilization increased disease severity 27 to 50% compared with non-fertilized and low (112 kg/ha) nitrogen fertilizer treatments in 2004. Seed inoculated with various levels of X.
axonopodis pv. allii did not result in persistent epiphytic populations of the bacterium or development of symptoms. The results of these studies suggest moderate rates of nitrogen fertilization may reduce severity of Xanthomonas leaf blight compared to higher rates, and that seed contamination by X. axonopodis pv. allii may be of limited importance for disease development under the semi-arid conditions of furrow-irrigated onion production in Colorado. Alternative strategies to manage this bacterial pathogen of onion were investigated. Under controlled conditions, bacteriophages did not impact disease severity. However, under field conditions, biweekly and weekly applications of bacteriophages reduced disease severity as well as or better than standard copper hydroxide-mancozeb treatments. Acibenzolar-S-methyl also successfully reduced disease severity with or without bacteriophages. Bacteriophages survived on onion leaf surfaces for at least 96 hours under greenhouse conditions or 72
hours at field locations. Resistant cultivars offer an efficient and safe method of pest management. A set of 180 onion cultivars and accessions was evaluated for resistance to Xanthomonas under growth chamber, greenhouse and/or field conditions. The least susceptible entries were 508402, 508406, 32141, 31831, and H-942; all but H-942 are Allium fistulosum materials. The most susceptible cultivars were 4469, 4331, Ringmaster, 4819 and SR4000ON. According to the disease severity continuum model utilized, 42 entries were considered less susceptible, 71 were moderately susceptible and 65 were susceptible to highly susceptible. Future research should explore more possiblities for resistant cultivars or accessions, as well as introgress wild or less-susceptible species such as A. fistulosum with A. cepa.
Impacts
Based upon these preliminary tests, onion seed sources should be screened for the presence and quantity of pathogenic bacterial contaminants that could be introduced into existing and new onion production areas and threaten the crop in that planted field as well as in adjacent fields planted to clean sources of seed. Confirmation of the pathogen in irrigation water also illustrates the importance of managing irrigation runoff water to reduce movement of the pathogen to downstream hosts. Weed management must be emphasized in and around onion fields so that these sources of bacterial contamination are managed, which in turn can reduce disease pressure on onions during the growing season and conceivably in future seasons. Alternative strategies such as bacteriophages and plant defense ativators may provide a safe and effective alternative for managing onion Xanthomonas leaf blight. Differences in resistance among onion cultivars and accessions could be utilized to improve
future releases and help growers select cultivars that are less susceptible to the pathogen and that require less protection with conventional pesticides and/or alternative biopesticides.
Publications
- Gent, D. H., and Schwartz, H. F. 2005. Effect of nitrogen fertilization and seed contamination on epiphytic populations of Xanthomonas axonopodis pv. allii and development of Xanthomonas leaf blight of onion. Plant Health Progress doi:10.1094/PHP-2005-0331-01-RS.
- 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.
- 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.
- Lang, J. M., Schwartz, H. F., and Gent, D. H. 2004. Alternative strategies for onion Xanthomonas leaf blight management. APS Annual Meeting, Poster Presentation, Anaheim, CA Phytopathol. 94:S56.
- Lang, J. M., D. H. Gent, and Schwartz, H. F. 2005. Differential response of onion cultivars to onion Xanthomonas leaf blight. Allium Improvement Newsletter 14: in press.
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Progress 01/01/04 to 12/31/04
Outputs
Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodis pv. allii, is an economically important disease in many onion producing regions. The relation of nitrogen fertilization and seed contamination level to pathogen epiphytic populations and subsequent disease development were quantified in field studies conducted over two years. No relationship among nitrogen fertilization treatment and epiphytic population development were observed, but excess nitrogen fertilization increased disease severity 27 to 50 percent as compared to fertilized treatments in 2004. Seed inoculated with any level of the bacterium did not result in persistent epiphytic populations of the bacterium or disease development. The results of these studies suggest moderate nitrogen fertilization may reduce Xanthomonas leaf blight severity, but seed contamination may be of little importance for disease development under semi-arid conditions in Colorado. A biological control program using
h-mutant bacteriophages has been evaluated in 2003 and 2004. Disease severity was reduced by the bacteriophage alone and with acibenzolar (plant defense stimulating compound).
Impacts
Based upon these preliminary tests, onion seed sources should be screened for the presence and quantity of pathogenic bacterial contaminants that could be introduced into existing and new onion production areas and threaten the crop in that planted field as well as in adjacent fields planted to clean sources of seed. Confirmation of the pathogen in irrigation water also illustrates the importance of managing irrigation runoff water to reduce movement of the pathogen to downstream hosts. Weed management must be emphasized in and around onion fields so that these sources of bacterial contamination are managed, which in turn can reduce disease pressure on onions during the growing season and conceivably in future seasons.
Publications
- Lang, J. M., Schwartz, H. F., and Gent. D. H. 2004. Alternative strategies for onion Xanthomonas leaf blight management. Phytopathol. 94:S56.
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Progress 01/01/03 to 12/31/03
Outputs
Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodis pv. allii, is an economically important disease in many onion producing regions. Resistance to Xanthomonas leaf blight has been identified in two short day onion cultivars, but these cultivars are no longer available commercially and resistance in long-day cultivars has not been reported. Germplasm accessions and cultivars of Allium cepa and A. fistulosum were screened in growth chamber assays to identify new sources of resistance to Xanthomonas leaf blight. Plants were pin-pricked at three equidistant locations on each leaf with a sterile needle bearing a bacterial matrix of X. axonopodis pv. allii from five day-old nutrient agar plates. Plants were subsequently incubated at 28/24 degrees Celsius day-night temperature regime and 100 percent relative humidity for 14 days, and lesion expansion from each pinprick was measured to quantify disease severity. Differences in susceptibility to Xanthomonas leaf
blight were observed, and could influence the effectiveness of integrated pest management programs. Xaa was recovered from 6.7 percent of seedlots screened (8 of 119); 5.6 percent of long-day cultivars and 15.4 percent of short-day cultivars. Methodology was developed for future studies with seed lots artifically inoculated with the pathogen. Naturally-occurring Xaa inoculum was recovered from various sources including irrigation water, onion crop debris, alternate hosts and weeds, in addition to the contaminated seed sources in northern and southern Colorado. Crop sources included alfalfa and volunteer onion plants. Weed sources included buffalobur, sandbur, spurred anoda, redroot pigweed, kochia, field bindweed, Canada thistle, and common mallow.
Impacts
Based upon these preliminary tests, onion seed sources should be screened for the presence and quantity of pathogenic bacterial contaminants that could be introduced into existing and new onion production areas and threaten the crop in that planted field as well as in adjacent fields planted to clean sources of seed. Confirmation of the pathogen in irrigation water also illustrates the importance of managing irrigation runoff water to reduce movement of the pathogen to downstream hosts. Weed management must be emphasized in and around onion fields so that these sources of bacterial contamination are managed, which in turn can reduce disease pressure on onions during the growing season and conceivably in future seasons.
Publications
- Gent, D. H., and Schwartz, H. F. 2003. Screening of onion germplasm for resistance to Xanthomonas leaf blight. APS Annual Meeting, Poster Presentation, Charlotte, NC Phytopathology Vol. 93:S29.
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