XANTHOMONAS LEAF BLIGHT OF ONION: BIOLOGY, EPIDEMIOLOGY, AND MANAGEMENT

David H. Gent, Howard F. Schwartz, Jillian M. Lang, and Carol A. Ishimaru

Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523-1177. Corresponding Author: Howard F. Schwartz. Email: Howard.Schwartz@ColoState.EDU
Phone: 970-491-6987

Xanthomonas leaf blight, caused by Xanthomonas axonopodis pv. allii (Xaa), is an emerging disease of onion worldwide. Basic knowledge of pathogen diversity, epidemiology, and disease management are unknown. Forty-three Xanthomonas spp. originally isolated from onion were characterized genetically, phenotypically, and pathogenically utilizing a polyphasic approach. Potential inoculum sources of the pathogen (weeds, alternate hosts, volunteer onion, irrigation water, and crop debris) were sampled for Xaa in Colorado in 2003 and 2004. Epiphtyic populations of Xaa on weeds and leguminous crops were quantified by leaf rinsing and subsequent plating onto modified MXP medium. Populations of the spontaneous rifampcin mutant Xaa strain RO177 were monitored in irrigation water and crop debris. A high level of genetic and phenotypic variability exists among Xaa strains, and is associated largely with strain region of origin. Pathogenic Xaa, either naturally occurring or strain RO177, were recovered from all sources evaluated, but were not found in onion growing regions of Colorado free from Xanthomonas leaf blight the previous year. In inoculated plots, Xaa strain RO177 was maintained throughout the growing season on leguminous hosts, including lentil and dry bean. Greater than 108 Xaa cfu per onion leaf were maintained throughout the winter and following spring in infected leaves on the soil surface, but decreased over one million fold in leaves buried 20 cm. Large Xaa populations (103 to 104 cfu per ml) were recovered consistently in irrigation tail water. Novel chemical and biological control strategies offer effective disease management strategies and should reduce grower reliance upon copper-based bactericides.

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