|Schwartz, Howard - COLORADO STATE UNIVERSITY|
Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2005
Publication Date: N/A
Interpretive Summary: Xanthomonas leaf blight of onion is an important disease of onion in Colorado and other regions of onion production, but few non-chemical management strategies have been developed for this disease. Field studies were conducted in small plots to determine how nitrogen fertilization and seed contamination with the Xanthomonas leaf blight bacterium effect disease development. The level of nitrogen fertilization did not effect how well the pathogen colonized the onion leaves, but in one season excessive fertilization increased the severity of disease 27 to 50% compared to recommended nitrogen fertilization rates. In this study, no level of seed contamination with the pathogen evaluated resulted in the establishment of the bacterium on onion leaves throughout the entire season, and disease symptoms did not develop on any plants. 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 semi-arid environments. Growers should be able to better manage Xanthomonas leaf blight of onion with fewer chemical inputs by using nitrogen fertilizer judiciously.
Technical Abstract: Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodis pv. allii, is a yield-limiting disease in Colorado, for which few effective management strategies are available. 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 rifamipicin-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 (>200kg/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 of development of symptoms even at 107 CFU/g seed. 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.