ALTERNATIVE STRATEGIES FOR MANAGING ONION XANTHOMONAS LEAF BLIGHT

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

Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523-1177. Corresponding Author: Howard F. Schwartz. Email: JillianL@lamar.colostate.edu Phone: 970-491-0256

Xanthomonas leaf blight, caused by Xanthomonas axonopodis pv. allii (Xaa) can be a devastating onion disease. An alternative management strategy is critical to reduce excessive use of chemical bactericides. A biological control program using h-mutant bacteriophages (Agriphage from Omnilytics, Inc.) has been developed and evaluated. In 2003 and 2004, 108pfu/mL phages were applied biweekly onto inoculated onion plots at three locations in Colorado. Both titer and interval of Agriphage applications were evaluated in 2004 at two locations. In 2003, disease severity was reduced by the Agriphage alone and with acibenzolar-S-methyl (Actigard, Syngenta, Inc.). In 2004, all treatments reduced disease severity, while ManKocide was no better than Agriphage alone in reducing rAUDPC at both locations. The 3 to 4 day phage interval was the most successful, while the 7-day interval was just as effective as ManKocide at reducing the rAUDPC. At ARDEC in 2004, the rAUDPC was reduced by all varied phage titer treatments compared to the untreated, but not by ManKocide. Phages with Actigard was the most consistently effective treatment in 2003 and 2004. Yield was not affected by any treatment at either location in 2004. Genetic resistance in onion germplasm can also be a valuable component in disease management. Over 150 onion cultivars and germplasm accessions were screened for resistance or tolerance to Xaa by stabbing the two youngest, fully extended leaves in three equidistant locations with a sterile needle bearing a bacterial matrix of Xaa. Plants were monitored for 12 days in a growth chamber at 28oC and 100% relative humidity, while each stab wound was measured for lesion progression. Cultivars were analyzed and ranked according to their calculated AUDPC.

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