IYS CHALLENGES TO THE ONION BULB INDUSTRY IN COLORADO.

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: Howard.Schwartz@ColoState.EDU Phone: 970-491-6987

Iris yellow spot (IYS), caused by the thrips-transmitted Tospovirus Iris yellow spot virus, is an emerging threat to onion worldwide and an immediate threat to sustainable and profitable production in the western U.S. Disease management strategies have not been developed for iris yellow spot. Field studies were conducted to determine the individual and combined effects of insecticide application, cultural practices, host genotype, and novel chemical treatments on disease incidence, severity, and onion yield and grade. Plant density was associated significantly with disease incidence with a moderately resistant cultivar, but not with two susceptible cultivars. In 2003 and 2004, disease incidence among more than 40 cultivars varied from 16 to 100 percent; red market class cultivars appear to be more susceptible to IYS. Cultivars with both iris yellow spot resistance and some degree of thrips feeding tolerance appear to be present within commercially-acceptable onion cultivars. In 2003, four applications of acibenzolar and imidacloprid reduced disease incidence 34 and 38 percent, respectively, with a corresponding 34 and 38 percent increase in jumbo grade bulbs. Other field studies in 2004 evaluated imidacloprid and acibenzolar spray timing, rate, interval, and their interactions with partial host resistance. Cultural practices such as planting density, crop rotation, sanitation of previously infected onions, use of clean transplants, and avoidance of plant stress will play key roles in the management of this disease and its insect vector. A multi-faceted management approach appears essential for effective IYS management.

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