INTEGRATED MANAGEMENT OF IRIS YELLOW SPOT VIRUS IN ONION

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, caused by the thrips-transmitted Tospovirus Iris yellow spot virus (IYSV), 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, disease incidence among 43 cultivars varied from 16 to 100 percent; red market class cultivars were uniformly susceptible to IYSV. 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. Ongoing field studies in 2004 are further evaluating imidacloprid and acibenzolar spray timing, rate, interval, and their interactions with partial host resistance. A multi-faceted management approach appears essential for effective IYSV management.

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