Iris yellow spot virus on Onion in Colorado

A new disease was found in September 2001 on greenhouse-produced onion transplants of cv. Colorado 6 grown in a field in Larimer County in northern Colorado. Symptoms included straw-colored, dry, tan, spindle- or diamond-shaped lesions on the leaves and scapes of onion plants. Infected plants were scattered (less than 5% incidence) throughout the outer perimeter of the sprinkler-irrigated field. Iris yellow spot virus (IYSV) in two collections each with 4 to 6 symptomatic onion plants was confirmed with western blot assays by James Moyer of North Carolina State University. Western blot showed a faint band from protein extracts of infected Nicotiana benethimiana. Western blot assay is the most definitive method of identification. IYSV can be mechanically transmitted to N. benethimiana, but it cannot be recovered and transmitted back to onion, and it is difficult to detect in infected onion plants. IYSV is a tospovirus that is transmitted by various species of thrips, including onion thrips and western flower thrips (1). The host range for this disease includes onion, leek, and iris. IYSV has been reported previously on onion in Israel, Brazil, and Idaho (2). There are no reports that this disease affects bulb quality or marketability; however, heavy losses of onion bulb production are reported (1). University and industry personnel in other onion-growing areas of the country are encouraged to monitor onion and other host fields for evidence and distribution of IYSV. References: (1) A. Kritzman et al. Plant Dis. 85:838, 2001. (2) L. Pozzer et al. Plant Dis. 83:345, 1999.