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POSTERS: Population biology and genetics

Characterization of Stemphylium spp. from spinach based on molecular data, host response, and azoxystrobin sensitivity
Kayla Spawton - Washington State University. Larry Stein- Texas A&M University, Tobin Peever- Washington State University, James Correll- University of Arkansas, German Sandoya- University of Florida, Michael Derie- Washington State University, Richard Raid- University of Florida, Gilberto Olaya- Sy

Stemphylium leaf spot of spinach causes losses under warm, humid conditions. The causal agent was described originally as Stemphylium botryosum based on anamorph and teleomorph morphology. As a result of increasing losses to this disease and evidence of diversity among Stemphylium isolates from spinach, molecular characterization, pathogenicity tests, and azoxystrobin resistance screening were performed on isolates sampled from symptomatic plants in Arizona, California, Florida, Oregon, Texas, and Washington as well as isolates from seeds grown in six countries, including the USA. The internal transcribed spacer 1 and 2 regions of ribosomal DNA, and regions of glyceraldehyde-3-phosphate dehydrogenaseand calmodulin were sequenced. Two spinach cvs., Mandolin and Viroflay, were inoculated with each isolate. Isolates clustered into two phylogenetic lineages: 1) isolates most similar to S. vesicarium,and 2) isolates previously identified as S. botryosum. The former caused small (? 5 mm diameter) lesions on Mandolin 2 days after inoculation, but no symptoms on Viroflay, even after 21 days. The latter produced larger (5 to 15 mm diameter) lesions on both cultivars within 10 to 14 days. Isolates resistant to azoxystrobin (EC50> 10 mg/liter) were detected, and mutations conferring resistance are being characterized. Differences in the biology, epidemiology, and management of these two causal agents of Stemphylium leaf spot of spinach are being investigated.