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Poster: Epidemiology: Population Biology Genetics


Thirty-three years of lettuce downy mildew and counting: a phenotypic and genotypic narrative
C. TSUCHIDA (1), F. Martin (2), R. Gil (1), O. Ochoa (3), I. Simon (3), R. Michelmore (3) (1) Department of Plant Pathology, University of California, Davis, U.S.A.; (2) USDA-ARS, Salinas, U.S.A.; (3) Genome Center, University of California, Davis, U.S.A.

Downy mildew, caused by Bremia lactucae, is the number one foliar lettuce disease in California and worldwide. This heterothallic oomycete pathogen is highly variable and can rapidly change to overcome resistant cultivars. We are characterizing the pathogen population to determine which sources of wild germplasm will provide effective resistance in California that minimizes dependency on chemical protectants and aids organic farmers. We have been monitoring and archiving isolates of B. lactucae in California since 1982, mostly on an opportunistic basis. In December 2013, we began monthly surveys to provide a more comprehensive data set on pathogen variability. We report the patterns in virulence phenotypes observed in the Californian B. lactucae population over the past 33 years and an increase in the B1 mating type. The Californian population has predominately been the B2 mating type and reproduces asexually. The occurrence of the B1 mating type indicates the potential for sexual reproduction with prevalent B2 isolates. Sexual progeny between recent B1 and B2 isolates are fit and highly variable and may explain the recent rise in novel virulence phenotypes. This finding necessitates a change in strategy to achieve more durable disease resistance. We have also sequenced the genomes of representative isolates collected throughout the three decades as a tool for understanding variation of B. lactucae in California and the world.