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New Populations of Sclerotinia sclerotiorum from Lettuce in California and Peas and Lentils in Washington

First, fifth, and sixth author: Department of Ecology and Evolutionary Biology, University of Toronto, Mississauga L5L 1C6. Second author: Center for Integrated Fungal Research, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Third author: Horticultural Crops Research Laboratory, USDA ARS, 3420 NW Orchard Ave., Corvallis, OR 97330. Fourth author: Department of Plant Pathology, University of California at Davis, 1636 East Alisal Street, Salinas 93905

Four populations of Sclerotinia sclerotiorum in North America were inferred previously, based on analyses of both rapidly evolving markers (DNA fingerprint and mycelial compatiblity), and multilocus DNA sequence spanning the range between fast and slow evolution. Each population was defined as an interbreeding unit of conspecific individuals sharing a common recent ancestor and arising in a unique evolutionary event. The present study applies this standard to extend characterization of S. sclerotiorum populations to the Western United States. Isolates of S. sclerotiorum (N = 294) were determined to represent three genetically differentiated populations: California (CA, lettuce), Washington (WA, pea/lentil), and Ontario (ON, lettuce). CA was the most diverse population yet sampled in North America. Clonality was detected in ON and WA. No DNA fingerprints were common among the populations. The index of association (I_A), based on fingerprint, was closer to zero (0) for CA than it was for the other populations. High diversity and lack of association of markers in California are consistent either with genetic exchange and recombination, or with large population size and high standing genetic variation. Intra- and interlocus conflict among three DNA sequence loci was consistent with recombination. The coalescent IGS genealogy confirmed subdivision and showed CA to be older than WA or ON. The Nearest Neighbor statistic on combined data confirmed subdivision among all present and previously defined populations. All isolates had both MAT1-1 and MAT1-2, consistent with uniform homothallism.

Additional key words: migration, MDIV, mating type, multilocus haplotype, MCG, population genetics, phylogeny, vegetative compatibility.