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Mycelial Incompatibility and Molecular Markers Identify Genetic Variability in Field Populations of Sclerotinia sclerotiorum. L. M. Kohn, Department of Botany, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6; E. Stasovski, I. Carbone, J. Royer, and J. B. Anderson. Department of Botany, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6. Phytopathology 81:480-485. Accepted for publication 6 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-480.

Sixty-three sclerotial strains of Sclerotinia sclerotiorum were obtained from transects in two fields of canola (Brassica napus) in Ontario. Mycelial pairings of the strains in all combinations on agar medium produced either an incompatible reaction in which a reaction line between the two strains developed in the interaction zone, or a compatible reaction in which no reaction line developed. The reaction line was a distinct discontinuity between the two strains, visible as a red line on the colony reverse in pairings made on medium amended with red food coloring. Among the 33 strains from the first field, six mycelial compatibility groups (MCGs) were recognized, the largest group including 19 strains. Among the 30 strains from the second field, many more MCGs were defined. In pairings of 10 monosporous strains from each of six apothecia collected along the transects, all sibling monosporus strains were compatible and no segregation for mycelial compatibility was observed among siblings. Analysis with three molecular markers indicated that each of the MCGs was genetically uniform. With one of these markers, each MCG was uniquely fingerprinted. This fingerprint was produced by a random fragment of nuclear DNA (approximately 4.5 kb) from S. sclerotiorum, pLK44.20, which when used as a cloned probe in Southern hybridizations of DNAs restricted with BamHI detected polymorphisms that corresponded exactly with strain groupings defined by mycelial compatibility. Southern hybridization of high molecular weight DNAs separated by pulsed-field electrophoresis showed that the repetitive element is located on at least five to six chromosomes. Another probe, plasmid pGP637, carrying the mitochondrial 24S rRNA gene from Neurospora crassa, in HindIII-digested DNA, produced six phenotypes. With the exception of phenotypic heterogeneity within one MCG, which had three phenotypes, only one phenotype was observed in strains from each MCG; each of four of the six phenotypes was shared by a group of MCGs. The third molecular marker was a length polymorphism in a segment of the small mitochondrial rRNA gene amplified by the polymerase chain reaction; of four phenotypes, each of two (fragment size of either 0.6 or 2.0 kb) was shared by a large group of MCGs. The data from this study demonstrate that a field population of S. sclerotiorum is genetically heterogeneous, and suggest that the distinct genotypes that compose the population are each conserved, increasing either by clonal asexual or by homothallic sexual reproduction.

Additional keywords: DNA fingerprinting, oilseed rape, Sclerotinia stem rot.