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White mold is a highly destructive disease caused by Sclerotinia sclerotiorum, a polyphagous fungus that infects several hosts including common bean grown that is widely cultivated in Brazil. The pathogenic populations are known to exhibit genetic variability but studies rarely used single ascospore-derived strains. In this study, 12 monoascospore strains originated from a same isolate (UFLA 54), but from different apothecia, were compared among them and with the mother strain with regards: days to first sclerotium formation, mycelial growth rate mycelial compatibility groups, microsatellite markers and mating type locus (MAT1-1 and MAT1-2). Their pathogenicity was assessed on three common bean lines. Cluster analysis of growth rates data separated UFLA 54 from all monoascospore strains, which shared same alleles in the microsatellite locus. Inversion of MAT1-1 region was found for both UFLA 54 and half of monoascospore strains. Aggressiveness of the monoascospore strains was not consistent across common bean lines. Our study reveals significant variability of S. sclerotiorum at the single isolate level. Keywords Phaseolus vulgaris. White mold. Aggressiveness. Morphological traits White mold is a fungal disease caused by Sclerotinia sclerotiorum (Lib.) de Bary, a necrotrophic and homothallic fungus capable of infecting several hosts and causing severe epidemics in important field crops including soybean, common bean and sunflower. White mold epidemics are reported worldwide, including Brazil, with yield losses estimated at 19% to 70% depending on the geographic region and year (

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The objective of this work was to evaluate the reaction of common bean (Phaseolus vulgaris) lines to Sclerotinia sclerotiorum, the aggressiveness of the isolates, and the inheritance of resistance to white mold, under two environmental conditions. The experiments were carried out under field and greenhouse conditions. In each one, four isolates were inoculated in 14 common bean lines using the straw-test inoculation method and severity on plants. No common bean line with a high level of resistance to white mold was identified, and most of the lines were moderately resistant to the pathogen. In general, the BRS Campeiro, BRS Radiante, BRSMG Talismã, and Ouro Negro cultivars stand out for resistance. Under field conditions, the UFLA 3 isolate discriminates well the lines, while, under greenhouse conditions, UFLA 26 and UFLA 92 are the most aggressive isolates. Furthermore, according to the genetic study, most of the observed variation can be attributed to horizontal resistance, although a small part is associated with vertical resistance.