A major gene controlling pathogenicity in Botryotinia fuckeliana (Botrytis cinerea)

Non-aggressive mutants were isolated from a single-ascospore strain of Botryotinia fuckeliana following ultraviolet and chemical mutagenesis of conidia and screening on French bean leaves (Phaseolus vulgaris). Crosses with reference strain SAS56 revealed one ultraviolet induced mutant (Mp97) in which the non-aggressive phenotype segregated 1:1 indicating control by a single gene with a major effect on pathogenicity. Further crosses showed that this gene, designated Pat1 , is not linked to Mbc1 (benzimidazole resistance), Daf1 (dicarboximide resistance), nit1 (nitrate non-utilising), or Sel1 (sodium selenate resistance). The pathogenicity of Pat1 strains was compared with various wild-type strains. Mutant strains produced small, restricted lesions on leaves of French bean and soybean (Glycine max) and slowly spreading lesions on rose (Rosa spp.) flowers. The mutant was essentially non-pathogenic on tomato stems at 20 and 25 degree C but produced a few invasive infections at 10 and 15 degree C. Tests of radial growth rate, sporulation and sclerotial production showed that Pat1 strains exhibit no gross unfitness or unusual nutrient requirement. Although in vivo studies showed substantially less total polygalacturonase activity in Pat1 lesions compared to those produced by wild-type strains, differences in polygalacturonase isozyme profiles were not correlated with the presence of Pat1 . Pat1 strains showed relatively low acid production in pH indicator medium. Microscopic examination showed that lesions induced by the Mp97 mutant on French and soybean leaves (4 days old), but not by its parent, were surrounded by a distinct staining zone of mesophyll cells suggesting a difference in host response. However, no differences in phytoalexin induction were found in a soybean assay.