A major gene controlling pathogenicity in Botryotinia fuckeliana (Botrytiscinerea)

Non-aggressive mutants were isolated from a single-ascospore strain of Botr yotinia fuckeliana following ultraviolet and chemical mutagenesis of conidi a and screening on French bean leaves (Phaseolus vulgaris). Crosses with re ference strain SAS56 revealed one ultraviolet induced mutant (Mp97) in whic h the non-aggressive phenotype segregated 1 :1 indicating control by a sing le gene with a major effect on pathogenicity. Further crosses showed that t his gene, designated Pat1, is not linked to Mbc1 (benzimidazole resistance) , Daf1 (dicarboximide resistance), nit1 (nitrate non-utilising), or Sell (s odium selenate resistance). The pathogenicity of Pat1 strains was compared with various wild-type strains. Mutant strains produced small, restricted l esions on leaves of French bean and soybean (Glycine max) and slowly spread ing lesions on rose (Rosa spp.) flowers. The mutant was essentially non-pat hogenic on tomato stems at 20 and 25 degrees C but produced a few invasive infections at 10 and 15 degrees C. Tests of radial growth rate, sporulation and sclerotial production showed that Pail strains exhibit no gross unfitn ess or unusual nutrient requirement. Although in vivo studies showed substa ntially less total polygalacturonase activity in Pat1 lesions compared to t hose produced by wild-type strains, differences in polygalacturonase isozym e profiles were not correlated with the presence of Pail. Pat1 strains show ed relatively low acid production in pH indicator medium. Microscopic exami nation 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 as say. (C) 1999 Academic Press.