A Detached Cotyledon Test for the Isolation of Mutants of Pyrenopeziza brassicae Defective in Pathogenicity Determinants

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Abstract
A detached cotyledon pathogenicity test was devised for the isolation of UV‐induced mutants of the hemibiotrophie ascomycete pathogen of Brassica spp., Pyrenopeziza brassicae defective in pathogenicity determinants. At least 95 % of cotyledons of suitable susceptible cultivars of oilseed rape (Brassica napus L. ssp. oleifera cvs Shogun or Bolko) showed symptoms of disease (white spore pustules [conidiomata] on the surface of the cotyledon) within three weeks of inoculation of cotyledons with 104 conidia of P. brassicae. The test allowed rapid screening of UV‐induced mutants with a low frequency of false negatives. From 1,700 survivors of UV mutagenesis tested, 20 non‐pathogenic mutants were obtained.