CHARACTERIZATION OF MUTATIONS IN THE BETA-TUBULIN GENE OF BENOMYL-RESISTANT FIELD STRAINS OF VENTURIA-INAEQUALIS AND OTHER PLANT PATHOGENIC FUNGI

All benomyl-resistant field strains of Venturia inaequalis, V. pirina, Monilinia fructicola, Sclerotinia homoeocarpa, and six species of Penicillium, except those with low resistance to benomyl, were found to contain a single base pair mutation in their beta-tubulin gene that resulted in an amino acid substitution in beta-tubulin. In V. inaequalis, codon 198, which encodes glutamic acid in a sensitive strain, was converted to a codon for alanine in a strain with very high resistance, to a codon for lysine in a strain with high resistance, or to a codon for glycine in a strain with medium resistance to benomyl. Codon 200 for phenylalanine was converted to a codon for tyrosine in a second strain of V. inaequalis with medium resistance to benomyl. Among field strains of other fungi, 14 had a glutamic acid to lysine, alanine, or valine substitution at position 198, and three had a phenylalanine to tyrosine substitution at position 200. Among seven benomyl-resistant strains with sensitivity to the N-phenylcarbamate fungicide diethofencarb, all had a glutamic acid to alanine or glycine substitution at position 198. A comparison of the codon changes in the beta-tubulin gene of field strains with laboratory-induced benomyl-resistant mutants of model fungi showed that mutations conferring field resistance represent a small subset of the mutations recovered in laboratory experiments.