Characterization and genetic analysis of field isolates of Botryotinia fuckeliana (Botrytis cinerea) resistant to dichlofluanid

Field isolates of Botryotinia fuckeliana were collected from naturally infected plants. Their responses to the multisite fungicide dichlofluanid in mycelium growth test fell into three phenotypic classes, characterized by the following EC sub(50) (and MIC) values in mu g/ml: sensitivity, 1-3 (6-10); low resistance, 3-10 (> 100); high resistance, 10-30 (> 100). The corresponding values obtained for these classes in a spore germination test were respectively: approximately equal to 0.05 (0.2), 0.05-0.1 (0.5), 0.5-1 (0.9-1.5). Resistant isolates were crossed with two sensitive and two resistant strains of appropriate mating type to determine the genetic basis of resistance. Distribution of resistance phenotypes in ascospore progeny indicated that a gene, named Dic1, was probably responsible for the low or high resistance of 14 mutants selectively collected from experimental plots of greenhouse-grown gerbera sprayed several times with dichlofluanid or tolyfluanid. A second gene, named Dic2, was probably responsible for the low resistance displayed by two isolates (from grapevine and from carnation) maintained in the laboratory collection. As a result of the investigation, the use of dichlofluanid in integrated management programmes against grey mould is discussed.