RESISTANCE TO FENARIMOL IN NECTRIA-HAEMATOCOCCA VAR CUCURBITAE

Genetic work with 51 fenarimol-selected strains of Nectria haematococca var. cucurbitae identified a polygenic system for resistance with at least nine chromosomal loci involved. The mutant genes, designated fen-1 to fen-9, gave low levels of resistance to fenarimol and to three other C-14 demethylation inhibiting (DMI) fungicides, namely triforine, imazalil, and triadimenol. Haploid strains carrying two fen mutations exhibit higher levels of resistance, indicating additivity of gene effects. All fen mutations appear to be pleiotropic, having more or less adverse effects on growth, sporulation, spore germination, pathogenicity, and tolerance of somewhat high temperatures. Accumulation of fenarimol in resistant strains was lower than in the wild type, suggesting that fen mutations code for a common resistance mechanism based on a permeability barrier. Various inhibitors of energy generation increased the accumulation level, indicating that accumulation is energy dependent and may be the result of passive influx and energy-dependent efflux. Lower accumulation in resistant strains is probably the result of increased efflux, as has been found with other fungi. A double mutant carrying the mutations fen-7 and fen-9 showed lower accumulation of fenarimol than a strain carrying the fen-7 only, indicating additivity of effects in this regard also. © 1991.