Resistance to triforine: A nonexistent problem?

Permanent resistance to triforine in Cladosporium cucumerinum was obtained after UV irradiation of spore suspensions and selection of resistant strains in the presence of triarimol or triforine. About 50 strains were examined for mycelial growth on malt agar, viability upon routine subculturing, growth and inhibition by triforine in thin-layer chromatographic bioassays, and virulence towards cucumber seedlings. In addition, sporulation and spore germination, as well as effects of ergosterol were investigated in a restricted number of strains. Although all strains differed considerably from each other in these characteristics, they were quite similar in a reduced overall-fitness. These triforine-resistant mutants of C. cucumerinum might, therefore, have a greatly reduced chance of survival in competition with the wild-type strain, as was actually demonstrated for two strains in greenhouse experiments with mixed-inoculations of cucumber seedlings. Such a reduced chance of survival might also explain why under practical conditions development of resistance to triforine has not been observed yet.The triforine-resistant C. cucumerinum strains showed cross-resistance to a number of systemic fungicides known or supposed to inhibit ergosterol biosynthesis, viz. Denmert, fenarimol, imazalil, triadimefon and triarimol. The practical implications and potential hazards of such cross-resistance for the use of already established fungicides are discussed.