The effect of fungicides on fungal antagonists of onion white rot and selection of dicarboximide‐resistant biotypes

The sensitivity of four fungal antagonists (Chaetomium globosum, Trichoderma harzianum, T. viride, Trichoderma sp.) to six fungicides was evaluated. The fungi were insensitive to captan, mancozeb and thiram but were sensitive to benomyl (EC50 < 0 3/μg/ml) and the two dicarboximides iprodione and procymidone (EC50 < 3 3/μg/ml). Conidia and mycelia of the four fungi were exposed to a series of ultraviolet irradiations in order to select biotypes resistant to benomyl and iprodione. Resistance to benomyl was not induced but a number of biotypes of each fungus were isolated from irradiated cultures that showed resistance to iprodione (EC50 values of up to 56·4, 177·8, 171·5 and 216·4/μg/ml for Chaetomium globosum, Trichoderma harzianum, T. viride and Trichoderma sp., respectively). With three exceptions, all selected biotypes retained their fungicide‐resistance after being extensively cultured on fungicide‐free medium. In general, growth of the fungicide resistant biotypes on MEA plates and in MYE liquid medium was equal to that of the corresponding wild‐type strains. One resistant biotype of Trichoderma sp. (C62UV3) grew significantly (P < 0·05) better both on solid and in liquid medium. This resistant biotype was also observed to have greatly enhanced conidial production on agar plates. All fungicide resistant‐biotypes retained the ability to antagonize the pathogen Sclerotium cepivorum in dual culture and, in two instances (A53UV1, A53UV5), exhibited greater antagonism as evidenced by the production of larger inhibition zones. Similarly, with two exceptions, the resistant biotypes retained the ability to control onion white rot disease caused by S. cepivorum in the glasshouse. In particular, two fungicide‐resistant biotypes (Trichoderma sp. C62UV3 and Chaetomium globosum A53UV1) reduced white rot of onion more effectively than did their respective wild‐type parents.