Benzimidazole resistance in Pseudocercosporella herpotrichoides: results of ADAS random surveys and fungicide trials in England and Wales, 1982–19841

Surveys of 59 and 528 randomly selected winter wheat and winter barley crops in 1982 and 1983 respectively, showed that benzimidazole-resistant strains of the eyespot fungus (Pseudocercosporella herpotrichoides) were common and widespread. The frequency of occurrence of resistant isolates, both in 1982 and 1983. was positively correlated with the number of benzimidazole fungicide applications since 1975. Resistance was much more frequent in R-type than in W-type strains of P. herpotrichoides. Fungicides or fungicide mixtures, applied to winter wheat at growth stage (GS) 31, were compared for control of eyespot at six sites in 1983 and one in 1984 where benzimidazole-resistant strains were readily detected prior to spraying. Carbendazim alone gave no control of eyespot and in some trials actually increased disease levels compared with the unsprayed control. Prochloraz, either alone or in mixture with carbendazim, was the most effective fungicide giving a moderate control of eyespot (33–42% reduction) and cost-effective yield increases. DPX-II6573, tested only in the 1984 trial, was as effective as prochloraz. The practical implications of these findings are discussed.