The incidence of benomyl tolerance in Verticillium fungicola, Mycogone perniciosa and Hypomyces rosellus in mushroom crops

SUMMARY Of the 229 isolates of Verticillium fungicola examined 63 % were found to show some benomyl tolerance and 53% were very tolerant having EDg, values in excess of 50 ppm. There was a tendency for the tolerant isolates to have a slower growth rate than the sensitive ones on unamended potato dextrose agar (PDA) and forty‐five isolates (19 %) showed growth stimulation at 5 ppm benomyl. No major differences were found in ten isolates of V. fungicola tested on benomyl, thiabendazole and thiophanate methyl agars or of twelve isolates compared on benomyl and carbendazim hydrochloride agars. Tolerant isolates of V. fungicola and higher levels of crop loss were linked in the east and north of the country but not in the south‐east. Tolerance was not found in sixty‐six isolates of Mycogone perniciosa or twenty‐four of Hypomyces rosellus. Benomyl was used on 89 % of the farms in the survey either applied to the casing (34%) or as a spray (55 %) or both (10%).