The mycelial growth of sixteen sensitive isolates of B. cinerea was strongly inhibited by less than 1·0 μ /ml of methyl-benzimidazol-2-ylcarbamate (MBC); ED 50 values for these isolates were between 0·06 and 0·19 μ /ml MBC. Twenty-six ‘fully tolerant’ isolates of B. cinerea grew well on agar media containing 10000 μ g/ml MBC, although their production of sclerotia was delayed and in some cases reduced by high concentrations of MBC. A further four ‘partially-tolerant’ isolates showed an intermediate level of tolerance to MBC with ED 50 values between 1·3 and 6·3 μ /ml. Isolates of other fungi tested were also clearly separable into sensitive and fully tolerant groups. Germination of conidia of sensitive isolates of B. cinerea was insensitive to MBC concentrations which completely inhibited mycelial growth, but germ-tubes were abnormal and did not develop further. Sensitive and tolerant isolates of B. cinerea were not morphologically distinguishable. Attempts to induce adaptation of sensitive isolates of B. cinerea to increased levels of MBC-tolerance were unsuccessful. Tolerant isolates showed a similar stability in their reaction to the fungicide.
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