Field isolates of Botrytis cinerea Pers. from tomato crops gave rise to two distinct morphological types after successive single spore isolations; type 1 (T1) characterised by regular, rapidly growing mycelium and type 2 (T2) characterised by sparse, often distorted, slow growing mycelium. The frequency with which isolates produced these morphological types varied considerably. The reaction to benomyl of both types obtained remained the same as that of the parent isolate through successive single spore isolations on fungicide free agar over 10 generations. Benomyl tolerant field isolates could not be distinguished from sensitive isolates on the basis of colony morphology. Examination of 188 isolates of B. cinerea collected from 10 different tomato crops showed that sensitive isolates tended to be more pathogenic to detached tomato cotyledons and grew faster in vitro than tolerant isolates. In vitro and in vivo studies revealed no difference in the competitive ability of benomyl tolerant and sensitive isolates in mixed culture. In a study of 17 Lancashire tomato crops all were found to contain benomyl tolerant strains of B. cinerea although in some cases benzimidazole fungicides had not been used on the nursery for up to three years. Examination of B. cinerea in tomato crops suggested that the majority of stem lesions arose from infected leaf scars and were non-aggressive. Side shoot stumps or scars were also liable to infection and the resulting lesions were more likely to become aggressive than lesions at leaf scars. Conidia were considered to be the major source of inoculum although lesions at leaf scars could take up to 61 days to develop after deleafing. This delay was attributed to latent infection. Artificial infection of the petiole with B. cinerea prior to deleafing greatly reduced the susceptibility of the resulting leaf scar to subsequent attack by B. cinerea. Extracts from infected stems were shown to delay the germination of B. cinerea conidia when compared to extracts from healthy stems. This inhibition of germination was attributed to a resistance factor (RF) produced by the fungus or host in response to infection
