With the help of active types of oxygen, extracellular enzymes and mechanical processes, Botrytis cinerea is capable of infecting plant tissue. Whereas no clear correlation was found between the activities of protease, pectolytic or other cell wall degrading enzymes and virulence, a positive correlation between pathogenicity and the intensity of active types of oxygen released was apparent in some isolates. It is assumed that these toxins result from the activity of glucose or xylose oxidases. Antioxidants inhibited the infection. It is concluded from these results, that active types of oxygen play a decisive role in the infection process.

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Various morphological and physiological characteristics, such as mycelial colour and appearance, mycelial growth rates, sporulation, pathogenicity on different hosts, activity of cell‐wall degrading enzymes, competitive ability and osmotic sensitivity of five dicarboximide‐resistant isolates of Botrytis cinerea were compared to those of their sensitive parent strains. There were only small reductions in mycelial growth rates of resistant strains as compared to the sensitive ones, whereas all of them showed greatly reduced rates of sporulation. With only one exception, the pathogenicity of the resistant isolates was reduced by varying extents compared with the sensitive parent strains. While the proteolytic activity of resistant strains tended to be higher, the activities of peclolytic enzymes were often lower than those of the sensitive isolates. In competition tests, resistant conidia often completely disappeared after a few passages on untreated plants. In only one of the resistant isolates tested so far was dicarboximide resistance related to a high osmotic sensitivity.

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