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The fungus Chondrostereum purpureum, the causal agent of silverleaf, when grown in liquid culture or on agar, secreted extracellular proteinases into the medium. Proteinase activity in the mycelium itself was barely detectable. At least 95% of caseinolytic activity in the extracellular fluid (ECF) was inhibited by chelating agents. Fluid dialysed against EDTA was activated by metal ions, confirming the presence of metalloproteinases. Caseinolytic activity of ECF was also largely abolished by sulphydryl compounds and reagents. Superdex G75 chromatography and SDS-PAGE analysis of the proteinases present in concentrated ECF indicated at least four components with proteolytic activity. In addition to the major metalloproteinases, a very low level of a chymotrypsin-like activity was detected as well as high levels of the exopeptidase, leucine aminopeptidase. A wide range of caseinolytic activity levels was found when 29 C. purpureum isolates from New Zealand were grown in liquid culture, but much less variation was observed on agar. Proteinase activity was found throughout the infection zone of silverleaf-susceptible plants but could only be detected at the wounding site of resistant cultivars. The possible roles of these proteinases in infection and growth of the pathogen are discussed.

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