Tolerance ofPhytophthora megaspermaIsolates to Metalaxyl

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Abstract
Thirty-five isolates of P. megasperma were tested in vitro for tolerance to the systemic fungicide metalaxyl (CGA 48988). Isolates were collected before commercial use of metalaxyl. Tolerance was measured by comparing isolate growth and oogonia and sporangia formation on amended and control media. Based on growth at 0, 1 and 10 .mu.g/ml metalaxyl, 15 isolates were highly sensitive, 8 were moderately tolerant and 12 were highly tolerant of the fungicide. No isolates in the highly sensitive group formed oogonia at 1 .mu.g/ml, whereas 6 isolates from the moderately tolerant group and 8 from the highly tolerant group did. The number of isolates that formed sporangia and the mean number of sporangia formed per isolate decreased with increasing fungicide concentration. At least 2 of 280 single-zoospore isolates were more tolerant to metalaxyl than their parents, suggesting that commercial applications of the fungicide may select for naturally occurring tolerant strains, which may in turn lead to loss of disease control.

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