Abstract
In 1975, spray applications of thiophanate-methyl and benomyl failed to control scab in several Japanese pear orchards. Mycelia of Venturia nashicola isolates, obtained from these orchards, developed normally on PDA supplemented with 50-800ppm of thiophanate-methyl or benomyl. Sensitive isolates did not grow even at 1ppm. However, conidia, formed in vitro, germinated in suspensions of these fungicides, irrespective of tolerant or sensitive isolate. Tolerant isolates exhibited vigorous germ tube elongation, but sensitive isolates produced only short germ tubes in these suspensions. In a protection test on Japanese pear treated with 100ppm of each fungicide, tolerant isolates developed sporulating lesions. Tolerance of V. nashicola to thiophanate-methyl and benomyl in the field was therefore demonstrated.
