A MICROSCOPIC EVALUATION OF THE SENSITIVITY OF VENTURIA-INAEQUALIS POPULATIONS TO STEROL DEMETHYLATION INHIBITORS

A method of examining the sensitivity of Venturia inaequalis populatio ns to sterol demethylation-inhibiting fungicides (DMIs) was developed. The sensitivity of the fungus was quantified microscopically by the a bility of conidia to produce appressoria, primary hyphae, subcuticular stromata, and conidiophores in vivo, on leaves from apple seedlings. One orchard that had never been treated with any fungicide was compare d with two orchards treated with DMIs for several years. Only conidia collected from the untreated orchard were inhibited after development of appressoria by bitertanol and flusilazol or during development of s ubcuticular stroma by fenarimol and pyrifenox at recommended concentra tions. Up to 40% of conidia collected from conventionally treated orch ards developed conidiophores similar to the water control, although tr eated with the recommended concentrations of the fungicides fenarimol and pyrifenox.