EFFECTS OF LEAF MATURITY AND CULTIVAR RESISTANCE ON DEVELOPMENT OF THE POWDERY MILDEW FUNGUS ON GRAPEVINES

The development of grape powdery mildew (caused by Uncinula necator) was compared on leaves of various maturities of resistant and susceptible grapevine (Vitis vinifera) cultivars growing in a vineyard and in a growth chamber. As leaves matured, colony hyphal length and the percentage of germinated conidia that developed hyphae decreased on both the resistant and susceptible cultivars. In both growth chamber and field experiments, significantly more germinated conidia developed hyphae on young leaves of susceptible cultivars than on young leaves of resistant cultivars. Temperatures of 20, 25 and 30.degree. C did not significantly affect the number of germinated conidia that developed hyphae or interact with cultivar resistance to affect growth of hyphae. Hyphal growth was greatest at 25.degree. C. Usually no signs of penetration or host response were associated with germinated conidia that did not develop beyond the formation of an appressorium, suggesting that mildew resistance may in part involve factors that inhibit or prevent penetration of epidermal cells.