Comparative Susceptibility of Various Grapevine Cultivars to the Powdery Mildew Fungus<i>Uncinula Necator</i>

Abstract

The susceptibility of various grapevine cultivars to <i>Uncinula necator</i> in California was assessed in two widely separated vineyards and by inoculating potted grapevines in a controlled environment growth chamber. Cultivars varied from very susceptible to resistant. Cultivars of <i>Vitis labruscana</i> and hybrids of <i>Vitis vinifera</i> x <i>Vitis rupestris</i> were resistant. Most, but not all, cultivars of <i>V. vinifera</i> were susceptible. No <i>V. vinifera</i> cultivar was immune. Under favorable conditions sporulating mildew colonies could sometimes be found on resistant cultivars of <i>V. vinifera</i>. Disease severity on the fruit was significantly correlated with that on the leaves. Observations of the generation time (period from deposition of conidia to production of conidia) under controlled environment allowed separation of very susceptible cultivars from susceptible ones. The results of growth chamber and field experiments were similar and showed that Thompson Seedless was very susceptible and that French Colombard was moderately resistant. The results suggest that chemicals used for mildew control can be reduced on some <i>V. vinifera</i> cultivars.