Powdery Mildew in Eastern Washington Commercial Grape Production: Biology and Disease Management

Powdery Mildew in Eastern Washington Commercial Grape Production: Biology and Disease Management

EM058E
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Michelle Moyer, Statewide Viticulture Extension Specialist, WSU Irrigated Agriculture Research & Extension Center, Prosser, WA, Gary Grove, Plant Pathologist, WSU Irrigated Agriculture Research & Extension Center, Prosser, WA
There are few plant diseases that have the combination of international distribution and importance. Grapevine powdery mildew is one such disease, and is present in almost all international viticulture areas where susceptible grape varieties are produced. This disease is predominately driven by weather and canopy microclimate conditions, and has a histroy of fungicide resistance development. Because of this, understanding it's biology and management is critical in maintaining control in the vineyard.
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Key Information

  • The grapevine powdery mildew fungus prefers mild temperatures with high humidity. Only the very early stages of development require free water.
  • High temperatures (>95°F) and low temperatures (<50°F) can debilitate or kill the fungus.
  • Fruit are susceptible to infection from pre-bloom up to three weeks post fruit-set (Eichorn-Lorenz [EL] Stages 15-31; BBCH Stages 55-75). In Vitis labruscana, ‘Concord’ berries have a somewhat shorter susceptibility window, although the rachis remains suscep- tible throughout the growing season.
  • The pathogen Erysiphe necator can quickly develop resistance to fungicides, so proper selection of materials, rates, and use patterns is critical in preventing control failures due to resistance development. Proper selection is also important in preserving fungicide chemistries. Cultural practices that reduce disease pressure mitigate the potential for resistance development.

Introduction

There are few plant diseases that have the same combination of international distribution and importance as grapevine powdery mildew (PM), which is present almost anywhere that susceptible grape varieties are grown. This disease, caused by the fungus Erysiphe necator, is believed to have originated in northeast North America, where the native grapevine species demonstrates a significant level of tolerance or resistance to this pathogen. However, the European wine grape species, Vitis vinifera, which did not evolve with this pathogen, is susceptible and severe symptoms can occur on fruit, foliage, and shoots of the plant when spread of the pathogen is extensive

(Figures 1-3). Severe cluster infections render the fruit unusable, and even modest infections can predispose fruit to secondary invasion by spoilage microorganisms and Botrytis bunch rot (BBR). Foliar infections can significantly reduce the photosynthetic capacity of the plant and, in severe cases, cause premature defoliation. Heavy, early season infections can predispose buds and canes to winter injury by compromising tissue integrity.

Figure 2. In severe cases, or shaded canopies, powdery mildew colonies can be found on the upper and lower leaf surfaces. On leaves exposed to the sun, colonies are likely to be found on the lower leaf surface only. Photo courtesy of Michelle Moyer.
Figure 1. Severe powdery mildew infection on clusters can cause fruit cracking and arrested development (no sugar accumulation). Infections of this visible nature generally are not noticeable until just before véraison, even though infections likely occurred around bloom. Photo courtesy of Gary Grove.

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