Grape -- Powdery Mildew
Cause: Erysiphe necator (formerly Uncinula necator), a fungal disease common to all areas of the PNW. The disease tends to be more severe on the westside of the Cascades but is a chronic problem in arid districts where over-the-canopy irrigation is used for early-season frost protection or watering. Vitis vinifera (European) cultivars commonly are susceptible to powdery mildew. Other hosts include Boston ivy, Virginia creeper, and Ampelopsis (porcelain berry). The fungus may overwinter as a group of thin threads called hyphae, inside the vine’s dormant buds and/or as small black bodies (chasmothecia) on the exfoliating bark of the vine.
Buds on new shoots can be infected 4 to 6 weeks after shoots start growing but not after bud scales become suberized. These new infected buds remain quiescent until the next growing season. The fungus infects developing buds during the growing season. Shortly after budbreak, the fungus becomes active and covers the emergent shoot with a large white mass of threads or mycelium (“flag shoots”). Flag shoots have rarely been observed in western Oregon or eastern Washington.
Chasmothecia on exfoliating bark release sexual spores during rainy weather above 50oF from budbreak through bloom. This weather also favors infection that results in individual powdery spots, called colonies, on the surface of leaves growing close to the bark.
Many asexual spores (conidia) are produced on the surface of powdery mildew colonies. Under optimal conditions of mild temperatures and high humidity, a single spore can germinate, infect the plant, and produce a new colony and a new crop of spores in 3 days. Temperatures over 85oF inhibit conidia germination. Free water from rain and/or irrigation can wash conidia off of a colony, burst conidia, or result in poor or abnormal germination of the conidia.
Grape berries are highly susceptible from the time calyptras (hoods) fall off to shortly after bloom when berries are about pea size (EL 29 to 31). Susceptibility of the fruit drops rapidly after that time. Grapes do not get new infections on fruit after 8% Brix but can still have sporulation up to 15% Brix. Leaves and canes, however, can be infected up to and past harvest.
Symptoms: Powdery mildew can attack all above-ground plant parts. In early stages, whitish or grayish patches are on leaves and, if severe, ultimately cover both surfaces. Colonies are more easily detected in full sunlight. Later in the season, the mildew darkens and is peppered with minute black dots (chasmothecia). On fruit, the fungus at first may look grayish or whitish but later has a brownish, russeted appearance. Infected fruit cracks and drops from the cluster. Even blossoms sometimes can be infected, causing them to dry up or fail to set fruit. When green shoots and canes are infected, the affected tissues appear dark brown to black in feathery patches. Patches later appear reddish brown on the surface of dormant canes.
Flag shoots are difficult to detect. Some young shoots may be covered with a large white mass of threads or mycelium. Others may have only a hint of thin threads on the shoot. Shoots generally are delayed in bud break and appear stunted and somewhat yellowed compared to healthy shoots.
