Powdery mildew, caused by the fungus Erysiphe necator, is an important disease of grapevines in Australia and worldwide. If not adequately controlled, it reduces yield and quality of grapes and the resulting wines. Many Australian wineries have thresholds for powdery mildew contamination, such that grapes that exceed 3-5% of the surface area affected (disease severity) may be rejected or downgraded.
Powdery mildew appears as a characteristic grey-white powdery growth on green tissue of the grapevine. Infections around time of flowering or shortly after cause the most damage to grapes. Main symptoms on leaves are irregular yellow blotches best seen on the upper surfaces and, on varieties like Chardonnay, with associated browning of the smallest (tertiary) veinlets on the under sides. The blotches soon show the typical white mildew as spots expand and merge across both surfaces of leaves. Eventually the whole surface is covered.
Powdery mildew is common across Australian vineyards, with some varieties being more susceptible than others. The disease develops in the absence of free moisture and generally progresses independently of weather, though mild, humid conditions are favoured and cooler temperatures may be limiting. Powdery mildew is driven by the amount of inoculum (spores) carried over from the previous season and present in buds, bark or leaf litter.
Control of powdery mildew in vineyards is largely by chemical fungicide sprays, with many vineyards being sprayed 6-7 times per season. With improved knowledge of disease epidemiology (ie how the disease spreads), of the need to time sprays appropriately, improved spray technology and access to an excellent array of fungicides, direct crop loss from powdery mildew tends to be minimal.
Powdery mildew assessment tools
The powdery mildew online resource and PMapp – available for Apple and Android devices – are designed to assist the diagnosis and assessment of powdery mildew on grape bunches.