How can grape powdery mildew be recognised?
Grape powdery mildew can produce whitish dusty growth on leaves, shoots and clusters. Diagnosis should consider the location of lesions, berry development and the vineyard’s disease history, rather than white colour alone.
Symptoms observed in a vineyard
1. Powdery growth on leaves and clusters Symptoms associated with Erysiphe necator may begin as pale leaf areas before whitish fungal growth becomes obvious. Cluster stems and berries can also be affected. Examine both surfaces of several leaves to distinguish disease from dust, spray deposits and other problems. Expanding irregular colonies differ from loose material simply deposited on a leaf. Whether a patch can be rubbed off is not a reliable diagnostic test by itself. Photographs and local professional inspection may provide a stronger basis for identifying the cause. 2. Lasting marks on berry skins After earlier infection, mature berries may retain brown, netlike scarring. Infection of young berries can interfere with growth and skin development, predisposing fruit to cracking and secondary rots. However, not every split grape has mildew; water fluctuations and cultivar-related factors can also cause cracking. Do not mistake an old skin scar for a new active infection. Comparing cluster photographs through the season helps establish whether the earliest changes appeared on leaves or berries and how damage developed before the current appearance. 3. Distinguishing downy mildew Grape downy mildew and powdery mildew are caused by different organisms. Downy mildew may produce oil-like leaf lesions and growth beneath leaves under suitable humid conditions, but appearance alone is not always sufficient for certainty. Continued powdery-mildew spread does not always require free water on the leaf surface. A lack of rain therefore does not mean risk has ended. Initial infection, overwintering and subsequent spread involve different conditions. Follow local weather and vineyard history together rather than treating the two diseases as interchangeable problems.
Protection throughout the growing season
4. Consider cultivar and training system Grape cultivars may differ in susceptibility. Dense, shaded canopies complicate inspection and coverage where authorised applications are needed. Organised shoots and balanced leaf area support air movement. However, suddenly removing every leaf around clusters can cause sunburn in hot districts. Assess rootstock and vigour alongside row spacing and crop load. A home pergola and a commercial vineyard row may need different maintenance programmes. The objective is manageable, balanced growth rather than maximum exposure of every berry to direct summer sunlight. 5. Early inspection and weather information Check new leaves and shoots regularly from budbreak, paying particular attention to areas affected in the previous year. Assessing disease only near harvest can miss opportunities for early protection. Temperature-based risk models are used in some regions, but a model developed for another climate should not simply be copied. Records of date, developmental stage and affected proportion support local advice. Distinguishing old marks from new active growth makes decisions more meaningful and helps determine whether current management is limiting additional infection. 6. Targeted, authorised management If protective treatment is justified, product choice, timing and intervals should follow local technical recommendations. Even materials such as sulphur require consideration of temperature, cultivar sensitivity and compatibility with other applications; do not improvise mixtures. Use only products authorised locally for the relevant grape use, following the label and harvest interval. Existing skin scars will not disappear afterwards. Assess success through fewer new infections and healthier clusters, rather than expecting one product to guarantee season-long protection irrespective of weather, growth and application conditions.