Sodium bicarbonate is deleterious to maintenance of soil pH and soil structure and may leave white foliar deposits, so numerous applications with resulting runoff should be avoided. Commercial formulations of neem oil will also control powdery mildew. Interveinal, angular purple, red, or brown spots appear on leaves, followed by leaf yellowing and abscission. Fruiting bodies of the fungus occasionally may be observed on the undersides of leaves. Downy mildew can be reduced by increasing air circulation through pruning and avoiding frequent overhead irrigation.
Downy mildew is most likely to cause problems in coastal areas of California.
Avoid overhead watering and prune back severely affected canes. During the winter collect and dispose of leaves remaining on the plants as well as those that have fallen off. Low levels of damage can be tolerated without significant losses. Preventive applications of fungicides can be used, but frequent applications may be needed and may not be justified in garden or landscape situations. Small black fruiting bodies are often present in spots on the upper sides of leaves.