Powdery mildew is a fungal disease that looks like a white powdery coating on susceptible leaves. It usually develops during periods of high humidity and cool temperatures and in shady areas with poor air circulation. The plants most often affected are currants, roses, caragana, cotoneaster and some bedding plants. It can also affect grass in shady places. Deciduous leaves become distorted and may die, while grass blades seem to tolerate the problem.
The fungal spores require an “obligate moisture period” in which to germinate and enter a leaf, so prevention of these moist conditions is key. Watering susceptible plants in the morning, not the evening, will reduce the amount of time that water remains on the leaves and thereby reduce the chance of infection, as will drip irrigation. Planting susceptible species in areas where they will receive both sun and adequate air circulation and preventing crowding will reduce powdery mildew. Where possible, plant mildew resistant cultivars.
At the first sign of powdery mildew, try spraying with a teaspoon of baking soda in a spray bottle of water. There are also fungicides available that will keep powdery mildew under control.