POWDERY MILDEW - Powdery mildew is caused by a fungus called Microsphaera penicilliata. Most species of woody shrubs and trees can be infected.  Plants that are in shady areas and lacking good air circulation are most susceptible. White, powdery areas form on new leaves and may eventually cover the entire leaf surface.  Look for powdery patches on the surface of new growth.  This fungal disease can cause the new growth to be curled and deformed; however, powdery mildew is usually just an aesthetic problem and will not harm plants in the long run.  In small trees, the loss of photosynthesis can weaken the plant and make it more prone to other diseases or pests. If there is very dry weather in the summer, there will be fewer problems with powdery mildew. The spores of the fungus are spread by water, so if there is not much rain the powdery mildew does not have a chance to grow or spread.  Only plants that are in more shaded areas and that get watered by sprinklers that wet their foliage are affected.This fungus is most common in late summer when the weather is warm and humid. The fungus overwinters on bud tissue. In the spring, the fungus grows on the leaf surface and sends fine threads (called haustoria) into the leaf cells.  These threads extract nutrients from the cells without killing them. To prevent powdery mildew: Avoid heavy doses of nitrogen fertilizer and excessive pruning.  Both these practices will encourage new succulent growth on which the fungus will thrive. Do not use overhead watering. Do provide your plants with good air circulation. When choosing plants, find types that are resistant to powdery mildew.