Powdery mildew can attack houseplants, grasses, flowers, vegetables, trees and bushes. Thousands of different fungi cause the easily recognizable disease. The fungi are host specific, meaning that although the powdery mildew may look the same, the fungus that attacks a lilac tree is not the same fungus that attacks the rose. Some plants have been developed to be resistant to powdery mildew disease. When purchasing plants, look for labels that identify these resistant varieties.
Powdery mildew looks like a talc on the top of the leaves. At first, it can sometimes be confused with common dust. The color is usually white or ash gray. Sometimes in badly infected plants, the mildew will show up on the undersides of the leaves and possibly even the stems.
Although the mildew looks like fine powder or talc, it is actually thousands of spores that can easily be transported from one part of the plant to another and even to other nearby plants. As the spores mature, the center may turn yellowish brown and then black.
The leaves of a plant with powdery mildew disease may eventually turn yellow and drop off. The plant may show distorted buds that refuse to open.
Most vegetation is susceptible to powdery mildew diseases, especially in hot and dry climates. The fungi grow and feed on dry surfaces of the plant, but in order for the spores of the fungi to germinate it need moisture. Plants that are crowded together and have little air circulation will provide the fungi the moisture it needs to germinate.
Stressed plants are especially susceptible to powdery mildew disease. The plant's age and health are important factors in the damage done by the fungus. Younger, more succulent growth areas of the plant are more prone to infection than older parts of the plant.
A mild infection of powdery mildew disease can be treated by rubbing the spores
from the affected surface areas of the plant.
Nitrogen fertilizer helps the plant develop succulent new growth that is most susceptible to the fungi that causes powdery mildew disease. Once the disease has been identified, stop feeding the plant nitrogen fertilizer.
Reduce the humidity around the plant that allows the spores to germinate by eliminating overhead watering. Prune the plants back to allow proper air circulation. Destroy all infected parts of the plant as soon as they are removed from the plant.