All plants are subject to disease. Some may suffer from only a few diseases while others, such as roses, can be damaged or killed by many.

Powdery mildew is a very common disease that infects many plants including houseplants, vegetables, flowers, shrubs, grapes and fruit trees.

Home gardeners easily identify powdery mildew. It appears as a dusty white or gray coating over leaves and other plant parts. This fungal growth can be partially removed by rubbing the leaves. Prompt recognition and control action can prevent severe damage to plants.

Powder mildew begins as small circular white spots. These spots grow and produce a continuous matt of mildew. Often leaf curling and twisting may be observed before the fungus is noticed.

Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, disfigured shoots and flowers. Although the disease is usually not fatal, it may hasten defoliation and the infected plant may become unsightly.

The fungi that cause powdery mildew are different for different plants. Although the fungi differ, all of the powdery mildew diseases are similar in appearance and require similar management practices.

In general each species of fungus will be limited in the number of types of plants that can be attacked. For example, the species of fungus infecting lilacs will not cause powdery mildew on roses.

Powdery mildew infects plants when temperatures are in the 60 to 80 degree range and will not occur during the hottest days of summer.

High humidity favors the growth of the disease. Overcrowding and shading tend to keep plants cool and promote higher humidity. These conditions are highly conducive to powdery mildew development.

The best way to limit powdery mildew damage is to avoid over crowding of plants and avoid planting in areas with poor air circulation.

Be sure to rake up and destroy all dead leaves. The spores sit over winter in the dead leaves and will re-infect plants the next spring.

Chemical controls are available. On vegetables use Daconil and Funginex and sulfur components are used on ornamentals.

About 10 years ago, Cornell University developed an effective control. Plants are sprayed weekly with a mixture of 1 tablespoon of baking soda and 2½ tablespoons of horticulture oil in a gallon of water.

Researchers in Brazil have found that weekly sprays of skim milk controlled powdery mildew and also acted as a foliar fertilizer. The solution used is 1 part skim milk and 9 parts water.

Do not use a higher concentration of milk or milk that contains fat. Higher milk concentration may cause an innocuous fungus to grow. Milk with a fat content may create odors.

In New Zealand, melon growers have been so successful using the skim milk solution on the plants that grape growers for wine are beginning to experiment with it on vines.

Pete Hale of Sarver is a Master Gardener with the Penn State Master Gardener program.