The generic term “mildew” is popularly used to describe powdery mildew, a fungal disease that strikes fruit trees and other plants. The disease is caused by several fungi, depending on the fruit. The conditions under which these fungi thrive and the symptoms of their infection are similar, as are their recommended treatment.
The fungi thrive in high relative humidity. A series of unconnected powdery white spots of fungi appear on the leaves and other parts of a tree. The spots come together to form a gray to white coating that looks like dust.
The spots usually appear late in the growing season. The fungus can appear as rough, corky spots on apples, apricots, nectarines and peaches. Powdery mildew can stunt and distort buds, leaves, growing tips and the fruit. The leaves often turn yellow and drop early. Mildew fungi remove nutrients from a tree, causing it to lose vigor and grow slowly.
During the growing season, white patches of infection produce spores that the wind blows to other parts of the tree and to neighboring trees. The fungus spends the winter as white patches of powder on infected plants and on fallen leaves and other plant debris; small black dots within these wintertime white patches are called cleistothecia. They release sexual spores in the spring that are blown by air currents and infect emerging leaves.
Plant fruit trees where they will get lots of sun and good air circulation. Applying too much fertilizer can cause mildew problems; slow-release fertilizers are best. Overhead sprinkling for long periods of time can wash the mildew off the trees. Do not plant apple or plum trees near rosebushes, because Sphaerotheca pannosa, a powdery mildew fungus, can jump from rosebushes to stone fruit.
Remove infected shoots and buds from apple trees when they are dormant. Infected buds look shriveled or flattened, and they have a fuzzy layer of mildew on their surface. Prune infected parts of trees when they are dormant.
Spray commercial horticultural oils on trees to eradicate infections of powdery mildew. Organic growers may use oils made from the neem plant or jojoba seeds. Oils should not be applied if the temperature is above 90 degrees F or the plants are stressed by drought.
Commercial sulfur compounds are marketed that can be made wet and contain surfactants that help them adhere to plants. If you spray a fruit tree with an oil, do not apply sulfur for two weeks. Sulfur can injure plants if it is applied when the temperature is near or over 90 degrees F. Never apply sulfur to apricot trees.
Spraying biological fungicides containing the bacterium Bacillus subtrilis on trees will inhibit or kill mildew fungi, although they are not as effective as horticultural oils or sulfurs.
Horticultural oils both protect trees from mildew and eradicate it. Oils work best when the tree is thoroughly covered and the mildew symptoms are mild to moderate.
Both horticultural oils and sulfur are best applied early in the growing season before the disease has developed. You may need to reapply them every 7 to 10 days as growing trees produce new plant tissue.
Susceptible Fruit Trees
Give special care and attention to varieties of fruit trees that are most susceptible to powdery mildew. Among apples, those include Gravenstein, Jonathan, Rome beauty and yellow Newton. Bing, black tartarian and rainier are susceptible cherries. Susceptible varieties of peaches include elegant lady, fairtime, Fay Elberta and summerset. Black beauty, Gaviota, Kelsey and Wickson are plums that may need special protection.