Powdery mildew is a fungal disease caused by Podosphaera leucotricha on apple trees. The mildew first appears as greyish, powdery patches on leaves and on apples, and it can result in the shrivelling and death of shoots and leaves. Fruit may become infected as well, and the mildew can create a patchy russeted pattern on the skin of the apple. Powdery mildew grows best in humid, warm conditions, but can survive in drier conditions as well; it doesn't need rain to spread. Prune and spray apple trees with fungicides to help control powdery mildew.
Prune infected buds and stems in winter, and destroy them--don't leave them at the base of the tree. Oregon State University recommends cutting below the infected portion by 2 to 3 inches, and also says that if you accidentally touch the mildewed area with the shears, you should sterilise them before cutting anywhere else. Use either rubbing alcohol or a mixture of bleach and water to sterilise the shears.
Prune the rest of the tree branches in spring so air can pass between them more easily. The University of Vermont says this will reduce humidity among the tree branches.
Spray the tree with a fungicide regularly, starting when the buds have appeared and are still closed tightly, up to the time when terminal growth stops for the season. Follow all safety precautions required by the manufacturer. The specific fungicide you need will vary according to environmental conditions. For example, the West Virginia University Extension Service says wettable sulphur can damage leaves when temperatures are hotter than 26.7 to 29.4 degrees Celsius. The university's Kearneysville Tree Fruit Research and Education Center notes that more frequent spraying, such as weekly, at lower amounts, is better than larger amounts of fungicide sprayed less often. For organic apples, the University of California's Statewide Integrated Pest Management Program recommends sulphur, a combination of sulphur and lime, and horticultural oils as controls. Contact your county's agriculture department or university extension for more guidance for your area.
Cultivate apple varieties that are more resistant to powdery mildew. All apples have some susceptibility, according to the University of Illinois Extension, but varieties such as Red Delicious, Winesap, and McIntosh have better resistance.
Prepare fungicides according to manufacturer directions, and wear safety goggles, gloves, a mask, and any other required safety gear. Oregon State University advises bathing after using the fungicide. Check with your county agriculture department or university extension to ensure local laws allow you to use the fungicide in question.