Powdery Mildew Control in Fort Collins

What is powdery mildew?

Powdery mildew is caused by a fungus and is one of the most common plant diseases. It affects almost all plants, but is most common in ash, lilac, cherry, apple, crabapple, pear trees, aspen and cottonwood trees. There are several types of powdery mildew, and they all cause similar damage, unsightly white or gray powder covers leaves, leaves may be deformed, turn yellow and fall from tree, but it is rarely fatal.

William M. Brown Jr., Bugwood.org

How to identify powdery mildew

Look for patches of white to gray powder on leaf surfaces in mid to late summer, more severe infections may grow to cover entire leaf. It is most often found on top of leaves, but it is also found under leaves, and on buds, shoots and flowers. Infected leaves may be deformed and fall prematurely. The powder is made up of the pinhead-sized fruiting bodies of the fungus, and starts off white, changing to yellow-brown and finally black.

Petr Kapitola, Central Institute for Supervising and Testing in Agriculture, Bugwood.org

Life cycle of powdery mildew

The fungus that causes powdery mildew overwinters as black fruiting bodies or fungal threads on leaf debris. In the spring, the fruiting bodies create spores that are carried by raindrops, wind and insects to the trees. The spores germinate and grow on the surface of the plant, and may spread over entire plant. The fungus forces feeding tubes into the plant to obtain its water and nutrients. The powdery mildew remains on the leaves and plant materials when they drop in the fall, where they overwinter.

Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org

How to control powdery mildew

Plant tree varieties that are resistant to powdery mildew, and plant them in sunny locations. Remove and destroy infected plant materials after the leaves drop in the fall to prevent reinfection next spring. Prune for better air circulation to reduce the humidity, and water in the morning to allow the plant to dry before night because the spores require high humidity to germinate, although this fungus doesn't require water to germinate, and the spores can be washed off with water. Powdery mildew prefers new growth, so avoid over fertilization especially in the late summer, to limit the new growth available to the fungus. Sulfur products, potassium bicarbonate, horticultural oils, milk and fungicides can all be used to control powdery mildew.

Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org

Other resources

• CSU Extension: Powdery Mildews
• UC IPM: Powdery Mildew on Ornamentals
• BugwoodWiki: Powdery Mildew
• APS: Identification of Powdery Mildew Fungi
• Oklahoma Coop Extension: Powdery Mildews of Ornamentals and Fruit, Shade and Nut Trees
• Wikipedia: Powdery mildew
• Growing a Greener World: Controlling Powdery Mildew

Mike Schomaker, Colorado State Forest Service, Bugwood.org