Powdery Mildew

Powdery Mildew is a fungus that infects developing shoots on apple varieties. It does little damage on apples of Delicious variety and does not require fungicides but heavy infections make it more difficult to market fruit and can interfere with photosynthesis. Below you will find information on how to identify this disease, how to avoid it, and how to treat it.

Cause:

• A fungus that infects developing shoots on some apple varieties

Symptoms:

• White powdery fungus that grows on the tops of leaves

• Primarily occurs on the lower branches

• Leaves may be twisted, distorted, or wilting

Timing:

• Infections occur when the relative humidity (RH) is greater than 70%

• Opportune temperatures for infection to occur fall in the range of 66-80°F (19-27°C)

• Occurs mainly in June and July

How to Avoid:

• Purchase trees that have a higher degree of resistance to PM (find suggested varieties here).

• Ensure proper ventilation around and within trees.

• Prune multiple times a year; you should be able to see some sunlight through the tree even with foliage on the tree.

How to Treat:

• A baking soda solution (4 teaspoons baking soda per gallon of water, with 1 teaspoon of liquid dish soap as a surfactant/adhesive agent) will work on contact if mildew begins to establish on new leaf growth in spring.

Why it Matters:

• Powdery mildew on most deciduous trees does little damage and does not require fungicides.

• A heavy powdery mildew infection makes it more difficult to market fruit and can interfere with photosynthesis.

Sources: MyIPM, Pennsylvania State University Extension, Cornell University Integrated Pest Management, Colorado State Extension, and Michael Phillips, “The Holistic Orchard”. Photo credit: Patrick L. Byers, Horticulture Specialist.