Aspen leaves heavily infected with powdery mildew Canadian Forest Service	Close-up of aspen leaf infected with powdery mildew Canadian Forest Service

Introduction

Powdery mildew is a common fungal disease that can appear on many types of trees and plants.  While it will not necessarily kill outright, the plant or tree will become more susceptible to other problems and its appearance and the amount of fruit it produces will become compromised.

It can occur in any geographic region, but is most prevalent in the arid regions of western North America.  Weather conditions are a determining factor in the severity of this disease and it will, if left untreated, persist year after year.

What does powdery mildew look like?

Powdery mildew will initially appear as light green to yellow spots.  Typically spidery or threadlike white patches develop peppered with minute black fruiting bodies.  The plant gives the appearance of having been dusted with flour or talcum powder.

Powdery mildew is most likely to attack in the spring and the fall when the weather is cool and the humidity is high.

Most powdery mildew develops as thin layers of mycelium on the plant's surface. Spores or resting bodies make up the bulk and are the primary means of dispersal.  Powdery mildew spores are carried by the wind and rain to new hosts.  Excess water on the plant's surface can kill spores and inhibit growth of mycelia, and both spores and mycelia are sensitive to extreme heat and direct sunlight.

Control

To lessen the chance of the disease occurring, plant in non-shaded areas.  Space plants providing enough aeration and growing room.  Prune and thin out branches and monitor for any signs of infection.  Collect infected leaves and remove.  Provide enough moisture, always watering in the morning or late afternoon.

Powdery mildew may also be treated with commercially produced fungicides.  These products may include sulfur, fenarimol, dinocap, triadimifon or benomyl.  Refer to the instructions on the package to determine method and timing of application.  Weather conditions such as temperature and humidity are important factors when determining when to apply fungicides.

Recommended Product

Safer's Defender Garden Fungicide

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