Powdery Mildew

Blumeria graminis

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  • Powdery mildews are common on many kinds of plants. Although they may look the same, each kind of plant is infected by a different species of powdery mildew fungus. Powdery mildew fungi on grasses will not infect lilacs, phlox, roses or other garden plants. Powdery mildew may appear quite suddenly, usually in shaded areas, and most commonly on Kentucky bluegrass. The grass blades may look as if they were dusted with flour. The white to gray powder is a combination of the mycelium and spores of the powdery mildew fungus. The mycelium grows over the surface of the leaf, absorbing nutrients from the plant. Later, the leaf may turn yellowish and begin to dry up and die, but the leaves often support the presence of the powdery mildew fungus for some time without significant injury. Powdery mildew is most common in turf from July to September, and occasionally in the spring. It occurs mainly during overcast periods of cool, moist weather. Spores can infect leaves in less than two hours, and new spores can be produced in abundance in less than a week. Air currents carry the spores to new grass plants. Disease development can be so rapid that powdery mildew may seem to appear very suddenly.


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