Powdery mildew of turf

What is powdery mildew of turf?

Powdery mildew of turf is caused by the fungus Blumeria graminis, called Erysiphe graminis in older guides. Penn State says it is most common on Kentucky bluegrass. The fungus lives on the leaf surface and feeds on leaf cells. It does not attack stems, crowns or roots.

Shade is the main trigger. Purdue says outbreaks start in shady, slow-growing turf with poor air movement, during cool, cloudy weather in spring and fall. High humidity helps, but unlike many lawn diseases it does not need wet leaves or long dew periods. UC IPM says it does best around 65°F.

Purdue says the disease rarely causes lasting damage, so its effects are mostly cosmetic. It also notes this mildew does not spread to other landscape plants. The long-term fix is more light and air, or a grass that handles shade better.

People and pets: Purdue says turf powdery mildew poses no risk to children or pets and does not infect other landscape plants.

Symptoms and signs

• White to light gray powdery coating on the upper side of grass blades
• Starts as small isolated spots that spread until whole blades look dusted with flour
• In shady spots, the whole lawn area can look dull white or gray
• Older infected blades may turn pale yellow
• Tiny dark brown to black specks may form within the white growth

Powdery mildew of turf identification: what to look for

• Symptoms: From a distance, shaded turf looks white or light gray. Up close, blades carry small white spots about 1/16 inch across that can merge and cover the whole leaf.
• Signs: Fine gray-white spore masses and fungal threads on the upper leaf surface. Penn State notes tiny dark brown or black specks may form in the white growth.
• Symptoms: Badly infected older leaves turn pale yellow, but plants rarely die.
• Timing: Cool, cloudy weather in spring and fall. Penn State says in Pennsylvania it generally shows in late summer and fall.

Disease cycle

The fungus survives winter as threads or spores in dead and living grass, or as tiny dark resting bodies in leaf litter. In cool, humid, shady conditions, airborne spores land on leaves and start new white spots. More spores form on those spots and blow, or get carried by mowers and feet, to nearby blades. UC IPM notes spores can also drift in from other grasses.

When to look: Look in shaded parts of the lawn during cool, cloudy stretches in spring and from late summer into fall.

Powdery mildew of turf look-alikes

A few things get mixed up with it:

• Turfgrass rust (Puccinia spp.): Rust also leaves a powder that rubs off, but it is orange or rusty brown, not white, and comes from raised pustules.
• Slime mold (Physarum cinereum): Slime molds make gray, white or colored crusty masses that sit on blades for a week or two and then dry up. They are not a leaf infection and can be washed off with a hose.

How to manage powdery mildew of turf

Cultural control

• Prune or thin trees and shrubs to let in sunlight and improve air movement.
• Avoid extra nitrogen in shady, mildew-prone spots; Purdue says excess nitrogen may raise the risk.
• Raise the mowing height where you can.
• Water deeply and less often, and keep fertility adequate but not heavy.

Chemical control

Penn State says fungicides are not needed except on highly managed turf. Purdue says to consider them only if the look is truly intolerable, and names the DMI group as most effective. Use only products labeled for this use; the label is the law.

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Overseed shady areas with shade-tolerant Kentucky bluegrass varieties, which Purdue says are less susceptible.
• Penn State says fine fescues and tall fescue are quite resistant, and Utah State suggests shade-adapted fescues for shady lawns.
• Avoid planting turf in heavy shade.

When to call a pro

Powdery mildew rarely needs a professional. For trees that shade the lawn, have pruning done by an ISA Certified Arborist. If you want a fungicide on high-value turf anyway, use a licensed pesticide applicator; see our pesticide applicator license guide.

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