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Mildew Timing Calendar

Powdery mildew and downy mildew sound alike but behave differently. Knowing which one you face tells you what weather to watch.

What is the difference between powdery and downy mildew?

Powdery mildews are fungi that grow on the surface of leaves and produce a white to gray powdery coating. Downy mildews are water molds that grow inside leaf tissue and produce fuzzy growth on the underside of leaves, with yellow or angular spots on the upper surface. They are not closely related, and they respond to different weather.

Identifying which you have is the first step, since the timing and the management differ.

A third group, often called rust or sooty mold, can look similar at a glance, so do not assume that every white or gray growth is mildew.

What weather favors each?

Downy mildew generally needs leaf wetness and cool to moderate temperatures, along with high humidity, so it often follows rainy, humid weather. Powdery mildew does not need free water on leaves to infect, and it often thrives in moderate temperatures with high humidity, such as warm days and cool humid nights, and in shaded or crowded plantings.

Specific conditions differ by crop and species, so check your extension service for the mildews that affect your crops.

Greenhouse and high tunnel plantings often stay humid, so powdery mildew can appear early in these settings and persist through the season.

When do mildews usually appear?

Powdery mildew often appears in mid to late season on cucurbits, grapes, roses, peas and many other crops, but can arrive earlier in protected culture. Downy mildew can appear whenever wet cool weather meets susceptible plants, and in some crops it arrives on wind-borne spores from other regions.

Do not rely on a date. Use weather and early scouting. Some states issue downy mildew alerts for crops such as cucurbits and basil when it is detected nearby.

• Powdery: white coating, often on upper leaf surfaces
• Downy: fuzzy growth under leaves, yellow spots above
• Watch humidity and leaf wetness
• Use local alerts when available

How do you scout for mildew?

Check both sides of leaves, starting with older and shaded leaves in the lower and inner canopy. Look for the first signs: small white spots for powdery mildew, and small yellow or angular spots with a gray fuzz underneath for downy mildew. Early detection leaves more options.

Take photos and send samples through your extension office for diagnosis, since several leaf problems resemble mildew.

Use a hand lens on a suspect leaf. Powdery mildew shows fine white threads and chains of spores, while downy mildew shows a gray to purple fuzz limited to the underside.

Which cultural steps help the most?

Resistant varieties are often the best tool, and many crops have varieties with improved resistance to one or both mildews. Space plants and trellis or prune to improve air flow and sunlight. Avoid overhead irrigation late in the day for downy mildew. Remove infected plant material and clean up residue after the season. Rotate where the disease survives in residue or on seeds.

Moderate nitrogen, since lush growth can raise susceptibility, and manage weeds that may carry the disease.

Plant varieties with different resistance in adjacent blocks if you can, since it reveals how each performs under your conditions.

What about protective products?

Protective products are labeled for each type of mildew, and they work best when applied before or at the first signs. The label is the law. Some products are specific to powdery or to downy mildew, so identification matters. Resistance management guidance is published by extension, pre-harvest intervals apply and pollinators must be protected, especially on crops such as cucurbits in bloom. Applicator licensing varies by state.

Ask your extension office or a certified crop adviser for current local recommendations.

Alternate between different protective products as the label and extension advice describe, since repeated use of a single kind speeds resistance.

What belongs in your mildew record?

Record the date of first symptoms, the weather in the preceding week, the crop stage, variety, the plant spacing and the percent of leaves affected. Compare across varieties and across blocks. For example, if one variety stays clean while the neighboring one is covered, that is useful evidence to carry into next year's variety choices.

Review the notes before ordering seed each year. Photograph the same leaves weekly to see how fast the problem moves.

Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.

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