Powdery Mildew vs Downy Mildew: How to Tell Them Apart

Powdery Mildew vs Downy Mildew at a Glance

Powdery mildew is a white, powdery coating that sits on the surface of leaves, stems and buds. Downy mildew shows as yellow blotches on top of the leaf, boxed in by the veins, with a gray, white or purple fuzz on the underside directly below. The two are unrelated organisms. Powdery mildews are true fungi that infect in humid air without any water on the leaf, and on cucurbits infection can occur anywhere from 50 to 90°F. Downy mildews are water molds that need extended leaf wetness and favor cool, wet weather.

	Powdery mildew	Downy mildew
Organism	True fungi	Fungus-like water molds (oomycetes)
First sign	Small, circular, powdery white spots	Pale green or yellow blotches on the upper surface
Where it grows	Upper and lower leaf surfaces, stems, flower petals	Fuzz on the leaf underside
Patch shape	Round, expanding and joining	Angular, limited by leaf veins
Weather	High humidity, no free water needed, 50 to 90°F on cucurbits	Cool and wet, extended leaf wetness
Common hosts	Squash, phlox, bee balm, zinnia, roses	Basil, impatiens, grapes, cucurbits, brassicas, lettuce
Survives in	Plant debris and infected plants	Plant debris, and soil for up to five years
Home garden products	Sulfur, horticultural oil, potassium bicarbonate	Copper, chlorothalonil

What Powdery Mildew Looks Like

Clemson’s fact sheet describes powdery mildew as a grayish-white, powdery mat visible on the surface of leaves, stems and flower petals. It begins as small, circular, powdery white spots that expand and eventually join. The University of Florida comparison notes that the growth can be anywhere on the leaf surface, which is why the patches are round and ignore the veins. University of Wisconsin Extension adds that both the upper and lower surfaces of leaves can carry the powder, so a white underside by itself does not rule powdery mildew out. What separates the two is texture and pattern: dry powder in round patches, with no yellow angular blotch above it.

Leaves may curl, twist or discolor before the white growth is obvious. Later they yellow, brown and dry, and flower buds can fail to open. On squash and other cucurbits, the white growth covers the upper leaf surfaces and stems, and infected leaves may be stunted and drop early while the fruit is usually not directly affected. For flowers and shrubs, see our guide to powdery mildew on ornamentals.

What Downy Mildew Looks Like

The first sign of downy mildew is a color change on the top of the leaf. The Royal Horticultural Society lists discolored blotches on the upper leaf surface, which may be pale green, yellow, purple or brown. Wisconsin describes small, green or yellow, translucent spots that later brown or bronze. Because the growth is limited by leaf veins, the blotches have straight sides and corners. On cucurbits they start as small yellow areas on the upper surface.

The confirming sign is on the other side. Turn the leaf over and look for a mold-like growth under each blotch, which may be white, gray or purple. It is easiest to see in the morning after a damp night, because many downy mildews produce their spores at night. Badly affected leaves shrivel and turn brown, or yellow and fall early. Basil is a common host. If a yellowing basil leaf has no fuzz underneath, why basil turns yellow covers the other causes.

Weather That Favors Each Disease

Recent weather is a useful second opinion on what you see. Powdery mildew fungi are unusual among leaf diseases because their spores do not require free water to germinate. A high level of relative humidity is enough. Clemson notes that spore production begins when daytime temperatures rise above 60°F and that the disease is less common during summer heat. On cucurbits, infection can occur between 50 and 90°F during dry weather with high relative humidity. Crowded plantings raise humidity inside the canopy and make it worse, which is why it often appears in a dry spell on plants that have never been rained on.

Downy mildew depends on water. Extended periods of leaf wetness are required for infection, and long stretches of high humidity or wet leaves overnight can lead to severe outbreaks. Cool, wet weather sets it off, so expect it in a rainy spring, a damp fall, or any bed watered from overhead in the evening.

Which Plants Get Which Mildew

Both diseases are more host specific than their names suggest. Each powdery mildew fungus has a limited host range. Clemson’s example is that powdery mildew on oak leaves is no cause for concern for nearby zinnias. Phlox, bee balm (Monarda) and zinnia are very susceptible, along with squash, roses and crapemyrtle. Conifers are not affected.

Downy mildew organisms are also fairly host specific. The organism that infects a rose is not the one that infects a grape. Common garden hosts include basil, impatiens, grapevines, melons and other cucurbits, brassicas, lettuces, onions and peas. Cucurbits get both diseases, so squash and cucumber leaves are where gardeners most often need to tell the two apart.

How to Treat and Prevent Powdery Mildew

Start with humidity. Space plants further apart to increase air flow, avoid overhead irrigation that raises humidity within the canopy, and remove fallen leaves, which can be a source of further infection. Clemson also advises against heavy nitrogen fertilizer, since it pushes soft new growth that is more susceptible. Where resistant varieties exist, as they do for most cucurbits, plant them.

For many plants powdery mildew is a cosmetic, non-lethal disease, and no spray is needed. On plants that lose leaves badly, fungicides work as protectants and have to go on early. Wisconsin gives an interval of every 7 to 14 days from bud break until humid weather subsides, and a home recipe of 1½ tablespoons of baking soda and 3 tablespoons of lightweight horticultural oil in 1 gallon of water. Sulfur, horticultural oil, potassium bicarbonate and neem oil all appear in Clemson’s table of home garden products, with two cautions from Clemson: do not apply sulfur or horticultural oil when temperatures exceed 90°F, and do not apply sulfur within two weeks before or after a horticultural oil treatment. Our comparison of neem oil vs horticultural oil explains how the oils differ.

How to Treat and Prevent Downy Mildew

Keep the leaves dry. Water at the base of the plant in the morning, avoid dense planting, improve air circulation by spacing and pruning, and remove affected leaves as they appear. At the end of the season, remove and destroy infected plant debris, because the organisms survive in debris and on infected plants. Some downy mildews also leave resting spores that are likely to survive in soil for up to five years, which is the reason the Royal Horticultural Society recommends crop rotation.

A product chosen for powdery mildew may do nothing here, since the two organisms are unrelated. Wisconsin lists fungicides containing chlorothalonil and copper, including Bordeaux mix, as labeled for downy mildew control in home gardens. Clemson notes that fungicides for cucurbit downy mildew are most effective when used preventatively, before symptoms appear, and advises planting varieties that are resistant to the disease. Whichever product you use, check that the label names both the disease and the plant.