Powdery Mildew vs Downy Mildew: How to Tell & Treat

Two of the most common foliar diseases in the garden share the word “mildew,” but they are caused by completely different organisms and call for completely different responses. Powdery mildew is a true fungus that coats the upper surface of leaves with a white, talcum-like dust and actually thrives in warm, dry conditions with high humidity but little surface moisture. Downy mildew is an oomycete (a water mold, more closely related to algae than to fungi) that needs cool, wet, humid weather and free water on the leaf to spread; it shows as yellow or angular blotches on top with a fuzzy gray-purple growth on the leaf underside. Misidentifying one for the other wastes time and money, because the fungicides and cultural fixes that knock back powdery mildew often do little against downy mildew. This guide shows you exactly how to tell them apart at a glance, what conditions trigger each, and how to identify and treat both so you can protect your cucurbits, grapes, roses, basil, impatiens, and lawns.

Feature	Powdery Mildew	Downy Mildew
Pathogen type	True fungus (Erysiphales)	Water mold / oomycete (Peronosporaceae)
Where it shows	Mostly upper leaf surface	Upper surface spots + fuzzy growth on underside
Appearance	White-gray powdery dust, talcum-like	Yellow/angular blotches; gray, purple, or downy fuzz below
Rub test	Wipes off; leaf green underneath	Does not wipe off; tissue stays discolored
Favored weather	Warm, dry days; humid; no leaf wetness needed	Cool, wet, humid; needs free water on leaves
Spread	Wind-borne spores, even in dry air	Splashing water, rain, overhead irrigation, dew
Key control	Sulfur, potassium bicarbonate, neem, airflow	Copper, oomycete-specific fungicides, resistant varieties

Powdery mildew

Powdery mildew is the disease most gardeners recognize on sight: a dusty white-to-gray coating that looks like someone shook flour over the leaves. It is caused by a group of true fungi (order Erysiphales), and unlike most fungal diseases it does not require free water on the leaf to infect. Instead, it thrives when days are warm (roughly 60–80°F), humidity is high, and air is stagnant — classic conditions in a crowded summer garden, a humid greenhouse, or a shaded bed with poor circulation. The fungus is an obligate parasite, meaning each species is fairly host-specific: the powdery mildew on your squash is a different species from the one on your roses or lilacs, so it generally will not jump between unrelated plants.

How to identify powdery mildew

Look first at the upper surface of older, shaded, lower leaves — that is where it usually starts. Early infections appear as small white powdery spots that expand and merge into a felt-like coating across the top of the leaf. The powder can also spread to stems, buds, and flowers. A reliable field test is the rub test: wipe a patch with your finger and the white dust comes off, leaving green (sometimes slightly yellowed) tissue underneath. Heavily infected leaves eventually turn yellow, curl, and dry out, and fruit may be stunted or sunscalded because the canopy thins. Plants that are especially prone include squash, pumpkins, cucumbers, melons, roses, lilacs, bee balm, phlox, zinnias, peas, and grapes. If you’ve seen the telltale white film on squash, our guide on squash leaves turning white walks through that exact scenario.

How to treat powdery mildew

Start with culture, because spray alone rarely wins. Increase airflow by spacing and thinning plants, pruning out the most heavily coated leaves (bag and trash them — do not compost), and clearing crowded interior growth so the canopy dries fast. Site susceptible plants in full sun, since shade and stagnant air invite the fungus. Avoid over-fertilizing with nitrogen, which pushes soft, susceptible new growth.

For active outbreaks, several low-toxicity sprays work well because powdery mildew lives on the leaf surface where contact products can reach it. Potassium bicarbonate (or baking-soda-plus-oil home mixes) shifts the leaf surface pH and kills exposed fungus on contact. Sulfur is a classic preventive and curative fungicide — just never apply it within two weeks of an oil spray or above ~85–90°F, as it can scorch foliage. Neem oil and other horticultural oils smother the fungus and offer some preventive coverage; neem’s dual pest-and-fungus action makes it a favorite. Begin treatment at the very first spots and reapply every 7–14 days, always covering both leaf surfaces. For prevention next season, choose mildew-resistant cultivars, which are widely available for cucurbits, phlox, and bee balm.

Downy mildew

Downy mildew looks superficially similar from across the garden, but it is biologically a different beast. It is caused by oomycetes (water molds) in the family Peronosporaceae — organisms more closely related to algae than to true fungi, which is why fungicides aimed at true fungi often miss them entirely. Downy mildew is a moisture lover: it needs cool, wet, humid conditions and a film of free water on the leaf surface to germinate and infect. That makes it a disease of spring and fall, rainy spells, heavy dew, fog, and overhead watering — the opposite of powdery mildew’s warm, dry preference. It spreads explosively in the right weather, with spores carried by splashing water and wind over long distances, and it can devastate basil, grapes, cucurbits, impatiens, spinach, lettuce, and brassicas in days.

How to identify downy mildew

The defining clue is to flip the leaf over. On the upper surface you’ll see pale yellow, angular blotches that are often boxed in by the leaf veins (giving a blocky, mosaic look), or general yellowing and browning. On the matching spot on the underside, you’ll find a downy, fuzzy growth that may be gray, white, purple, or brown depending on the host and humidity — this fuzz is the pathogen sporulating, and it is usually most visible in the cool of early morning when moisture is present. Unlike powdery mildew, downy mildew does not wipe off cleanly, and the underlying tissue is genuinely discolored and dying, not just dusted. On basil, leaves yellow and the undersides look dirty-gray; on grapes and cucurbits, the angular yellow patches between veins are a classic giveaway; on impatiens, plants suddenly drop leaves and collapse. Because the symptoms can resemble nutrient deficiency or other leaf spots, the white-fuzz-on-the-underside check is your most reliable confirmation. If you’re chasing down yellowing for other reasons, our overview of pumpkin leaves turning yellow covers the broader differential.

How to treat downy mildew

Downy mildew is harder to cure than powdery mildew, so prevention and fast action matter most. Because the pathogen depends on leaf wetness, your first line of defense is moisture management: water at the soil, never overhead, and do it in the morning so any splashed foliage dries quickly. Space and prune for maximum airflow, stake or trellis sprawling plants off the ground, and avoid working among wet plants (you’ll spread spores on hands and tools). Remove and bag infected leaves and whole plants promptly, and clean up debris at season’s end since some species overwinter in residue.

For chemical control, the products differ from powdery mildew. Copper-based fungicides are the main organic option and work preventively — they protect healthy tissue but can’t reverse existing infection, so apply before or at the very first sign and reapply on schedule, especially after rain. Conventional gardeners may use oomycete-specific fungicides (the active ingredients targeting water molds are different from those for true fungi), so read labels for “downy mildew” specifically rather than assuming a generic fungicide works. Most importantly, lean on resistant varieties: downy-mildew-resistant basil, cucumber, and spinach cultivars have been bred precisely because the disease is so hard to control once established. In wet regions, treat resistant varieties plus airflow plus dry foliage as your real cure — the spray is just a backstop.

Conditions and prevention that apply to both

Although the two diseases favor opposite extremes of moisture, the same cultural backbone reduces both. Crowding and stagnant air are the universal enemy: generous spacing, thinning, and pruning let leaves dry and slow the buildup of humid microclimates that powdery mildew loves and the leaf wetness that downy mildew requires. Full sun and good drainage help. Sanitation matters for both — remove infected foliage as soon as you spot it and clean up debris at the end of the season. Rotate susceptible crops (cucurbits, basil, brassicas) to a new bed each year so soilborne and residue-borne inoculum can’t build. And start scouting early: walk the garden weekly, check leaf undersides, and treat at the first spots, because both diseases are far easier to hold back than to reverse once they’ve taken over a canopy. For more on spray choices, see our comparison of neem oil vs insecticidal soap.

The verdict: which one do you have?

Run three quick checks. First, where is the growth? White dust mainly on the upper surface points to powdery mildew; fuzzy growth on the underside with yellow blotches on top points to downy mildew. Second, does it rub off? Powdery mildew wipes away to reveal green tissue; downy mildew does not, because the tissue itself is dying. Third, what’s the weather been? Warm, dry days with humid air favor powdery mildew, while cool, wet, drizzly spells with heavy dew favor downy mildew. Once you’ve identified it, match the treatment: sulfur, potassium bicarbonate, neem, and airflow for powdery mildew; copper, oomycete-specific fungicides, dry foliage, and resistant varieties for downy mildew. Get the ID right and you’ll stop wasting sprays — and save the plant.