Pathogen: Erysiphe, Golovinomyces and Podosphaera species (powdery mildew fungi)
Also called: powdery mildew, white mold on leaves
Plant Powdery Mildew
The leaf disease that doesn't need rain
Plant Powdery Mildew At-a-Glance
Plant Group
Shade Trees
Flowering Shrubs
Evergreen Shrubs
Roses
Perennials
Plant Part
Leaves
Flowers & Buds
Stems & Twigs
Is Spraying Worth It?
Sometimes Worth It
Disease Type
Fungal
Active Season
Early Spring
Late Spring & Early Summer
Mid to Late Summer
Fall
Damage Potential
Cosmetic Only
Weakens Over Time
Plant Powdery Mildew Quick Answers
What Plant Powdery Mildew Looks Like
From a Few Feet Away
From a few feet away, infected plants look like someone sprinkled them with baby powder or flour. It usually starts as scattered white spots on the lower leaves, then spreads until whole leaves, stems and flower buds are coated in a gray-white film. Badly infected leaves turn yellow or brown and drop early.
Symptoms by Plant Part
Leaves. Small, round white to gray spots appear first, often on the upper surface of lower leaves. They enlarge and merge into felt-like patches that can cover the whole leaf. Young leaves infected while still growing curl, twist or cup. Over time, infected leaves yellow, brown and fall early. Some plants show leaf curling and discoloration even before the white growth is obvious.
New shoots and buds. New growth is hit hardest. Shoot tips may be stunted and distorted, and infected flower buds may open poorly or not at all.
Stems. Green stems can carry the same white coating, especially on herbaceous perennials and on rose canes.
How it looks on common hosts:
- Garden phlox and bee balm: heavy white coating on lower leaves first, often followed by yellowing and late-summer leaf drop.
- Roses: white felt on young leaves, buds and canes; new leaves curl and pucker.
- Purple-leaved ninebark, flowering dogwood and some other shrubs: leaves may turn purple to red around the infection, sometimes with little visible white growth.
- Oaks: repeated infection of shoot tips can cause dense clusters of stunted shoots, called witches’ broom.
- Lilac: white coating on leaves in late summer and fall, after the leaves have done most of their work for the year.
Signs of the Fungus
The white powder is the fungus — a thin mat of threads topped with chains of spores. It shows up as small, round, white to gray spots that spread and merge until whole leaves look dusted with flour. It’s usually heaviest on the upper side of the leaf, but it can cover both sides, stems, buds and flowers. Late in summer or in fall, look closely with a magnifier: tiny round dots, orange at first and then black, often appear inside the white patches. Those are the fungus’s winter survival structures.
Could It Be Something Else?
Common Lookalikes
Downy mildew is the one most often mistaken for powdery mildew, and it’s a different organism that needs different treatment. Its fuzzy, grayish growth sits mainly on the underside of leaves, usually below yellow patches on the top, and it favors cool, wet weather (about 50–70°F) — the opposite of powdery mildew’s dry spells.
Sooty mold also coats leaves, but it’s black, not white, and it grows on the sticky honeydew dripped by aphids, whiteflies and scale. It’s a sign of an insect problem, not a leaf infection.
Field Checks Before You Treat
- Check which side of the leaf it's on. Powdery mildew is usually thickest on the upper surface. Fuzzy growth mainly on the underside, under yellow patches on top, points to downy mildew.
- Look with a hand lens. Powdery mildew spores sit in short chains. Downy mildew's spores sit on tiny branched stalks that look like miniature trees.
- Feel for stickiness and look for insects. A black coating on leaves that feel sticky, with aphids, whiteflies or scale nearby, is sooty mold.
- Think about the weather. Powdery mildew spreads in dry spells with cool, humid nights. A problem that took off during a cool, rainy stretch is more likely downy mildew.
- Look for black dots late in the season. Tiny dark specks inside the white patches in late summer or fall confirm powdery mildew.
Plants at Risk
Commonly Affected Plants
Garden phlox, bee balm, lilac, crape myrtle, roses, zinnia, flowering dogwood, ninebark, euonymus, and shade trees such as oak and sycamore (London plane). Many other ornamentals get it too, including dahlia, hydrangea, azalea, rhododendron, spirea, photinia and snapdragon.
Resistant Choices
“Resistant” means much less mildew, not immunity. Good choices extension programs name:
- Garden phlox: ‘Robert Poore’ and ‘David’ (among the most resistant), plus ‘Delta Snow’, ‘Eva Cullum’, ‘John Fanick’, ‘Shortwood’ and ‘Katherine’. Most annual phlox resists it well. Be wary of spotted phlox ‘Alpha’, ‘Omega’ and ‘Delta’ — often sold as resistant, but fairly susceptible in Southeast trials.
- Bee balm: ‘Marshall’s Delight’, ‘Jacob Cline’, ‘Colrain Red’ and ‘Cambridge Scarlet’.
- Zinnia: Profusion, Zahara and Pulcino series, and narrowleaf zinnia (Zinnia angustifolia).
- Crape myrtle: the hybrids with Native American names, such as ‘Natchez’, ‘Muskogee’, ‘Tuscarora’, ‘Sioux’, ‘Acoma’ and ‘Hopi’.
- Dogwood: kousa dogwood (‘Milky Way’, ‘National’), kousa × flowering dogwood hybrids (‘Aurora’, ‘Constellation’, ‘Celestial’), and flowering dogwood ‘Jean’s Appalachian Snow’ and ‘Cherokee Brave’.
- Roses: ‘Mister Lincoln’, ‘Queen Elizabeth’, ‘Iceberg’, ‘Julia Child’, ‘Double Delight’, and rugosa roses.
- London plane tree: ‘Columbia’, ‘Liberty’, ‘Exclamation!’ and ‘Yarwood’.
- Euonymus: variegated types resist it better than solid green ones.
Why It Shows Up
Conditions That Trigger It
Powdery mildew is the dry-weather leaf disease. Unlike almost every other leaf disease, it doesn’t need rain, dew or wet leaves to infect — and extended water on leaves actually inhibits it. What it does like is mild to warm days, roughly 60–80°F, followed by cool, humid nights. High humidity at night helps the fungus form spores; dry daytime air helps those spores blow away to new leaves. That’s why it so often explodes in late summer and early fall, when days are dry and nights turn cool. Very hot weather slows it down: temperatures above about 95°F can suppress the fungus.
Several site conditions make it worse:
- Shade. Shaded plants stay cooler and more humid, and they never get the sun and heat that hold the fungus back.
- Crowding and still air. Dense clumps and tightly packed beds trap humid air around the leaves.
- Too much nitrogen. Heavy feeding pushes soft, fast new growth — exactly what the fungus infects most easily.
- Young growth. New leaves, shoot tips, suckers and water sprouts are far more susceptible than mature leaves.
- Dry spells. Drought-stressed plants are more vulnerable, and dry weather is when the disease spreads fastest.
How It Survives and Spreads
Powdery mildew survives winter in two ways. On woody plants such as roses, oaks, sycamores and euonymus, it hides inside buds or forms tiny black survival structures on leaves and stems. On many perennials, those structures sit on fallen leaves and dead stalks. In spring, they release spores that infect the first new leaves.
From then on, the fungus grows on the leaf surface and sends small feeding structures into the outer leaf cells. It keeps producing new spores that the wind carries to other leaves and nearby plants of the same kind, starting a new round of infection within days when conditions are right. Each powdery mildew fungus infects only a limited range of plants, so it spreads within a kind of plant, not across your whole yard.
Where It Hits Hardest: Regional Notes
Powdery mildew turns up in every US region. It’s especially relentless in the dry-summer West, where warm, dry days and cool nights last all season and there’s little summer rain to wash spores off. In the humid East, Midwest and South, it tends to peak in spring and again in late summer and fall, when cool nights follow dry days, and it’s less common in the heat of midsummer.
Plant Powdery Mildew Diagnostic Profile
Check what you see against the full set of signs. The more that match, the more likely this is your problem.
What You’ll See
Symptom Color
Yellow
Brown
Red, Pink or Purple
White or Gray
Visible Signs
White Powdery Coating
Plant Symptoms
Leaf Spots
Powdery or Sooty Coating
Curled or Distorted Leaves
Yellowing
Browning & Dieback
When and Where
Weather Triggers
Warm & Humid
Warm Days, Cool Humid Nights
Drought or Heat Stress
Spread Pattern
Lower Leaves First
Scattered
Controlling Plant Powdery Mildew
Most powdery mildew can be managed — or simply tolerated — without spraying. If an infection doesn’t noticeably reduce growth or flowering, it’s fine to leave it alone.
- Give susceptible plants sun. Full sun keeps leaves warmer and drier and exposes the fungus to the light and heat that suppress it. Don’t plant phlox, bee balm, roses or zinnias in shade.
- Space for air movement. Plant at the spacing for the plant’s mature size so air can move through and around it.
- Thin crowded perennials. In spring, thin garden phlox to 4 to 5 strong stems per clump. Fewer stems let in light and air where the disease starts — on the lower leaves.
- Open up shrubs. Prune crowded interior branches, and remove suckers and water sprouts as soon as they appear; their soft growth is especially susceptible.
- Water at the soil, not over the leaves — with one exception. Watering at the base is the safest default, because wet leaves invite other diseases such as black spot and rust on roses. But water itself works against powdery mildew: on plants where it’s the only problem, a mid-morning rinse of the leaves that dries quickly can wash spores off. Water deeply during dry spells either way; drought-stressed plants get hit harder.
- Go easy on nitrogen. Fertilize based on a soil test. Excess nitrogen makes soft, lush growth the fungus loves.
- Pick off the first infected leaves. Check susceptible plants weekly from spring on, and remove the first spotted leaves before the fungus spreads.
- Clean up in fall. Cut infected perennials down after frost, prune infected stems at the end of the season, and rake up fallen leaves. This removes the survival structures that start next spring’s infections.
Sometimes worth it — but usually not. Fungicides make sense only for prized plants with a history of severe infection, such as roses, crape myrtles, purple-leaved ninebark or a favorite stand of phlox. On mature shade trees and on shrubs like lilac that get mildew late in the season, spraying does nothing for the plant’s health and isn’t worth the cost.
Timing is everything. Fungicides protect healthy leaves; they won’t clear up an existing infection. Start before symptoms appear on plants with a history of the disease, or at the very first spot. Once most leaves are covered, it’s too late to spray. Cover both sides of the leaves, repeat at the label’s interval (often 7 to 10 days while conditions favor the disease), and rotate between different FRAC groups to avoid resistance.
Homeowner options:
| Active ingredient | FRAC group | How it works | Key cautions |
|---|---|---|---|
| Sulfur | M2 | Prevents new infections; doesn’t cure existing ones | Not near or above 90°F, not on drought-stressed plants, not within 2 weeks of an oil spray. Injures sulfur-sensitive plants such as viburnum. |
| Horticultural oil, neem oil | Not classified | Clears mild to moderate infections | Not above 90°F, not on water-stressed plants, never within 2 weeks of sulfur. Can injure cedars, cryptomeria, hickories, certain maples, junipers, redbud and smoke tree. |
| Potassium bicarbonate | Not classified | Clears early infections | Can burn leaves badly when the disease is already advanced. |
| Bacillus (biological) | BM02 | Protects leaves | Less effective than oils or sulfur. |
| Chlorothalonil | M5 | Protects leaves only | Must go on before infection. |
| Thiophanate-methyl | 1 | Systemic | Rotate; resistance develops with repeated use. |
| Myclobutanil, propiconazole, tebuconazole | 3 | Systemic; protect new growth and slow early infections | Rotate with a different group. |
The systemic products (groups 1 and 3) can be sprayed less often than sulfur or chlorothalonil. To protect pollinators, don’t spray plants in bloom while bees are active — spray late in the evening.
Removal, Disposal and Replanting
Powdery mildew rarely justifies removing a plant. Remove and replace only a plant that’s heavily infected year after year and no longer worth growing — and replace it with a resistant variety or a different kind of plant. The fungus doesn’t survive in the soil, so the same spot is safe to replant; if it’s shady and crowded, give the new plant more sun and space.
Infected leaves, stalks and prunings go to yard-waste pickup or the trash. Home composting only works if your pile reliably heats above 140°F; most backyard piles don’t, and the fungus’s survival structures can make it through to next spring.
When to Call a Pro
Send a sample to your state’s extension plant diagnostic clinic if you can’t tell powdery mildew from downy mildew (they need different treatments), if leaves turn purple or red with no white growth, or if a valuable plant keeps declining despite good care. Collect fresh leaves showing early and advanced symptoms and check the clinic’s instructions before mailing. Call an ISA Certified Arborist for large trees — for example, an oak with dense clusters of stunted shoots (witches’ broom) — or whenever spraying would require equipment a homeowner can’t safely use.
Frequently Asked Questions
These answers are synthesized from plant pathology research and diagnostic guidance published by university extension services and plant disease clinics.
Baking soda alone is unreliable. Clemson suggests mixing 3 tablespoons each of horticultural oil and potassium bicarbonate or baking soda per gallon of water for better control, with the same heat and sulfur cautions as oil sprays. Potassium bicarbonate is the more reliable of the two. None of the extension sources used for this page recommend milk sprays.
Because that’s when it thrives. Unlike most leaf diseases, powdery mildew doesn’t need wet leaves to infect. Dry days help its spores blow around, and cool, humid nights help it form new ones.
On some plants, such as purple-leaved ninebark and flowering dogwood, leaves turn purple to red around the infection, sometimes with little visible white growth. If you’re not sure, a diagnostic clinic can confirm it.
No. Powdery mildew on lilac usually appears late in the season, after the leaves have done most of their work for the year. It looks bad but doesn’t hurt the shrub, so there’s no need to spray. Rake up the fallen leaves.
Scientific Authority
This profile is built on plant pathology research and diagnostic guidance from university extension services and plant disease clinics. We prioritize accurate identification and cultural control first, recommending fungicides only where research shows they are justified, and naming them by active ingredient rather than brand.
Primary Resources
- UC IPM — Powdery Mildew on Ornamentals (Pest Notes)
- University of Minnesota Extension — Powdery Mildew in the Flower Garden
- University of Minnesota Extension — Powdery Mildew on Trees and Shrubs
- Clemson Cooperative Extension HGIC — Powdery Mildew on Landscape and Garden Plants (HGIC 2049)
- Clemson Cooperative Extension HGIC — Phlox
- Penn State Extension — Powdery Mildew
- UC IPM — Sooty Mold