How to Prevent and Treat Powdery Mildew on Flowers

What Powdery Mildew Is and How to Stop It

Powdery mildew is a fungal disease that covers leaves, stems and buds in a white, flour-like coating. To treat it on flowers, pick off the first spotted leaves, then spray every 7 to 14 days with horticultural oil or a bicarbonate and oil mix, or with sulfur on growth that is still clean, keeping oil and sulfur for days below 90 F. To prevent it, give plants sun and moving air, go easy on nitrogen, and replace repeat offenders with resistant varieties such as ‘David’ phlox or ‘Marshall’s Delight’ bee balm.

The disease is caused by many closely related fungi, and each has a limited host range. Clemson’s fact sheet makes the point with an example: mildew on oak leaves is no cause for concern for nearby zinnias. Colorado State University Extension adds that the fungi are host specific and cannot survive without the proper host plant. What mildew on one plant does tell you is that the weather suits mildew generally, so the phlox, bee balm, zinnias and roses in the same garden deserve a look. If the patches are yellow and angular with fuzz underneath, you are looking at a different disease, and powdery mildew versus downy mildew shows how to separate the two.

How to Recognize It Early

The first sign is small, circular, powdery white spots that expand and join as the infection grows. University of California IPM notes that the spots develop on both leaf surfaces, and Wisconsin’s horticulture extension lists stems as well. On some plants the leaves curl, twist or discolor before any white growth is visible.

Later symptoms are harder to reverse. Leaves turn yellow or brown and drop, and leaves or shoots can twist or distort. Check susceptible plants at least once a week once daytime temperatures rise above 60 F, which is when Clemson says spore production begins, and look inside dense clumps where air moves least.

The Weather That Brings It On

Powdery mildew behaves differently from most leaf diseases because it does not need wet leaves. UC IPM states that moderate temperatures of 60 to 80 F and shade encourage it, and that temperatures above 95 F may suppress the fungus. Colorado State reports the disease as severe in spring and fall, when warm weather of 70 to 80 F favors it, with infection increasing as relative humidity rises to 90 percent. Clemson describes the favorable pattern as cool days and humid nights.

Free water works against the fungus. According to UC IPM, water sitting on plant surfaces for extended periods inhibits spore germination and kills spores. Humid air around a dry leaf is the condition the fungus wants, and a crowded, shaded bed on a still evening supplies exactly that.

The fungi carry over from year to year on the plants and their litter. On perennial hosts such as roses, UC IPM says mildew survives as strands inside buds or as small round fruiting bodies, and Wisconsin notes survival in plant debris. Wind carries the spores to new hosts in spring.

Cultural Practices That Prevent It

Prevention means removing shade, still air and soft growth. Extension sources agree on five practices.

1. Plant in sun. UC IPM’s first recommendation is to locate plants in sunny areas as much as possible.
2. Open up the planting. Wisconsin advises spacing plants further apart to increase air flow and lower humidity, and Colorado State recommends selectively pruning overcrowded growth.
3. Hold back nitrogen. Clemson explains that nitrogen stimulates young, succulent growth, which is more susceptible to infection. UC IPM suggests a slow-release fertilizer, and Colorado State warns against late-summer nitrogen in particular.
4. Remove infected growth. Clemson recommends taking off infected leaves when symptoms are first noticed and pruning out infected stems at the end of the growing season. UC IPM adds pruning out infected buds during the dormant season.
5. Dispose of the debris. Wisconsin’s options are to burn it where local rules allow, bury it, or hot compost it. A cool, slow pile is not on that list, and our comparison of hot and cold composting explains the difference.

Watering advice differs between sources, and the difference is worth understanding. Colorado State and Clemson say to avoid overhead watering, since it raises humidity around the foliage. UC IPM points out that overhead sprinkling can reduce the spread of mildew because it washes spores off the plant, provided it is done mid-morning so the leaves dry rapidly. Both positions lead to the same rule for a flower bed: never leave the canopy damp and humid into the evening. Water at the base as routine, and if you rinse foliage, do it in the morning.

Resistant Varieties by Name

For a plant that mildews every year in a good site, replacement is the lasting fix. These cultivars are listed as resistant by UC IPM or Clemson.

Plant	Resistant varieties	Listed by
Garden phlox	’David’, ‘Robert Poore’, ‘Natasha’, ‘Delta Snow’	UC IPM, Clemson
Bee balm	’Marshall’s Delight’, ‘Jacob Cline’, ‘Colrain Red’, ‘Blaustrumph’	UC IPM, Clemson
Zinnia	Profusion and Zahara series, Pulcino, African varieties	UC IPM, Clemson
Rose	Rosa rugosa types, ‘Iceberg’, ‘Julia Child’, ‘Double Delight’	UC IPM, Clemson
Crape myrtle	’Natchez’, ‘Tuskegee’, ‘Muskogee’, ‘Tuscarora’, ‘Hopi’, ‘Catawba’	UC IPM, Clemson

Resistant does not mean immune, and a resistant phlox still belongs in sun with air around it. Tags and catalogs usually state mildew resistance, so check for one of these names before buying a replacement.

Treating an Active Infection

Start by removing the worst leaves and stems, then protect what is left. Clemson says that for fungicides to be effective, they must be applied as soon as symptoms are noticed.

Oils. UC IPM names petroleum-based horticultural oils and plant-based oils such as neem oil as the products to use on an infection that is already present, and Colorado State also lists neem oil for home gardens. Do not apply oil when the temperature is above 90 F, and never within 2 weeks of a sulfur application, which UC IPM warns may injure plants. Our comparison of neem oil and horticultural oil covers how the two differ.

Sulfur. Sulfur has been used against powdery mildew for centuries, but UC IPM says it is effective only when applied before the disease appears. Use it to protect clean growth, and expect little from it on leaves that are already white. It carries the mirror-image limits: not when the temperature is near or above 90 F, and not within 2 weeks of an oil spray.

Bicarbonates. Potassium bicarbonate is sold as a ready-made fungicide, and UC IPM lists it among the products that eradicate existing mildew. Baking soda works when combined with oil, and the published home recipes differ. University of Wisconsin Extension cites 1 1/2 tablespoons of baking soda with 3 tablespoons of lightweight horticultural oil in 1 gallon of water. Colorado State gives 1 tablespoon of baking soda with 2.5 tablespoons of oil per gallon. Clemson uses 3 tablespoons each of oil and either bicarbonate per gallon. Start with one of the weaker mixes and test it on a few leaves before spraying a whole plant.

Conventional fungicides. Wisconsin lists chlorothalonil, copper, mancozeb, myclobutanil and thiophanate-methyl among the active ingredients registered for powdery mildew. Clemson recommends rotating between products so the fungus is less likely to develop resistance.

Whatever you choose, one spray is not a treatment. Colorado State and Wisconsin both give an interval of 7 to 14 days, and Wisconsin extends the program from bud break until humid weather subsides. Colorado State’s stated reason for repeating is continuous protection. Read the product label for the plants it may be used on and for its own temperature limits.

When to Leave It Alone

University of Wisconsin Extension describes powdery mildews as cosmetic, non-lethal diseases for many plants, while noting that phlox, bee balm and zinnia can suffer severe leaf loss. Mildew that arrives on established perennials at the end of the season, after flowering, lands on stems that are about to be removed anyway. Cut them down at the end of the growing season as Clemson recommends for infected growth, clear the debris, and the plant starts clean in spring. Hot weather also helps: above 95 F the fungus may stall without any spray.

Save the spraying for plants where mildew arrives early enough to spoil the bloom or strip the leaves in midsummer. If one plant needs treatment from June onward every year, the site is too shaded or too still for it. Move it into sun, or plant a resistant variety from the table in its place.