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Powdery Mildew

Plant disease

Description​

Powdery mildew is a group of fungal diseases, not a single species. Different fungi infect different hosts, including roses, squash, cucumbers, grapes, and many ornamentals. They produce a pale, powder-like growth, usually most noticeable on leaves but sometimes present on stems, buds, flowers, or fruit. The fungi draw nutrients from living plant tissue. Heavy infections can reduce photosynthesis, slow growth, spoil flowers or fruit, and cause leaves to yellow or drop early. Powdery mildew often develops when days are warm, nights are cool, and air circulation is poor. Unlike many other fungal diseases, its spores do not need a film of water on leaves to germinate. Some powdery mildew fungi survive between growing seasons on living buds or plant debris. The disease is rarely fatal to an established plant, but repeated or severe infections can weaken it and substantially reduce a crop.

Symptoms​

  • White powdery coating
  • Yellowing leaves
  • Curled or cupped leaves
  • Distorted new growth
  • Stunted growth
  • Leaves dropping

How to Identify​

Look for white to gray, flour-like patches on the leaf surface. They may begin as small circles, then spread across leaves and onto stems or other above-ground parts. Affected leaves may yellow, curl, or become distorted. The coating can rub off, but fungal growth will remain or reappear. Downy mildew more often produces yellow, sometimes angular, patches on upper leaf surfaces and fuzzy growth underneath; it is a different disease. Dust or dried spray residue should not continue spreading to new leaves. Mealybugs form distinct cottony clusters, often near leaf joints, and individual insects can be seen on close inspection.

Prevention​

Choose resistant varieties when available, and give plants the spacing recommended for their mature size. Place sun-loving plants where they receive adequate light; avoid crowding and prune selectively to improve airflow. Water consistently to reduce plant stress, applying water to the soil when practical. Wet leaves are not required for powdery mildew infection, so keeping foliage dry alone will not prevent it. Avoid excessive nitrogen fertilizer, which encourages tender growth that may be susceptible. Inspect plants regularly, especially when days are warm and nights cool, so you can act before the disease spreads. Remove badly infected fallen leaves and other debris, and follow crop-rotation advice where practical. Because powdery mildew fungi often specialize in particular hosts, an infection on one kind of plant will not necessarily infect another.

Treatment​

Start by removing the worst-affected leaves, if the plant can spare them, and dispose of them; avoid stripping a plant of most of its foliage. Improve spacing and airflow, and correct water or fertilizer stress. Existing white patches will not turn healthy again, so judge progress by whether new growth stays clean. For a spreading infection, use a product labeled for powdery mildew on that specific plant, following its directions and repeat interval. Potassium bicarbonate sprays and horticultural oils are options for many garden plants; good coverage is important. Oils can burn foliage in hot weather or on stressed plants. Do not apply sulfur close to an oil treatment: follow both labels’ required separation periods. Sulfur and other labeled fungicides can help protect uninfected growth, but are generally more effective when started early than after a severe outbreak. Follow label restrictions for edible crops and avoid spraying when pollinators are visiting flowers.

Plants Affected​

Acorn Squash, Adzuki Bean, American Basswood, American Sycamore, Apple, Artichoke, Azalea, Barberry, Barley, Basswood, Bitter Melon, Black Cherry, Black-eyed Pea, Black-eyed Susan, Bluebonnet, Borage, Bottle Gourd, Bottlebrush Buckeye, Buckwheat, Bur Oak, Burdock Root, Burning Bush, Butterfly Bush, Butternut Squash, California Poppy, Callicarpa, Calycanthus, Caragana, Catalpa, Chayote, Chervil, Chestnut Oak, Chestnut-leaved Oak, Chinkapin Oak, Chokeberry, Chokecherry, Chrysanthemum, Cilantro, Clematis, Coneflower, Coreopsis, Coriander, Cornus, Cosmos, Crabapple, Cucumber, Cumin, Daisy, Dandelion Greens, Delicata Squash, Delphinium, Deutzia, Dill, Dogwood, Elderberry, Euonymus, European Hornbeam, Fennel, Fenugreek, Flowering Dogwood, Forget-me-not, Gerbera Daisy, Goldenrod, Grapes, Guar Bean, Hamamelis, Honeysuckle, Hornbeam, Horse Gram, Hyacinth Bean, Hydrangea, Itea, Ivory Gourd, Jerusalem Artichoke, Kabocha Squash, Kousa Dogwood, Lablab Bean, Lambsquarters, Laurel Oak, Ligustrum, Lilac, Live Oak, London Plane, Lonicera, Lupine, Magnolia, Mahonia, Mango, Marigold, Mint, Morning Glory, Moth Bean, Mulberry, Mung Bean, Nasturtium, Ninebark, Northern Catalpa, Norway Maple, Oak, Oats, Oregano, Overcup Oak, Pacific Dogwood, Paeonia, Parsnip, Pattypan Squash, Pea, Peony, Petunia, Philadelphus, Photinia, Physocarpus, Poppy, Post Oak, Potentilla, Privet, Pumpkin, Queen Anne’s Lace, Red Mulberry, Red Oak, Red Twig Dogwood, Ribes, Rice Bean, Ridge Gourd, Rosa, Rose, Rosemary, Rye, Sage, Salsify, Sambucus, Sand Cherry, Scarlet Oak, Scorzonera, Serviceberry, Shasta Daisy, Shumard Oak, Smokebush, Snake Gourd, Snap Pea, Snapdragon, Snow Pea, Snowberry, Southern Catalpa, Spaghetti Squash, Spirea, Strawberry, Sumac, Sunchoke, Sunflower, Swamp Chestnut Oak, Sweet Pea, Sword Bean, Sycamore, Sycamore Maple, Symphoricarpos, Syringa, Tarragon, Trumpet Vine, Urad Bean, Viburnum, Water Oak, Watermelon, Weigela, Wheat, White Mulberry, White Oak, Willow Oak, Winter Melon, Witch Hazel, Yardlong Bean, Zinnia, Zucchini

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