Powdery mildew on Plants: Causes, Treatment & Prevention

Symptoms

Look for white powdery spots on the leaf surfaces. Check both sides of the leaf, because the spots develop on both leaf surfaces and expand as the infection grows. The same circular powdery white spots can appear on stems, fruit or flowers. Later, round black specks may form within the mildew as it matures or in fall.

Leaves can turn yellow or brown and fall off, which exposes the plant or fruit to sunburn. In severe infections, leaves yellow and wither. Leaves or shoots may twist or become distorted. New growth can come in dwarfed and distorted. On infected oaks, you may also see an abnormal witches'-broom growth.

Susceptible varieties of rose, crape myrtle and euonymus are especially prone to the disease, so check their leaves for these spots. Fruit crops such as grape, apple, raspberry and peach are also listed as hosts, so check fruit and stems as well as leaves. A plant showing several of these signs together fits the pattern described for powdery mildew.

Causes

Powdery mildew is caused by infection with fungi of the order Erysiphales, which belongs to the phylum Ascomycota. Moderate temperatures of 60 to 80°F (16 to 27°C) and shady conditions are most favorable for its development. Warm, dry weather in that range favors the disease. Crowded, poorly ventilated growing conditions also favor it.

The fungus can germinate and infect leaves without free water on the leaf surface. Cool days combined with humid nights favor establishment and spread. Hot weather works against the fungus, since temperatures above roughly 90 to 95°F (32 to 35°C) can suppress its growth.

Too much nitrogen causes lush foliage and shade, providing conditions for fungal growth. New growth is the most susceptible to infection. The fungus overwinters, which is why removing plant debris, especially in fall, reduces it. The listed hosts include woody plants such as rose and oak, and crops such as squash and grape.

Diagnosis

No look-alike problems are named in the cited material, so this check rests on the signs and conditions of powdery mildew itself. The signature sign is white powdery spots on leaf surfaces that expand as the infection grows. Yellowing is listed here as a result of severe infection, so look for the white spots before settling on powdery mildew. Round black specks within the mildew, as it matures or in fall, are another sign to look for.

Next, compare the conditions around the plant with the known causes. Powdery mildew is favored by shade, crowding and poor ventilation. It is also favored by temperatures of 60 to 80°F (16 to 27°C) and by cool days with humid nights. Dry leaf surfaces do not rule it out, since the fungus can infect without free water.

Then look at where the spots sit. Spots on stems, fruit or flowers as well as leaves match the circular powdery white spots described for this disease. Twisted, dwarfed or distorted new growth is another listed sign. Hosts include rose, apple, grape, cucumber and zinnia, so a spotted leaf on one of these plants fits the pattern.

Treatments

Grow plants in sunny locations rather than shade. Prune excess foliage and space plants properly to give good air circulation. Fertilize properly and avoid excess nitrogen, which causes lush, shaded foliage that favors the fungus. Choose resistant varieties or disease-resistant cultivars where you can. Avoid overcrowding, overwatering and poorly drained soil, and remove weeds around plants.

Replace overhead watering with drip or soaker irrigation, or water in early morning if you can't avoid overhead watering. You can also wash spores off infected plants with overhead sprinkling. Remove infected leaves, branches, flowers or fruit, and do not compost the diseased material. Prune out small infestations, and remove infected buds during the dormant season. Remove plant suckers, since new growth is most susceptible to infection. Sterilize garden tools between infected and uninfected plants, and clear plant debris, especially in fall, to reduce fungal overwintering. Maintain compost piles properly so heat destroys fungal spores.

Sulfur products, especially ready-to-use formulations with soap-like surfactants, are used to prevent infection. Horticultural oil or plant-based oils such as neem oil can control mild to moderate infections. Do not combine oil with sulfur, and do not use oil above 90°F (32°C). Potassium bicarbonate products are used to eradicate existing infections. Bacillus-based fungicides are another option, though research shows they are generally less effective than oils or sulfur. Copper- or phosphorus-based fungicides are also listed, but results are not guaranteed and copper can harm some plants.

Chemical options include phosphonate systemic fungicides, which move through the plant. Myclobutanil-containing products act as an eradicant and protectant against both powdery mildew and rust. Apply protectant fungicides to susceptible plants before disease develops or in its earliest stages. The product label is the legal instruction, so follow it for the crop and site you are treating. Consult your local extension for product recommendations.

Prevention

Sunny locations remove shade, one of the conditions most favorable to the disease. Space plants properly and prune excess foliage for good air circulation, since poor ventilation favors the fungus. Feed plants properly and avoid excess nitrogen, because lush, shaded foliage favors the fungus.

Water at the base with drip or soaker irrigation instead of overhead watering. If overhead watering can't be avoided, do it in early morning. Keep plants uncrowded, avoid overwatering and poorly drained soil, and keep weeds down.

Choose resistant varieties or disease-resistant cultivars when you plant. In fall, remove plant debris to reduce fungal overwintering. Sterilize garden tools between use on infected and uninfected plants. Remove plant suckers, because new growth is most susceptible to infection.

Plants most often affected

Rose, crape myrtle, sycamore, oak, euonymus, aster, azalea and begonia are all listed hosts. Fruit and vegetable hosts include apple, grape, melon, pumpkin, squash, cucumber, bean, pea, raspberry and peach. Rhododendron, zinnia and london plane tree are on the list too. The disease can be serious on woody species such as rose, crape myrtle, sycamore and oak.

Powdery mildew on Plants

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