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White powdery growth on leaves and shoots can be a sign of powdery mildew. This disease affects many plants, and one of several fungi can cause it. Manage powdery mildew by growing resistant plant varieties and altering the growing environment. In some situations, fungicide treatments might be required for susceptible plant species.
Symptoms can vary by plant species.
- White powdery spots develop on both leaf surfaces and expand as the infection grows.
- Leaves turn yellow or brown and fall off, exposing the plant or fruit to sunburn.
- Leaves or shoots can twist or distort.
- The fruiting parts of vegetables are usually not affected, but apples, grapes, and stone fruits can develop web like russet scars or corky areas.
Powdery mildew is common in warm, dry conditions.
- Unlike many diseases, powdery mildew doesn’t require moist conditions to grow.
- Moisture during the spring inhibits growth.
- Moderate temperatures (60° to 80°F) and shade encourage the disease.
Alter the growing environment to make plants less susceptible.
- Grow plants in sunny locations.
- Provide good air circulation by pruning excess foliage.
- Fertilize properly because too much nitrogen causes lush foliage and shade, providing conditions for fungal growth.
Plant resistant varieties.
Some plants have resistant or less susceptible varieties such as:
- Ornamentals: crape myrtle, rose, London plane tree, rhododendron, and zinnia.
- Fruit: apple, raspberry, and peach.
- Vegetables: melon, pumpkin, squash, cucumber, bean, and pea.
Consider nonchemical methods.
- Wash spores off infected plants with overhead sprinkling. To prevent other disease problems, do this midmorning so moisture dries rapidly.
- Prune out small infestations and remove infected buds during the dormant season. Quickly remove infected materials so you don’t spread spores.
What about pesticides?
- Some infections may require fungicides.
- Control mild to moderate infections with horticultural oil or with plant-based oils, such as neem oil. Do not use oils if you have applied sulfur or the temperature is above 90°F.
- Prevent infections with sulfur products, especially ready-to-use products with soap like surfactants. These products are not effective after the disease appears. Repeat applications might be necessary as new leaves grow.
- Other fungicides are available. Many must be applied before you see the first sign of disease.
Pest Notes: Introduction
Powdery mildew is a common disease on many types of plants. There are many different species of powdery mildew fungi (e.g., Erysiphe spp., Sphaerotheca spp.) and each species only attacks specific plants. A wide variety of vegetable crops are affected by powdery mildews, including artichoke, beans, beets, carrot, cucumber, eggplant, lettuce, melons, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes, and turnips. Powdery mildews generally do not require moist conditions to establish and grow, and normally do well under warm conditions; thus they are more prevalent than many other leaf-infecting diseases under California’s dry summer conditions.
Identification and Damage
Powdery mildew first appears as white, powdery spots that may form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit. These spots gradually spread over a large area of the leaves and stems. An exception is one of the powdery mildews that affects artichokes, onions, peppers, and tomatoes: it produces yellow patches on leaves but little powdery growth.
Leaves infected with powdery mildew may gradually turn completely yellow, die, and fall off, which may expose fruit to sunburn. On some plants, powdery mildew may cause the leaves to twist, buckle, or otherwise distort. Powdery mildew fungal growth does not usually grow on vegetable fruits, although pea pods may get brownish spots. Severely infected plants may have reduced yields, shortened production times, and fruit that has little flavor.
Life Cycle
All powdery mildew fungi require living plant tissue to grow. Year-round availability of crop or weed hosts is important for the survival of some powdery mildew fungi. Special resting spores are produced, allowing overwinter survival of the species that causes the disease in cucurbits, lettuce, peas, and certain other crops.
Most powdery mildew fungi grow as thin layers of mycelium (fungal tissue) on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the white, powdery growth visible on the plant’s surface and are produced in chains that can be seen with a hand lens; in contrast, spores of downy mildew grow on branched stalks that look like tiny trees.
Powdery mildew spores are carried by wind to new hosts. Although humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of some powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Moderate temperatures (60° to 80°F) and shady conditions generally are the most favorable for powdery mildew development. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight.
The best method of control is prevention. Planting resistant vegetable varieties when available, or avoiding the most susceptible varieties, planting in the full sun, and following good cultural practices will adequately control powdery mildew in many cases (Table 1). However, very susceptible vegetables such as cucurbits (cucumber, melons, squash, and pumpkins) may require fungicide treatment. Several least-toxic fungicides are available but must be applied no later than the first sign of disease.
Resistant Varieties
In some cases, varieties resistant to powdery mildew may be available. If available, plant resistant varieties of cantaloupe, cole crops, cucumber, melons, peas, pumpkins, and squash. If you plant more susceptible varieties, you may need to take control measures.
| Hosts | Fungus species | Controls |
|---|---|---|
| cucumbers, endive, lettuce, melons, potato, pumpkin, squash | Erysiphe cichoracearum | resistant varieties of lettuce, cucumber; water sprays; fungicides if necessary on squash and pumpkin |
| broccoli, Brussels sprouts, cauliflower, and other cole crops; radicchio, radishes, turnips | Erysiphe cruciferarum | not usually required |
| tomatoes | Erysiphe lycopersici | fungicides if necessary |
| peas | Erysiphe pisi | resistant varieties; sprinkler irrigation |
| carrots, parsley, parsnips | Erysiphe heraclei | tolerant varieties |
| beets | Erysiphe polygoni | tolerant varieties |
| artichoke, eggplant, peppers, tomatillo, tomatoes | Leveillula taurica | rarely required; fungicides if necessary |
| beans, black-eyed peas, cucurbits, okra | Sphaerotheca fuliginea | resistant varieties for some; fungicides if necessary |
Cultural Practices
Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Overhead sprinkling may help reduce powdery mildew because spores are washed off the plant. However, overhead sprinklers are not usually recommended as a control method in vegetables because their use may contribute to other pest problems.
Fungicide Application
In some situations, especially in the production of susceptible cucurbits, fungicides may be needed. Fungicides function as protectants, eradicants, or both. A protectant fungicide prevents new infections from occurring whereas an eradicant can kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Once mildew growth is extensive, control with any fungicide becomes more difficult. The products listed here are for home garden use. Commercial growers should consult the UC Pest Management Guidelines.
Fungicides
Several least-toxic fungicides are available, including horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also have some protectant activity.
Oils
To eradicate mild to moderate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil, or one of the plant-based oils such as neem oil or jojoba oil (e.g., E-rase). Be careful, however, to never apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer; always consult the fungicide label for any special precautions.
Sulfur
Sulfur products have been used to manage powdery mildew for centuries but are only effective when applied before disease symptoms appear. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (e.g., Safer Garden Fungicide) However, sulfur can be damaging to some squash and melon varieties. To avoid injuring any plant, do not apply sulfur when air temperature is near or over 90°F and do not apply it within 2 weeks of an oil spray. Other sulfur products, such as sulfur dust, are much more difficult to use, irritating to skin and eyes, and limited in terms of the plants they can safely be used on. Copper is also available to control powdery mildew but is not very effective.
Biological Fungicides
Biological fungicides (such as Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, destroys fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.
How to Use
Apply protectant fungicides, such as wettable sulfur, to susceptible plants before or in the earliest stages of disease development. The protectant fungicides are only effective on contact, so applications must provide thorough coverage of all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions are conducive to disease growth.
If mild to moderate powdery mildew symptoms are present, the horticultural oils and plant-based oils such as neem oil and jojoba oil can be used to reduce or eliminate the infection.
References
Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3332.
Gubler, W. D., and D. J. Hirschfelt. 1992. Powdery Mildew. In Grape Pest Management. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3343. pp 57-63.
McCain, A. H. 1994. Powdery Mildew. HortScript #3, Univ. Calif. Coop. Ext. Marin County.
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