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White powdery growth on leaves and shoots can be a sign of powdery mildew disease.
This disease affects many plants and may be caused by several fungi. Manage powdery mildew by using resistant plant varieties and altering the growing environment. In some situations, fungicide treatments may be required for susceptible plant species.
Symptoms can vary by plant species.
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- White powdery spots develop on both surfaces of leaves and expand as the infection grows.
- Leaves gradually turn yellow or brown and fall off, exposing
the plant or fruit to sunburn.
- In some cases, leaves or shoots twist or distort.
- Vegetable fruits are not usually affected, but apples, grapes
and stone fruits may develop weblike russet scars or corky areas.
Powdery mildew is common under warm, dry conditions.
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- Unlike many diseases, powdery mildew does not require moist conditions to grow and growth is inhibited
by water in the spring.
- Moderate temperatures (60º to 80ºF) and shade favor
the disease.
Alter the growing environment to make plants less susceptible.
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- Grow plants in sunny locations.
- Provide good air circulation by pruning excess foliage.
- Don’t overfertilize with nitrogen because lush foliage
and shade favor the disease.
Plant resistant varieties.
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Highly susceptible plants with resistant or less susceptible varieties include:
- Ornamentals: crape myrtle, rose, London plane tree, rhododendron,
zinnia.
- Fruit: Apple, raspberry, peach.
- Vegetables: Melons, pumpkins, squash and cucumbers, beans and
peas.
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Consider nonchemical approaches.
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- Sprinkle infected plants with water. To prevent problems with other diseases, do this midmorning so
moisture dries out rapidly. Adding a little soap to the water can increase control.
- Prune out small infestations and remove infected buds during
the dormant season. Be sure to quickly remove infected materials from the garden and not spread spores
to new areas.
Fungicides may be needed for susceptible varieties of some plants.
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Plants often requiring treatment include apples, caneberries, grapes, roses, and cucurbits.
- Control mild to moderate infections with horticultural oil
or plant-based oils such as neem oil or jojoba oil, or potassium bicarbonate fungicides. Don’t
use oils where sulfur applications have been made or when it is above 90ºF.
- Prevent infections with wettable sulfurs, especially those
in ready-to-use products that are formulated with soaplike surfactants. These products are ineffective
if applied after the disease appears. Repeat applications may be necessary.
- Several other fungicides are available for some plant types
but most must be applied before you see the first fungal growth.
Minimize the use of pesticides that pollute our waterways. Use nonchemical alternatives or less toxic pesticide products whenever possible. Read product labels carefully and follow instructions on proper use, storage, and disposal. |
