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Pests in Gardens and Landscapes: Quick Tips

Powdery mildew on melon leaves.

PPowdery mildew on a rose.

Powdery mildew on euonymus.

White powdery growth on leaves and shoots can be a sign of powdery mildew. This fungal disease affects many plants. Manage powdery mildew by growing resistant plant varieties and improving the growing conditions. In some situations, fungicide treatments might be required for susceptible plant species.

Symptoms can vary by plant species but common signs include:

• 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 weblike 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.

Change where you grow plants so they're less susceptible.

• Grow plants in sunny locations.
• Provide good air circulation by pruning excess foliage and properly spacing plants.
• Fertilize properly because too much nitrogen causes lush foliage and shade, providing conditions for fungal growth.

Plant resistant varieties.
You can find resistant or less susceptible varieties for:

• 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. Reduce the spread of spores by disposing of infected plant parts in the compost, green waste, or trash receptable. Properly maintained compost piles should kill spores.

What about pesticides?

• Prevent infections with sulfur products, especially ready-to-use products with soaplike surfactants. These products are not effective after the disease appears. Repeat applications might be necessary as new leaves grow.
• Fungicides may be necessary for some infections.
• 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.
• Other fungicides are available. Many must be applied before you see the first sign of disease.

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.