How to get rid of powdery mildew on plants

Powdery mildew is a fungal disease that looks like someone dusted your leaves with flour. It shows up on squash, cucumbers, peas, roses, lilacs, phlox, bee balm, and many other plants. It peaks in mid to late summer.

Powdery mildew seldom kills a plant. But it weakens plants, cuts your harvest, and makes leaves die early.

How to identify powdery mildew

• White to gray powdery spots on the tops of leaves, then the undersides and stems
• Spots that spread and join until the whole leaf looks coated
• Leaves that turn yellow, curl, brown, and die
• Older leaves hit first

Rub a spot with your finger. Powdery mildew wipes off, at least at first.

That sets it apart from downy mildew. Downy mildew shows up as yellow patches on top of the leaf, with gray or purple fuzz underneath.

What causes powdery mildew

Many different fungi cause powdery mildew. Most of them infect one plant or one plant family. The type on your lilac will not spread to your squash.

Unlike most fungal diseases, powdery mildew does not need wet leaves to spread. The spores travel on the wind. They do best with:

• Warm days and cool nights
• High humidity
• Shade and crowded plants with poor airflow
• Lots of soft new growth from too much nitrogen

How to get rid of powdery mildew

Step 1: Remove the worst leaves

Cut off the worst leaves at the first sign. Put them in the trash or your city green bin, not your home compost. Clean your pruners between plants.

Do not strip a plant bare. Leaves feed the plant. Take the worst ones and treat the rest.

Step 2: Spray

Sprays work best early, before the plant is covered. Most of them stop new spots better than they clear old ones. Spray every 7 to 14 days, or as the label directs. Coat both sides of the leaves.

• Potassium bicarbonate. Sold in products like GreenCure and MilStop, it kills the fungus on contact. It is a common choice for organic gardens.
• Baking soda. Sodium bicarbonate works like potassium bicarbonate, but it is weaker. A common mix is 1 tablespoon of baking soda and 1/2 teaspoon of liquid soap in 1 gallon (3.8 L) of water. Repeated use can build up sodium in the soil, so do not use it all season long.
• Neem oil and horticultural oils. They smother the fungus and slow its spread. Do not spray above 90°F (32°C), in direct sun, or within 2 weeks of a sulfur spray.
• Sulfur. This old fungicide works best as a preventive. Do not use it within 2 weeks of an oil spray, or on hot days. Some squash and melon varieties are sensitive to sulfur, so read the label.

Test any new spray on a few leaves first and wait a day.

Step 3: Fix the conditions

• Thin crowded plants and prune for better airflow.
• Water at the soil, not over the leaves, and water in the morning.
• Hold back on high-nitrogen fertilizer.
• Water during dry spells. Plants under drought stress get hit harder.

Powdery mildew on squash and cucumbers

Squash, zucchini, pumpkins, and cucumbers often get powdery mildew by late summer. If the plants have already given you most of their harvest, you can let it go.

To keep plants producing longer, start spraying when you see the first spots. Earlier is better.

Next year, choose resistant varieties. Look for “PM” or “PMR” in the variety description or catalog codes. Plant in full sun with plenty of space between plants.

These posts help you grow healthier squash family crops:

• Zucchini plant care
• How to grow cucumbers
• Companion plants for zucchini

Peas get powdery mildew in warm weather too. That is one more reason to plant peas early in spring.

Powdery mildew on flowers and shrubs

Lilacs, phlox, bee balm, roses, peonies, and zinnias all get powdery mildew. On lilacs and peonies, late-summer mildew is a cosmetic problem more than a health problem.

Choose resistant varieties when you can. Garden phlox ‘David’ and bee balm ‘Jacob Cline’ are both known for good mildew resistance.

Clean up fallen leaves in fall. Cutting back peonies in fall helps too. Here is when to cut back peonies. For roses, see how to grow roses.

How to prevent powdery mildew next year

• Plant resistant varieties.
• Give plants full sun and room to breathe.
• Rotate vegetable crops where you can.
• Clean up infected leaves and plant debris in fall. Many powdery mildew fungi survive winter on old plant material.
• Start preventive sprays early on plants that get it every year.

For other common garden problems, see my guide to garden pests.