Powdery Mildew Season

Warm days followed by cool nights signal the beginning of powdery mildew season. It's showing up here and there throughout my garden.

Powdery mildew is identified as a white or gray powderlike coating on the surface of plant foliage. If left untreated, the powdery mildew fungus commonly causes leaf drop, poor growth, and reduced production of fruits, flowers, and vegetables. One infected leaf can produce 100,000,000 spores that travel on the wind to spread the disease to other susceptible plants throughout your garden.

The first signs of powdery mildew here in Northern California's coastal valleys occur when the summer fog rolls in. Unlike most fungal foliar diseases, powdery mildew does not require that the leaf surface actually be moist in order to grow, but it does require high humidity. The infection first appears as small, discolored patches on leaves, which soon grow and spread to cover the entire surface. Eventually, the leaf will either curl and yellow or turn brown and die. My poor zucchini plants seem to suffer the most from powdery mildew, which ruins the huge leaves.

Susceptible Plants
Plants most susceptible to powdery mildew are apple trees, beans, cape myrtle, hydrangea, peas, roses, zucchini, squash, pumpkin, and zinnia, to name a few. However, powdery mildew can occur on most plants under the right conditions.

Mildew Controls
There are several things you can do to combat powdery mildew:

1. Choose resistant cultivars.

2. Don't over fertilize with high-nitrogen fertilizers. It pushes tender new growth, which is especially prone to attack.

3. Water in the morning hours instead of the evening. By allowing the foliage to dry during the warm daylight hours, you reduce the chance of mildew spores germinating on damp leaves.

4. When you initially plan and plant your garden, leave room between plants. Maximum air circulation is mandatory for preventing fungus attacks.

5. Prune woody shrubs so that the center of the plant is open, once again allowing for maximum air circulation.

6. Keep the area under your plants clean and raked up. Fungus spores travel in water that is splashed back onto the lower foliage. If diseased leaves have fallen to the ground and are left there, every time you water, you are reinfecting your plants.

7. If you have a plant that is affected every single year, get rid of it! There are plenty of nice plants out there looking for a home.

Remedies for Mildew
Fungicides are useful in combating powdery mildew; however, always try the least toxic method first. This just makes good sense for controlling any pest in your garden, be it weed, bug, or fungus. Here are some simple, organic remedies for controlling powdery mildew:

1. Apply sulfur dust to both the upper and undersides of the leaves as directed on the product label. Sulfur doesn't turn back the existing infection, but it does keep the next generation of spores from germinating.

2. Mix a solution of 1 teaspoon baking soda in 1 quart of warm water in a small plastic spray bottle. Add 1/2 teaspoon of either liquid dish soap or insecticidal soap to help the solution cling to the foliage. Spray infected plants thoroughly on both sides of the leaves every 5 to 10 days.

3. Make a solution of one part alcohol-based mouthwash and three parts plain water in a spray bottle and apply the mix to the leaves daily.

Powdery mildew spores die at 30F and below. Discard, rather than compost, infected foliage in the fall to prevent the spread of the disease the following season. The fungus spores would simply love to overwinter in a nice, warm compost pile.

Photo by Kim Haworth/National Gardening Association

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