In the Garden:
Northern California Coastal & Inland Valleys
August, 2000
Regional Report

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Powdery mildew on pumpkin leaves.

Powdery Mildew Season

It's powdery mildew season. Warm days followed by cool nights are the perfect growing environment for this fungal disease. I can see it all over my garden.

Show Me the Mildew

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 usually occur when the summer fog rolls in. It 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. 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. Here comes the list:

1. Choose resistant cultivars.

2. Don't over fertilize with high-nitrogen fertilizers. Tender new growth 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 Mildews

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

1. Apply sulfur dust to both the upper and under sides of the leaves. Sulfur doesn't kill the existing spores, but it does keep the next generation 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.


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