In the Garden:
Northern California Coastal & Inland Valleys
August, 2013
Regional Report

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Powdery mildew frequently coats the leaves of pumpkins by late summer.

Powdery Mildew Season

It's powdery mildew season. Warm days followed by cool nights are the perfect growing environment for this fungal disease. Identify powdery mildew by the white or gray powder-like coating on the surface of the leaves. While rarely fatal to plants, the powdery mildew can cause 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. But don't worry that your phlox infecting your pumpkins. Powdery mildew is caused by several different species of closely related fungi, with each species infecting only particular kinds of plants.

The first signs of powdery mildew usually occur when the summer fog rolls in. It starts 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 especially susceptible to powdery mildew include apple trees, beans, crape 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

  • Choose resistant cultivars.

  • Don't over fertilize with high-nitrogen fertilizers. Tender new growth is especially prone to attack.

  • Prune out and destroy any infected leaves as soon as you see them.

  • Allow plenty of room for good air circulation between plants. Water in the morning so foliage dries quickly. High humidity favors powdery mildew.

  • 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. Clean up plant debris well at the end of the season to reduce the amount of overwintering fungi.Don't add infected plant material to your compost pile.

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 problem, be it weed, bug, or fungus, in your garden. Here are some remedies for controlling powdery mildew:

  • Spray preventatively with a product containing sulfur, potassium bicarbonate, or the biological fungicide Serenade at 7-14 day intervals. Follow all label instructions and precautions.

  • Make a homemade preventative spray by mixing 2 1/2 TBSP of horticultural oil in one gallon of water, then add 4 tsp. of baking soda. This changes the pH, making it less suitable for the germination of powdery mildew spores. (Test both commercial and homemade bicarbonate sprays on a small section of plant before treating the entire plants, as these materials can injure some plants.)

  • To control an active infection, treat with horticultural oil or neem oil.

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