This fungal disease is a white or gray, powdery, growth on the lower leaf surface and flower buds of zinnias, crape myrtles, and many vegetables, phlox, lilac, melons, cucumbers, and many other plants. It is common during cool, humid, cloudy days. Leaves turn yellow on the top. Controls include compost tea, baking soda spray, potassium bicarbonate spray, neem, garlic, and horticultural oil. PureGro Pest and Disease Control (to clean plants) seems to be the best immediate cure. Garrett Juice plus garlic tea is best for long-term results. Treat soil with horticultural cornmeal and use the entire Sick Tree Treatment for serious problems. This disease can cause long term weakness when it occurs early in the growing season. This common fungal disease is increased by humidity but actually deterred by water. Mildew forms a white to grayish powdery growth, usually on the upper surfaces of leaves. Small black dots contain spores that are blown by wind to infect new plants. Leaves will become brown and shrivel and fruits ripen prematurely and have poor texture and flavor. Prune plants to improve air circulation. Thoroughly compost infected plants before spores form and spread.
Crape Myrtle Powdery Mildew
Powdery mildew attacks crepe myrtles and other plants especially during humid weather. The control is to apply horticultural cornmeal to the root zone at 2 lbs. per 100 sq ft and spray the foliage with Garrett Juice plus either potassium bicarbonate or cornmeal juice. Cornmeal juice is made by soaking a pound of cornmeal in 5 gallons of water. Put the cornmeal in a nylon bag made from old panty hose or whatever to keep the larger particles from getting in the water. This cornmeal juice can be used alone or mixed with compost tea or Garrett Juice.
Potassium bicarbonate Fungicide
Mix 4 teaspoons (about 1 rounded tablespoon) of potassium bicarbonate into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Potassium bicarbonate is a good substitute for baking soda. There are commercial EPA registered as well as generic products available.
Baking Soda Fungicide
Mix 4 teaspoons (about 1 rounded tablespoon) of baking soda and 1 tablespoon of horticultural oil into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Avoid over-using or pouring on the soil. Potassium bicarbonate is a good substitute for baking soda. Citrus oil and molasses can be used instead of horticultural oil.
Q: I have always battled powdery mildew in my garden. This year, we have gone organic and have had no problems except with powdery mildew. I put horticultural cornmeal on the ground and spray Garrett Juice once a week. I have even tried baking soda spray a few times, but nothing seems to work. The mildew is spreading like crazy, and my pumpkins, zucchini and yellow squash are dropping fruit or rotting. Please tell me what else to do. M.M., Stephenville, TX
A: Soak some cornmeal in water and spray the plants with cornmeal juice. Put a cup of horticultural cornmeal or whole ground cornmeal in 5 gallons of water. Let the solution soak for at least an hour, then strain the solids out with cheesecloth or pantyhose. Spray the plants thoroughly.
Q: How can I "doctor" my crape myrtles the natural way? They have a powdery white substance on the leaves. B.C., Waco
A: My Sick Tree Treatment is the solution, but a couple of additions are necessary. Some crape myrtle varieties and cultivars have more resistance to powdery mildew and other diseases, but all will benefit from aerating the root zone and applying these amendments: compost, lava sand, Texas greensand, dry molasses, horticultural cornmeal and cedar mulch. A thin layer of shredded cedar is all the mulch that's needed. Never pile mulch onto the trunks of trees.
Also, make sure the trees in question are not planted too deeply before adding amendments. If they are too deep (root flare and root ball top are covered with soil), hire a certified arborist to use an air spade to remove the excess soil.
The last part of the Sick Tree Treatment is to spray the foliage, trunk and ground with compost tea or Garrett Juice.