Patsy asks: The leaves on my squash and tomato plants have a gray film and yellow spots on them. I’m hoping this is not a serious problem. What is it and what should I do about it?
Tropical Gardener answer: Your symptoms match those of the fungus called powdery mildew, which appears like a gray powder on leaves, stems, buds and flowers of affected plants. The yellow spots are often the result of a powdery mildew attack. Check the photos with this article to see if they show the symptoms you describe.
The many species of powdery mildew exist are host specific. The different fungi will only spread to plants of the same species. Its spores often remain in the soil and multiply when conditions are ideal.
Powdery mildew is encouraged by warm, humid weather but can multiply even in dry somewhat shady conditions. It does not need free moisture to grow. The spores and fungal growth are sensitive, however, to direct sun and heat over 90 degrees. In ideal conditions, the fungus produces lots of dry, powdery spores that are easily dispersed by air currents.
Powdery mildew is unattractive and can cause plant stress but it is not usually fatal. If it covers a lot of leaves, it can impair photosynthesis and weaken the plant. It can also cause infected leaves to fall prematurely.
You can start treating powdery mildew by removing infected plant parts but not more than one-third of the foliage. Dispose of these away from plants of the same species. Further treatment can be done with a few nontoxic ingredients. Baking soda (sodium bicarbonate) will slow the fungus’ growth when mixed into water containing a wetting agent and a sticking agent. Insecticidal soap and neem oil are good choices, though a liquid hand soap and light cooking oil can be used.
To a gallon of water add 1 teaspoon of soap, 1 tablespoon of oil and 2 tablespoons of baking soda. Mix thoroughly and apply with a sprayer to all infected plant parts. Apply this mixture late in the day, out of direct sun to prevent burning the leaves. Weekly applications can rid the plant of the disease followed by twice monthly applications to prevent the fungus’ return.
Relatively safe chemicals can also be effective. Wettable sulfur works well as does potassium bicarbonate (organic product is Kaligreen). Sulfur should not be applied in temperatures over 90 degrees or within two weeks of products containing oil. Serenade is a bacterium (Bacillus subtilis) that helps prevent powdery mildew from infecting a plant and is registered for organic use in Hawaii. Be sure to read the label and follow the directions.
Planting vulnerable plants far apart and pruning them to provide good air circulation and reduce interior humidity can help prevent powdery mildew. Some resistant plant varieties are available and are recommended where conditions favoring the fungus exist.
For more information, check out this University of Hawaii College of Tropical Agriculture and Human Resources publication at https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-98.pdf.
Diana Duff is a plant adviser, educator and consultant living on an organic farm in Captain Cook. Some questions that appear here were originally directed to Kona’s Certified Master Gardeners. You may contact them with questions at konamg@ctahr.hawaii.edu.
Gardening Events
Monday: “Hawaii Tropical Fruit Growers meeting on soil,” 7-9 p.m. with soil specialist Bob Shafer and Deano from Organic Matters Hawaii at 81-6393 Mamalahoa Highway in Kealakekua. Info: Brian Lievens at 895-8753 or greenwizard@hawaii.rr.com.
Farmer Direct Markets
Wednesday: “Hooulu Farmers Market,” 9 a.m. to 2 p.m. at Sheraton Kona Resort and Spa at Keauhou Bay
Wednesday: “Sunset Farmers Market,” 2 p.m. to sunset at the north makai corner of the Kmart parking lot
Saturday: “Keauhou Farmers Market,” 8 a.m. to noon at Keauhou Shopping Center
“Kamuela Farmer’s Market,” 7 a.m. to noon at Pukalani Stables
Sunday: “South Kona Green Market,” 9 a.m. to 2 p.m. at Amy Greenwell Garden in Captain Cook
Tuesday–Saturday: “U-Pick greens and produce,” 10 a.m. to 4 p.m. Tropical Edibles Nursery in Captain Cook
Plant Advice Lines
Anytime: konamg@ctahr.hawaii.edu
Tuesdays and Thursdays: 9 a.m. to noon at UH-CES in Kainaliu, 322-4892
Mondays, Tuesdays and Fridays: 9 a.m. to noon at UH-CES at Komohana in Hilo, 981-5199 or himga@hawaii.edu
