Prevent powdery mildew on plants

  • Powdery mildew affects many vegetable species including tomato, beans and cucurbits. (Scot Nelson/UH CTAHR)
  • Powdery mildew appears on many different plants but is host specific and will not transfer from one species to another. (Scot Nelson/UH CTAHR)

Cheetah asks: I lost most of my last bean crop to powdery mildew. I want to try growing beans again. How can I prevent and treat this disease?

Tropical Gardener answer: This disease is common here and though it has been covered previously I have been asked about it quite often lately. So, here is the usual treatment recipe and a few new recommendations.


The species of powdery mildew that appears on beans is caused by the fungal organism Erysiphe polygoni. Its dry, powdery spores are dispersed on the air. Many different species of this disease exist, but since they are host specific, it will usually only transfer to other plants of the same species.

Despite the name, this disease can appear in dry environments. It is one of the few fungal diseases that does not require free moisture on the leaf surface. The optimal environment for the development of this disease is cool, humid nights followed by hot, dry days. Welcome to West Hawaii.

The best way to control most plant diseases is to prevent them or catch and treat them early. Several management practices can help you prevent and control powdery mildew.

Planting your crops far apart in full sun with good air circulation is best. Removing weeds or other close plants can help lower the humidity in the area. Overhead watering can wash off spores but it should be done early in the day so the moisture can dry in the sun. Watering sparingly early in the day and ensuring that the soil drains well and does not hold water can also keep the humidity low. Pruning off any affected leaves or fruit but no more than 1/3 of the plant’s foliage can limit the spread of the disease. Dispose of affected material away from your garden area.

Planting varieties that are resistant to powdery mildew can lower its incidence. Go to To find tables that list vegetable varieties that are resistant to lots of different diseases including powdery mildew. These tables also list the seed companies that sell the resistant seeds. It’s a great resource.

Many remedies can both prevent and control powdery mildew. A spray made from the following recipe is recommended both as a preventive and a control treatment. To be completely effective, it should be applied very early in the appearance of the disease.

• 1-2 tablespoon baking soda (Sodium bicarbonate. Its alkalinity retards the fungus’ growth)

• 1-2 tablespoon cooking oil or a light horticultural oil (as a sticker). Neem oil has fungicidal as well as insecticidal properties so it can discourage insects as well as helping control fungus growth.

• 1 teaspoon liquid soap or an insecticidal soap (wetting agent helps combine/ spread ingredients.

Safer insecticidal soap works well.)

Mix in 1 gallon of water and apply to infected foliage late in the day so as not to fry the leaves in hot sun. This can be applied weekly until all signs of the disease are gone. It can also be used as a prophylactic treatment by spraying every other week to keep fungal diseases at bay.

Homemade and organic treatments against powdery mildew abound. You can find some that include milk, compost tea, garlic, oils and some additional references at

Several fungicides can control of powdery mildew on beans. Be sure to follow the application instructions on their labels.

Wettable sulfur works well as does potassium bicarbonate (the ingredient in an organic product called Kaligreen). Sulfur should not be applied in temperatures over 90 degrees or within two weeks of any product containing oil. Serenade is a commercially available product that contains a bacterium (bacillus subtilis) that helps prevent powdery mildew and is registered for organic use in Hawaii.

Be aware that some of these products can be very acid or highly alkaline. Frequent applications can change the pH of your soil. If you are applying products regularly, alternate between alkaline and acidic treatments. This can also prevent the fungus from becoming resistant to the products.

More about powdery mildew can be found in the UH publication at

Email plant questions to for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column

Diana Duff is a plant adviser, educator and consultant living in a dryland forest north of Kailua-Kona.

Gardening Events

Thursday: “Local Work Group for Kona Soil and Water Conservation District” from 3-5:30 p.m. at Yano Hall in Captain Cook for agricultural producers. Discussion on our local natural resources challenges to submit to NRCS for funds allocation. For more information contact Mary Robblee at 339-9042

Saturday: “Work Day at Amy Greenwell Garden” from 9 a.m. to 12:30 p.m. Meet at the Garden Visitor Center across from the Manago Hotel in Captain Cook. Volunteers will be able to help with garden maintenance and are invited to bring a brown bag lunch. Water and snacks provided. Call Peter at 323-3318 for more information.

Next Saturday: West Hawaii Master Gardeners present “Garden Success In Hawaii” from 9 a.m. to noon. at the County Extension Office in Kainaliu. Plant sale, info on rat lungworm and fire ants, keiki activities and more. For more information call 322-4895.

Next Sunday: “Web Marketing for Farmers” from 3-6 p.m. at the Farm Bureau Office (above the old Food Basket) in Honalo. Topics include website creation and maintenance, using social media, advertising on the web and more. Free to Farm Bureau members. $10 for non-members. Bring a laptop or tablet for hands-on help. Space is limited. Registration is required at

Farmer Direct Markets

Wednesday: “Hooulu Farmers Market,” 9 a.m. to 2 p.m. at Sheraton Kona Resort &Spa at Keauhou Bay

“Sunset Farmers Market,” 2-6 p.m. in the HPM parking lot, 74-5511 Luhia St. in Kailua-Kona (across from Target)

Friday: “Pure Kona Market”, 9 a.m. to 5 p.m. at Amy Greenwell Garden in Captain Cook

Saturday: “Keauhou Farmers Market” 8 a.m. to noon at Keauhou Shopping Center

“Kamuela Farmer’s Market,” from 7 a.m. to noon at Pukalani Stables

“Waimea Town Market,” from 7:30 a.m. to noon at the Parker School in central Waimea

“Waimea Homestead Farmers Market,” from 7 a.m. to noon at the Waimea Middle and Elementary School Playground

Sunday: “Pure Kona Green Market,” 9 a.m. to 2 p.m. at Amy Greenwell Garden in Captain Cook

“Hamakua Harvest,” 9 a.m. to 2 p.m. at Highway 19 and Mamane Street in Honokaa

Plant Advice Lines



Tuesdays &Thursdays: 9 a.m. to noon at UH-CES in Kainaliu – 322-4892

Mondays &Fridays: 9 a.m. to noon at UH CES at Komohana in Hilo 981-5199 or

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