Thursday | December 14, 2017
About Us | Contact | Subscribe

The garden plague of powdery mildew

Powdery mildew (PM) is a menace to many gardeners. The disease will turn large zucchini leaves white then brown. It will cause a whitish cast on many common garden plants such as tomatoes, beans, cucumbers, mustard, peas and collards. Infected leaves often turn yellow to brown and may shrivel up and die, making the plant unproductive.

The white fungal growth can develop both on the upper and the lower surfaces of the leaf, and sometimes, on flowers and fruit. The cottony-like threads of this fungus travel along the surface of the leaf, occasionally sending “roots” down into the leaf tissue in order to obtain nutrients.

There are many different types of powdery mildew fungi, but fortunately, they are rather host specific. This means that one particular mildew fungus will only infect those plants in a particular genus or family; the PM on the beans will not attack papaya, and the PM on mangos will not attach tomatoes.

This fungus likes temperatures between 60 and 80 degrees F, and is sensitive to temperatures above 90; cool days and warm nights favor the disease.

Spores of the pathogen are dispersed readily by wind. Even though this disease flourishes with high humidity, wet leaves can actually inhibit germination of the fungal spores, thus preventing infection. Because of this, the disease should be minimized during periods of heavy rainfall. Also during this time, the numerous spores that are on the leaves will be washed away.

Control — Stopping powdery mildew in its earliest stages of development provides the best control. This can be done by sanitation — remove and destroy infected parts of the plant, by planting in the sunniest locations, providing good air circulation through pruning, and avoiding excessive applications of fertilizer.

Most important, choose plant varieties (vegetables, fruit trees and ornamentals) which are tolerant or resistant to the powdery mildew fungus. For more information about seeds from locally developed vegetable varieties, contact the University of Hawaii at Manoa:

Note: Some of the resistant varieties will exhibit powdery mildew symptoms, but the disease is less severe.

Fortunately there are a number of relatively safe and effective materials to use against this fungus — wettable sulfur, horticultural oils, including neem, and Kaligreen (potassium bicarbonate a relative of baking soda). Cucurbits (melon, squash, cucumbers) can be sensitive to sulfur. Do not apply when the temperature is near or over 90 degrees and do not apply within 2 weeks of an oil spray. Use these materials in the earliest stages of disease development for best results. Before spraying, it would help to remove the leaves that are heavily infested. Do not dispose of them on the ground since they are loaded with fungal spores.

A biological fungicide called Serenade is a bacterium that helps prevent the powdery mildew from infecting the plant. Powdery mildews can attack healthy plants, but older plants that are less vigorous and plants that are stressed are more susceptible to infection.

I was digging in my compost pile the other day and came across a long worm, longer than an earthworm. Most unusual, it had a flat head. Do you know what kind of worm is this and is it harmful? — W.S.

These worms are called land planarians or flatworms; they have a hammer or disk shaped head. Some can reach 10 inches, even as long as 20. They are predacious, feeding on other worms, slugs and each other. Unfortunately, they are voracious predators of earthworms. It has been reported that they can destroy an entire worm population in a worm farm in a matter of days.

They reproduce very quickly. If you covet your earthworms, it would be a good idea to get rid of any flatworms you come across. Much like slugs, they hide in dark, cool, moist areas during the day while feeding and moving at night. For more information about compost and other garden topics, see

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at


Rules for posting comments