Powdery mildew thrives in late summer and fall gardens

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Last Updated: 
August 31, 2007

CORVALLIS – Are your ornamental or vegetable plants covered with a whitish powdery substance? Your plants may be suffering from one of a set of fungal diseases commonly known as “powdery mildew.”

Often rampant in home gardens in western Oregon in the late summer and early fall, powdery mildew disease first appears as whitish spots forming on the surface of leaves, shoots and sometimes on flowers and fruit. These patches may gradually spread over a large area of an infected plant.

Powdery mildews rank among the most economically significant diseases of food and ornamental plants in the Pacific Northwest. Damage from these diseases can cause dead patches on foliage, making them look “ratty” by the end of the season, according to Melodie Putnam, chief diagnostician with the Oregon State University Extension Service Plant Clinic.

In addition, powdery mildews may affect plants by enabling decay organisms to enter fruits through fungi-damaged tissue.

Growers of wine grapes and cherries heavily manage, through pruning and soil fertility management, to reduce powdery-mildew outbreaks. Fortunately, effective cultural and chemical control practices are available for nearly every plant species susceptible to powdery mildew.

To help home gardeners and nursery personnel identify and cope with this disease, Oregon State University’s Online Guide to Plant Disease Control offers an array of information on this group of diseases on both ornamental and food crops. To learn more, go to: http://plant-disease.ippc.orst.edu/index.cfm

OSU’s North Willamette Research and Extension Center has put together a web page on powdery mildew as well, with several photos of the disease. Go to: http://oregonstate.edu/Dept/nurspest/powdery_mildew.htm

The symptoms begin as spots of whitish coating on leaves. Leaves infected with powdery mildew may gradually turn completely yellow, die and fall off, exposing vegetable fruits such as your pumpkin to sunburn. On some plants, powdery mildew may cause the leaves to twist, buckle, or otherwise distort. Powdery mildew fungal growth does not usually grow on the vegetable fruits themselves.

The whitish coating is made up of asexual spores of a fungus. If you have a hand lens, you can see them as long, chainlike strands. These spores are carried by wind to new hosts.

Severely infected plants may have reduced yields, shortened production times, and fruit that has little flavor.

Moderate temperatures (60 to 80 degrees) and shady conditions generally are the most favorable for powdery mildew development. Hot and sunny weather (90 degrees or above) reduces powdery mildew spores and fungal growth.

But powdery mildews do not need water droplets to germinate, explained Putnam. The warm days and cool nights of late summer and early fall result in high humidity in the garden, which encourages powdery mildews.

All types of powdery mildew fungi require living plant tissue to grow. Year-round availability of crop or weed hosts is important for the survival of some powdery mildew fungi. Some types of this disease may overwinter as resting spores.

To avoid the use of chemical pesticides, the best method for controlling this fungal disease is prevention. Plant resistant vegetable varieties if you can find them. Seed packages or veggie start tags often contain disease resistance information. Avoid the most susceptible varieties. Plant your vegetable garden in the full sun. Provide good air circulation by keeping weeds at bay and planting your plants far enough apart for good airflow. Avoid applying too much fertilizer. A good alternative is to use an organic or slow-release fertilizer.

Pumpkins, squash, cucumbers and melons are very susceptible to powdery mildew disease. Next year, if you really want to keep powdery mildew at bay, you’ll have to watch closely for the first signs of it. There are several “least-toxic” fungicides on the market, but they must be applied no later than the first sign of disease.

“Powdery mildew on woody plants is of no great concern, since it doesn’t significantly affect the overall health of the plant,” said Putnam.

On non-woody plants, such as garden vegetables and flowers, there are “least toxic” substances that kill the spores, including potassium bicarbonate products such as Kaligreen. But these have to be applied repeatedly, she said.

Basic garden sanitation, such as removing diseased leaves, also helps reduce the spread and effects of the powdery mildew. Dispose of the infected plant parts in the garbage. Do not put infected debris into the compost pile.

Author: Carol Savonen