Powdery mildew is a common problem in local vegetable gardens. What is powdery mildew, why does it occur and what can you do about it?
Here are some basics to help you control this pervasive garden pest.
Powdery mildew is caused by a group of fungi that typically lives on the outer surface of the plant and derives nutrition from the plant.
The fungi produce a white, powdery coating on leaves, buds, shoots and flowers. The powdery coating is made up of spores. The spores are wind blown to other parts of the same plant or other plants of the same species.
Powdery mildew may start out as individual spots on different plant parts, but these spots gradually spread over a large area of the leaves and stems.
Infected leaves are frequently distorted in shape and smaller than normal. On severely infected plants, leaves become yellow, shrivel and die.
Because the fungi can survive the winter in the buds of infected plants, the first infections of powdery mildew often are found on the tips of new shoots in the spring.
Powdery mildew affects a large number of vegetable garden plants, including artichokes, beans, beets, carrots, cucumbers, eggplants, lettuce, melons, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes and turnips. Powdery mildew fungi are host-specific; for example, the mildew on squash will not spread to peas or tomatoes, and vice-versa.
Powdery mildew spores require no external moisture for germination, as do many other fungi, so this disease often is found during dry weather. The disease is more prevalent in crowded plantings with poor air circulation; in damp, shaded areas; and where nitrogen content in the soil is high. Powdery mildew grows quickly when days are warm and nights are cool, favoring moderate temperatures (65 to 85 degrees) and humid air.
Most powdery mildews grow on the surface of the plants (not internally in plant tissues) and can be controlled. Preventing the infection, however, is the best approach and includes the following steps:
• Select disease-tolerant varieties, where available.
• Avoid over-fertilizing which encourages susceptible new growth.
• Space plantings according to recommendations for the specific crop and prune to ensure good air circulation.
• Gather and destroy fallen leaves.
If your plant develops powdery mildew, clip out the infected areas and dispose of them. Composting will not kill the fungus, so all infected plant parts should go into the trash. Clean tools used on infected plants to avoid spreading it to other plants. Spraying an infected plant with water can reduce the amount of powdery mildew because the spores rupture when in water.
Several fungicides are effective in managing powdery mildew; the fungicide must be registered for the specific host plant. Since the fungus is growing only on the surface, fungicide applications after the disease is visible can be effective. As with all pesticides, “the label is the law.” Be sure to follow all label directions if a chemical control is used.
At the end of the gardening season, remove and dispose of all infected plant material from the garden. Infected plant parts left in the garden will be a source of spores for re-infection next season.
Pearl of wisdom
Although powdery mildew usually manifests as a white, powdery coating on leaves, buds, shoots and flowers, on certain plants including artichokes, onions, peppers and tomatoes, it sometimes produces yellow patches on leaves but little powdery growth.
For more
Get gardening information at the Brown Bag Series at the Clallam County Courthouse. On Oct. 13, veteran Master Gardener Rita Cirulius and WSU Extension Sustainability coordinator Meggan Uecker discuss alternatives to burning landscape debris.
Judy English is a WSU-certified Clallam County Master Gardener.
