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Be on the Lookout for Diseases in the Garden

Sunday, April 7, 2024

The month of April is an exciting time in the garden. It’s warm enough to get plants in the ground and the landscape is coming alive with new growth. It is also the month for many diseases to appear, including powdery mildew, leaf spot and crown rot, which are forms of fungus.


Fungi grow in high humidity, low light and during mild temperatures that occur in April and into May. While many diseases of ornamentals can be treated with fungicides, gardeners must follow the directions on the label. Keep in mind that although the diseases are caused by different pathogens, management strategies often are similar within disease groups.


It’s a good idea to have any visible diseases properly diagnosed. Check with the Oklahoma State University Extension county office for help.


When shopping for pesticides, check the label to make sure it lists the plant being treated before applying it. Use only the recommended application rate and don’t spray more often than directed on the label.


Powdery mildew looks like a white coating on leaves and occurs on many plants including lilacs, roses and crapemyrtles. Increasing air circulation around plants can help reduce the severity of powdery mildew. It’s best to water plants in the morning and avoid getting the foliage wet whenever possible. In some cases, fungicides may be necessary. There are many cultivars of susceptible plants available that are resistant to powdery mildew.


Pine trees can become infected with tip blight, which kills infected branch tips if not treated. Look for resin on needles of affected branches.


Crabapple and apple trees are susceptible to cedar-apple rust and may be sprayed after orange jelly galls have formed on nearby cedars. The best way to prevent cedar-apple rush is to plant disease-resistant cultivars. The county OSU Extension office or the local garden center can help in selecting those varieties.

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