The Q&A Archives: powdery mildew

Question: I have a bed with 10-12 bee balm plants. Two have powdery mildew. Can you recommend a fungicide or another way to get rid of it and prevent it from spreading? thanks.

Answer: Powdery mildew generally will not kill the plant; however, repeated infections can reduce plant vigor and predispose the plant to other disease or environmental problems. Repeated infections can also lower the aesthetic value of the plant. The easiest way in which to avoid powdery mildew infection is to plant varieties that are resistant to mildew. Since many varieties of landscape flowers and shrubs differ in their resistance to powdery mildew, check the descriptions in seed catalogs, labels on ornamental plants in the nursery, or ask about disease susceptibility of the plant you want to purchase.

Cultural practices can also affect the severity of infection on plants in the garden and landscape. For those plants such as bee balm, phlox, zinnia, roses, and lilacs that are prone to severe infections of powdery mildew, plant them on sites that are well-drained and that have good air circulation. Practices such as pruning plants and surrounding vegetation to allow more sun and air penetration into the plant, avoiding overcrowded plantings and providing adequate but not excessive rates of fertilization will aid in reducing the severity of powdery mildew infections. At the end of the season destroy and discard plant parts that are severely affected with mildew to reduce the chances for infection next year.

On very susceptible plants it may be necessary to use fungicides to maintain the aesthetic qualities of the plant. There are several fungicides such as chlorothalonil, copper, sulfur, triadimefon and triforine labeled for powdery mildew control on landscape ornamentals. These products, however, need to be applied at regular intervals through the season. Typically, applications can begin at the first sign of infection and applied as directed by the label thereafter and as the weather conditions warrant. Therefore, it is important that the plants be monitored through the season. However, on extremely susceptible plants it may be necessary to start fungicide applications before budbreak and continue every 2 weeks thereafter.

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