Learn what powdery mildew on plants looks like and what causes this fungal disease. A garden expert explains the best treatments for powdery mildew.
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What Causes Powdery Mildew?
“My red rhododendron looks as if it has a layer of ash or white powder on it (above). What should I do?” asks Birds & Blooms reader Steven Graves of Aptos, California. Horticultural expert Melinda Myers says, “Your rhododendron plant has powdery mildew. Leaves infected with powdery mildew look as if they’ve been dusted with flour. Plants that are healthy otherwise will survive this disease. But it does affect appearance and reduce vegetable productivity.
In severe cases, the powdery fungal threads cover the leaf surface, blocking light from reaching the leaves. These leaves may turn yellow and drop off the plants. Give plants plenty of room and sunlight; sufficient light and airflow reduce the risk of the fungal disease.
Powdery mildew is most common on overcrowded plants and those growing in shade. If the shade hasn’t increased and plants are not overcrowded, this is likely a response to the weather.”
Learn how to spot the signs of aster yellows disease on plants.
Powder Mildew Treatment to Try
Melinda says, “If this disease is an annual problem in your garden, it may be time to take action. Plant something a bit shorter in front of the susceptible specimen. The shorter plant masks the unsightly mildewed leaves but allows you to enjoy the blossoms.
Do a thorough cleanup and discard infected leaves in fall to reduce the source of future disease next year. Stop powdery mildew before it starts by choosing mildew-resistant varieties whenever possible.
Plants suffering yearly infections and severe leaf drop may benefit from treatment. Treat the plants with Summit Year-Round Spray Oil or other organic lightweight horticulture oil. These create a barrier between the leaf and disease. Synthetic fungicides labeled for this purpose can be applied preventively at the first sign of the disease. As always, read and follow label directions, whether using organic or synthetic chemicals.”
Powdery Mildew Prevention
“How do you treat and prevent powdery mildew on phlox plants?” asks reader Kelly Groth of Winona, Minnesota.
Melinda says, “Consider replacing your mildew-susceptible phlox with one of the resistant varieties. This is the easiest way to manage this disease.
If you want to keep the plant, consider removing one-fourth of the stems in spring to increase air flow and light penetration, which helps reduce the risk of powdery mildew. Make sure plants receive full sun and are not overcrowded. Consider planting something shorter in front of the phlox to mask the diseased leaves but not the flowers.
As you already know, the phlox will survive the disease and return next year. Remove the diseased leaves in fall to reduce the source of infection for next season. Lightweight horticulture oils are an organic option for treating this disease. As always, read and follow label directions, whether using organic, natural or synthetic products.”
Next, learn how to treat black spot and rose rosette disease.
