How can I eliminate white mildew on Coreopsis leaves?

Credit: Andrea_44

Q: For the past two years, my tall yellow leafed coreopsis has been growing fine to just about to mature height (3 feet) and then the leaves became all white-looking. It really detracts from the beauty of the plant and its little yellow flowers.

Is there anything I can put on it as it grows or when the white starts to appear so I can eliminate it?

Thanks for any help you can give.

There are a number of fungal organisms that produce powdery mildew on “Tickseed” (Coreopsis grandiflora, C. lanceolata, C. verticillata, etc). Powdery mildew is favoured by high relative humidity at night, creating new spores. Low humidity during the day spread the spores. Temperatures of 70 to 80 °F are optimum for rapid disease development.

Water on leaves tends to inhibit spore germination for most powdery mildew. Syringing plant leaves during the daytime when humidity is low can inhibit infection and suppress powdery mildew to some degree.

Coreopsis may vary in susceptibility to powdery mildew infection. Some cultural prevention tips include providing good air circulation, avoiding excessive shade, providing adequate levels nutrition, keeping late evening watering to minimal, not allowing plants to dry out too much, cleaning-up old plant debris at the end of season, and dividing plants often, etc.