Thanks for contacting "ask extension" with your question about your dahlia.
I suspect that your dahlia is suffering from Powdery mildew which is one of the most common diseases of ornamental plants. The disease is easily recognizable as a white to gray powdery growth on leaves and sometimes the stems and flowers, especially the leaves that are closest to the ground. It may have little or no effect on the plant other than its appearance or it may cause infected leaves to discolor, distort in shape, wither and defoliate. Unlike most fungi, powdery mildew only colonizes the surface of the plant. Severity of symptoms depend upon the species of host plant and the particular species of powdery mildew. The age of a plant and environmental factors also play a role in the spread of the fungi and unfortunately, dahlias can be particularly susceptible to the disease. Powdery mildews thrive in high humidity (greater than 95%) moderate temperatures (68-86 F) and low light or shady areas. This plant disease is more prevalent in the spring and fall when day and night temperatures vary widely; spores are spread by wind or splashing rain (including sprinkler irrigation).
Epidemics that seem to develop overnight are often the result of undetected low level infections that have spread spores. Good plant sanitation is key to containing the spores and fungi from spreading. Removing the plant or pruning out the diseased leaves and disposing of them in a plastic bag will prevent spores from spreading. Choosing plant varieties that are bred for resistance, avoiding overcrowding of plants and overhead irrigation, providing good air movement and keeping relative humidity levels low also keep the infection at bay. If your plants are in a greenhouse, thoroughly cleaning between crops, avoiding excessive application of nitrogen fertilizer and eliminating all weed hosts and volunteer plants will also prevent the spreading of spores to healthy plants.
I hope this helps! Please contact us if you have further questions.
Deb K & Laurie W.