Powdery Mildew of all my Dahlias!

Asked September 22, 2016, 6:39 PM EDT

Help! I have lovely dahlias and I love my garden. I have powdery mildew on most of the plants. I sprayed with Neem oil and it helped a bit but it came roaring back as I had to care for my husband who had a heart attack. Should I cut down all my plants except leave a little stalk so I can see where they are planted when digging up time? Do Ineed to do anything to cure the mildew on my tubers? What about the soil do I need to amend the soil in a special way? Please help!

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1 Response

Unfortunately, this seems to be a good year for powdery mildew. Powdery mildew is a ubiquitous group of fungi, that are often plant specific. Powdery mildew is seen on plants when conditions are right (warm, moist). The common species for dahlias is Golovino orontii or G. spaciceus. The best way to treat powdery mildew when you see it, is to remove any affected parts of the plant and place in the trash. This will help minimize the spread of the fungus to the unaffected parts of the plant, and will help decrease the spores that overwinter. If you choose to use a fungicide, please follow directions for both dilution and when and how to spray on the label. Three fungicides that are approved for home use are: Ortho Rose Disease Control @ 1 Tbsp/gallon; Safer Garden Fungicide (ready to use); and Spectracide Immunox @ 1 oz/gallon. The disease is very difficult to treat once it is established in your plants.

To prevent powdery mildew: 1. Plant resistant cultivars (Dahlianova or Dahlianova hypnotica) 2. Space plants for excellent air circulation. 3. Remove and destroy any severely affected plants, prune away affected plant parts. These plants/ leaves should be removed to the garbage.

This article, Powdery Mildew on Dahlias gives more information on powdery mildew on dahlias in our area. This article, How to Grow Dahlias, gives information on growing dahlias, including storing them over winter.