Q: I had powdery mildew on my dahlias this summer. Can I save the bulbs for next year without getting it again? If so, how should I store and treat them?
A: This question proved to be surprisingly difficult to answer. We couldn't find any scientific sources that talked about whether powdery mildew can overwinter on dahlia tubers, so we asked plant pathologist Michelle Grabowski, a University of Minnesota Extension educator who specializes in plant diseases. She couldn't find an answer, either, so she submitted the question to a group of plant pathologists who specialize in ornamental plants, and they didn't know, either! Apparently scientists haven't studied this question.
Michelle suggested that if your tubers appear healthy, and you like the variety, you should go ahead and keep them, but keep an eye on them after you plant them in spring to see if powdery mildew develops again.
"If the very first shoots to come up have powdery mildew, I would be suspicious of overwintering in tubers, but if the plants are large before the disease starts, then spores were likely blown into the garden from other areas," she said.
We do know that powdery mildew overwinters on plant stalks and leaves, so you'll want to be sure to clear any dead foliage from your garden. Trim the dead foliage off your tubers and store them in sawdust or peat moss in a container that allows air to circulate - not a closed plastic bag. Keep them in a cool, dry place.
Powdery mildew flourishes in humid weather. You can help prevent it next summer by spacing your plants far enough apart that air can circulate, allowing them to dry more quickly after a rain or morning dew. Avoid overhead watering. Don't overfertilize, because that can cause a flush of new, tender growth that is particularly susceptible to powdery mildew. Remove nearby weeds that may harbor the spores.
If the disease does strike again, remove the affected plant parts promptly to reduce its spread. Don't compost infected plant debris. Seal it up in a plastic bag to prevent the spores from spreading.
You may have better luck in the future if you plant a dahlia cultivar that is resistant to powdery mildew, such as Dahlianova or Dahlinova hypnotica. But in a bad year, even a resistant variety can be overcome.