Powdery mildew is a fungus that lives in soil. It’s unlikely to kill your dahlias, but it can infect their leaves/stems and affect the plants’ ability to photosynthesize. It won’t stop your dahlias from blooming, but it will certainly slow them down.
Air Circulation is Important
Preventing powdery mildew is easier than treating it. We’ve found that the best way to protect dahlia plants from a powdery mildew infection is to make sure that they aren’t crowded and that they have plenty of air circulation.
The worst problem we’ve ever had with powdery mildew at our farm happened the year we decided that a 24-inch aisle spacing was enough for our double rows of dahlias. We wanted to pack more plants into a smaller space, and we compromised on airflow opportunities for our dahlias. We absolutely regretted the way this prevented the wind movement necessary for keeping fungal disease out of our dahlias.
Airflow is important, especially in high humidity climates and environments, because it keeps your foliage dry. Crowded plants can’t dry out properly from overhead watering or even morning dew, and wet foliage is the perfect environment for powdery mildew to proliferate.
Powdery mildew spores overwinter in soil, and can splash up onto wet plants in the growing season to start infections, so removing and discarding any infected plant material at the end of the season instead of allowing it to lie on the soil in fall/winter will help a lot with prevention next season.
Also, keeping your dahlias as healthy as possible will help your plant to fight off powdery mildew, so don’t forget to fertilize and irrigate regularly.
How to Respond to a Powdery Mildew Problem
If you are dealing with a powdery mildew infection right now, there are a few things you can do to help your situation.
- Remove all infected leaves and discard from your plants before the fungus spreads.
- Consider stripping all leaves from the lower third of your plants to increase circulation immediately, even in mid-season.
- Spray neem oil on your plants to prevent the spores from spreading. Tip: You may want to add a surfactant to your neem oil (if it doesn’t already have one in it) to help the oil coat the leaves thoroughly. A few drops of household dish soap for every gallon of neem oil would make a handy surfactant. HOWEVER, we highly recommend spraying in the evening to avoid the neem oil sitting in the sun on your plants’ leaves, which will burn them. Spraying in the evening also helps you to avoid coating pollinators with the neem oil.
- Overhead water only with a gentle method that will not cause spores in the soil to splash onto your plants. We recommend watering with a soaker hose, drip irrigation or other non-splashing method to avoid infecting your plants. You can also top dress the ground under your dahlia plants with fresh/sterile soil, compost, or another mulch to create a barrier between soil with existing powdery mildew fungus in it and your plants.
Powdery mildew is a tricky problem to solve, so please share your growing region + any tips in the comments below if you find something that works in your garden!