Mildew in Ninebark 'Diablo' shrub
My 12- to 15-year-old Diablo Ninebark shrubs have had downy mildew for 3 or 4 years. Each year I prune branches out that have mildew. I disinfect my pruners between cuts, clean up as much as I can around the bushes, and throw all the leaves and branches in the garbage. The problem becomes more severe each year. These bushes get lots of airflow around and through them. I'm keeping the centers open for air flow.
I am wondering if I can give the bushes a hard pruning this spring before they leaf out, and if that will help control the mildew. How far down can I prune them -- is a foot from the ground too much? Or would I be better trying to control the problem with a fungicide. My thought is that if I get rid of as much of the mildew as I can, I would have an easier time controlling it with a fungicide. I have not yet used a fungicide.
Richland County North Dakota
Ninebark shrubs are susceptible to powdery mildew. Fortunately, powdery mildew is not fatal to the plant but it does affect its appearance. Powdery mildew is more likely to occur when the humidity is high. It appears that you are doing what you can to increase ventilation around the plant and lower humidity.
With ninebarks, I would try renewal pruning because they can get overgrown very easily. Essentially, you would remove 1/5 to 1/3 of the oldest canes. The oldest canes should be thicker and a duller color than the younger canes. You would make the pruning cut 2 to 3 inches above the ground. After 3 to 5 years, the entire shrub will have been pruned. March and early April are a great time to prune ninebark. However, you will have fewer flowers because you will have removed a portion of the canes.
Please see figure 7 in our Pruning Trees and Shrubs publication on the following website for an illustration of renewal pruning:
Fungicide can be used to prevent powdery mildew infections. However, keep in mind that fungicides are a preventative treatment. They do not cure infections so you need to start treatment before you notice an outbreak of powdery mildew and continue to treat at recommended intervals until the humidity drops due to drier weather. Fungicidal products with the active ingredient chlorothalonil are commonly available at gardens centers. Please follow the labeled instructions.