powdery mildew treatment?

Asked June 30, 2017, 3:13 PM EDT

Hello! I got a great response about my mealy bug problem...turns out they are harmless leafhoppers!! Thanks! I have several plants that are struggling with powdery mildew. Usually my tall phlox gets this later in the summer, but it is covered along with my verbena bonariensis and one of my peonies. I have used a fungicide spray (Daconil (chlorotholonil) two bottles so far) and I am not noticing that it looks to be improving or going away. I also cut out some of the phlox for better air flow and make sure not to water the leaves of the plants. What do you advise? Should I be removing the impacted leaves? Is there a preferred spray or another method to stop the spread and save the plants?

Howard County Maryland

1 Response

Perennial phlox, although a beautiful plant, is very susceptible to powdery mildew. The best defense is to give the plants good air circulation. Space them so they are allowed to grow without being crowded and water thoroughly during establishment, and later during dry periods. Avoid overhead irrigation which raises the level of relative humidity within the plant canopy. If powdery mildew is noticed on a few leaves, simply removing them will help with control. At the end of the growing season, prune out infected stems and remove fallen leaves which can serve as a source of further infection.
Fungicides are preventatives, not curatives. You can spray with a fungicide or a horticultural oil product labeled for powdery mildew and needs to done as soon as you notice the mildew during the growing season.

Some varieties are more susceptible to powdery mildew than others, so I would suggest planting only those varieties that are classified as mildew resistant. This is the best recommendation.

Peonies - Powdery mildew has become a bigger problem on peonies the past few years. Make sure there is adequate sunlight and good air circulation to reduce humidity levels. Allow proper plant spacing for the same reasons. Pruning for better air circulation also may help. Practice good sanitation.

Cut down all infected foliage this fall as soon as frost turns it brown (or before if the mildew has killed it.) Get rid of this infectious material. Next spring, monitor your peonies, and as soon as you see one infected leaf, pull it off. You will have to decide it you want to spray a fungicide or a horticultural oil labeled for powdery mildew or tolerate it. Fungicides are preventative--not curative. They will halt further infection, but cannot restore infected leaves to health. There are no resistant peony varieties on powdery mildew. Here is our website info on powdery mildew