Q. I had mildew on my peonies for the first time this year. The phlox have it every year but never the peonies. What would cause it? Thanks Marilyn Dowson – Canada
A. I bet you are wondering if the disease spread from your phlox to your peonies. It is doubtful. The mildew you are referring to is powdery mildew to which most phlox are highly susceptible. Powdery Mildew is a common name used to refer to many different pathogenic fungi that attack ornamental and crop plants and appear as a white cast on the surface of leaves. In the case of phlox the species is Erysiphe cichoracearum . As a general rule different species of Powdery Mildew have very specific hosts and a few have a larger host range. Although Erysiphe cichoracearum has a relatively large host range the hosts are considered limited to plants in the Aster Family (Asteraceae/Compositae) (e.g. chrysanthemum, dahlias, phlox, sunflowers and zinnia). A probable scenario is that your peonies have another fungal disease called botrytis (Botrytis cinerea) or grey mold. If you find infection of the flowers then this is a good indication of a botrytis infection. Both fungal diseases are exacerbated by lack of air circulation and humid conditions. The fungal spores need some moisture present on the surface of the plant to germinate and grow. It could be from overhead watering or even from dew that forms overnight and doesn’t evaporate due to poor air circulation. So anything you can due to change the environmental conditions will help. Sanitation (removal) of infected leaves is also helpful since you reduce the spores available next season. Both diseases can be controlled with copper based fungicides.