Good morning,

I should have gotten this email out right after the downy mildew one. Yes – we have 2 different kinds of mildews.  Again, Margery Daughtrey, Cornell’s Mistress of Mildews (in only the best possible way), responds:

Powdery mildews are NOT the same thing as downy mildews.  Powdery mildews are true fungi [whereas downy mildews are water molds and have higher need for a wet environment]. Powdery mildews thrive with high humidity, so they can trouble your crops even in a well-managed greenhouse. 

The white colonies of powdery mildew are most often noticed on the top surface of leaves, but they can begin on the undersurface where only a clever scout will find them.  Although all powdery mildews look alike, these are really caused by a large group of fungi that are fairly host-specific, sometimes affecting plants within one family and sometimes affecting plants in several families.

The greenhouse crops most often seriously troubled by powdery mildew include African violet, begonia, calibrachoa, gerbera, hydrangea, nemesia, petunia, rose, rosemary, torenia, verbena and zinnia. Poinsettias can get powdery mildew also, but one rarely sees this disease today (and let’s keep it that way). Many herbaceous perennials get a powdery mildew disease, the most notorious of these being phlox and monarda.  A large number of plants in the aster family are also prone to powdery mildew, as are columbine, delphinium, peony and scabiosa. Tree and shrub species can get powdery mildew too—such as lilacs, which almost always develop powdery mildew by late summer, and London plane trees, apples, pears and ninebark.

Controlling powdery mildews is all about finding resistant cultivars to grow, trying not to overcrowd plants, and using the appropriate fungicides.  These include strobilurins and DMIs, as well as bicarbonates and horticultural oil. See Cornell Guidelines for more information on materials for powdery mildew control –https://www.cornellstore.com/books/cornell-cooperative-ext-pmep-guidelines

Looking for resistant species and cultivars of woody plants?  Check the fact sheets, here:https://nysipm.cornell.edu/agriculture/ornamental-crops/disease-and-insect-resistant-ornamental-plants/

From University of California:

http://ipm.ucanr.edu/PMG/PESTNOTES/pn7493.html

A quick comparison – remembering that they don’t always look like the pictures.

IPM Images https://www.ipmimages.org/search/action.cfm?q=5077003

Have a great, mildew free, week!