Issue 13, July 30, 2012 Powdery mildews are one of the most
easily recognized diseases in the landscape. Unfortunately, they
are equally as common and can be a problem on annuals,
perennials, shrubs, and even trees. Powdery mildews flourish
when the days are warm to hot, the nights are cool, and humidity
is high. They are often more severe on crowded plants growing in
the shade where air circulation is poor. Powdery mildews are
caused by fungal pathogens. Many different species are
responsible. The species are grouped in six closely related
genera. Most powdery mildews are fairly host specific. Those who
are curious about matching a host to the likely powdery mildew
involved should consult Table 1 of RPD No. 617: Powdery Mildews
of Ornamentals (Adobe PDF). As mentioned, the disease symptoms
are easily recognized and these symptoms also appropriately
match their disease name because they first appear as
distinctive white powdery patches on shoots, buds, flowers, or
stems (first image). The disease progresses t