PPDL Picture of the Week
August 14, 2017
Powdery Mildews-NOT ALL THE SAME
Gail Ruhl, Senior Plant Disease Diagnostician, Purdue Plant and Pest Diagnostic Lab
Powdery mildew (PM) is
the name given to a group of diseases with a grayish-white, powdery coating of
spores and fungal mycelia visible on the surface of leaves, stems, flower
petals and fruit. Powdery mildew is caused by several closely related fungi, each
having a limited host range. In other words, observing powdery mildew on lilac
leaves should not cause concern about spread to nearby zinnias.
All PM fungi are
obligate parasites, meaning that they must grow and reproduce on living tissue.
PM survives from one season to the next as spherical, thick-walled, fruiting
bodies, called chasmothecia (previously called cleistothecia), on the bark of
branches and stems of woody, perennial hosts and also on fallen, infected leaf
debris beneath plantings. In spring or early summer, airborne spores from
overwintering chasmothecia infect susceptible leaves to once again begin the
infection cycle.
The reason we often see
powdery mildew infections in the absence of rain events is that all PM species
can germinate and infect susceptible tissues in the absence of free water. In
fact, water on plant surfaces for extended periods of time will actually
inhibit germination and can kill the spores of most PM fungi! We do not
recommend this as a control measure because moisture on leaves promotes the
growth of most other diseases.
The best method of
control for PM is prevention. Cultural practices that will decrease the
severity of PM in the landscape include avoiding planting those cultivars which
are highly susceptible and alleviating high humidity by spacing plants far
enough apart to allow good air movement to quickly dry the foliage. Although
chemical control is seldom warranted in the home landscape for powdery mildew,
preventative fungicide sprays are available for use on prized ornamentals and vegetables that
require protection.
See these publications
for more information:
https://www.extension.purdue.edu/extmedia/bp/bp-5-w.pdf
https://www.extension.purdue.edu/extmedia/bp/id-414-w.pdf