Plant Pathology Fact Sheet
Powdery Mildew of Vegetables
Ken Pernezny, Don Maynard and W. M. Stall, Professor- Plant Pathology, Ever-
glades Research & Education Center, Belle Glade; Professor, Horticultural Sci-
ences, Gulf Coast REC, Bradenton, and Professor-Horticultural Sciences Dept.,
University of Florida, Gainesville. 1985, Revised March 2002.
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Powdery mildew is a serious disease of
beans, southern peas, okra, squash, cucumbers,
muskmelons, honeydews, and pumpkins in
Florida. The disease occurs also on English peas
and is found on strawberries and watermelons
in the state. Powdery mildew of okra, squash,
cucumbers, muskmelons, honeydews, pump-
kins, and watermelons is caused by the fungus
Podasphaera xanthii (=Sphaerothecafulginea) or, oc-
casionally, Erysiphe cichoracearum. The fungus
Erysiphe polygoni causes powdery mildew of
beans, southern peas, and English peas. Pow-
dery mildew of strawberries is caused by the
fungus Sphaerotheca macularis.
The disease is found mainly on the older
leaves and stems of plants. Yields of many of
the infected vegetables are reduced due to pre-
mature foliage loss. In honeydew and musk-
melon severe leaf infection usually results in
lower fruit sugar content and subsequent re-
duction of fruit quality. In a few crops direct
damage to the marketable produce occurs.
Symptoms and Disease Development
The fungus is usually first noted as
subtle, small, round, whitish or yellow spots
on leaves (Fig. 1) and sometimes stems. The
spots enlarge and coalesce rapidly and a white
mass resembling talcum powder becomes evi-
dent on the upper surface of older leaves (Fig.
2) or other plant parts (Fig. 3). Young leaves
are almost immune. A large part of the talc-like
powder on the leaf surface is composed of
spores. These spores are easily blown by winds
to nearby susceptible plants.
Heavily infected leaves yellow, then be-
come dry and brown (Fig. 4). Extensive prema-
ture defoliation of the older leaves can ensue if
the disease is not controlled. Yield reduction
from defoliation is proportional to the severity
and length of time plants are infected. Severe
economic losses can occur in beans when pods
are infected. The pods develop purplish spots
and become distorted. (Fig. 5).
Powdery mildew fungi can reproduce
under relatively dry conditions. Increased hu-
midity can increase the severity of the disease,
and infection is enhanced during periods of
heavy dew. Unlike downy mildew, powdery
mildew can and does become severe during
periods of low rainfall in the winter and spring
months in Florida.
It is not known for certain how the
fungus survives between crop seasons. The
fungus is thought to survive on wild cucurbit
and other weeds year round
Crop rotation and many other cultural
practices seem to have little effect on the inci-
dence and development of powdery mildew.
However, healthy, vigorous leaves and stems
are less prone to infection. Plants under nutri-
tional stress in most cases will develop pow-
dery mildew much sooner than plants the same
age grown under a good nutritional program.
Tolerance or resistance to powdery mil-
dew is available in some vegetable crops. For
example, most commercial varieties of slicing
and pickling cucumber varieties grown in
Florida have acceptable levels of resistance.
Most varieties of cantaloupe used in Florida,
have some tolerance to powdery mildew. Tol-
erance to powdery mildew is available in a few
of the most recently introduced summer squash
and zucchini varieties. It is expected that most
varieties released in 2000 or thereafter will have
some level of tolerance to powdery mildew of-
ten combined with an array of virus resistance.
In decorative pumpkins and acorn squash, the
situation is similar to that in summer squash,
i.e. there are few resistant varieties to powdery
mildew available now but many varieties re-
leased in the future should have this desirable
trait. Documented tolerance to powdery mil-
dew in butternut squash and watermeon vari-
eties has not been noted. Growers can obtain
information on resistance of specific crop vari-
eties from seed industry personnel and the Uni-
versity of Florida Cooperative Extension Ser-
In addition to resistance, economic
control can be achieved with chemicals.
Under low disease pressure, some materials
applied for downy mildew control may give
satisfactory control of powdery mildew.
However, under moderate to heavy mildew
disease pressure, specific fungicides are
recommended. Check with your county agent
for updated registrations, recommendations,
and harvest limits.
Figure 1. Early powdery mildew on leaves of snap
Figure 2. Advanced leaf symptoms of powdery mildew on yellow squash.
Figure 3. Powdery mildew on petiole (leaf stalk) of cucumber.
Figure 4. Squash leaves with various degrees of powdery mildew severity.
Figure 5. Powdery mildew on snap bean pods. (photo used with permission of R.T.