Powdery
mildew (PM) is a menace to many gardeners. The disease will turn large zucchini
leaves white and later brown. A whitish cast will appear on many common
garden plants such as tomatoes, beans, cucumbers, mustard, peas, and collards. Infected
leaves often turn yellow to brown and may shrivel making the plant
unproductive and eventually die.
The white fungal growth can develop both on the upper and the
lower surfaces of the leaf, and sometimes, on flowers and fruit. The cottony
like threads of this fungus travel along the surface of the leaf, occasionally
sending “roots” down into the leaf tissue in order to obtain nutrients.
Many different types of powdery mildew fungi are host specific. This means that one particular mildew fungus will only
infect those plants in a particular genus or family; the PM on the beans will
not attack papaya, and the PM on mangoes will not attack tomatoes.
The
fungus likes temperatures between 60 and 80 degrees F being sensitive to
temperatures above 90. Cool days and warm nights favor the disease. Spores of
the pathogen are dispersed readily by wind. Even though this disease flourishes
with high humidity, wet leaves can actually inhibit germination of the fungal
spores, thus preventing infection. Because of this, the disease should
be minimized during periods of heavy rainfall. Also during this time, the numerous
spores that are on the leaves will be washed away.
Control – Stopping powdery mildew in
its earliest stages of development provides the best control. This can be done by
sanitation: remove and destroy infected parts of the plant. Other measures of control are as follows: plant in the
sunniest locations, provide good air circulation through pruning, and
avoid excessive applications of fertilizer.
Fortunately
there are a number of relatively safe and effective materials to use against
this fungus - wettable sulfur, horticultural oils, including neem, and
Kaligreen (potassium bicarbonate a relative of baking soda). Serenade
is a biological fungicide, a bacterium, that helps prevent the powdery mildew
from infecting the plant. Cucurbits (melon, squash, cucumbers) can be sensitive
to sulfur. Do not apply when the
temperature is near or over 90 degrees and do not apply within 2 weeks of an
oil spray. Use these materials in the earliest stages of disease development
for best results. Before spraying,
it would help to remove the leaves that are heavily infested. Do not dispose of them on the ground since
they are loaded with fungal spores. Powdery mildews can attack healthy plants;
but older plants, less vigorous plants and those that are
stressed are more susceptible to infection.
Most
important,
choose plant varieties (vegetables, fruit trees and ornamentals) which are
tolerant or resistant to the powdery mildew fungus. For more information about
seeds from locally developed vegetable varieties, contact the University of
Hawai‘i at Manoa: http://www.ctahr.hawaii.edu/seed/seeds.asp. Or, contact your
local university cooperative extension office. Note: Some of the resistant
varieties will exhibit powdery mildew symptoms, but the disease is less severe.
Photos: Cornell University, Colorado State University and USDA
Photos: Cornell University, Colorado State University and USDA