| Powdery mildew is caused by a fungus and is seen
as a light gray or whitish powder on the surface of
leaves. It occurs following warm days and cool
nights often being seen in the fall and spring. The
disease is considered more unsightly than harmful.
Death of the plant is rare.
Symptoms and Diagnosis
Even though each species of powdery mildew attacks
only a narrow range of hosts, there are 11,000
species of the powdery mildew fungi, and many
ornamentals are hosts. Superficial powdery patches
appear on leaf surfaces, young stems, flowers, and
even fruit. The powder is composed of mycelium and
colorless chains of spores on upright stalks. Later
there may be dark "pepper-like" spots among the
powder (the spots are spore-producing bodies). As
the disease progresses, leaves may be dwarfed, curl,
turn yellow, and drop off. Flowers may be deformed.
Fruit crops may be reduced, with the fruit misshapen
and covered with powdery patches.
Life Cycle
Powdery mildew commonly winters over as
mycelial mats in dormant buds or on plant stems
and fallen leaves. During humid and warm spring
weather (with cool nights), infected buds open and
spread conidia (spores) to new host tissue. These
conidia do not require free moisture in order to
germinate as many fungi do. A new generation may
be produced every 72 hours, if conditions are right.
Integrated Pest Management Strategies
1. Purchase resistant varieties. This is the best
strategy for avoiding powdery mildew.
2. Pruning. Prune out diseased tissue and destroy it.
Gather and destroy dead leaves and stems in the
fall. This is the source of next season’s inoculum.
3. Keep plants in good vigor. Space the plants for
good air circulation. Give the plants plenty of
sun, as too much shade seems to make the
problem worse. Try to avoid overhead watering
and do not handle the plant when the foliage is
wet as this will spread the disease. Water early in
the day.
4. Fungicides. Use fungicides as a preventive
treatment before a rain for maximum protection.
Spray both sides of leaves and flowers well with
sulfur, mancozeb, maneb, chlorothalonil
(Daconil), cinnamaldehyde (Cinnamite), or
copper-containing fungicides. | 
