Disease Problems - Powdery Milew

SYMPTOMS

The leaves of young shoots are cupped or distorted and covered with a white, powdery mat. The leaf tissue may have a lot of red pigmentation. Flower buds may also be affected and will not develop properly. The basal shoots (water sprouts) that develop from the base of the tree are commonly attacked by this disease.

DESCRIPTION OF PEST

Powdery mildews are visible to the unaided eye as a thick, white layer of fungal strands (mycelium) and chains of spores (conidia). Toward the end of the growing season, the mycelium may turn tan or brown and there may also be small, dark spheres (cleistothecia) that serve as overwintering structures. There are many species of powdery mildew, but they are very host-specific. In other words, the powdery mildew that affects Zinnias will not affect Crapemyrtles, etc.

distorted leaves

Distorted leaves often accompanied by
red pigmentation is common with
powdery mildew.

LIFE CYCLE

This powdery mildew survives the winter as mycelium within buds, or rarely as cleistothecia. As new growth begins and the weather warms, the mycelium begin to grow across the surface of the leaves and succulent stems. The powdery mildew obtains its energy from the plant tissue by producing branched, root-like structures (haustoria) that penetrate the leaves. Many conidia are produced in chains that can spread the disease to other parts of the plant. Moisture is needed for the conidia to germinate and infect the plant, but dry weather will enhance the proliferation of mycelium. Unlike many other types of fungus, powdery mildews cannot survive on dead plant tissue. This means that they do not kill the plant, but can severely stress the plant which may then succumb to other problems.

water sprouts

Water sprouts are very susceptible
to powdery mildew.


MANAGEMENT

CHEMICAL
There are many fungicides labeled for control of powdery mildew. They usually must be reapplied at one to three week intervals, depending on fungicide, to provide adequate protection.

BIOLOGICAL

The best defense against powdery mildew is to select resistant varieties. All of the varieties include on this web site are resistant to this disease. On existing, susceptible varieties, proper pruning to increase air circulation and removing basal shoots will minimize the problem. Avoid excessive nitrogen applications that promote over-succulent growth

chains of conidia

A magnified view of chains of conidia.

cleistothecia

Cleistothecia are rarely seen with this
species of powdery mildew. They are
about the size of a mustard seed.

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