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Plant Diseases

Powdery Mildew on Dogwood

The flowering dogwood (Comus florida) is a very popular native tree and has few equals when

it comes to providing a spectacular show of spring color. However, this wonderful tree is not

without its problems. Dogwood borers, errant lawnmowers, and Discula anthracnose have

limited the life of many a dogwood. Now a new emerging fungal disease is disfiguring this tree

and is being noticed by landscape maintenance personnel and homeowners alike. Powdery

mildew, caused by the fungus Microsphaera, is widespread. Wherever the fl owering dogwood is grown, powdery mildew is increasing in occurrence and severity.

Symptoms

Powdery mildew infections are occurring as early as mid-May and can be identifi ed by the

patches of white powdery fungal growth on the upper surface of newly emerging leaves. This

infection will often cause the new growth to be twisted or deformed. Older, infected leaves

have green-brown or green-purple blotches that progress into dark brown to tan dead patches

as the summer progresses. The white fungus grows throughout the summer and fall. It is

diffi cult to evaluate the effect powdery mildew infections have on landscape trees but the loss of photosynthesis and water due to leaf infections could be weakening trees. This weakening could make infected trees more susceptible to dogwood borers or Botryosphaeria canker disease. The aesthetic effects are obvious; the disease reduces the attractiveness of the trees.

Recent observations from areas with nursery production indicate that the disease is capable of

reducing growth of very small trees that are used for rootstock and bud wood.

Control

There are several things that landscapers and gardeners can do to prevent powdery mildew:

❚ For existing trees discourage the disease by avoiding heavy nitrogen fertilizer use, heavy

overhead watering and excessive pruning. These practices produce succulent growth that is more easily infected by the fungus.

❚ Provide a thin layer of mulch over the root system, prune out dead branches, and provide good air movement so the foliage dries quickly.

❚ Planting disease resistant cultivars is the best long-term solution to powdery mildew control.

❚ Plant pathologists and horticulturists have identifi ed a number of disease resistant and

moderately resistant cultivars of fl owering dogwood. Ratings have also been taken of mildew

resistance of Kousa dogwood and the new dogwood hybrids, Cornus ruteriensis.

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