UCONN IPM:Nursery:Diseases:Phytophthora Dieback of Rhododendron

Phytophthora Dieback of Rhododendron

Phytophthora dieback is caused by the fungus Phytophthora cactorum. This disease affects rhododendron, azalea, and Japanese andromeda. It was first reported from Maryland in the 1930s. It has been found in the eastern United States and Europe, where it has become increasingly prevalent in the last 15 years. This disease is favored by hot, wet weather. It is usually thought of as a nursery disease. Succulent young growth is much more susceptible than the older leaves. Woody stems are not usually affected.

Symptoms. On leaves, round spots appear on new growth. They are watersoaked at first. Then within 3 to 5 days the spots become olive-colored, and then chocolate-brown. The spots are brittle. The leaves curl down toward the stem, but they remain attached to the stem.

If the leaf spots reach the midrib of the leaf as they grow, the fungus will grow into it, and down the petiole until it reaches the stem. On the stem, a diamond shaped canker is formed. This will continue to grow until it completely surrounds the stem. The portion of the stem above the canker then dies, and turns completely brown.

On older plants, the fungus may grow down the stem into a healthy leaf, causing a narrow brown band to appear along the midrib. This band will expand in a V-shaped pattern, with the widest portion at the base of the leaf. These leaves usually fall off within 1 to 2 weeks.

Prevention.

It is important to keep leaves dry to prevent this disease.
Avoid overhead watering.
Avoid watering early or late in the day.
Avoid excessive nitrogen fertilizer, especially in mid-summer. Fertilizing at this time causes flushes of susceptible young growth when the weather is favorable for disease development.
Prune out diseased branches well below the diseased area.
Covering the bases of container-grown plants with 2-2 1/2" of gravel is effective in keeping the spores of the fungus from splashing from the bases of the plants onto the leaves and starting infections there.
Avoid planting rhododendrons near lilac, as this fungus causes a lilac disease as well.
There are a few varieties that are tolerant of this disease.
See current recommendations for chemical control measures.

By Pamela S. Mercure, IPM Program Assistant, University of Connecticut

References.

Benson, D.M. and H.A.J. Hoitink. 1986. Phytophthora Dieback in Compendium of Rhododendron and Azalea Diseases. D. L. Coyier and M. K. Roane, eds. APS Press, St. Paul, MN. pp. 14-16.

Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants, Fifth Edition. John Wiley and Sons, New York.

Sinclair, W. A., H. H. Lyon, W. T. Johnson. 1987. Diseases of Trees and Shrubs. Comstock Publishing Associates, Ithaca, New York.

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

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