UCONN IPM Nursery:Diseases:Rhododendron Wilt

Rhododendron Wilt

Rododendron wilt, otherwise known as Phytophthora root rot, is caused by the fungus Phytophthora cinnamomi. This fungus infects rhododendron, azalea, mountain laurel, blueberry, heather, Irish-heath, Leucothoe, spike-heath, wintergreen, Arbutus unedo, bear-berry, and Pieris. This disease was first reported in 1929 in New Jersey. It is now found in most parts of the world where these plants are grown. The fungus is thought to have originated in Asia or Australia. It was first isolated from cinnamon trees, for which it was named. This disease usually affects 1 to 2 year old plants, especially those grown in containers. It can also occur on field grown plants in poorly drained soils.

Symptoms. The leaves become pale, droop, roll downward parallel to the midrib, and wilt. The leaves eventually die. Death may occur in as few as 14 days in susceptible young plants, or death may not occur for up to a year in older plants or less susceptible varieties. Individual branches may die. These branches usually have a canker at their base if the plant is young.
Underground, small roots become infected first and turn brown. The roots die, although the plant may grow more roots if the plant is older and established. The fungus grows into the stem and when the bark is cut away, the region where the bark attaches to the wood is dark brown.
Some less susceptible cultivars may not wilt until all the roots have died. The youngest leaves may turn yellow between the veins, near the midrib. These areas eventually turn brown. Large, well-established plants often have few aboveground symptoms.

Prevention.

In the Nursery.
- Use disease free cuttings that have not had contact with soil.
- Use sterile soil for planting.
- Use only water from wells or water that has been chlorinated; not surface water or water that has been recycled from previous waterings.
- Place pots on a well-drained surface, not plastic, to avoid allowing plants to stand in puddles.
- Remove leaves and pruning debris from the area around the plants.
- Growing in composted hardwood bark has been reported to suppress the growth of the fungus.
- See current recommendations for chemical control measures.
At home.
- Plant in acidic (pH 5.0-5.5), well-drained soil.
- In heavy soils, plant in raised beds or on a slope to improve drainage.

Resistance Available. Some varieties of Rhododendron are resistant, including Caroline, Professor Hugo de Vries, and Red Head. A few others are moderately resistant. Some azaleas are also resistant to this disease, including Formosa, Fakir, Corrine Murrah, and the Indica hybrids. It is important to use resistance whenever acceptable, particularly in sites where P. cinnamomi is known to exist.

By Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998.

References.

Refs: H.A.J. Hoitink, D.M. Benson, and A.F. Schmitthenner. 1986. Phytophthora Root Rot in Compendium of Rhododendron and Azalea Diseases. D. L. Coyier and M. K. Roane, eds. APS Press, St. Paul, MN. pp. 4-8.

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.

The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.