Symptoms. The symptoms of phytophthora root rot vary with the cultivar. Some cultivars fail to grow or grow very slowly with pale green foliage and may die after several years. Others suddenly wilt and die within a few weeks. Roots are reddish-brown, brittle, and often limited to the upper portion of the media in a container or very close to the soil surface (upper 2 inches). The reddish-brown discoloration advances to the larger roots and eventually to the lower part of the main stem.

Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as in heavy clays or poorly drained soils, etc. The disease is common and severe in areas where run-off water, rain water from roofs, etc. collects around plant roots. Setting woody plants deeper than the soil level in the container, over-watering plants, or long periods of heavy rain also favor disease development, especially in shallow soils with underlying rock or compacted hard pans.

Prevention/Control. Phytophthora root rot must be prevented, as chemicals are ineffective in controlling this disease after above-ground symptoms appear. See Pest Identification: Diseases and Nematodes for a discussion of general prevention and control measures.

Rhododendrons are susceptible to stunt nematode (Tylenchorhynchus claytoni) damage. Leaves turn yellow and plants are stunted and gradually die. They fail to respond to fertilizer and water.

There are no chemicals (nematicides) available to control nematodes on rhododendrons established in the landscape. Good growing conditions--such as mulch, partial shade, fertilizer (applied in accordance with a soil test report), and the application of water during dry periods is recommended. If plants die from nematode damage, nematode-free plants should be purchased and planted in an area free of stunt nematode.

RHODODENDRON PETAL BLIGHT
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

The North Carolina Agricultural Chemical Manual should be consulted for current fungicide recommendations. Petal blight can be controlled by applying a labeled fungicide to the soil 1 to 2 weeks in advance of flowering of early cultivars or sprayed on thoroughly just as flower buds begin to open. A second application may be made if disease has been severe in past years. Later blooming varieties should be sprayed as they begin to show flower color. Blighted flower heads should be picked and removed from the garden before the petals fall to the ground.

TWIG BLIGHT OF RHODODENDRON
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

Twig blight can be controlled on cultivars that are not severely stressed nor highly susceptible by promptly pruning out and disposing of the diseased branches. It is important to check carefully to ensure all the discolored stem tissue has been removed, making sure that correct pruning practices are followed. Fungicides are of little value.

PHYTOPHTHORA DIEBACK ON RHODODENDRON
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

Symptoms. Lesions first appear as chocolate-brown spots on young, expanding foliage and stem. Infected areas may appear water-soaked at first; later, leaves dry out and may drop off. The fungus grows from initial lesions down the stem into older leaves. Lesions on mature leaves are characteristically wedge-shaped, extending from the petiole toward the leaf margin. Infection spreads rapidly in warm days. The fungus progresses slowly in older woody stems but can kill plants of any size.

Factors Favoring Infection and Spread of Dieback. Phytophthora dieback is favored by hot, wet weather during the summer months. The fungus is spread between plants and on infected plant parts by spores in splashing water. Free water on the leaves is needed for infection by the spores. Infections occur on the lower surface of expanding new leaves. Overhead irrigation often increases infections due to the spread of the fungus by splashing water.

Excessive use of nitrogen fertilizer, which increases plant succulence, particularly during August, also increases plant susceptibility to Phytophthora.

Diagnosing Rhododendron Dieback Caused by Botryosphaeria and Phytophthora.

• mostly in landscape
  
• entire branch or section of plant wilts quickly (1 to 2 days) and dies
  
• attacks older wood
  
• entire leaf rolls downward parallel to midrib
  
• entire leaf turns gray-green then brown
  
• leaves remain attached to dead stem
  
• brown discoloration in wood on one side of stem
  
• moves fast in old stems
  
• branches die at any time

Phytophthora:

• mostly in container nurseries
  
• attacks individual shoots
  
• attacks succulent leaves and stems
  
• leaf does not roll
  
• dark brown to black discoloration progressing in V-shaped pattern from midrib to leaf margins
  
• mature leaves drop from stem quickly
  
• brown discoloration in wood
  
• moves fast in succulent stems
  
• shoots usually die during growth flush

Control of Phytophthora Dieback.

Although this disease is more commonly seen in the nursery, the following practices are recommended if it occurs in the landscape:

• On larger rhododendron plants, dieback can be selectively pruned out if it is detected in the early stages. All brown, discolored wood, as well as any diseased leaves and stems should be promptly removed (pruned back to healthy branches) and destroyed.

• If overhead sprinklers are used, late afternoon irrigation should be avoided to allow the foliage to dry before night. If plants are watered by hand, foliage should not become wet.

• Plants should not be over-fertilized. Dieback develops and spreads more rapidly in succulent tissue.

• Spacing should be provided between plants to promote better air circulation around plants to dry foliage. Lower branches should be pruned to allow better air circulation under the plant; this will help the soil surface dry under the plant.

Fertilizer injury. Rhododendrons have fine roots that grow very close to the soil surface. It is easy to burn the roots with fertilizer. Several light applications during spring and summer are better than one heavy application. The fertilizer should be spread around the drip-line of the foliage.

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