Wilt and root rot of rhododendron and azalea are symptoms of a disease caused by the soil-inhabiting water mold fungus Phytophthora. Several different spe of Phytophthora have been reported to cause similar symptoms, but Phytophthora cinnamomi and Phytophthora parasitica are recovered most frequently from diseased plants in Virginia. In addition to rhododendron and azalea, Phytophthora species also frequently infect many cultivars of boxwood, andromeda, mountain laurel, blueberry, camellia, juniper, and yew.

Symptoms

In the early stages of disease, symptoms consist of retarded growth, slight drooping of the foliage and off-color foliage. Infected broadleaf species wilt during the heat of the day (Fig. 1) and recover at night. Roots become discolored and die (Fig. 2). Dark or reddish brown discoloration may extend up into the wood of the lower stem (Fig. 3). Severely affected plants wilt permanently and turn brown.

 

Fig. 1. Wilting of susceptible plants in the field. (Photo by R. C. Lambe)

Fig. 2. Root rot on rhododendron caused by Phytophthora cinnamomi. (Photo by R. C. Lambe)

Fig. 3. Internal discoloration of the wood in the stem near the ground line following root infection. (Photo by M. A. Hansen)

Environmental Factors Favoring Disease

Disease development is favored by high soil moisture and soil temperatures of 80°F and above. Infected plants growing on sandy soils are generally not as seriously affected by the disease, whereas those growing on poorly drained soils wilt and die. On well-drained soils, only feeder rootlets are rotted, but when drainage is poor, the main roots and stem become discolored and infected plants wilt and die. Many areas of Virginia have soil that is heavy in clay, poorly drained, and conducive to Phytophthora root rot. Infected plants may also be brought into Virginia from other states. These plants may appear healthy when received, but wilt from root rot when exposed to local conditions of high soil moisture and high temperatures. Plants with root systems impaired by Phytophthora root rot may also be more sensitive to subsequent periods of drought.

Losses from Phytophthora root rot in commercial operations have decreased due to the increased use of well draining mixes, such as 100% pine bark. In general, root rot occurs less frequently on plants grown in mixtures that have an air volume of 20-25%.

Control

Home Landscape

Planting sites should be well-drained. Growers often misinterpret wilt as a sign that a plant needs water and respond by applying more water. This reaction should be avoided in sites where plants are clearly receiving adequate water. The use of drain tiles to prevent rain water from collecting on the surface is suggested. Special care should be taken to set plants high so that the soil line is not more than one inch above the upper roots. Where subsoils are known to be poorly drained, planting on raised beds should be considered. After planting, the soil should not be mounded up around the base of the stem because this increases susceptibility to disease.

Chemical Control

Chemical control of Phytophthora root rot in established plantings of rhododendrons and azaleas is difficult. However, the spread of the fungus from diseased plants to adjacent healthy plants may be checked by drenching soil around the healthy plants with a fungicide containing mefenoxam (e.g. Subdue MAXX), metalaxyl (e.g. Subdue), fosetyl-Al (e.g. Aliette), etridiazole (e.g. Truban) or etridiazole + thiophanate methyl (e.g. Banrot). Follow label rates and treat at four-week intervals during the summer when environmental conditions are favorable for disease. For further details on chemical control, consult the current Virginia Pest Management Guide for Home Grounds and Animals (VCE Publication 456-018) or the Virginia Pest Management Guide for Horticultural and Forest Crops (VCE Publication 456-017), http://pubs.ext.vt.edu/456/456-017/456-017.html. For information on the proper use of pesticides and fungicides, refer to any current VCE pest management guide.

In locations where plants have died from Phytophthora root rot in the home landscape, it would be advisable to replant with a cultivar that has resistance to the disease. A list of cultivars of azalea and rhododendron that are reported to have moderate to excellent resistance to Phytophthora root rot is provided below.

Propagation

Before sticking a new crop of cuttings, remove old rooting media from the propagating benches. To reduce the possibility of pathogen infestations originating from soil on the floor of the greenhouse, propagate on raised benches. Flats, baskets, and greenhouse benches can be treated with a disinfectant, such as Greenshield or Physan 2.0, to eradicate plant pathogens from surfaces. Propagating tools should be steam-sterilized or soaked in a commercial disinfectant. If a commercial disinfectant is not available, use household bleach containing sodium hypochlorite (1 part bleach to 9 parts water). Propagating benches should also be surface-sterilized between each set of cuttings prior to filling with sterile rooting media.

Containers

Containers should be placed on well drained sites, preferably on rock or gravel, so that any surface water contaminated with Phytophthora zoospores, which are motile in water, cannot enter the container through the bottom. Growing areas can be crowned to facilitate rapid runoff of water. If the water used for irrigation of containers is recycled, the hazard exists that the water may become contaminated with zoospores of Phytophthora spp. Chlorination will eliminate pathogens from the water.

Field

Some growers have learned to reduce disease losses in heavy soils naturally infested with Phytophthora species by growing plants on hills in rows or on well drained, raised beds. It is suspected that most of the field spread of Phytophthora spp. results from movement of fungal zoospores in water. Because it is possible to introduce Phytophthora spp. into fields through an irrigation system that draws and recirculates water from a catch basin or irrigation pond, chlorination of the water may be necessary to eradicate fungi.

Resistance

Many different rhododendron hybrids develop symptoms of Phytophthora root rot under natural conditions in Virginia. Field experiments with artificially inoculated plants have shown many well known hybrids and cultivars to be susceptible to P. cinnamomi, the most common species of Phytophthora found on azaleas and rhododendrons in Virginia. However, some species and hybrids that have resistance to P. cinnamomi have been identified in experimental trials (Tables 1-3). Although performance of these species and hybrids in a given site may vary, resistant plants have a better chance of survival where Phytophthora diseases have been a problem in the past. Be sure to check with your nursery personnel for any newly introduced resistant cultivars that may not be listed below.

 

Table 1. Rhododendron hybrids with resistance to P. cinnamomi
Rhododendron Hybrids Highly Resistant to P. cinnamomi Caroline Martha Isaacson Pink Trumpet Professor Hugo de Vries Red Head		Rhododendron Hybrids Moderately Resistant to P. cinnamomi Bosley Dexter 1020 Brickdust Broughtonni Aureum Disca Dr. A. Blok Dr. Arnold W. Endtz English Roseum Lucky Strike Madame Carvalho Mr. C. B. Van Nes Mrs. A. T. de la Mare Prize Rocket (Shammarello) Van Veen Wilbrit

 

Table 2. Rhododendron species with resistance to P. cinnamomi
Rododendron Species Highly Resistant to P. cinnamomi R. davidsonianum 'Serenade' R. delavayi R. glomerulatum R. hyperythrum R. lapponicum R. occidentale R. pseudochrysanthum R. poukhanense R. quinquefolium R. sanctum R. simsii R. websterianum		Rhododendron Species Moderately Resistant to P. cinnamomi R. aberconwayii R. charitopes R. ciliatum R. hemitrichotum R. nitens R. oldhamii R. ponticum I R. ponticum II R. racemosum R. rigidum R. serphyllifolium R. shwelliense R. simiarum R. spiciferum R. yumnanense

 

Table 3. Azalea cultivars with resistance to Phytophthora root rot**
Alaska (R) Chimes (I) Corrine Murrah (BA) Eikan (S) Fakir (GD) Formosa (I) Fred Cochran (N) Glacier (GD) Hampton Beauty (P) Higasa (S) Merlin (GD) Morning Glow (K) New White (I) Pink Gumpo (S) Pink Supreme (I) Polar Seas (GD) Rachel Cunningham (BA) Redwing (I) Rose Greeley (G) Shin-ki-gen (S) Sweetheart Supreme (P)		**Key to letters in ( )'s BA - Back Acres K - Kurume R - Rutherford G - Gable N - NC State University S - Satsuki GD - Glen Dale W - Whitewater I - Indian P - Pericut

Selected References

Benson, D. M. and F. D. Cochran. 1980. Resistance of evergreen hybrid azaleas to root rot caused by Phytophthora cinnamomi. Plant Disease 64: 214-215.

Hoitink, H. A. J. and A. F. Schmitthenner. 1975. Resistance of rhododendron species and hybrids to Phytophthora spp. root rot. American Rhododendron Society Bulletin 29: 37-4.

Adapted from previous publication by R. C. Lambe and C. R. Drake

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