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.
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.
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%.
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.
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.
Rhododendron Hybrids Highly Resistant to P. cinnamomi
Rhododendron Hybrids Moderately Resistant to P. cinnamomi
Bosley Dexter 1020
Rododendron Species Highly Resistant to P. cinnamomi
R. davidsonianum 'Serenade'
Rhododendron Species Moderately Resistant to P. cinnamomi
Corrine Murrah (BA)
Fred Cochran (N)
Hampton Beauty (P)
Morning Glow (K)
New White (I)
Pink Gumpo (S)
Pink Supreme (I)
Polar Seas (GD)
Rachel Cunningham (BA)
Rose Greeley (G)
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
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
Commercial products are named in this publication for informational purposes only. Virginia Cooperative Extension does not endorse these products and does not intend discrimination against other products which also may be suitable.
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Interim Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 1, 2009