Problems Associated With Resistance of
Rhododendrons to Phytophthora Wilt
H. A. J. Hoitink and A. F. Schmitthenner
Dept. of Plant Pathology, Ohio Agricultural Research & Development Center History
Rhododendrons have long been a popular plant in the southeastern and western regions of the United States. In Ohio and other east central and northern states, however, recent interest has made rhododendron culture one of the largest among woody ornamentals.
The most important disease of rhododendron is rhododendron wilt, first described by White in New Jersey in 1927. The disease is known as "wilt" or "root rot" because its outstanding symptom is wilting of foliage followed by death of the plant. On the West Coast, the name 'stiff neck' has been locally used to designate the disease.
Rhododendron wilt has previously been thought to be caused by a single pathogen, Phytophthora cinnamomi. Three years of studies in Ohio by plant pathologists, however, have shown that at least two other Phytophthora spp. also cause the disease. In addition, data have been obtained which show that considerable differences in resistance exist among commercial rhododendron hybrids.
Cause of Disease
P. cinnamomi, P. citricola and a third as of now identified only as a Phytophthora spp. cause wilt. Wilt caused by P. citricola is as severe as wilt caused by P. cinnamomi in young plants. P. citricola may become more important in the northern regions of the United States where rhododendrons are grown since it forms spores that are more persistent than those of P. cinnamomi. On the other hand, wilt caused by P. cinnamomi has become increasingly severe throughout the U. S. These Phytophthoras are classified as water molds. They thrive under high moisture conditions. The importance of the third unidentified Phytophthora spp. in the wilt syndrome is presently being investigated.
Our work also has shown that strains of P. cinnamomi and P. citricola differ in their ability to cause disease. Strains of P. cinnamomi that were isolated from Viburnum and azalea cause a mild root rot on rhododendron. Similar strains of P. citricola isolated from plants other than rhododendrons cause a mild root rot only. These data indicate that crop rotation in the field. even azalea with rhododendron, will tend to limit severity of disease.
Factors Affecting Wilt
a) Environmental - Since the early work on rhododendron wilt by White it has been known that high moisture favors disease. The Ohio studies show that high temperature also favors disease. Wilt is most severe at temperatures of 80°F and above. Once a plant becomes infected under high soil moisture conditions it never recovers. This is in contrast to plants infected with Pythium where recovery often occurs after the soil dries.
Rhododendron wilt has increased during the last decade. This is concurrent with the increased practice of container growing of nursery stock. This can be explained by the fact that temperatures of media in containers during the growing season frequently are higher than those of the surrounding soil. In addition, control of soil moisture in containers requires greater skills than control of soil moisture in field grown plants under irrigation.
b) Resistance - During the past 2 years evidence has accumulated that difference in resistance exists among commercial rhododendron varieties. Twenty 2-year-old container plants of several hybrids were inoculated with a highly virulent strain of P. cinnamomi. Three months after inoculation. roots of all plants were washed and a relative resistance rating was thus obtained. Data indicate that 'English Roseum' is resistant to wilt. It is of interest that this hybrid also shows a high degree of field resistance to Phytophthora cactorum which causes die-back in a mild and late summer. On the other hand, 'Roseum Elegans' which has been used as the standard variety throughout this research is very susceptible. Since 'English Roseum' also has excellent horticultural characteristics it should probably be grown on a larger scale than it is at present.
Unfortunately, none of the good red, purple or white hybrids were resistant. In addition to 'English Roseum', 'D. A. Koster' and 'Van der Hoop' were also resistant. Hybrids 'Album Elegans', 'Purpureum Elegans', 'Roseum Superbum', and 'V. W. Poelman' had some resistance. The following were susceptible: R. maximum, R. carolinianum, 'America', 'Nova Zembla', 'Catawbiense Boursault', 'Boule de Neige'. 'Roseum Pink', 'Roseum Elegans', 'Catawbiense Alba', 'Lemon Ice', 'Cunningham's White', 'Parson's Gloriosum', 'Edward S. Rand', 'Lee's Dark Purple', 'Caractacus Deep Crimson'. 'Dr. Rutgers', 'Amphion', 'H. C. Dresselhuys', 'Bettex', 'P. den Ouden', and 'Blue Peter'. Additional studies are required to show that 'English Roseum', 'D. A. Koster', and 'Van der Hoop' are resistant to other Phytophthora species and strains that cause wilt.
c) Control - A 3-year study has shown that during propagation wilt can be controlled when proper sanitary procedures are followed strictly. Monthly drenches with Dexon (35%), mixed at the rate of 10 ounces per 100 gallons of water and applied at the rate of 1 pint per square foot, inhibit growth of Phytophthora in the cutting bench. After transplanting, however, neither drenches with Dexon nor Terrazole mixed with the planting media will control the wilt disease once the pathogen is present under conditions that favor disease. For control of wilt in liners, sanitation and proper drainage are the only sure control method available at present.
Plans for the Future
Presently, additional hybrids are being examined for wilt resistance; in all, approximately 150 rhododendron species will be tested for their resistance to wilt. So far this study has been limited to Ohio. Rhododendrons shipped across the country may appear healthy, especially if they were grown under well drained conditions. However, if such plants are placed under high levels of nutrition and moisture to force growth, severe wilt often occurs. Due to such shipment of apparently healthy nursery stock, strains of the wilt disease can be rapidly be introduced into an area where it previously did not exist. This study therefore should reach beyond Ohio. A nationwide cooperation of nurserymen and rhododendron breeders in this project is essential for successful introduction of resistant types.