QBARS - v27n3 Some Environmental Factors Affecting Rhododendron Wilt

Some Environmental Factors Affecting Rhododendron
Wilt Caused by Phytophthora Cinnamomi
Michael J. Trombino Jr., Rhode Island

The fungus, Phytophthora cinnamomi causing wilt or basal rot, is one of the largest threats to the production of certain clones of Rhododendrons. Rhododendrons, are most susceptible at the age of one, two and three years.
Several studies have been made to find general characteristics of the fungus (2), also studies dealing with specific relationship of the fungus to the environment. Established techniques of inoculation (1), and studies involving aspects of Phytophthora incidence in Rhododendrons, have been used as aids in this experiment. In 1927 White (2) studied Phytophthora wilt of rhododendron and described the general characteristics of the disease. He found that an optimum temperature of 25° - 27.5°C obtained for growth of the organism and that temperatures above 35°C were lethal.
The fungus is spread through the soil during transplanting, irrigating, or by its own gradual growth. Its court of entry is through roots or stem bases.
The plants yellow and develop dullness in the leaves. The root system turns brown and decays. A brown streak is eventually visible in the stem cambium and phloem.
Suggestions for controlling the disease include the manipulation of temperatures so that the plants are grown at a temperature below that of the optimum for the fungus, until the young plants are established.
Since the amount of damage caused by Phytophthora cinnamomi varies greatly from nursery to nursery, it may be hypothesized that improper cultural practices might be implicated in disease incidence.
In an experiment to consider the aspect of control by cultural methods, twenty-seven three-year-old container grown plants each of R. 'Nova Zembla' and R. 'English Roseum', were repotted in a mix consisting of equal parts by volume of peat and perlite and sand in six-inch No. 10 fruit cans. The cultivars were chosen because R. 'Nova Zembla' has been reported to be susceptible to Phytophthora cinnamomi and R. 'English Roseum' has been reported to be resistant. (1)
One day after repotting the plants were inoculated with P. cinnamomi mycelial suspension. The fungus was previously isolated from Rhododendron plants at a local nursery. The method of inoculation was that described by Hoitink (1) and consisted of growing mycelium in an enriched pea broth medium in three-ounce bottles. When the mycelial suspension showed sufficient growth, one bottle was then applied to each plant. A handful of media and roots was lifted out of the top of the containers and the suspension was poured into this depression. Fresh peat and perlite medium was used to restore the level in the container. The amount of roots removed was not enough to damage the plant, but wounded the roots sufficiently to allow access of the fungus. The plants were then given approximately 1000 ml. of water and left on the bench in a heated greenhouse for one day.
On Nov. 24, 36 of the plants were placed in two separate Wisconsin Water Baths. Water temperature was controlled by an automatic thermostat while a pump circulated the water around the containers. The baths were filled with enough water to cover the bottom one-third to one-half of the containers which ensured a constant temperature around the area of the roots, where the inoculum was placed.

The first bath treatment held 18 plants, nine of each clone, with a constant temperature of 90°F ± 4°. The second treatment also held 18 plants with nine of each clone at a constant temperature of 80°F ± 4 degrees. The remaining 18 plants (third treatment), were placed on a bench at room temperature which never exceeded 75°F, nor dropped below 44°F. The average temperature was 70°F and although the minimum was 44°F, it usually did not drop below 66°F.
Three calibrated tube thermometers were placed in the water bath and in the containers, to the depth of the roots and these were moved, on a daily basis, to different vicinities of the water bath or to different containers.
On Nov. 30, the first symptoms of wilt were evident. Treatment one (90°F) had six plants that showed wilt. Treatment two (80°) had two plants and treatment three (75°) had none. Within two weeks nine plants were dead in the first treatment, three in the second and none in the third.
Eight days later symptom development was complete. No further wilting occurred prior to the termination of the experiment on Jan. 17.
On Dec. 20, a totally wilted R. 'Nova Zembla' was taken from one of the baths, in order to attempt to re-isolate the fungus. Isolations made from the stem and roots, used stem chips and root segments about ¼ inch in length and placed in media of PDA in Petri dishes. The dishes showed positive results in two days. Two uninoculated plants of each clone were placed in the 90°F bath for over three weeks. These two factors emphasize the presence of the fungus as the causal agent.
R. 'English Roseum' showed no loss of plants in any of the three treatments. However, R. 'Nova Zembla' had 100% loss of plants at 80°F, no losses occurred for the 70°F treatment.
he data presented show a definite correlation between temperature and the incidence of Phytophthora wilt. Temperatures 70°F and below are apparently disadvantageous for the spreading or growth of the fungus in clones of R. 'Nova Zembla' and R. 'English Roseum'. A 10° raise in temperature from 70°F gave a 33% increase in R. 'Nova Zembla' plants lost. A 20° raise in temperature from 70°F resulted in a 100% loss or 2/3 more than at 80°F. This may suggest a logarithmic progression, but it cannot be proven with the data from this experiment.
Soil temperature of 80°F and above should be avoided, and temperatures from 50 ¹ F to 70°F should be explored to find the temperatures least productive to P. cinnamomi growth and yet nearest to the optimum for Rhododendrons.
A further study was made of several plants grown and inoculated at 70°F.
Sixty days after inoculation, these plants were placed in the 90°F treatment. They did not succumb to the disease after an exposure to 90°F for five weeks. Apparently the fungus can enter the roots for only a short time after inoculation.
This study indicates a definite clonal variation in susceptibility to P. cinnamomi and clonal variation in the influence of temperature on susceptibility.

*Student. Work done as a special project in a cooperative program between the Departments of Plant and Soil Science and Plant Pathology-Entomology under the direction of J. J. McGuire, University of Rhode Island.

1. Hoitink, H. A., Ohio Report on Research and Development, Vol. 54 No. 6, November-December, p. 83-84.
2. While, Richard P., 1937, Rhododendron Wilt and Root Rot, New Jersey Agricultural Experiment Station Bulletin No. 615, p. 1-31.