Rhododendron Wilt and Root Rot
Paul E. Nelson
Department of Plant Pathology
Cornell University, Ornamentals Research Laboratory
Farmingdale, L. I., N.Y.
The rhododendron wilt and root rot disease has been known since 1927 and still can cause serious losses to rhododendron growers today. Both young and mature established plants can be attacked.
The earliest symptoms of this disease consist of a dull yellowish-green color of the foliage and affected foliage soon wilts permanently. Roots of such plants will be decayed, especially the fine feeder rootlets. The base of the stem at the soil level and below will be brown in color. When the bark above the base of the stem is peeled away reddish-brown streaks will be observed extending for various distances up the stem. In cases where the disease ha alis reached the current season's growth, brown sunken cankers may be connected with these discolored streaks.
This disease is caused by a fungus, called Phytophthora cinnamomi. This fungus is a member of a large group of fungi known as water molds because free water is required for the germination and initial growth of their spores. Fungi are plants without chlorophyll and therefore must live either as parasites (obtain food from living plants or animals) or saprophytes (obtain food from dead materials such as plant or animal residues). Fungi grow by means of thread-like structures called mycelium. The mycelium bears other structures called spores which may vary in size and shape and are responsible for the reproduction of the fungus. The rhododendron wilt and root rot fungus has three types of spores which help to spread the disease, to build up the fungus population in the soil and to enable the fungus to survive during periods when environmental conditions are unfavorable for its growth. This fungus is a soil inhabitant and is present in many different types of soil. When conditions become favorable the fungus will grow into the rootlets and main roots of the plants. The small rootlets are killed first and eventually the larger roots are invaded and die gradually slowly eliminating the entire root system. This may be a slow process and take several months depending on environmental conditions. In the later stages of the disease the fungus grows up the stem in the region of the cambium, destroying cambium and phloem tissue. In current season's growth the fungus may grow into all the tissues of the stem producing a brown, sunken canker frequently girdling the stem.
Environment can have a great deal of influence on the prevalence of this disease. Soil moisture, soil acidity and soil temperature are environmental factors that can influence disease development.
Excessive soil moisture favors the growth of this fungus and therefore the development of the disease. This condition can result from such things as over-watering, planting in heavy soils, and failure to provide proper drainage at the time of planting.
A soil pH of 4.0 to 7.0 favors the development of this disease. However, the lower the pH (more acid) from about pH 1.5 on, the less favorable it is for disease development. Therefore, it is wise to) keel) soil as acid as is conducive to good growth of Rhododendrons.
Exposure to freezing temperatures in the upper 4 to 6 inches of the soil may kill some of the mycelium and spores of this fungus. This factor may account for the sporadic occurrence of the disease. After a severe winter the fungus population may be at a low level and require several months or a year to build up to a point where severe disease symptoms result.
Phytophthora cinnamomi has a wide range and attacks many different kinds of plants. A partial listing of these plants is given in Table 1.
Table 1. Partial list of plants susceptible to Phytophthora cinnamomi
R. californicum Fir Birch Azalea R. carolinianum Cedar Chestnut R. catawbiense Cypress Beech Camellia R. indicum Juniper Walnut Heather R. maximum Larch Plane Japanese Holly R. mucronulatum Incense Cedar Peach Viburnum R. ponticum Spruce Plum Laurel R. caucasicum Pine Oak Andromeda (var. 'Boule de Neige') Yew Black Locust Arborvitae
Control of this disease consists primarily of proper cultural practices such as planting in well drained soils and avoiding excessive irrigation, keeping the soil acid, around pH 4.0, and avoiding replanting in soils where previous plantings have been killed by this fungus. If it is necessary to replant in infested soil fumigate the soil before planting with a suitable soil fumigant such as Vapam. Apparently no resistance to this disease has been found in the rhododendron species commonly grown today.
White, Richard P., 1937, Rhododendron Wilt and Root Rot, J. Agric. Expt. Sta. Bull., 615, 32 p.
Thorn, William A., and George A. Zentmeyer, 1904, Hosts of Phytophthora Cinnamomi, Rands, Plant Dis. Reptr., 38:47-52