Diseases of Rhododendrons and Azaleas
By Robert D. Raabe
Department of Plant Pathology
University of California, Berkeley
A paper presented at the Fall meeting of the California Chapter of the A.R.S.
In discussing the diseases of rhododendrons and azaleas, it is well to remember that these plants are grown throughout a large portion of the country. As the climate varies somewhat from region to region, the diseases found also vary and though the diseases to be considered here are primarily diseases of concern in California, some of them are found in other parts of the country.
Rhododendrons and azaleas do best when planted in a fairly light, well-drained porous soil into which has been incorporated an abundant amount of acid-forming organic matter such as peat moss or leaf mold. The soil should have a good water-holding capacity and yet should be porous enough so that water will move through it and so that satisfactory aeration will result in the root area. Since many of the California soils are quite heavy and have a high water holding capacity, special consideration should be given since these conditions, if not corrected, are important factors in the successful cultivation of these plants.
Water-mold Root Rot
One of the diseases which may result from extremely wet conditions is known as wilt, root rot, or water-mold root rot. This disease may be incited by one or more fungi which are commonly called water molds. Under conditions which favor their development, they will attack the roots and cause them to become brown and decayed. Usually young plants are attacked and affected plants show a wilting and a dull green color in the leaves. This is followed by a leaf drop and permanent wilt. Upon examination, the portion of the main stem at the soil line will be browned and a cut into this area will reveal only dead bark and wood.
Since this disease usually attacks only young plants, and then only when the soil is excessively wet, the control is quite easy. As was mentioned before, the plants should be planted in a well-drained soil. This does not mean that a hole or pit should be dug in a heavy soil and then filled with a soil mixture of the desirable type. Such a hole acts as a pocket to catch the water and provides the favorable conditions which favor disease development. If it is necessary to plant in heavy soils, it is best to plant the rhododendrons and azaleas where the ground is sloping so the excess water will drain off. Then either the light soil in which the shrubs are to be planted can be placed on the sloping ground or if a hole is dug, a trench can be cut or a tile put in from the bottom of the hole to the surface of the slope on the downhill side. Both of these methods will greatly reduce the conditions favorable to the disease. Avoidance of planting too deeply and the adjustment of soil pH to between 4.5 and 5.5 also help to control the disease.
Armillaria Root Rot
Another disease which is favored by wet conditions is Armillaria root rot which is incited by the well-known oak-root fungus. Plants infected with this fungus become sparse in foliage, the leaves droop and turn yellow. Upon close examination, black, shoestring-like strands may be found on the outsides of the larger roots. Large white fans or plaques of the fungus are visible beneath the bark of the portion of the main stem below the soil level and beneath the bark of the larger roots.
Once established this fungus is hard to control. In an infested area, plants call be removed and the soil treated with carbon bisulfide. In garden areas where plants are close together, this is hard to do since such a treatment will kill all the plants in the treated area. Exposing the crowns of infected plants may help to prolong the life of the plant since the fungus can not exist under dry conditions. However, since it thrives under wet conditions, improving the drainage and avoiding too much water will help to prevent the plants from becoming infected.
Rhizoctinia Cutting and Graft Decay
Another disease problem that arises from the soil is cutting and graft decay which is incited by a soil fungus, Rhizoctonia solani. This fungus primarily attacks the tops of the plants in moist cutting or grafting frames. The fungus grows over the leaves and stems as a cobwebby growth and causes a decay of the infected plant parts. Under drier conditions, the fungus may cause a rot of the bases of the cuttings. Control is brought about by using sterilized soil or sand for cutting beds. The plants should be grown as dry as possible. If the disease does break out, a drench of thiram will help to control it.
In addition to the soil diseases which have been mentioned, there are also a number of problems connected with the soil but which are non-parasitic in nature. This means that although the plant may be diseased, it is the result of a soil condition or cultural practices rather than the result of infection by a disease-producing organism.
One of the very common diseases of this type is known as chlorosis. In this disease, the leaves, and particularly the ones at the tips of the new growth turn pale green to yellow. Frequently only the margins of the leaves are a lighter color. Chlorosis is brought about by a condition in the soil which results in the unavailability of iron and can be corrected temporarily by spraying the leaves with iron sulfate (⅛ part by weight to 100 parts of water) about once a month. Sprinkling sulfur on the soil beneath the plant will also help. A better control is to incorporate organic matter such as peat moss or leaf mould into the soil.
Salt injury is another problem which is found in this area. The symptoms include a browning and death of the tissues around the margins of the leaves. Leaves may become chlorotic or yellow and on some plants, small dark or purplish warty growths or bumps may appear on the under surfaces of the dark leaves. This type of injury is much more common where the plants are kept in containers. The control is to give the plants an occasional heavy watering to leach away the excess salts which have accumulated in the soil. If the plants are in small containers and can be handled easily, soaking them overnight in a tub of water will be effective.
A trouble that is more prevalent some years than others is sun scald. This is found more on rhododendrons than azaleas and appears as large, brown areas in the leaves. When planting, care should be taken to see that the plants will receive the proper amount of shade in the hot part of the day. Plants in cans or boxes which can be moved around should be given extra attention to be sure that they do not get moved to a location where they receive too much sun for, in a matter of several days, they may be injured. In addition, it is known that some species can stand more sun than others and this should be considered when setting oat plants. Although sun scald may not be too serious, the injuries do present a bad appearance and also they allow the entrance of certain fungi which are weak parasites and normally would not harm the plants. However, once gaining a foothold, they are capable of damaging the plants.
Pestalotia Leaf Spots
There are two fungi belonging to the genus Pestalotia (Pestalotia macrotricha and P. rhododendri) which are found associated with injuries resulting from sun scald, insects and infections incited by other fungi. These two fungi produce spots on the foliage which are silver-gray on the upper surface and light brown on the lower surface. The upper surface may be dotted with small black spots which are the fruiting bodies of the organisms. The silvery spots may also be produced on the stems The control is brought about indirectly by such practices as planting in desirable locations in regard to sun and controlling insects and disease-producing fungi.
Lophodermium Leaf Spot
Another leaf spot, which is not very bad but may occur in California, is known as the Lophodermium leaf spot. This disease derives its name from the fungus inciting the disease, Lophodermium rhododendri, and results in the production of large, silvery-white spots with reddish margins. The upper surface of the spots is covered with prominent, black, scale-like fruiting bodies of the fungus. On these conspicuous fruiting bodies are scattered small, white cushion-like patches which are spore masses of the fungus. The lower surface of the infected area is chocolate brown.
A disease of azaleas known as azalea leaf scorch is sometimes found in this area and particularly in areas where there is a lot of fog. This disease is incited by a fungus known as Septoria azaleae and produces dark, reddish-brown angular spots on the leaves and causes them to fall prematurely. Although this disease is found only on azaleas, there is a similar disease on rhododendrons incited by Septoria solitaris. The control for these fungi and for the Lophodermium leaf spot fungus is to spray with either a 2-2-50 Bordeaux mixture or ferbam at about two-week intervals as the new leaves expand during the growing season or when the diseases appear. Destroying the fallen leaves also helps to prevent the spread of the fungi.
A leaf disease of rhododendrons which is of minor importance but which has been found in California is leaf rust. In the East, it is found primarily on seedlings of Rhododendron ponticum and the hybrid varieties. The disease appears as red spots on the tops of the infected leaves. On the undersides of these spots are found orange-red pustules which consist of the spores of the fungus. Control is brought about with the use of dusting sulfurs.
Azalea Leaf Gall
One of the more common diseases and one which occasionally may be fairly serious is the azalea leaf gall, incited by Exobasidium vaccinii. Infection by this fungus causes the leaves to become thickened and fleshy either wholly or in part. The infected areas turn pale green, white or pink. The flower parts of some species may turn into a hard, fleshy, waxy, irregular gall. The infected parts later become covered with a white bloom which consists of the spores of the fungus. The control is quite simple and is effected by removing and destroying the affected parts. Spraying with a weak Bordeaux (3-1-50) has also been recommended.
A closely related fungus has been reported as causing considerable damage on Rhododendron californicum. Infection by the fungus causes a distinct witches' broom to be produced. The leaves become yellowish-white and covered on the lower surface with a dense white fungus growth. Removal and destruction of infected branches controls the disease.
Azalea Petal Blight
Turning to the flower blights, there is only one which is serious. At the present time, it has been reported only on azaleas in nurseries in California but the spread to not only garden azaleas but to the native azaleas is a potential threat. The disease, known as azalea flower spot or Ovulinia flower blight, is incited by a fungus which attacks only the flowers of azaleas and some of the related plants. The disease is found primarily in the southeastern part of the country where it is particularly severe on azaleas of the Kurume and Indian types.
The disease appears as small pale spots on the inner surface of the petals of colored varieties and as small brown spots on the white varieties. These spots enlarge and produce a collapse of the entire flower which then may fall or may cling to the leaves or stems. Inside these diseased flowers, small, dark, flat or cupped structures of the fungus develop. These are the resting bodies of the fungus and serve to carry it through adverse conditions. They fall to the ground with the flowers and remain there until the next spring when they germinate to produce fruiting structures which produce spores. These spores are forcibly ejected and carried by the wind to where some of them will land on azalea flowers and initiate new infections. An interesting point about this fungus is that it is so constructed that weather conditions which favor the flowering of the plants likewise favor the germination of the resting bodies so that the infection will be insured. Once the flowers are infected, spores of another type are abundantly produced which, if carried by wind or splashing water to nearby plants, can infect them too. Control consists of avoiding overhead watering while the plants are in flower and spraying with zineb. The sprays should be put on three times a week during the rainy season because of the rapid flower expansion and succession of bloom. Removal and destruction of diseased blossoms are also helpful. Covering the beds where the disease has appeared previously with several inches of peat moss may prevent the production of spores from the resting structures.
In summary, it might be stated that on the whole, the leaf and flower diseases are not as important as the diseases resulting from soil problems or infection by soil organisms. Many of the problems resulting from soil troubles can be avoided by proper preparation so that good drainage, aeration and a suitable pH will result. This will not only favor plant growth but will have an adverse effect upon the disease-producing organisms. Regard for the pH of the soil is important, for although both rhododendrons and azaleas will do well at pH levels near neutrality providing all other conditions are favorable, they do better at pH levels on the acid side between 4.5 and 5 5.
The use of the new soil conditioners to improve the soil structure may be very effective if their purpose is kept in mind. They are merely compounds which help to aggregate the soil particles and improve the physical structure of the soil. They do not act as sources of nutrients, though they may help to make for better utilization of nutrients already in the soil.
The troubles discussed here do not include all of the diseases which might occur but instead include the more important ones. If you have troubles which do not fit any symptoms given or some which you can not diagnose, it is suggested that you contact the office of the Farm Advisor in your county for help.