Diseases and Control for Rhododendrons, Azaleas and
Arthur I. Coyle, Houston, Texas
Much has already been written by competent authors regarding diseases and control of the same for most garden plants, including the Rhododendron family. Outstanding garden books such as Clement Gray Bower's "Rhododendrons and Azaleas," David G. Leach's "Rhododendrons of the World" and Frederic P. Lee's "The Azalea Book" are a "must" for any serious gardener who is interested in the genus rhododendron. Many excellent articles on rhododendron diseases and controls by competent authorities have been published from time to time by The American Rhododendron Society and The American Horticultural Society. Two other books of considerable merit, which bear mention, are "Soil, the 1957 Yearbook of Agriculture" and "Plant Diseases, the 1953 Yearbook of Agriculture" by the United States Department of Agriculture. Both of these U. S. D. A. publications contain many fine articles by the foremost authorities on horticulture. All of these books have been most helpful to us when growing rhododendrons and companion plants out of their natural environment.
We have found from personal observation that soil fertility and the types of fertilizer used do have a direct bearing on the cause and control of many plant diseases. In many cases this is also true for rhododendrons.
Root Rots and Wilt Diseases
The southern half of the United States is plagued with more types of root rot fungi than are normally found in the northern part of the country. The most serious of the pathogens which inhabit the soil and which may attack garden plants are, of course, the vascular wilts. Plants so affected seldom recover.
The most serious of all of the wilt diseases is from one of the so-called water mold, Phytophthora cinnamomi. This root rot fungus attacks many different species. The disease has been reported in California where thousands of avocado trees have become infected and either have died or will do so.
The "little leaf" disease of pine trees which is caused by the Phytophthora cinnamomi fungus has infected pine trees on several million acres in the southeastern part of the United States. It has been estimated that over 200 million board feet of pine trees are being killed each year. P. cinnamomi is most prevalent in wet and heavy soils. Soils which are loose and well drained seldom become infested with P. cinnamomi.
We know of only one of the broad leaf type rhododendrons which appears, from our observations, to actually be immune to P. cinnamomi. One of our large 'Nova Zembla' rhododendrons died of the wilt. We removed the dead 'Nova Zembla' and placed a 'Gomer Waterer' in the same hole and it has flourished ever since. Some rhododendrons are more resistant to P. cinnamomi than others. Still others appear to have absolutely no resistance to this disease.
Our approach to controlling P. cinnamomi is to use raised beds with the bottom of the root ball placed on gravel at least six inches above the regular grade of the surrounding area. The planting beds contain a 50/50 mixture of Perlite and European peat moss, and this mixture is pushed up around the root ball. A treatment of the rhododendron roots with Dexon monthly, according to directions, during the warm months is effective. If Pine sawdust is used along with peat moss, the sawdust may be infected with P. cinnamomi and it should be thoroughly treated with Dexon prior to planting. Azaleas are not usually affected by this fungus. Rooted azalea cuttings may become infected, so it is well to treat small nursery azaleas with Dexon.
The Rhizoctonia solani fungus is not the vascular wilt type but its action is to attack the root ends or the bark at the ground level. R. solani attacks both small and large rhododendrons and may be very serious when attacking small plants. The better the planting medium the more serious this disease can be. Fortunately, Rhizoctonia solani can be readily eliminated from the soil by treatment with Terraclor Super-X at springtime and in the fall of the year.
The Pythium fungi which causes damp off diseases can be very serious for azalea and rhododendron seedlings. This fungus is present in nearly all moist soils but may be controlled with Terraclor Super-X.
Azaleas are known to have been attacked by Verticillium wilt. Little can be done to combat this pathogen other than to feed the plants with low nitrogen fertilizer such as a 4-10-8 formulation. Drenching the soil with four teaspoons of Lindane per gallon to kill off all soil or mulch insects is helpful. The leaves of a plant infected with the Verticillium wilt turn yellow and usually fall from the lower branches first. We are not aware that this pathogen attacks the broad leaf rhododendrons.
Leaf Spots and Stem Blights
The chemical treatment for leaf spots and stem blights has been covered so well by David Leach in his book "Rhododendrons of the World" and other competent writers that we have little to add.
We use Ferbam and Zineb to control all leaf spots and blights. These two chemical fungicides do an excellent job on leaf spots. The blights such as the Rhododendron Die-Back Blight (Phytophthora cactorum) are hard to handle. Most of the trouble encountered in growing rhododendrons in a hot, humid area such as the Gulf Coast, is from P. cactorum. Some of the rhododendron varieties which we have experimented with die from blight in a very short time, even though the plants are well covered at all times with Ferbam or Zineb or a 50/50 mixture of both. P. cactorum is most active on the Gulf Coast while it is an unknown factor at Oklahoma City. Improving the nutrient content in the rooting medium is very necessary if P. cactorum is to be controlled for many rhododendron varieties. Many of the blights which strike rhododendrons, azaleas and their companion plants are the result of poor or improperly balanced minerals in the soil. The leaf spots on rhododendrons are the result, in many cases, of unhappy roots or too much shade. Changing the planting location may be very helpful toward eliminating leaf spots or blight troubles.
A shortage of any of the chemical elements necessary for growth of plants may cause diseases in the plants.
Nutrition and its Effects in Plant Disease
Excess nitrogen causes thin cell walls and ready entry of pathogens into the plant. A deficiency of nitrogen causes slow root growth. The ratio of nitrogen to potassium in the fertilizer should not be over a 1 to 2 ratio for young plants growing in the South, and never over a 1 to 1 ratio for rhododendrons with several years' growth. We add excess nitrogen only when the leaves tend to turn slightly yellow. Never add nitrogen alone to infertile soil. To do so is just asking for an invasion by the Pythium and Phytophthora cinnamomi fungi into the plant's root system.
We have used high nitrogen liquid fertilizer for forcing young azaleas into fast growth in heavily Dexon treated soils, which did protect the roots from the root rot fungi, and were then plagued with an invasion by the Azalea leaf scorch fungus, Septoria azaleae. This fungus appears to actually feed on nitrogen which has been sprayed on the azalea leaves. Heavy and frequent applications of Zineb and Ferbam were necessary to prevent complete defoliation.
Liberal application of phosphorus prevents many seedling diseases by causing fast root development. The ratio of nitrogen to phosphorus is important. A deficiency of phosphorus causes dark purple leaves on azaleas which can be corrected by adding superphosphate.
Many plant diseases have been prevented by the correct use of potassium. A deficiency of potassium causes increase of carbohydrate and inorganic nitrogen within the plant. Potassium has been reported to increase plant resistance to rust. Some have postulated that potassium is necessary for catalyzing cell activity. Potassium causes thicker cell walls which resist fungi and causes plants to be better able to resist long seasons of dry weather. Light sandy soils tend to be readily leached of potassium. The addition of organic matter to the soil helps retain fertilizers in the root area. We use potassium sulfate for replacing any deficiency.
A shortage of calcium may cause die-back or death of plants. Too much lime raises the pH of the soil and can cause death of the plant. We use dolomite or gypsum for correcting calcium deficiency. Shade trees are very susceptible to calcium deficiency. Calcium is reported to control plant diseases by neutralizing the toxins produced by the vascular wilt type fungi.
A deficiency of magnesium can be noted in azaleas by the lower leaves which turn purplish red with yellow areas and then fall off. We spray plants with magnesium sulfate to correct any deficiency.
Organic matter should be added to sandy soils in order to retain boron which is readily leached from light soils. A deficiency of boron causes thin cell walls which makes it easy for soil pathogens to enter into the plant. It has been reported that there are fifteen different functions of boron in a plant. The boron requirements are small, not over 20 parts per million. Excessive amount of boron will kill the plant. Some potash fertilizers contain too much boron.
Acid soils tend to leach copper. Some peat soils are deficient in copper. A deficiency causes die-back of the limbs and discoloration of the leaves. We have noted that one of our 'Blue Peter' plants is affected by copper deficiency in European peat moss. This was corrected by spraying the plant and planting medium with a Bordeaux mixture.
Deficiencies of zinc occur along the Gulf Coast and elsewhere. Too much phosphate may cause a shortage of zinc because insoluble zinc phosphate is formed. We have noted a rosette of leaves an 'White Pearl' when growing in European peat moss. Zinc sulfate spray on leaves and planting medium corrected this problem. For more information see U. S. D. A. books "Soil" and "Diseases."
This is so well covered in David Leach's book "Rhododendrons of the World" that we have nothing to add. All of the other elements necessary for plant growth such as molybdenum, sulfur, silicon, etc. are fully described in the above mentioned two U. S. D. A. books on "Soil" and "Plant Diseases."