Topics for Discussion at Round Table on Rhododendron Diseases
Freeman A. Weiss, Annadale, Minn.
Plant Pathologist, U. S. D. A., retired
1. Ovulinia azaleae, flower blight
This is the first recorded serious flower disease of cultivated azaleas, observed in South Carolina in 1931. Its introduction from Japan was on rooted Kurume azaleas prior to 1920, and its first appearance on cultivated azaleas in the United States, is attributed to that source. It has been reported since then in the Atlantic and Gulf Coast states from N.Y. to Fla. and Tex., in Calif., and also in England; on cultivated rhododendrons also in Calif., Md., S.C.
On all known hosts its occurrence is limited to flowers alone, there being no evidence of infection in any other parts. The pathogen is first detected as a minute stalk having a terminal spore bearing cup or disc, appearing from soil in early spring, but attached to a resting stage (sclerotium) derived from a fallen flower of the previous year. Only a few thousand spores capable of infecting azalea flowers are produced in each cup, are discharged intermittently for several days, and are distributed by air currents. When they happen to land on an azalea flower each spore can produce an infection that quickly enlarges from a pinhead speck to a blotch, and finally to a general soft blight on which secondary spores are produced in numbers hundreds of times greater than the total spore population of the cup from which it emerged only days before. These are widely distributed by air currents, by spattering of rain, and flower-visiting insects. An azalea garden in full bloom can become almost a floral disaster within two weeks.
The first attempts to protect flowers from quick destruction were diverted by the lack then of fungicides that could be sprayed on flowers without risk of direct injury or leaving an unsightly residue. It was not until the production of organic sulfur fungicides of the dithiocarbamic acid series began, notably zinc ethylene bisdithiocarbamate (common name zineb) in the early 1940's, that any spray material was available that was adaptable to use on flowers and really effective against azalea flower blight. A liquid product known as Dithane D-14, (nabam) to which zinc sulfate is added before use, and its equivalents in wettable powder form, which need no additive, are still the most widely used spray materials to protect azaleas from flower blight. They seem not to have been really tested or proved to be effective as a ground spray, prior to azalea bloom to destroy the pathogen in its weakest link quantitatively, i.e. the spore stage that produces the initial flower infection each spring. The first direct attempt at controlling this stage was the introduction of pentachloronitrobenzene (PCNB), a fungicide or retardant known to be effective against other fungi that are dependent on soil-borne sclerotia as a primary infective agent, into the antibiotic fungicide, cycloheximide (known commercially as Acti-Dione RZ). This provided a new approach to a neglected phase of the problem of controlling azalea flower blight. This, and other combination sprays that include PCNB are now under investigation, and appropriate emphasis is being placed on direct control of the initial effective stage to reduce the burden and cost of frequent repetition of flower sprays.
The importance of determining whether flower blight infection is occurring, or has become established, among native azaleas and rhododendrons near garden plantings of the cultivated forms, to the extent of complicating local control, is emphasized.
2.Botrytis einerea, gray mold, flower blight
This has long been regarded as a cosmopolitan flower disease of azaleas and rhododendrons, both native and cultivated, as a sequel to frost damage or to prolonged cold and wet weather. It has also been reported as causing shoot tip blight, and blighting of seedlings and cuttings under outdoor conditions, following weather damage. Gray mold is generally considered as a cosmopolitan pest injuring different plants and parts under adverse environmental conditions. It is something of a different situation to find that it is now being regarded as a direct cause of flower blight of azaleas and rhododendrons, especially in the South (Ala., Fla., S.C.), in Md. and elsewhere in the North.
The symptoms differ from those typical of Ovulinia blight in these respects: there is no soft or limp blight which on squeezing will reduce the flower tissue to juice; the color is darker, usually a dull brown; the affected tissues are covered with a fuzzy gray or brown mold, with typical Botrytis spores visible by hand lens; if sclerotia appear at all in the corolla tissue they are usually globose in form, dull black in color, instead of flat or curved, and shiny, as in Ovulinia blight. It sometimes appears to be a virulent infection, not just mold following other injury. The sclerotia produce only mycelial growth when placed on culture media, no reported development of stalked spore cups (apothecia) and ascospores.
Possible prevention by flower spraying is uncertain. Flowers seemingly well protected against Ovulinia blight when sprayed with Dithane or Actidione RZ may be infected later by Botrytis. Protection is sometimes reported after spraying with Thylate (tetramethylthiuram disulphide) and which is also sometimes used as a protectant against Ovulinia blight: but few reports of definite comparison of this and other flower sprays under conditions that eliminate frost damage as a factor are as yet available.
In situations where local environmental conditions are a greater factor than basic host-parasite relations in determining which plant diseases are really significant, the problem of control becomes one of giving advantage to the host, not the parasite. Control of Botrytis blight of azalea and rhododendron flowers is a case in point. If sudden chills and dampness give advantage to the parasite the gardener should seek to improve the host's environment, not to find a spray that will demolish the enemy. Experienced gardeners know that on frosty nights topographic hollows get the lowest temperatures first, and are often poorly drained in both soil and air moisture. Botrytis as a natural enemy of garden plants would gain advantage; azaleas would lose. In this instance, preventive or corrective measures are more a problem for garden planners and keepers than of plant pathologists. Botrytis damage to these flowers is probably more a matter of environmental adjustment in favor of the host (except weather!) than it is of controlling the fungus.
3. Phomopsis sp., dieback
This form of decline and eventual loss of lateral or terminal shoots, sometimes extending into main branches or trunks to the root crown, is not a disease of recent origin as effecting azaleas and rhododendrons under cultivation. It may have increased in damage to plants in particular areas or in some years. It has recently been receiving more attention and serious study by plant pathologists who specialize on plants of ornamental interest. A thorough investigation of this subject is now in progress at Clemson University under the direction of Dr. Luther Baxter, Jr. and his student aid, Sharon Brown. At a recent meeting of the Southern Division of the American Phytopathological Society, Miss Brown presented a report on this study now in its second year. Some of the details under discussion here and the use of some illustrative material are available through the courtesy of these investigators, and part of the field work has been done at Magnolia Garden near Charleston, S.C. The most important detail presented here is that a fungus belonging to the form genus Phomopsis has been isolated from typically diseased specimens, grown in culture media under laboratory conditions, and its pathogenicity confirmed by experimental inoculations. Its connection with a perfect stage belonging to the genus Diaporthe has been indicated. Actual infection seemed only to follow some previous injury to the surface tissue. Heretofore reports from field collections of specimens of twig blight, dieback, sometimes canker have been recorded as occurring on both azaleas and rhododendrons, from sources as diverse as Mass. to Tex. and Calif., including native and cultivated plants, and usually assigned to Phomopsis, sometimes to Diaporthe.
The typical dieback of "Indian" azaleas as known at Magnolia Garden was described as "hardening of the arteries" many years ago because its most distinctive symptom, differing from "environmental" dieback (shading out, freeze injury, starvation), was the presence of brown streaks in the normally white or green wood of healthy plants. These streaks could be found in small shoots, larger branches, and main trunks, always associated with some external indication of declining health.
Experience in coping with this malady demonstrated the necessity of pruning out all tissues showing these streaks, clear to the root crown if necessary for complete elimination. Renewal main shoots from the ground level usually were free of streaks, and often continued growth to normal size and bloom, indicating that root trouble was not the cause of streaking, wilt, or dieback. Recognition of this has now become the standard pruning principle at Magnolia Gardens in combating dieback, whether on only an isolated azalea or in a group planting.