Research on Chrysomyxa Rusts Attacking Rhododendrons
Charles J. Gould, Plant Pathologist
Western Washington Research and Extension Center
Washington State University, Puyallup, Washington
I wish to thank Dr. Maksis Eglitis, Mr. Worth Vassey and Mr. Wm. Scheer who have assisted me in this research, and the American Rhododendron Society and many rhododendron growers who have generously donated plants and seeds.
Paper presented at the Annual Meeting of The American Rhododendron Society, Tacoma, Wash., May 14, 1966.
Our interest in this group of rhododendron diseases started with the discovery of the European rhododendron rust (Chrysomyxa ledi v. rhododendri) in Washington in 1954. This was the first record of the rust in the United States. This fungus was later discovered in California, Oregon, and British Columbia, with evidence that it may have been present as early as 1947 in one of the other states.
In an effort to control this disease, a series of fungicidal tests were made, but infection was so sporadic that the tests were inconclusive. A special grant was subsequently obtained from the Medical and Biological Fund (Initiative 171) of Washington State University for research on the life histories and host ranges of rusts attacking rhododendrons. Results of some of the 365 experiments are discussed here. More details will be available in three technical articles being prepared for publication.
At least four rusts can attack rhododendrons in Washington. One Chrysomyxa piperiana, is very common on the native Rhododendron macrophyllum but occasionally attacks a few other species and varieties. The European rust attacks many cultivated rhododendron varieties and native species of Ledum but not Rhododendron macrophyllum. Two rusts that normally occur on Ledum spp. may also attack rhododendrons. They are: Chrysomyxa ledi var. glandulosi, most often found on Ledum glandulosum var. glandulosum in the mountains, and C. ledicola which usually attacks both Ledum groenlandicum and L. glandulosum var. columbianum, hosts occurring at relatively low altitudes. In addition to the above rusts, we have collected some samples with intermediate-sized spores. These may be different races or species than the above. The European rust is particularly interesting because it normally occurs on rhododendrons but cannot infect R. macrophyllum, yet does infect members of another genus Ledum.
Most of these rusts are widespread. However, the European rust is mostly confined to areas immediately adjacent to the Pacific Ocean. The European rust has been found in a few inland areas of western Washington but only on very susceptible rhododendron varieties growing in moist locations with very poor air drainage.
Fig.66. Chrysomyxa rust spots on a leaf of R. occidentale.
Life Cycle of Chrysomyxa Rusts
The typical life cycle of the native rusts is as follows: uredospores are the most common spores usually seen. These spores may develop at any time of the year under mild moist conditions and can re-infect other leaves or other plants of susceptible rhododendrons and ledums. A teliospore stage develops on infected plants under certain conditions in the spring. The teli ospores produce basidiospores, which infect spruce. On spruce they form plus or minus pycnia, which, if cross fertilized, produce aeciospores. One of our principal concerns is that hybridization between different rusts may possibly occur at the pycnial stage, giving rise to more virulent strains. The aeciospores infect rhododendrons or ledums. The only spore that can re-infect the host on which it develops is the uredospore. Since the rust can survive in leaves of evergreen rhododendrons and ledums, the alternate host (spruce) is not essential for its survival. We have found all five stages under natural conditions and also have produced most of them experimentally in the greenhouse or growth chambers for the native rusts, but not for the European rust. Apparently the latter rust requires much colder temperatures for development of telia than occur naturally or than we, have provided artificially.
Spores—Their Germination, Longevity and Infection
The uredospores of all these rusts begin to germinate within two or three hours as soon as they get favorable conditions. Germination is best at mild temperatures (59-68° F), a high relative humidity and weak light (50-75 foot candles). A few spores may germinate at temperatures near freezing or as high as 90-95° F. With one exception, uredospores of the different rusts germinate easily in the laboratory on artificial media. However, uredospores of the native rhododendron rust (C. piperiana) require high rates of chelating compounds, such as citric acid, cysteine or EDTA for optimum germination in the laboratory. Aeciospores of this species also need such compounds for good germination.
Succulent young leaves of rhododendrons and ledums are most susceptible to infection, although very young leaves are somewhat resistant. Even old leaves may be infected under optimum conditions. Two to three months are required after infection for new uredial pustules of C. piperiana to develop, but only three to four weeks for the other rust species.
Uredospores can survive only a few days at warm temperatures and at high relative humidity. They may live for several weeks under dry conditions near freezing and for over a year if stored dry at -104° F.
Basidiospores can infect only very young needles of spruce. They cannot stand drying or high temperatures. Observations indicate that basidiospores ordinarily do not infect spruce located more than a few hundred yards from diseased rhododendrons or ledums.
Susceptibility of Spruces and Rhododendrons
The European (Norway) and Asiatic spruces tested were either highly resistant or immune to our native rusts. The most susceptible American spruces were: Sitka, Colorado Blue and the albertiana variety of white spruce.
Our studies of susceptibility of Rhododendron species and cultivars are just getting underway. Most cultivars appear to be resistant or immune to the native rhododendron rust. However, several cultivars and species are highly susceptible to the European rust, including 'Racil', 'Pink Drift', 'Bibiani', 'Romany Chai', 'Lady Roseberry' and R. occidentale. Reports indicate that there may be different clones of certain group varieties which vary in their resistance to the European rust.
These fundamental studies were made in order to determine the factors necessary for infection and subsequent disease development. Studies on disease control are planned for the near future. However, some of the information already obtained may be of help in preventing or reducing infection.
When rust becomes a serious problem, very susceptible rhododendron varieties should be discarded to facilitate disease control. If it is essential that such susceptible varieties be retained for any reason, they should be planted as far as possible from other varieties and separated by a barrier (windbreak, etc.). Removing and destroying all diseased leaves will help reduce spread of rust spores.
Susceptible varieties should be planted in an open and well-aerated location. Ledums should not be grown near rhododendrons. Avoid planting spruces nearby or, if they are an essential part of the landscaping plan, at least avoid the highly susceptible types mentioned above and substitute either European or Asiatic spruces.
Some commercial growers have used Acti-dione (cycloheximide) sprays with partial success. The rates used are usually 2 to 10 ppm. However, rhododendron varieties vary considerably in their susceptibility to injury, some being injured by 2 ppm and others tolerating rates as high as 10 ppm. The blossoms and young leaves are most susceptible to spray damage.
We still have to clarify a few aspects of the life histories of these rust fungi. As soon as the major points are better understood, control measures will be studied. These will involve: (1) testing the effectiveness of fungicides and (2) testing rhododendron species and varieties for resistance in order to: (a) recommend resistant rhododendrons to home owners for planting and (b) provide plant breeders with information on sources of resistance so the latter can be incorporated into new varieties.
Incidentally, the first tests on resistance will be with the rust fungi but later we also expect to test species of rhododendrons for resistance to other serious pathogens. About 250 species of rhododendrons have been collected for this purpose. However, we are still lacking certain species which have been used frequently in making crosses. They are listed below. If anyone knows of a source of cuttings, small plants, or selfed seed, we would appreciate the information.
Rhododendron Species Needed For Research (* needed most) aperantum facetum maddenii bullatum *forrestii meddianum boothii formosum megeratum *campylocarpum *griffithianum *neriiflorum catacosmum haemaleum sperabile caucasicum haematodes sulfureum chaetomallum hodgsonii valentinianum chrysodoron hookeri veitchianum ciliicalyx indicum venator crassum keysii virgatum dalhousiae kyawii wightii delavayi *lacteum xanthostephanum didymum lindleyi zeylanicum diphrocalyx litiense And one species from each of the following Series: Camelliiflorum,
Lepidotum, Vaccinioides, and Ovatum.