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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 32, Number 4
Fall 1978

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Mycorrhizal Fungus Strikes Rhodies!
Ted Van Veen, Portland, Oregon
Reprinted from the Portland Chapter Newsletter

        Drought, freeze, aphids, root weevil, Phytophthora. And now I learn my rhododendrons are coming down with an obscure case of Endotrophic arbuscular-vesicular mycorrhiza. What next! (I read on). Soil-borne spores form on hyphae connected to the Endophytic mycelial system and penetrate host root-cells. (Doesn't sound good.) After initial infection by germ spore tube (sounds worse) a network of fungal hyphae ramify the rhododendron's root cortex. Heaven forbid!
        Then symbiosis sets in. I know about silicosis and halitosis, but what in tarnation is symbiosis? Quickly I flip the pages of my horticultural glossary. Well now that may not be too serious. In fact, it might not be bad at all. "A mutually beneficial association of two different kinds of organisms". Isn't that interesting.
        It seems the fungus causing those arbuscles, or pockets in the root system might be doing our rhododendron some good after all. Even though removing carbohydrates for survival, the fungus is kind enough to assist our plant in absorbing nutrients more efficiently. This is particularly true of phosphorus and certain trace elements, which move slowly in the soil. Just think, an infection of fungi performs this wonderful process.
        Fertilizer requirements can be lessened by this mycorrhizal relationship. But when artificial soil mixes are used, or when we sterilize our growing media by fumigation, mycorrhizae must be reintroduced either directly, or more slowly by nature. Colonization of the spores in the laboratory often is quite difficult, but nature takes care of this through the millions of spores in each mushroom, the fruiting structure for additional mycorrhizae. Experts can identify the mycorrhizae species in the soil by the kind of mushrooms growing there.
        One of Dr. Roy Watling's Mycological Taxonomist at the Royal Botanic Garden in Edinburgh, Scotland, primary interests is the study of mushrooms found in Rhododendron soils.
        A healthy mycorrhizal association is an energy saver because it reduces fertilizer production. Of greater interest today, would be the reduced water usage. Healthier plants require less fertilizer and thereby need less water to assimilate the nutrients.
        Like "Rhododendron" that word "mycorrhiza" is not so mysterious when we understand its makeup. "Myco" means fungus, and "rhiza" means root. This root fungus appears to be indispensable for the survival and well-being of the host plant. The beneficial organism also is a fighter. Diseases, such as Phytophthora root rot, don't dare come around when this fellow is present. He is an excellent antibiotic.
        It is surprising to me, how little is known about the mycorrhiza/plant relationship. Apparently there are thousands of different species of this minute fungi, and to complicate things a bit, different plants have their preferences - some delight in more than one species association.
        Some mycorrhizal work is being done at the U. S. D. A. laboratories in Corvallis by Dr. Robert Linderman. The U.S. Forest Service and the timber industry have completed more of these studies than anyone. Knowledge of mycorrhizae is partially responsible for the much faster tree growth in our forests today.
        Last year the American Rhododendron Society established a Research Foundation for which funds are being solicited, to create an endowment to pay for research of this type. At the national meeting of the A. R. S. Board of Directors in Portland, February 20, a $500 research grant was approved for rhododendron mycorrhizal study in New England. This is a small beginning, but definitely a step in the right direction. As monies become available they will be used to further our understanding of the mycorrhizal influence on our rhododendrons. Healthier and more disease-resistant rhododendrons will be the reward for all of us. I feel better already.


Volume 32, Number 4
Fall 1978

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