DIAGNOSIS of FOREST DISEASES DR. GEORGE H. HEFTING (L
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DIAGNOSIS OF FOREST DISEASES BY DR. GEORGE H. HEFTING (l) Diagnosis is well defined by Webster as "the art of recognizing a disease from its symptoms." In animals many diseases can be identified and their course predicted (prognosis) without knowing the cause. Such is the case with cancer, gout, mononucleosis, and many other familiar diseases for which the cause is not known, In the case of plant diseases, none of which are psychosomatic, diagnosis virtually requires determining the cause, Having told you what diagnosis is, I would like to tell you--with respect to forest pathology--one thing that it isn't. It is not, for most of us, a field to which the brave slogan, "Do it yourself" applies. If today's diagnosing physician had to trade places with a forest pathologist here are some of the things that he would discover: 1. Instead of one host species, Homo sapiens, he has to deal with 150 or more species. 2. The patient cannot say where, when or how it hurts. 3. He has no sophisticated techniques such as x-ray, electrocardiogram, blood sugar, and urine analysis, etc., at his disposal. 4. He is expected to cope with everything from awart to a cataract to leukemia. 5, He has over a thousand potential pathogens to consider among the causes of plant disease. Now you see where we are and why we need all the joint effort we can get from each other. Now we will look at how we go about diagnosing plant diseases. The sequence of steps is usually as follows: 1. What is the host plant? Genus and species? Exotic or native'/ In its normal environment or not? 2. 1-Jhat is the host organ involved? Do the symptoms indicate disease of the organs you have in the specimen at hand, or that the probably injury was to organs not represented·/ For example, all you see may be a dying of foliage, but the cause might well be a root rot which you do not see. 3. What are the symptoms? Use visual, hand lens, dissection microscope, sections, and examination by compound scope. Do they suggest a pathogen or a non-biotic influence such as cold, drought, or air pollution. (1) Principal Research Scientist, Southeastern Forest Experiment Station, Asheville, North Carolina. - 2 - 4. Examine for signs (structures of a pathogen) such as fungus fruiting bodies, felts of mycelium, bacterial ooze, etc. 5. what are the diseases of this host? (See USDA Plant Disease Index, Hepting's Diagnosis of Diseases, Boyce, Westcott, and other books.) Do your symptoms and signs tally with those of known diseases well enough to stop at this point in diagnosis? If not, then: 1. Culture from plant tissue (from margins of lesions, from vascular dis colorations, from fruiting-bodies.) 2. Incubate the specimen or the culture to obtain well-developed growth and fruiting of fungi, or to obtain bacterial ooze. Don't incubate longer than a few days in the laboratory because of molding. One can incubate on water and straw agar outside for weeks, and sometimes it takes outdoor conditions of diurnal variation in light, temperature, and humidity to bring a fungus to normal fruiting. Is a primary pathogen involved at all? Leaf pathogen, vascular fungus, canker fungus, etc. If there is no evidence of a pathogen you may have an abiotic disturbance or the pathogen may not occur in the plant parts avail able to you. If a potential pathogen is uncovered, can you identify it, using the literature and making herbarium comparisons? Here is where you stop doing it yourself. You may think you have identified a Nectria sp., or a known rust, or a pathogenic Fusarium, or a needle blight fungus. That's fine, but now, unless you are very sure of your identification, send your specimen or culture for confirmation to whomever has expertise in the taxonomy of this group. There are several helpful tests and smaller general publication on tree diseases that the practitioner should have at hand. The main one are: Anonymous. 1960. Index of plant diseases in the United States. U. s. Dept. Agri., Agriculture Handbook 165. 531 p. Baxter, Dow Vawter. 1943. Pathology in forest practice. 618 P• New York: John Wiley and Sons, Inc. Boyce, John Shaw. 1961. Forest Pathology. Ed. 3, 572p. New York: McGraw-Hill Book, Inc. Hepting, George H. 1964. Diagnosis of disease in American forest and shade trees. Part I. Diseases of coniferous species. 378 p. Asheville, N. C.: U.S. Dept. of Agri., Forest Service. ( Hepting, George H. 1966. Diagnosis of disease in American forest and shade trees. Part II. Diseases of non-coniferous species. 371 p. Asheville, N. C.: U.S. Dept. of Agri.,Forest Service. - 3 - r Peace, T. R. 1962. Pathology of trees and shrubs. 753p. Londcn: Oxford Univ. Press. Pirone, P. P. 1948. Maintenance of shade and ornamental trees. Ed. 2, 436 P• New York: Oxford Univ. Press. westcott, Cynthia. 1960. Plant disease handbook. Ed. 2, 825 p. New York: D. Van Nostrand Go. Other smaller publications are the bulletins by \Jaterman and Marshall on shade tree diseases, by Renting and Fowler on eastern forest diseases, and the Forest Pest Leaflet series of the U.S. Forest Service. Anyone doing much diagnostic work also should be aware of the monographs on certain genera. These include Booth on Nectria, Lohman and Watson on Nectria, Arthur and Cummins on rusts, Fun1c on Galiciopsis, Guba on Pestalotia and Monochaetia, Groves on Dermea, Groves on Tympanis, Darker on the Hypoderma taceae or needle cast fungi, Hahn on PeaysoyphA, Hunt on QP-ratooystis tn cluding most wilts and blue stain fungi, Waterhouse on Phytophthora, Middleton on Pythium, Lohman and Cash on Atropellis, Miller on Hypoxylon, Chupp on Gercospora, and Trappe on mycorrhizal fungi. Other useful publication are Christie's text on nematodes, Dowson 1s on bacteria, and Gill's monograph of the dwarfmistletoes. Examples of leading laboratories for the identification of certain organisms are Berkeley, California, for Fusarium spp.: Riverside Galifornia,for Phytophthora; Purdue for rusts; Syracuse for rot fungi; Delaware, Ohio, for elm fungi; Wisconsin for bacteria; N. G. State and Cornell for nematodes; Quebec for Nectria; and Beltsville for fungi in general and for wood rot fungi in culture. The Forest Service has experts on many important organisms, and we should make full use of men such as w. A. Campbell on Pythiums, Ruehle on nematodes, Toole on hardwood fungi, Hodges on cedar and nursery fungi, Powers and Gzabator on Southern tree rusts, and Verrall on products fungi. Finally, try to see the area of forest involved, especially if information is needed on the soil, the air, associated hosts, etc. In this way you can also see examples of the whole sick plant, or many sick plants. A warning I can't overstress is to watch for false leads, and guilt-by association, especially in dealing with secondary needle and twig fungi. A bad diagnosis can lead to a lot of trouble and expense. It can lead to the wrong treatment, to loss of valuable trees, litigation, descrediting of the pathologist, and other unpleasant situations. If the case is important it is better to say you don't know the cause than to guess at it. New techniques such as the use of selective media and living trap plants (e.g. apple and avocado for Phytophthora cinnamomi,) and such as new types of mi croscopy including phase, dark-field, incident light, x-ray diffraction and electron, mean that everyone can't have the knowledge and equipment to deal with every problem. The answer? Do your best; don't ge ashamed to ask for help, and seek confirmation if there is any question of correctness. REDUCING LOSSES CAUSED BY FOREST DISEASE.'! BY DR. JOHN s. BOYCE, JR. (1) A conventional approach to the subject of reducing losses cause by forest diseases is to describe methods for controlling each major disease. I am not going to do this today. Instead, I invite you to consider some other ideas. We can begin by observing that reduction of disease losses is a nacessary part of forest management even though it may be overlooked in practice. The peculiar nature of the forest and its diseases dictates that disease control be accomplished through silviculture where it is possible. Direct control treatments are properly regarded as emergency measures because of their high cost. Occasionally there may be outbreaks where promptly taken direct steps may result eventually in much lower damage losses. Hopefully, this is the case with oak wilt in some of the so-called 0 fringe areas" in the East. It is worth stressing again and again that the annual increment of the managed forest has a relatively low value. This makes it important to consider carefully the adoption of any control measure that is not part of silvicultural practice. It is fortunate that most diseases do not increase rapidly to damaging epidemic proportions as is frequently the case with insect outbreaks. Usually there is sufficient time to delineate the disease problem and then to deal with it deliberate ly as part of silviculture. What I have said so far has been so obvious that I have almost hesitated to say it. Yet I have said it because I do not think that the reduction of disease losses through silviculture is being given the emphasis in practice that it deserves. The impact of most of our major forest diseases can probably be lessened by adjust .. ing silviculture to take into account their characteristics. In this connection let us look briefly at an important southern disease, fusiform rust.