The Diagnosis of Plant Diseases of Complex Etiology

This review examines the thesis that the phenomenon of "one cause-one disease" is rare in nature, and that procedures, therefore, are needed that clarify situations where each of several factors contributes substantially to a disease. Diseases that can be readily and correctly diagnosed as the result of a specific cause are seen as examples where one factor predominates over other causal factors. Such factors are contained within the plant's environ­ ment, i.e. all those factors that influence the plant's ability to grow, re­ produce, and survive. Thus, water, nutrients, insects, and weeds as well as fungi, bacteria, nematodes, and viruses may be involved in the etiology of a disease. The problem is to determine the relative importance of the various factors in their contribution to disease, and their relationships to each other. The solution to such a problem is both a diagnosis and an hypothesis requiring controlled experiments to test its usefulness. The product may be a means of controlling disease. There are numerous examples in the literature where difficulty in diagno­ sis has been ascribed to complex etiology (5, 15,47, 60, 81, 87, 112, 138). Furthermore, it is evident from titles in plant pathology journals that an increasing amount of research is being done on the influence of two or more pathogens on the same plant and on the relationships between pathogens, plants, and their environments. Thus, there appears to be an awareness of the complex nature of disease etiology. Some have emphasized the rarity of the single pathogen-host relationship (106) and pointed out that disease complexes may be far more important than has previously been recognized