Fungal Parasitism of Woody Plant Roots from Mycorrhizal Relationships to Plant Disease

Descriptive and developmental studies of both pathological and mycorrhizal root-infecting fungi involve detailed knowledge of the growth and function of root systems in the natural environment. The results of extremely virulent pathogens or of extensive ectomycorrhizal development are immediately ob­ vious even to the untrained eye. The effects of casual obligate biotrophs or of weak endomycorrhizal infections may escape observation even by the special­ ist. The variable form and extensive branching of root systems have often discouraged investigation of the morphogenetic effects of root-inhabiting fun­ gi. Pathologists have concentrated attention on the anatomical aspects of penetration and host response or on dramatic growth abnormalities, while the mycorrhizal research workers have concentrated on characteristics of mycor­ rhizal structures or quantitative assessments of the degree of infection. Investi­ gators in both disciplines have largely ignored the effects on root growth and branching or on alterations of normal root anatomy. Biotrophs that occupy the mid-portion of the continuum of fungus/root association from mutualism to pathosism have often been ignored by both patRologists and mycorrhizal workers. Such parasites may cause no detectable disturbance of host physiolo­ gy and so are neglected by the pathologist; and because they confer no physiological benefits and form no organized structures, they are similarly ignored by mycorrhizal workers. Harley (31) has discouraged the consideration