This 1970 book was published as a replacement text to the Biology of Root-Infecting Fungi by the same author. The subject had advanced so rapidly since the previous book was published, that the creation of a new title was far more effective than the release of a second edition of the first one. The book is intended to be read through, rather than used solely for reference. Pathogenic root-infecting fungi are a substantial threat to the welfare of all crop plants, and are therefore of enormous economic importance. The subject matter is treated both from the viewpoint of its economic significance and from its biological characteristics; thus the approach is a broadly based one and in addition to the plant-pathological aspects the reader is made aware of the complex microcosmic world of the soil. This book will continue to be of great historical interest to all scholars of Pathogenic root-infecting fungi.