It was shown in the work here reviewed that Venturia inaequalis (Cke.) Wint., the Ascomycete that incites apple scab, has advantages similar to those of the eight-spored Neurosporas for related genetic and physiologic studies, and therefore affords exceptionally favorable material for investigation of basic problems of infectious disease such as the inheritance of pathogenicity, the origin and stability or plasticity of biotypes or races with different pathogenic capabilities, and the nature of the disease reaction between pathogen and suscept. Some specific advantages shown for this fungus for such studies are: (A) It freely infects young, vigorous, unwounded tissues and may live for many weeks in intimate association with living host cells. (B) It has many biotypes with different pathogenic capabilities to different host species or varieties. (C) It can be cultured throughout its life cycle and cross-bred at will in vitro. (D) It can be maintained for many years in vitro with satisfactory stability of t...