The Relationship of Genes for Pathogenicity and Certain Other Characters In

EiVTURIA inaequalis (Cke.) Wint., the ascomycetous pathogen of apple scab, Yhas been shown to have adaptations similar to the eight-spored Neurospora species for use in genetic analyses (KEITT and LANGFORD 1941; SHAY and KEITT 1945; KEITT et al. 1948; BOONE and KEITT 1956). I t has, in addition, two very important advantages over Neurospora: 1) I t is pathogenic and can be studied in vivo; and 2) it is uninucleate in its vegetative stage and, thus, is free from the confounding effects of heterocaryosis. Pathogenicity in V . iizaequalis is variable and each instance of virulence that has been studied adequately has been shown to be controlled primarily by a single gene (KEITT 1952; SHAY and HOUGH 1952; SHAY and WILLIAMS 1956). Many physiological races could be recognized on the basis of pathogenicity on apple varieties, but, so far, there has been no advantage in naming more than three (SHAY and WILLIAMS 1956). The purpose of this paper is to present the relationships of the pathogenicity genes differentiating the three recorded races of the organism. A preliminary account has been published (SHAY and WILLIAMS 1953).