VENTURIA INAEQUALIS (CKE.) WINT. I. A GROUNDWORK FOR GENETIC STUDIES

A BETTER understanding of the genetics of pathogenic microorganisms is urgently needed for the advancement of knowledge of infectious diseases and their control (cf. Stakman, 1940a). Pathogen and suscept are of co6rdinate importance in determining disease reaction. Each is variable under the influences of heredity and environment, and adequate understanding of variability is a limiting factor in researches on basic phenomena in the development and the control of infectious diseases. The most fruitful approach to the understanding of variability of organisms has been through genetic studies. For many years most investigators of genetic problems relating to plant diseases and their control have naturally tended to place major emphasis on experimentation with the suscept. The genetics of higher plants was better understood, and selection and breeding of the suscept offered the most direct approach to the urgent practical problem of obtaining resistant varieties. With recent advances in the field of genetics of microorganisms, however, needs for more attention to the pathogen are becoming increasingly apparent. Its variability must be adequately considered if conclusions regarding disease reaction, including disease resistance, are to be trustworthy. Furthermore, well selected pathogenic microorganisms are far better adapted than the higher plants and animals for many types of experimentation on basic phenomena of infectious diseases. In our present state of knowledge the fungi seem to offer the richest, and for many purposes the best adapted, material for experimental work on the genetics of pathogenic microorganisms. The phytopathogenic fungi offer exceptional advantages for such studies, because of their great number and diversity and on account of the nature and diversity of their suscepts (cf. Riker, 1939). Unusual opportunity exists, therefore, in the field of plant pathology, both for the selection of well adapted material and for economy of experimentation. Substantial progress has already been made in genetic studies of phytopathogenic fungi (Craigie, 1940; Rodenhiser, 1940; Christensen, 1940; Stakman, 1940b). Major attention thus far has been given to the rust and the smut organisms. These pathogens have many exceptionally favorable adaptations for such work. However, other fungi offer some important advantages that they lack. The investigations of Dodge and his co-workers (Shear and Dodge, 1927; Dodge, 1927, 1928, 1931, 1936a, 1936b, 1939, 1940; Wilcox, 1928), Lindegren and his co-workers (Lindegren, 1932a, 1932b, 1933, 1934a, 1934b, 1936; Lindegren and Lindegren, 1 Received for publication July 28, 1941. This work was supported in part by a grant from the Wisconsin Alumni Research Foundation. 1937), and others have greatly advanced knowledge of the genetics of a group of non-pathogenic Ascomycetes, and have drawn attention to some rare adaptations of certain types of Ascomycete for genetic study. The apple scab pathogen, Venturia inaequalis (Cke.) Wint. is an Ascomycete that seems well adapted for genetic studies in relation to fundamental pathological problems. Its parasitism has many features in common with that of the rust and the smut fungi. It is capable of infecting the normal, unwounded host and living for weeks or months in intimate association with living host tissues, from which it efficiently derives nutriment and in which it incites pathological processes (Nusbaum and Keitt, 1938). It comprises many biotypes with different pathogenic capabilities (Wiesmann, 1931; Palmiter, 1932, 1934; Schmidt, 1935, 1936a, 1936b, 1937; Kuthe, 1935). It has a stage of sexual reproduction, which provides a mechanism for combinations (Keitt and Palmiter, 1938; Keitt, Palmiter, and Langford, 1938). Each of the eight haploid nuclei derived through three successive divisions from the zygote nucleus of the ascus becomes the initial nucleus of an ascospore; and, as is shown herein, the entire haplQid progeny stemming from a single meiosis mav be obtained, with record of nuclear descent, by isolating the eight spores of the ascus in their serial order and studying the characters of the isolates. V. inaequalis offers an advantage lacked by most, if not all, of the rust and the smut fungi in that it is strictly haploid throughout the parasitic stage; therefore, it affords opportunity for study of the effects on parasitism of one set of genes without the complications that arise in diploid or dicaryotic phases of organisms (cf. Dodge, 1939). Furthermore, as is shown in the present paper, it is readily cultured and bred in vitro; and, since the vegetative cells are uninucleate (Backus and Keitt, 1940), it seems free from the possibility of heritable variation because of heterocaryosis. With some disadvantages, its suscepts offer the advantage of many diverse species or varieties that can be propagated vegetatively. The present paper reports studies designed to lay a groundwork for further genetic investigations of V. inaequalis in relation to basic pathological problems. It is concerned largely with development of methods and with studies of cultural behavior, sexual compatibility, and pathogenicity of four sets of monoascosporic isolates, each set comprising eight isolates from a single ascus with record of the serial order of the spores. This work is an outgrowth of previously reported studies of heterothallism and variability in V. inaequalis, in connection with which pertinent literature has been reviewed (Keitt and Palmiter, 1938). Brief preliminary reports have