Inheritance of Pathogenicity in Venturia inaequalis (Cke.) Wint.

Abstract

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 the characters studied. (E) It is haploid throughout the parasitic phase and in its vegetative stage in vitro, thus permitting study of effects of a single set of genes. (F) The ascus contains the complete progeny stemming from a single meiosis, with survival of all nuclei in an orderly arrangement that permits tracing the line of nuclear descent by isolating the ascospores in their serial order and studying the derived cultures. (G) The vegetative cells are uninucleate and free from important problems of heterocaryosis. (H) The fungus responds well to mutagenic agents. (I) Materials and methods have been developed for correlated studies on its genetics, nutrition, and pathogenicity. A limitation on the use of this material in many places is the necessity for adequate greenhouse facilities with suitable control of temperature and humidity. Only two main types of pathogenic reaction were encountered with monosporic wild-type lines of the pathogen, "lesion" and "fleck." The lesion reaction was typically pathogenic with abundant sporulation of the fungus, the fleck non- or slightly pathogenic with sporulation lacking or sparse. With a given apple variety the fungus lines were referred to as lesion or fleck according to the reaction incited. In lesion x fleck crosses pathogenicity to a given variety ordinarily was conditioned by a single lesion/fleck gene pair, which segregated alternatively in the first or the second nuclear division in the ascus. Each lesion/fleck gene pair studied conditioned pathogenicity to one group of apple varieties and not to another group. Six such gene pairs were found and occurrence of many more was hypothesized. Two of these gene pairs were shown separately to control pathogenicity to one apple variety; the fleck gene of each pair was epi` static to the lesion gene of the other. The working hypothesis was advanced that critical single-gene-controlled biochemical reactions condition pathogenicity of V. inaequalis. The probability was recognized that studies involving a greater range of isolates and varieties would reveal cases of more complex inheritance of pathogenicity. The importance of guarding against errors in interpretation of the factorial basis for inheritance of pathogenicity on account of modifying factors in the parental lines or of inadequate control of experimental materials or conditions was shown. It was concluded that the species V. inaequalis comprises a great number of biotypes with differential pathogenicity to various apple varieties and Malus species and that a major source of such biotypes in nature is genetic combination. Cultures of the fungus were carried in vitro for some 15 years without evidence of loss or gain in pathogenicity. The Wisconsin research program has been broadened to include correlated studies on the genetics, nutrition, and pathogenicity of V. inaequalis, some early results of which have been reviewed elsewhere (Keitt, 1952b).