AN INVESTIGATION OF NUCLEAR NUMBER IN ALTERNARIA SOLANI

Abstract

VARIATIONS IN isolates of Alternaria solani (Ell. and Mart.) Jones and Grout have been reported by many workers. Variations in pathogenicity of isolates of this fungus have been a serious handicap in screening for early blight resistances in existing lines of tomatoes, as well as in the development of new early blight resistant varieties. More intelligent work could be done in breeding for resistance if the pathogenic variation of the organism were understood. Heterocaryosis has been suggested as a mechanism of variation of A. solani. Neergaard (1943) reported that one isolate segregated into conidial, mycelial, and intermediate types. Brock (1950) and Henning and Alexander (unpublished) separated pathogenically different strains from isolates derived from single conidia. Stall (1954) reported that pathogenic differences existed among isolates from different germ tube tips from the same conidium. A cytological investigation of A. solani was undertaken to obtain additional information regarding the variation which occurs in this pathogen. Such an investigation aids in appraising the genetic composition of cultures derived from any particular vegetative structure, and could aid in finding techniques to minimize variations in culture. MATERIALS AND METHODS.-In this study, a sporulating isolate of A. solani was used. This isolate resembled wild type isolates in all apparent respects except that it sporulated without special cultural treatments. A modification of the cellophane-square method described by Hrushovetz (1956) was employed to stain conidia at various stages of development. Cellophane was cut itno 1 cm. X 2 cm. sections and sterilized in an approximate 3 per cent chlorine solution for 15 min. The sterilized cellophane sections were placed aseptically on Difco PDA in Petri dishes. Mycelia from mass transfers grew over the cellophane sections. When sporulation occurred on the cellophane, the sections were fixed and stained. Since nuclei could not be observed in pigmented cells in preliminary cytological work, a method was devised to bleach the pigment in cell walls. In developing a satisfactory method, water solutions of many chemicals were screened for pigment-bleach-