PRELIMINARY STUDIE IN ASCOCHYTA PISI LIB.

Abstract

Foliage infection of pea by Ascochyta pisi Lib. is accomplished by direct penetration of the cuticle without the formation of an appressorium. Subsequent colonization is at first subcuticular, then intercellular, resulting in the collapse and death of the host cells. No haustoria are formed. Resistance may be due in part to the cuticular barrier as well as to a physiologic factor which limits the colonization of the host once penetration has been effected. Pycnidial formation is symphogenous. Spores are produced singly from conidiophores that grow out from the inner wall cells of the pycnidium. Each spore is two-celled, and each cell contains but one nucleus. A single nuclear division occurs early in the ontogeny of the spore so that most spores even prior to germination have binucleate cells. Spore production is marked by the almost complete depletion of the protoplasmic content of the fungal mass; as a consequence vegetative development of the mycelium ceases and the focus of infection becomes localized as a leaf spot. It is suggested that the production of this discrete type of lesion by A. pisi and certain other pycnidiaceous leaf-spotting fungi is, mainly, a function of the developmental rhythm of the fungus, and, accordingly, is not the expression of a specific host-parasite interaction.