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Etiology

Perithecial Development and Nuclear Behavior in Pyricularia. H. Yaegashi, Department of Plant Pathology, North Carolina State University, Raleigh 27607, Present address of senior author: The Tohoku National Agricultural Experiment Station, Omagari 014-01, Japan; T. T. Hebert, Department of Plant Pathology, North Carolina State University, Raleigh 27607. Phytopathology 66:122-126. Accepted for publication 16 August 1975. DOI: 10.1094/Phyto-66-122.

In fertile matings of isolates of Pyricularia grisea, perithecia originate as small masses of thicker-than-normal hyphae wound together. Cells of these hyphae are uninucleate in early stages. Binucleate ascogenous hyphae appear later and give rise to asci by crozier formation. Nuclear fusion in the young ascus is followed by three nuclear divisions, resulting in eight nuclei around which ascospores are delimited. Two additional nuclear divisions occur in the ascospores, and septa are formed between daughter nuclei following each division. Mature ascospores have four uninucleate cells. Most cells of the mycelium, conidia, and conidiophores are uninucleate, although more than one nucleus was occasionally seen in cells of old hyphae. The single nucleus of the young conidium divides, and a two-celled conidium is formed. The nucleus of the apical cell divides again, which completes the formation of the three-celled mature conidium. Chromosomes were most easily counted in the first nuclear division after ascospore delimitation. The haploid number of chromosomes was usually found to be six, although occasionally only five could be distinguished. No differences in nuclear behavior were observed between P. oryzae and P. grisea in the asexual state.