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The Ultrastructure of Aphanomyces euteiches During Asexual Spore Formation. H. C. Hoch, Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 62:149-160. Accepted for publication 24 August 1971. DOI: 10.1094/Phyto-62-149.

The ultrastructure of Aphanomyces euteiches was studied to determine what cytological changes occur during the transition from the vegetative state through the formation of the primary spore, its extrusion and subsequent encystment. Cleavage of the sporangial protoplasm commenced with aggregation of the cytoplasm toward each nucleus, and with the withdrawal of the plasmalemma from the hyphal wall to delineate the primary spores. As the spores formed, the central vacuole evaginated and emptied its contents into the space developed between the plasmalemma of the primary spores and the hyphal wall. Much of the tonoplast of the evaginated central vacuole became part of the plasmalemma surrounding the primary spores. Simultaneously, plasmalemmasomes progressively lost their compact nature as the plasmalemma pulled away with the protoplast from the hyphal wall. Certain electron-opaque vesicular inclusions increased in size and became striated after induction of asexual sporulation in the hyphae. Other vesicles with an electron-opaque cortex and center became apparent as the protoplasm was delimited into primary spores. Vesicles concentrated at the developing cyst wall of the extruded primary spore, possibly contributing to its deposition. Centrioles were not observed until after induction of asexual sporulation. Flagella were not observed during the development of the primary spores in the hyphae or in the newly encysted primary spores. This indicates that A. euteiches is not diplanetic as previously described.