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Fine Structure of Phytophthora infestans During Sporangial Differentiation and Germination. P. R. Elsner, Research Associate, Department of Botany and Plant Pathology, Iowa State University, Ames 50010; Gail E. VanderMolen(2), J. C. Horton(3), and C. C. Bowen(4). (2)(3)(4)Graduate Student, Associate Professor, and Professor, respectively, Department of Botany and Plant Pathology, Iowa State University, Ames 50010. Phytopathology 60:1765-1772. Accepted for publication 7 July 1970. DOI: 10.1094/Phyto-60-1765.

Sporangial formation, differentiation, and germination were studied in Phytophthora infestans. The cytoplasm of about 90% of the mature sporangia cultured at 20 C contained cleavage vacuoles and well-developed flagella. The sporangia appeared ready for cytokinesis and indirect germination. About 10% of the mature sporangia retained cytoplasm similar to that of vegetative hyphae and had neither basal bodies nor flagella. When sporangia were induced to germinate at 12 C, about 90% released zoospores. The remaining 10% did not germinate. When sporangia from the same lot were induced to germinate at 20 C, however, approximately 10% germinated directly to form germ tubes. Sporangia with germ tubes always had cytoplasm similar to that of immature sporangia. The other 90% did not germinate at 20 C. At the ultrastructural level, about one-third of nongerminating sporangia contained disorganized flagella, indicating resorption. We suggest that flagellar resorption occurs regularly at higher temperatures before sporangia germinate directly, and is a means of increasing spore longevity. We consider mature sporangia containing cleavage vacuoles and flagella to be differentiated from cytoplasm in vegetative hyphae. Flagellar resorption is one evidence of the dedifferentiation which takes place prior to direct germination at warm temperatures.

Additional keywords: ultrastructure, potato late blight fungus, spore germination.