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Inhibition of Soybean Hypocotyl Elongation by Rhizoctonia solani. C. R. Grau, Former research assistant, Department of Botany and Plant Pathology, Iowa State University, Ames, 50011, Present address of senior author: Department of Plant Pathology, University of Wisconsin, Madison 53706; C. A. Martinson, associate professor, Department of Botany and Plant Pathology, Iowa State University, Ames, 50011. Phytopathology 69:706-709. Accepted for publication 16 January 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-706.

Rhizoctonia solani reduced rates of hypocotyl elongation in some soybean cultivars and consequently delayed seedling emergence in artificially-infested soil:sand:vermiculite and vermiculite growth media. Inhibition of hypocotyl elongation differed with soybean cultivars, isolates of R. solani, and temperature. Maximum reduction occurred at 25 C, but none at 30 C. Short, swollen hypocotyls with a brownish discoloration were the symptoms associated with the condition; however, lesion formation on the lower region of the hypocotyl was not associated closely with it. Reduced hypocotyl elongation and increased time for seedling emergence was not related to the effect of temperature on the initial rate of germination. It was theorized that seed coat invasion by R. solani near the meristematic region in the hypocotyl hook of the seedling influenced hypocotyl elongation. Removal of the seed coat before planting increased the rate of hypocotyl elongation compared with that of seedlings with seed coats present. Water extracts from R. solani-infected seed coat tissue but not from healthy tissue reduced hypocotyl elongation.

Additional keywords: Glycine max, Thanatephorus cucumeris, seedling diseases.