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Cytology and Histology

Relation Between Infection by Rhizoctonia solani and R. oryzae and Disease Severity in Rice. D. S. Marshall, Former graduate research assistant, Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Experiment Station, Baton Rouge 70803, Present address of senior author: Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907; M. C. Rush, professor, Department of Plant Pathology and Crop Physiology, Louisiana State University Agricultural Experiment Station, Baton Rouge 70803. Phytopathology 70:941-946. Accepted for publication 28 March 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-941.

Histological aspects of infection by Rhizoctonia solani and R. oryzae on rice cultivars differing in resistance levels were found to be identical. Both pathogens formed two infection structures, infection cushions and lobate appressoria. A highly significant correlation was detected between the formation of infection cushions and lobate appressoria (r = 0.977). Penetration pegs produced from these structures penetrated the plant surface directly. Stomatal penetration was infrequent and no infection structures were observed. Initial penetration occurred on the outer surface of the rice sheath. There were highly significant correlations between disease severity ratings of the cultivars and both infection structure formation (r = 0.994) and culm invasion (r = 0.935). The fungi failed to penetrate past the outermost sheaths of cultivars having low disease severity ratings and thus did not colonize the culms.