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Determination of Losses in Soybeans Caused by Rhizoctonia solani. H. Tachibana, Research Plant Pathologist, Crops Research Division, ARS, USDA, Iowa State University, Ames 50010; D. Jowett(2), and W. R. Fehr(3). (2)(3)Associate Professor of Statistics, and Associate Professor of Agronomy, respectively, Iowa State University, Ames 50010, (2)Present address: College of Environmental Sciences, University of Wisconsin, Green Bay 54305. Phytopathology 61:1444-1446. Accepted for publication 12 July 1971. DOI: 10.1094/Phyto-61-1444.

A method for visually scoring disease severity was evaluated to determine losses in soybeans caused by Rhizoctonia solani. In different fields infected with the fungus, two cultivars, Amsoy and Hawkeye 63, were evaluated to determine whether the method was valid. Plots 3.1 m long were selected and scored for disease severities of 0, 1, 2, 3, 4, and 5 (ca. 0, 5, 15, 30, 60, and 75 to 100% plants killed, respectively). Data were obtained on total cm killed, total cm barren, plants killed early, mid-, and late season, stand, and yield. The validity of the scoring method was indicated by absence of interaction between the cultivars or fields and the score when the data were analyzed statistically. The relationships with score were linear for five of seven characters, and midseason kill generally had the closest relationship. Total cm killed was closely related to score, but total cm barren was not. Total plants, like cm barren, did not affect scoring; thus, both are considered inaccurate guides in visual scoring methods to determine losses from R. solani in soybeans. Indications were that the fungus is able to reduce yield as much as 48% in Amsoy and 42% in Hawkeye 63 in small plots.

Additional keywords: rating method, effect upon yield.