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Effect of Red Leaf Blotch on Soybean Yields in Zambia. L. E. Datnoff, Former Graduate Research Assistant, International Institute of Tropical Agriculture, PMB 5320, Oyo Road, Ibadan, Nigeria. D. M. Naik, and J. B. Sinclair. Plant Pathologist, International Institute of Tropical Agriculture, PMB 5320, Oyo Road, Ibadan, Nigeria, and Professor, Department of Plant Pathology, University of Illinois at Urbana-Champaign, 1102 S. Goodwin Avenue, Urbana 61801. Plant Dis. 71:132-135. Accepted for publication 18 September 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0132.

Red leaf blotch of soybean (Glycine max) caused by Pyrenochaeta glycines (= Dactuliophora glycines) has increased in disease incidence and severity as hectarage planted to soybeans has increased in Zambia. Field plots in Zambia consisting of six soybean cultivars were either unsprayed or sprayed with the fungicide triphenyltin acetate. Disease severity, vertical disease progress, defoliation, and the area under the disease progress curve (AUDPC) for unsprayed cultivars ranged from 14.0 to 24.1, 90 to 100, 2.4 to 6.2, and 653.9 to 1,322.5%, respectively. The cultivars Jupiter and Tunia had lower disease severity, vertical disease progress, and AUDPC values than Geduld, Magoye, Oribi, or Sable. Defoliation varied among cultivars. Disease severity was not significantly different between the early- to medium- and late-maturing cultivars when evaluated at corresponding growth stages. Yield differences and 300-seed weight between unsprayed and sprayed plots ranged from 6.5 to 37.1 and 15.9 to 25.8%, respectively. Yield losses were due to reduced seed size. Seed weight was negatively correlated with all disease parameters studied.