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Reaction of Two Maize Synthetics to Anthracnose Stalk Rot and Northern Corn Leaf Blight Following Recurrent Selection for Resistance to Diplodia Stalk Rot and European Corn Borer. K. A. Nyhus, Former graduate assistant (now corn breeder, Garst Seed Company, Woodburn, IN 46797); W. A. Russell(2), W. D. Guthrie(3), and C. A. Martinson(4). (2)C. F. Curtiss distinguished professor in agriculture, Department of Agronomy, Iowa State University, Ames 50011; (3)Research entomologist, Corn Insects Research Unit, Agricultural Research Service, U.S. Department of Agriculture, Ankeny, IA 50021; (4)Associate professor, Department of Plant Pathology, Iowa State University, Ames 50011. Phytopathology 79:166-169. Accepted for publication 13 August 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-166.

Two maize (Zea mays) synthetics, BSAA and BSBB, were recurrently selected for resistance to Diplodia (Diplodia maydis) stalk rot (DSR) and leaf feeding caused by the first-generation European corn borer (Ostrinia nubilalis) (ECB), based on the reaction of S1 lines to artificial inoculations of D. maydis and artificial infestations of the ECB. This study was conducted to determine if plant factors contributing to DSR and ECB resistance also conferred resistance to anthracnose stalk rot (ASR) caused by Colletotrichum graminicola and northern corn leaf blight (NLB) caused by Exserohilum turcicum. Highly significant linear improvements in ASR resistance were observed over cycles (C0 to C4) of selection in both synthetics. These improvements mirrored the gains reported previously for DSR resistance in BSAA and BSBB and suggested that a genetic correlation exists between DSR resistance and ASR resistance in these populations. NLB severity ratings were recorded on six dates throughout the growing season. A natural logarithm transformation was used to describe the disease progress curve for each of the C0 to C4 populations of each synthetic. Linear regression of lnNLB ratings on lnDATE (days after inoculation) accounted for more than 97% of the variation among entries when averaged over replications. Our results showed no concomitant improvement in NLB resistance over cycles of selection for ECB resistance, contradicting previous reports that 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-one (DIMBOA), a known biochemical factor in leaf-feeding resistance, confers resistance to NLB.