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Disease Resistance in Five Cycles of ‘BS9’ Corn Synthetic Selected for Resistance to Two Generations of European Corn Borer. J. R. Klenke, Former graduate assistant, Department of Agronomy, Iowa State University, Ames 50011, Present address: Asgrow Seed Company, Ames, IA 50010; W. A. Russell(2), W. D. Guthrie(3), C. A. Martinson(4), and W. L. Pedersen(5). (2)C. F. Curtiss distinguished professor in agriculture, Department of Agronomy, Iowa State University, Ames 50011; (3)Research entomologist, Corn Insects Research Unit, USDA-ARS, Ankeny, IA 50021; (4)Professor, Department of Plant Pathology, Iowa State University, Ames 50011; (5)Assistant professor, Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 77:735-739. Accepted for publication 24 October 1986. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1987. DOI: 10.1094/Phyto-77-735.

A corn (Zea mays) synthetic, BS9, was evaluated to determine if S1 recurrent selection for resistance to European corn borer, Ostrinia nubilalis, resulted in correlated changes in resistance to Exserohilum turcicum, causal organism of northern corn leaf blight, or resistance to Diplodia maydis, causal organism of Diplodia stalk rot. On a 1-to-9 scale (1 = highly resistant and 9 = highly susceptible), first-generation borer damage ratings decreased from 3.6 to 2.4, and second-generation damage ratings decreased from 6.5 to 4.3 in the populations BS9C0 and BS9(CB)C4, respectively. Percentage of leaf area infected with leaf blight was rated six times after inoculation with E. turcicum; ratings were transformed to the natural logarithm (lnNLB), and dates of ratings were transformed to the natural logarithm of days after first inoculation (lnDAY). Linear and quadratic components of the regression of lnNLB on cycles of selection were significant, resulting in a significant increase in leaf blight infection from the C0 to C4 population of BS9. Ratings expressed as percentage of leaf area blighted, in terms of the linear regression, increased from 4.6 and 5.8% at day 16 to 38.9 and 41.7% at day 71 for BS9C0 and BS9(CB)C4, respectively. Selection for borer resistance did not affect Diplodia stalk-rot ratings. Selection for resistance to one pest, therefore, cannot be expected to increase the resistance to other pests, although DIMBOA (2,4-dihydroxy-7-methoxy-(2H)-1, 4-benzoxazin-3(4H)-one) content in a corn plant has been correlated with resistance to a wide range of pests.

Additional keywords: corn disease, corn stalk rot, Exserohilum turcicum, Helminthosporium turcicum, maize, Ostrinia nubilalis.