Recurrent selection for resistance to first-generation European corn borer and Diplodia stalk rot in two maize synthetics

Nyhus, Kris
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Four cycles of recurrent selection, based on the evaluation of S[subscript]1 lines, were conducted in two maize (Zea mays L.) synthetics, BSAA and BSBB, for resistance to first-generation European corn borer (Ostrinia nubilalis Hubner) (ECB) and to Diplodia stalk rot (DSR) caused by Diplodia maydis (Berk.) Sacc. Artificial infestations of ECB and inoculations of D. maydis were used to assess direct response to selection and to evaluate the effect of genetic resistance on other agronomic characters. Significant progress in the populations per se was observed for resistance to DSR in BSAA and for resistance to DSR and to ECB in BSBB. On a nine-class scale (1 = highly resistant, 9 = highly susceptible) ECB ratings were reduced from 2.9 to 2.6 in BSAA and from 5.8 to 2.9 in BSBB (C0 to C4). DSR ratings were reduced on a six-class scale (1 = resistant, 6 = susceptible) from 3.6 to 2.1 in BSAA and from 4.1 to 2.7 in BSBB (C0 to C4). The genetic variation among 100 unselected S[subscript]1 lines from each of the C0 and C4 populations for ECB ratings was reduced by a factor of three and five for BSAA and BSBB, respectively. Improvement in rind strength over cycles of selection was significant in the BSBB populations only; low and generally nonsignificant correlations between DSR ratings and rind puncture readings (among S[subscript]1 lines) were observed. Stalk lodging was reduced significantly (P < 0.01) from 16.3% to 2.9% in BSAA and from 23.4% to 6.3% in BSBB. These improvements in pest resistance and stalk quality over four cycles of selection were associated with 11.6 and 4.8% reductions in plant height and 18.8 and 21.0% reductions in grain yield for BSAA and BSBB, respectively. Ear length was the main component contributing to grain yield reductions. The level of resistance achieved in the improved populations of BSBB was sufficient to eliminate grain yield losses caused by ECB infestations and D. maydis inoculations. A correlated response in resistance to anthracnose stalk rot, caused by Colletotrichum graminicola (Cesati) Wilson, was observed in both synthetics, but improved resistance to northern corn leaf blight, caused by Exserhilum turcicum (Pass.) Leonard & Suggs, was not evident. Additional genetic studies suggested that the response of yield and yield components to recurrent selection may be explained by gene frequency changes resulting from selection.