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Virginia M. Moore and William F. Tracy

Corn earworm (Helicoverpa zea) is a costly and destructive pest in sweet corn (Zea mays) production. A recurrent selection experiment was conducted to assess the feasibility of selecting a sweet corn population for longer husks without shortening the ears and to determine whether direct selection for longer husks confers improved resistance to the corn earworm. The initial population was derived from crosses between long- and tight-husked inbred lines (A684su, A685su, and A686su) originally selected for resistance to the european corn borer (Ostrinia nubilalis) and a high-quality commercial hybrid parent (‘Super Sweet Jubilee Plus’). Three cycles of recurrent full-sib family selection were conducted from 2014 to 2016, with a selection intensity of ≈10% per cycle. In 2017, the cycles of selection were evaluated and compared with common commercial cultivars for husk extension, ear length, rate of corn earworm infestation, and extent of corn earworm damage. Ears were artificially infested with corn earworm eggs suspended in an agar solution. The population was successfully selected for longer husk extension without shortening the ears, but the increase in husk extension did not correspond to an increase in corn earworm resistance. Further evaluation is needed to determine whether long husk extension is ineffective as a source of corn earworm resistance, whether it is more effective in combination with other resistance traits, whether greater differences in husk extension are needed to confer biologically significant differences in corn earworm resistance, or whether improved inoculation methods could improve differentiation between genotypes.

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Reid R. Rice and William F. Tracy

Excellent table quality is an essential characteristic of commercial sweet corn (Zea mays) and commonly held paramount as a selection criterion. As a consequence, breeding for improved agronomic performance in sweet corn has been limited in comparison with United States dent corn breeding efforts. The narrowness of genetic diversity within modern sweet corn germplasm suggests potential exists for yield enhancement through new heterotic combinations and introgression of sources of improved agronomic performance. The objective of this study was to examine the results of incorporating nonsweet germplasm in the development of improved temperate sweet corn cultivars. Five inbreds derived from crosses between nonsweet germplasm and temperate supersweet (shrunken2, sh2) inbreds were crossed with three temperate sh2 testers to make 15 experimental hybrids. The hybrids were evaluated in four environments with three replications per environments. Experimental entry Wh04038V × Tester2 yielded 18.1 Mg·ha−1 in 2009 and 16.6 Mg·ha−1 in 2010, significantly out-yielding the top producing commercial control, ‘Overland’, in both years. An additional six entries derived from exotic-by-temperate crosses yielded significantly more than all commercial checks in 2009. Four specific experimental entries consistently exhibited superior resistance to root lodging, northern corn leaf blight (Exserohilum turcicum), and Maize dwarf mosaic virus (MDMV) compared with ‘Marvel’ and ‘Supersweet Jubilee Plus’. Ten of the 15 experimental entries exhibited similar quality for flavor relative to ‘Marvel’ and ‘Overland’, however ‘Supersweet Jubilee Plus’ outperformed all entries for both flavor and tenderness, suggesting that while incorporation of nonsweet germplasm in sweet corn breeding programs may provide valuable contributions for yield and agronomic performance, flavor and tenderness must be carefully regarded.

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Bruce G. Abedon and William F. Tracy

Research was conducted to evaluate correlated effects of full-sib phenotypic recurrent selection for resistance to common rust (Puccinia sorghi Schw.) on ear quality traits in three sugary1 (su1) sweet corn (Zea mays L.) populations: Minn11, Minn14, and NECDR. Cycles 0, 1, 2, and 3 of each population were evaluated in both rust infested and nonrust infested environments. Generally, selection for rust resistance resulted in significant, but minor, decreases in ear and kernel size. Changes in specific traits varied with population. The nonsignificant cycle × environment interaction indicates similar responses occurred in all environments evaluated. Based on these results, selection for ear quality traits need not accompany selection for resistance to common rust if maintenance of ear quality is desired.

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John A. Juvik, Maurice C. Jangulo, John M. Headrick, Jerald K. Pataky and William F. Tracy

Ten cycles of simple mass selection for increased field emergence and kernel weight in a population of shrunken2 (sh2) maize affected various kernel and seedling traits. Ten of 29 variables measured were intercorrelated and were included in the first principal factor of a principal component, factor analysis. The eight variables in factor 1 that increased with cycles of selection were: emergence and kernel weight (the two selection criteria) plant height 41 days after planting (a measure of seedling vigor), uniformity of stand, total starch content per kernel total carbohydrate content per kernel, concentration of starch, and starch content per kernel. The two variables in factor 1 that decreased were: conductivity of electrolytes that leached from imbibing seeds and symptomatic infection of kernels by fungi. Factor 1 was a “seed and seedling quality” factor. The other 19 variables formed five principal factors that primarily were “sugar,” “pericarp,” and “asymptomatic fungal infection” factors. These five factors and the variables from which they were formed, were not affected by selection. These results suggest that seed and seedling quality factors can be improved by selection in a sh2 population without affecting sweetness or tenderness. These results also suggest that although selection for increased emergence and kernel weight lowered the incidence of symptomatic infection by fungi, the population was not improved specifically for resistance to Fusarium moniliforme Sheldon.

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Pedro Revilla, William F. Tracy, Pilar Soengas, Bernardo Ordás, Amando Ordás and Rosa Ana Malvar

The genes sugary1 (su1) and shrunken2 (sh2) are commonly used to produce sweet and super-sweet corn (Zea mays L.), respectively. In this work we compare corn borer [european corn borer (ECB) (Ostrinia nubilalis Hbn.) and pink stem borer (PSB) (Sesamia nonagrioides Lef.)] susceptibility in seven pairs of su1 and sh2 near-isogenic sweet corn inbreds (101t, C23, C40, C68, Ia453, Ia5125, and P39) and the relationship between corn borer resistance and vegetative phase transition. The seven pairs of near-isogenic inbreds were evaluated under corn borer infestation during 3 years in northwestern Spain. Differences among inbreds were significant for most of the traits, although resistance was partial. Ia5125su1 and C40su1 were the most resistant inbreds. Differences between a few pairs of near-isogenic su1 and sh2 strains were significant for some vegetative phase change and corn borer damage-related traits. Generally su1 strains flowered earlier, had a shorter juvenile phase, fewer PSB, and more ECB larvae than sh2 strains. However su1 and sh2 strains did not differ significantly for most traits related to phase transition and corn borer damage; notably ear damage was not significantly different between su1 and sh2 strains. These results suggest that theoretical and practical results of sweet corn (sugary1) breeding for corn borer resistance could be capitalized for super-sweet corn (shrunken2) breeding.

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John A. Juvik, Gad G. Yousef, Tae-Ho Han, Yaacov Tadmor, Fermin Azanza, William F. Tracy, Avri Barzur and Torbert R. Rocheford

This study was conducted to identify the chromosomal location and magnitude of effect of quantitative trait loci (QTL) controlling sweet corn (Zea mays L.) stand establishment and investigate the impact of dry kernel characteristics on seedling emergence under field conditions. Genetic and chemical analysis was performed on two F2:3 populations (one homozygous for su1 and segregating for se1, the other homozygous for sh2 endosperm carbohydrate mutations) derived from crosses between parental inbreds that differed in field emergence and kernel chemical composition. A series of restriction fragment-length polymorphism (RFLP) and phenotypic markers distributed throughout the sweet corn genome were used to construct a genetic linkage map for each population. F2:3 families from the two populations were evaluated for seedling emergence and growth rate at four locations. Mature dry kernels of each family were assayed for kernel chemical and physiological parameters. Composite interval analysis revealed significant QTL associations with emergence and kernel chemical and physiological variables. Improved emergence was positively correlated with lower seed leachate conductivity, greater embryo dry weight, and higher kernel starch content. QTL affecting both field emergence and kernel characteristics were detected in both populations. In the su1 se1 population genomic regions significantly influencing emergence across all four environments were found associated with the se1 gene on chromosome 2 and the RFLP loci php200020 on chromosome 7 and umc160 on chromosome 8. In the sh2 population the RFLP loci umc131 on chromosome 2 and bnl9.08 on chromosome 8 were linked to QTL significantly affecting emergence. Since seedling emergence and kernel sugar content have been shown to be negatively correlated, undesirable effects on sweet corn eating quality associated with each emergence QTL is discussed. Segregating QTL linked to RFLP loci in these populations that exert significant effects on the studied traits are candidates for molecular marker-assisted selection to improve sweet corn seed quality.