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J.R. Hotchkiss, P. Revilla and W.F. Tracy

Cold tolerance useful for sweet corn improvement may be present in open-pollinated (OP) cultivars. Cold tolerance in sweet corn is the ability to germinate, emerge, and grow under low temperatures. The cold tolerance of 35 open-pollinated sweet corn populations and controls was measured by growing the entries under 14 °C day/10 °C night in growth chambers. The same entries were grown under warm (24 ± 2 °C) conditions in a greenhouse. Traits measured included percent and time to emergence, seedling color, and seedling root and shoot dry mass. Respective repeatability estimates calculated from mean squares were 0.08, 0.33, 0.33, 0.50, and 0.60 for these traits. Entries were ranked separately in each environment based on their performance using a rank-summation index. Differences in cold tolerance existed among the entries. Emergence ranged from 75% to 100% among the entries, with a mean of 90.9%. Time to emergence ranged from 16.2 to 21.9 d, with a mean of 18.2 d. Root and shoot mass ranged from 0.07 to 0.27 g/plot and 0.07 to 0.24 g/plot, respectively. Correlations among the traits measured were favorable, permitting simultaneous improvement. The rankings between the warm and cold environments were significantly correlated (r = 0.67***), indicating that some entries that performed well under low temperatures also performed well under warm conditions.

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P. Revilla, J.R. Hotchkiss and W.F. Tracy

Many sweet corn hybrids germinate poorly and have low seedling vigor in cold soils. Sources of cold tolerance and an understanding of its inheritance would benefit sweet corn production. Our objective was to determine the genetics of cold tolerance among open-pollinated progenitors of modern sweet corn. Six open-pollinated sweet corn cultivars were used as parents of a diallel. The 15 crosses plus reciprocals, parents, and checks were evaluated in cold chambers. Growing conditions were 14 hours with light at 14 °C, and 10 hours without light at 10 °C. Days to emergence, percent emergence, shoot dry weight, and root dry weight were recorded. The experiment was repeated in the greenhouse under warm conditions. Variation for cold tolerance was present among the crosses and cultivars. The variation was primarily due to general combining ability (GCA) effects, with specific combining ability (SCA) effects and reciprocal effects being significant for seedling dry mater. `Howling Mob' had significant favorable GCAs for all cold tolerance traits and resulted in the most cold-tolerant hybrids. `Country Gentleman' and `Stowell's Evergreen' were the slowest emerging parents. Days to emergence under cold conditions was not correlated to days to emergence under warm conditions. The correlations between root weight (cold) and root weight (warm) and shoot weight (cold) and shoot weight (warm) were significant, positive, and relatively large. In this material it appears that seedling vigor under warm conditions could be used to predict seedling size under cold conditions.

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R.A. Malvar, P. Revilla, P. Velasco, M.E. Cartea and A. Ordás

The pink stem borer (PSB) (Sesamia nonagrioides Lefebvre) and the European corn borer (ECB) (Ostrinia nubilalis Hübner) are the major insect pests of corn (Zea mays L.) in Mediterranean countries, although larvae of other insects can also cause damage. The objective of this research was to evaluate the effect of sweet corn hybrids, planting time, and environment on damage by various insects. Data were recorded on the number of larvae of each of the aforementioned pests and damage produced in the ear 20 days after pollination and in the ear and stem when plants were dry. PSB was the most abundant pest, followed by ECB. Other insects, such as Mythimna unipuncta (Haworth) and Helicoverpa armigera (Hübner) were rarely found in sweet corn plants. ECB was constant over time, PSB had larger seasonal variation, and M. unipuncta and H. armigera were highly variable over time. There were significant differences among planting dates and growing cycles for damage traits in each environment. Interactions among hybrids, planting dates, and environments were significant. Dry ears were damaged more than fresh ears and stems had more larvae than ears. The economic value of the crop was seriously affected because most fresh ears had some damage, and seed production would be severely affected by PSB.

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P. Revilla, V.M. Rodríguez, R.A. Malvar, A. Butrón and A. Ordás

Four sweet corn (Zea mays L.) heterotic patterns have been reported among sugary1 (su1) open-pollinated cultivars: two among sweet corn cultivars (`Golden Bantam' × `Country Gentleman' and `Golden Bantam' × `Stowell's Evergreen'), one related to the `Reid' × `Lancaster' field corn heterotic pattern (`NE-HY-13A' × `NE-HY-13B'), and one related to the northern × southern Spain field corn heterotic pattern (`EPS31' × `EPS32'). The objective of this research was to compare the performance of sweet corn heterotic patterns. The four crosses and their seven parents were evaluated in 2 years, at two environments in northwestern Spain, as well as in a cold chamber. `Golden Bantam' × `Stowell's Evergreen' and `Golden Bantam' × `Country Gentleman' had poorer agronomic performance and better ear quality than `NE-HY-13A' × `NE-HY-13B' and `EPS31' × `EPS32'. `Golden Bantam' × `Stowell's Evergreen' had the best cold tolerance in the cold chamber. Earliness, emergence, and early vigor were higher for `EPS31' × `EPS32', related to the northern × southern Spain field corn heterotic pattern. Heterosis was positive and significant for several traits for `Golden Bantam' × `Stowell's Evergreen' and `Golden Bantam' × `Country Gentleman,' while heterosis was not significant for `NE-HY-13A' × `NE-HY-13B' and `EPS31' × `EPS32'. The objective of capitalizing on the `Reid' × `Lancaster' and the northern × southern Spain field corn heterotic patterns for improving sweet corn has failed. We believe that heterosis is lost when field corn heterotic patterns are combined with sweet corn, due to incompatibility in gene combination among sweet and field corn genetic backgrounds.