Heterotic patterns in sweet corn are weakly defined. Most sweet corn inbreds are descended from three open-pollinated cultivars: `Golden Bantam', Stowell's Evergreen', and `Country Gentleman'. Heterotic and phylogenetic relationships among these three cultivars and others are not clearly known. This investigation was designed to investigate the heterotic patterns among some historically important open-pollinated sweet corn cultivars: `Country Gentleman', `Golden Bantam', `Lindsey Meyer Blue', `Stowell's Evergreen', `Howling Mob', and `Pease Crosby'. The 15 possible hybrids from the diallel cross plus the 6 parents were grown in midspring and late summer plantings. Heterosis and combining ability effects were estimated for 13 traits. Hybrid × planting date interactions were significant for most of the main traits, hence, planting dates were analyzed separately. Average midparent heterosis for grain yield was 29.17% in the first planting date and 57.04% in the second planting. Midparent heterosis for yield and plant height were highest for hybrids with `Country Gentleman' as a parent. `Stowell's Evergreen' when crossed to `Pease Crosby', `Lindsey Meyer, and `Golden Bantam' exhibited high heterosis. The two late-maturity cultivars `Country Gentleman' and `Stowell's Evergreen' had higher general combining ability than the four early-maturity cultivars for most traits. Specific combining ability was seldom significant. Yield of `Country Gentleman' hybrids averaged over all crosses and planting dates was the highest. These data indicate a strong heterotic pattern—`Country Gentleman' × `Pease Crosby', `Golden Bantam', and `Lindsey Meyer Blue'—and a weaker one—`Stowell's Evergreen' × `Pease Crosby', `Golden Bantam', and `Lindsey Meyer Blue'.
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