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Jerald K. Pataky, Mirian Gonzalez, James L. Brewbaker, and Frederik J. Kloppers

Resistance to Puccinia sorghi Schwein. based on the Rp1-D gene has been used successfully in North America for the past 15 years to control common rust on sweet corn (Zea mays L.). The objective of this preliminary research was to examine rust reactions of Rp-hybrids grown for processing in the midwestern United States against biotypes of P. sorghi virulent against Rp1-D. In Sept. 1999, isolates of P. sorghi virulent on corn with the Rp1-D gene were collected throughout the midwestern United States. Rust reactions of 41 Rp-resistant, processing sweet corn hybrids and nine non-Rp hybrids were evaluated during the 1999-2000 season in Argentina, Hawaii, Mexico, and South Africa, where populations of P. sorghi are virulent against Rp1-D. Sporulating uredinia were observed on all hybrids in all locations. Although rust reactions varied among locations, mean standardized scores of nine non-Rp hybrids that were included in the trial as controls ranked nearly the same as in previous trials. Thirteen hybrids with standardized scores above 0.25 were more susceptible than the hybrid with the lowest mean rust rating, `Green Giant Code 27'. Thirty-two hybrids were intermediate in reaction to P. sorghi virulent against Rp1-D. Reactions were moderately resistant for nine hybrids with mean standardized scores below -0.50, including two moderately resistant, non-Rp hybrids (`GG Code 27' and `GG Code 6') that were included as controls. Additional trials are necessary to confirm reactions of these hybrids. If the Rp-hybrids that were moderately susceptible or susceptible in this trial are infected by P. sorghi virulent against Rp1-D, secondary inoculum will be abundant and infection will be severe if the weather is wet.

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Amy Barr, Mark Bennett, and John Cardina

The objective of this study was to ascertain if stand establishment of sh2 sweet corn (Zea mays L.) would benefit from variable planting depths determined by the use of geographic information systems (GIS). Spring and fall research plots were established in a field [80 × 20 m (262 × 66 ft)] containing Crosby silt loam and Kokomo silty clay loam soil series in Columbus, Ohio. Three sh2 sweet corn cultivars (Starship, Skyline, and Confection) were planted at three depths on the two soil types in the fall study, with an additional transition soil added in the spring. Emergence counts as well as soil moisture and temperature were monitored. In the spring, sites were also sampled for nutrient levels and soil compaction. Significant variability was found within the field with respect to soil moisture, temperature, nutrient levels, and compaction. Seedling emergence fluctuated with average soil moisture increasing in blocks with up to 24% moisture and then leveling off. Daily minimum soil temperatures impacted stand establishment. Although heat units accumulated faster on the Crosby soil, emergence was slower and less complete on these soil series than on Kokomo soil series. Further investigation determined that although temperatures of the Crosby soil were 3 to 4.5 °C (5.4 to 8.1 °F) warmer during the day than the Kokomo soil, temperatures on the Crosby soil averaged 2 °C (3.6 °F) cooler at night. Analysis of emergence patterns and field variability was performed on ArcView mapping software. Although `Skyline' planted at 2 cm (0.8 inches) had the best emergence overall, final stand would have been increased with `Skyline' planted variably at 2 and 4 cm (1.6 inches). Mapping the field under these different scenarios showed that although area with less than 70% stand would exist with a 2-cm uniform planting depth, the entire field would have a stand of 70% or greater with variable planting depth using a high vigor seedlot.

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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.

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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.

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T.E. Dickert and W.F. Tracy

Heterosis in corn (Zea mays L.) usually results in earlier flowering, larger plants, and increased yield. In extremely early sweet corn the effect of heterosis on flowering time may be reduced or eliminated due to developmental and physiological requirements for vegetative growth before the transition to reproductive phase. The objective of this study was to determine the level of heterosis and the combining ability for flowering time and other agronomic traits in a diallel cross of six very early open-pollinated sweet corn cultivars. The diallel was grown in 1995 and 1996. Hybrids and parents averaged over hybrids differed for silk date, plant height, ear height, 10-ear weight, ear length, and 100-kernel weight but did not differ for row number and ear width. Heterosis for silk date was significant, but the difference between parents and hybrids was very small, 0.5 day. No hybrids were earlier than the earliest parent, and average midparent heterosis was -0.8%. In contrast midparent heterosis was significant and relatively high for 100-kernel weight (10.0%), ear length (12.9%), ear height (8.6%), plant height (9.0%), and 10-ear weight (28.2%). The traits with low heterosis had very high general combining ability/specific combining ability ratios while these ratios were much smaller in traits with high heterosis. Heterosis for many of the traits, including 10-ear weight, was higher than published values. Conversely, heterosis for flowering time was small, compared to other traits in this study and to published values for silk date, indicating that this extremely early germplasm may be at or near the limit for flowering time under the photoperiod and temperatures typical of summer in Madison, Wis. (43.05°N, 89.31°W).

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Thomas F. Morris, George Hamilton, and Sara Harney

There is little published data to support current recommended plant populations of 11,500 to 17,500 plants/acre (28,600 to 34,600 plants/ha) for fresh market sweet corn (Zea mays L.) in the northeastern United States. The plant population likely affects marketable yield and recovery of nitrate. Residual soil nitrate is of concern because of the potential for nitrate contamination of water supplies. Our objectives were to determine the effect of plant population on the yield of sweet corn grown for fresh market without irrigation and on the amount of nitrate in the surface 1 ft (30 cm) of soil at harvest. Seven main-season sweet corn varieties were planted in a total of eight experiments in 1995, 1996, and 1997. Seven experiments were in Connecticut and one was in New Hampshire. All but one of the varieties were standard (su) or sugary enhanced (se) varieties. The experimental design was a randomized complete block with four replications, and the treatments consisted of 12,000, 16,000, 20,000, 24,000, and 28,000 plants/acre (29,600, 39,500, 49,400, 59,300, and 69,200 plants/ha). The yield of marketable ears was classified based on the length of the ears. The results suggest that the current recommendations for plant population in the Northeast US may be too low. Populations of 20,000 and 24,000 plants/acre produced consistently greater yields of ears greater than 7.0 inches (178 mm) long. Soil nitrate-N concentrations at harvest were about 8 mg·kg-1 lower with 16,000 plants/acre or greater, compared with 12,000 plants/acre, which suggests that populations of 16,000/acre or greater should decrease the potential for nitrate contamination of water supplies in the fall, winter, and early spring.

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T.K. Hartz and J. Caprile

Sweet corn (Zea mays L.) cultivars carrying the sh2 mutation show poor seed vigor under stressful field conditions, requiring higher seeding rates to ensure stand establishment. The effects of sodium hypochlorite seed disinfestation, solid matrix priming (SMP), and seed-coating with Gliocladium virens Miller, Giddens & Foster to enhance emergence of sh2 sweet corn in controlled-environment cold stress tests and field trials were investigated. In combination with a chemical fungicide seed treatment (captan, thiram, imazalil, and metalaxyl), SMP significantly improved the percentage and rate of seedling emergence of `Excel' and `Supersweet Jubilee' in a cold stress test (in soil for 7 days at 10C, then 15C until emergence) but was inconsistent under field conditions, improving emergence in only one of four field trials. Sodium hypochlorite disinfestation was ineffective. Compared to a film-coated control, coating seeds with G. virens strain G-6 was highly effective in increasing emergence in two of three cultivars tested in cold stress tests in two soils, while strain G-4 was generally ineffective. In field trials, G-6 treatment significantly increased emergence over that of nontreated seed but was inferior to conventional fungicide treatment and conferred no additional benefit in combination with fungicide treatment. Overall, no seed treatment evaluated was an economically viable alternative for or supplement to chemical fungicide treatment. Chemical names used: cis-N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide (captan); tetramethyl-thiuram disulfide (thiram); 1-[2-(2,4-dichlorophenyl)-2-(2-propenyloxy)ethyl]-1H-imidazole (imazalil); N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-alanine methyl ester (metalaxyl).

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Eric Simonne, Robert Boozer, and Amarat Simonne

White sweet corn (Zea mays L.) is widely grown in the southeastern United States. Although `Silver Queen' has been a popular variety in that region for over 20 years, many other varieties are now available. Selecting a variety for commercial or home production is a complex decision because varieties vary considerably with regard to field performance, ear characteristics, and eating quality. Because limited information is available on overall evaluation of sweet corn varieties, the objectives of this study were to 1) evaluate field performance, ear characteristics and eating quality of selected white sweet corn varieties, 2) globally compare varieties using an overall rank-sum index (ORSI), and 3) determine if `Silver Queen' is still the best variety or if it benefits from name recognition. Significant differences among varieties were found for most of the attributes evaluated. When a variety needs to be selected on the basis of a single group of attributes, our results suggest that the best varieties for field performance, ear characteristics and eating quality were `Even Sweeter' and `Treasure', `Silver Queen' and `Rising Star', and `Silverado', respectively. When ranks for all attributes were pooled together, the ORSI for all varieties fell within the 40 to 60 median range for ORSI. These results suggest that while marked differences between varieties can be found for a selected attribute, overall all selected varieties showed similar potential for commercial production. Panel response on sweet corn variety names and the rate of correct blind identification of `Silver Queen' suggested that while it is still among the best varieties, `Silver Queen' did benefit from name recognition.

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Jabe E. Warren and Mark A. Bennett

Drum priming enhances seed performance without the waste and additional materials associated with conventional osmotic or matric priming techniques. Sweet corn (Zea mays L.) se (`White D' Lite') and sh2 (`WSS-4948') endosperm seeds were hydrated using drum priming at 25 °C for 6 hours. During each cycle, 125-g seed samples were exposed to 1.6, 3.2, 4.8, or 6.0 mL of distilled water and then rotated in a drum for 1 hour to ensure uniform uptake. At the end of this period, samples of 100 seeds (each) were removed and moisture content was determined. Drum priming hydrated all seedlots gradually, with increasing time required at reduced water levels for individual seedlots to achieve the desired moisture content (25% to 30%). Drum priming may provide a better alternative to conventional systems of priming.

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C.L. Treat and W.F. Tracy

Goss's wilt is a bacterial wilt and blight that may cause yield losses up to 50% or greater in sweet corn. Ten hybrids from a diallel cross of five sweet corn (Zea mays L.) inbreds were analyzed for resistance to Goss's wilt (Corynebacterium michiganense ssp. nebraskense Schuster, Hoff, Mandel, and Lazar) in 1987 and 1988. The inbreds used to make the diallel were widely used historically and were chosen on the basis of adaptation and relative maturity. Three hybrids were resistant and seven intermediate, while the field corn controls were extremely susceptible. General combining ability (GCA) and specific combining ability (SCA) sums of squares accounted for 94% and 6% of the variation among crosses, respectively. GCA was highly significant (P ≤ 0.01), while SCA was nonsignificant. Year differences were nonsignificant, but date of rating and hybrid × year interaction effects were significant (P 0.05). Resistance to Goss's wilt is available in sweet corn, and recurrent selection should be effective if improvement in resistance is desired.