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Pablo Velasco, Rosa A. Malvar, Ana Butrón, Pedro Revilla, and Amando Ordás

Pink stem borer (Sesamia nonagrioides Lef.) is one of the most important insect pests of corn (Zea mays L.) in southern Europe. The objectives of this work were to determine the level of resistance in different sweet corn inbreds and to identify sources of resistance to ear feeding by the pink stem borer. Twenty-eight sweet corn (su1 and su1se1) inbreds and four resistant field corn (Su1Se1) inbreds were evaluated for ear resistance at different sowing dates, under two methods of artificial infestation. There were significant differences between infestation methods for ears with damaged grain, husks, cobs, and shanks. The inbred×infestation method interaction was significant for general appearance of the ear. The most resistant inbreds were identified by using mean comparisons and principal component analysis of ear damage traits. All inbreds were damaged. Hence, resistance was incomplete and in need of improvement. EP59, H3, I5125, IL767b, and V7726 were the most resistant sweet corn inbreds, which did not differ significantly from A635, the most resistant field corn inbred. General appearance of the ear appears to be a good indicator of pink stem borer resistance and can be used in preliminary evaluation. Variability exists in the resistance of these sweet inbreds to the pink stem borer and the use of field corn inbreds may not be necessary in the improvement of resistance, although further research is needed to determine if the sources differ in the pertinent genes conferring resistance.

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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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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.