First Report of Colletotrichum graminicola Causing Maize Anthracnose in Galicia, Northwestern Spain

Maize (Zea mays) is one of the most important crops worldwide, and fungal diseases are responsible for major losses in food production. Anthracnose caused by Colletotrichum graminicola can infect all maize tissues, although stalk rot and seedling blight cause more significant economic damage (Munkvold and White, 2016). Anthracnose stalk rot is characterized by a distinctive external blackening of the lower stalks resulting in large black streaks, while the pith turns dark brown and has a shredded appearance. Like most stalk rots, the most obvious symptom is a sudden death of plants before grain maturity, and plant lodging. Symptoms commonly appear late in the season, suspicious maize stems of cultivar Tuy exhibiting symptoms of anthracnose stalk rot were collected from a field in Pontevedra, Galicia, Spain (Geographical coordinates: 42°23'27.1" N - 8°30'46.3" W) between June and December of 2022. Stem samples, approximately 50 mm2, were dissected and surface-disinfected for 90 seconds in 20% sodium hypochlorite (v/v) and rinsed three times in sterile distilled water. The samples were transferred to one half-strength acidified potato dextrose agar (PDA) supplemented with ampicillin (100 µg/mL) and lactic acid 90% (1.5 mL/L) and incubated for 5 days at 25 ºC (Sukno et al. 2008). Single spores were transferred to fresh PDA plates to obtain pure culture isolates. A total of six isolates were obtained, and among them, two were selected for further characterization (SP-36820-1 and SP-36820-3). Colonies grown on PDA have dark gray aerial mycelium with orange-colored spore masses. Conidia are falcate, slightly curved, tapered toward the tips, and are produced in acervuli with setae, measuring 37.65 to 24.84 x 8.02 to 4.67 µm, respectively (n = 100). These morphological characteristics are in agreement with C. graminicola previously described by Bergstrom and Nicholson (1999). Isolates were grown in potato dextrose broth (PDB) for 3 days at 25 ºC and total genomic DNA was extracted using a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA, USA). The internal transcribed spacer region of rDNA and the manganese-type superoxide dismutase gene (SOD2) were amplified using primers ITS4/ITS5 (White et al. 1990) and SOD625/SOD507 (Fang et al. 2002) and consequently sequenced. GenBank BLAST analysis revealed that the sequences were 100% identical to strains of C. graminicola. All sequences were deposited in GenBank (see e-Xtra 1 for accession numbers). To confirm Koch's postulates, plants of a derivative of maize inbred line Mo940 (developmental stage V3) were placed horizontally in a tray for inoculation and 20 droplets (7.5 µL) of a suspension of 3 x 105 conidia per milliliter were placed on the surface of the third leaf. The trays were closed to retain moisture and incubated overnight at 23ºC. The next day, the plants were returned to a vertical position and incubated in a growth chamber at 25ºC with 80% humidity and a light cycle of 16 h of light and 8 h of dark (Vargas et al. 2012). After four days inoculated leaves presented brown elongated lesions with necrotic centers consistent with C. graminicola infection, whereas control plants remained asymptomatic. The strains reisolated from infected leaves were morphologically identical to the original isolates. To our knowledge, this is the first report of Colletotrichum graminicola causing maize anthracnose in Spain. Recently, maize anthracnose was also reported in Bosnia and Herzegovina and China (Duan et al. 2019; Cuevas-Fernández et al. 2019), suggesting the pathogen's geographic range is increasing, which may be a threat to maize cultivation in locations with optimal humid conditions for disease development.

Traditional Foods From Maize (Zea mays L.) in Europe

Maize (Zea mays L.) is one of the major crops of the world for feed, food, and industrial uses. It was originated in Central America and introduced into Europe and other continents after Columbus trips at the end of the 15th century. Due to the large adaptability of maize, farmers have originated a wide variability of genetic resources with wide diversity of adaptation, characteristics, and uses. Nowadays, in Europe, maize is mainly used for feed, but several food specialties were originated during these five centuries of maize history and became traditional food specialties. This review summarizes the state of the art of traditional foodstuffs made with maize in Southern, South-Western and South-Eastern Europe, from an historic evolution to the last research activities that focus on improving sustainability, quality and safety of food production.

Identification of single nucleotide polymorphisms (SNPs) for maize cell wall hydroxycinnamates using a multi-parent advanced generation intercross (MAGIC) population

Higher hydroxycinnamate content makes maize tissues more recalcitrant to damage by insects, less digestible by ruminants, and less suitable for biofuel production. In a Genome Wide Association Analysis (GWAS) study carried out in a maize MAGIC population, we identified 24 SNPs associated with esterified cell wall-bound hydroxycinnamates, that represented 15 Quantitative Traic Loci (QTL). We identified new genomic regions associated to cell wall bound hydroxycinnamates in maize stover that could have an impact on their content across different genetic backgrounds. The high resolution QTL described in this study could be valuable for addressing positional mapping of genes involved in hydroxycinnamate biosynthesis and could uncover genes implicated in the esterification of hydroxycinnamic acids to the arabinoxylan chains that are poorly understood. However, we found that genetic correlation coefficients between hydroxycinnamate content and economical important traits such as saccharification efficiency, animal digestibility andi pest resistance were low to moderate, so modify specific hydroxycinnamates to indirectly improve cultivar performance will be unsuitable.

Genetics of Germination and Seedling Traits under Drought Stress in a MAGIC Population of Maize

Drought is one of the most detrimental abiotic stresses hampering seed germination, development, and productivity. Maize is more sensitive to drought than other cereals, especially at seedling stage. Our objective was to study genetic regulation of drought tolerance at germination and during seedling growth in maize. We evaluated 420 RIL with their parents from a multi-parent advanced generation inter-cross (MAGIC) population with PEG-induced drought at germination and seedling establishment. A genome-wide association study (GWAS) was carried out to identify genomic regions associated with drought tolerance. GWAS identified 28 and 16 SNPs significantly associated with germination and seedling traits under stress and well-watered conditions, respectively. Among the SNPs detected, two SNPs had significant associations with several traits with high positive correlations, suggesting a pleiotropic genetic control. Other SNPs were located in regions that harbored major QTLs in previous studies, and co-located with QTLs for cold tolerance previously published for this MAGIC population. The genomic regions comprised several candidate genes related to stresses and plant development. These included numerous drought-responsive genes and transcription factors implicated in germination, seedling traits, and drought tolerance. The current analyses provide information and tools for subsequent studies and breeding programs for improving drought tolerance.

A worldwide maize panel revealed new genetic variation for cold tolerance

A large association panel of 836 maize inbreds revealed a broader genetic diversity of cold tolerance, as predominantly favorable QTL with small effects were identified, indicating that genomic selection is the most promising option for breeding maize for cold tolerance. Maize (Zea mays L.) has limited cold tolerance, and breeding for cold tolerance is a noteworthy bottleneck for reaching the high potential of maize production in temperate areas. In this study, we evaluate a large panel of 836 maize inbred lines to detect genetic loci and candidate genes for cold tolerance at the germination and seedling stages. Genetic variation for cold tolerance was larger than in previous reports with moderately high heritability for most traits. We identified 187 significant single-nucleotide polymorphisms (SNPs) that were integrated into 159 quantitative trait loci (QTL) for emergence and traits related to early growth. Most of the QTL have small effects and are specific for each environment, with the majority found under control conditions. Favorable alleles are more frequent in 120 inbreds including all germplasm groups, but mainly from Minnesota and Spain. Therefore, there is a large, potentially novel, genetic variability in the germplasm groups represented by these inbred lines. Most of the candidate genes are involved in metabolic processes and intracellular membrane-bounded organelles. We expect that further evaluations of germplasm with broader genetic diversity could identify additional favorable alleles for cold tolerance. However, it is not likely that further studies will find favorable alleles with large effects for improving cold tolerance in maize.

Association mapping for maize stover yield and saccharification efficiency using a multiparent advanced generation intercross (MAGIC) population

Cellulosic ethanol derived from fast growing C4 grasses could become an alternative to finite fossil fuels. With the potential to generate a major source of lignocellulosic biomass, maize has gained importance as an outstanding model plant for studying the complex cell wall network and also to optimize crop breeding strategies in bioenergy grasses. A genome-wide association study (GWAS) was conducted using a subset of 408 Recombinant Inbred Lines (RILs) from a Multi-Parent Advanced Generation Intercross (MAGIC) Population in order to identify single nucleotide polymorphisms (SNPs) associated with yield and saccharification efficiency of maize stover. We identified 13 SNPs significantly associated with increased stover yield that corresponded to 13 QTL, and 2 SNPs significantly associated with improved saccharification efficiency, that could be clustered into 2 QTL. We have pointed out the most interesting SNPs to be implemented in breeding programs based on results from analyses of averaged and yearly data. Association mapping in this MAGIC population highlight genomic regions directly linked to traits that influence the final use of maize. Markers linked to these QTL could be used in genomic or marker-assisted selection programs to improve biomass quality for ethanol production. This study opens a possible optimisation path for improving the viability of second-generation biofuels.

Dissecting the genetics of cold tolerance in a multiparental maize population

We identify the largest amount of QTLs for cold tolerance in maize; mainly associated with photosynthetic efficiency, which opens new possibilities for genomic selection for cold tolerance in maize. Breeding for cold tolerance in maize is an important objective in temperate areas. The objective was to carry out a highly efficient study of quantitative trait loci (QTLs) for cold tolerance in maize. We evaluated 406 recombinant inbred lines from a multi-parent advanced generation intercross (MAGIC) population in a growth chamber under cold and control conditions, and in the field at early and normal sowing. We recorded cold tolerance-related traits, including the number of days from sowing to emergence, chlorophyll content and maximum quantum efficiency of photosystem II (F_v/F_m). Association mapping was based on genotyping with near one million single nucleotide polymorphism (SNP) markers. We found 858 SNPs significantly associated with all traits, most of them under cold conditions and early sowing. Most QTLs were associated with chlorophyll and F_v/F_m. Many candidate genes coincided between the current research and previous reports. These results suggest that (1) the MAGIC population is an efficient tool for identifying QTLs for cold tolerance; (2) most QTLs for cold tolerance were associated with F_v/F_m; (3) most of these QTLs were located in specific genomic regions, particularly bin 10.04; (4) the current study allows genetically improving cold tolerance with genome-wide selection.

Warm Season Grain Legume Landraces From the South of Europe for Germplasm Conservation and Genetic Improvement

Currently, there is a high concern from consumers regarding food quality, with emphasis on the origin of food sources. We here review the current situation of beans (Phaseolus spp.) and cowpea (Vigna unguiculata (L.) Walp.) landraces in the South of Europe focusing on morpho-agronomic and genetic diversity and physiological adaptation to the different agrosystems, including the symbiotic association with rhizobia. Despite the reduction in the production and consumption of grain legumes in Southern Europe, the variability of common bean, runner bean and cowpea landraces in this region is adequately preserved ex situ in germplasm banks and in breeder collections in Portugal, Spain, Italy and Greece; however, on-farm (in situ) conservation in isolated areas mainly in gardens and small fields for farmers own consumption and local markets is not guaranteed. This variability can be used for the genetic improvement of varieties, which will result in environmental-friendly improved legumes for a sustainable production in the South of Europe as well as in other regions of the World.

Genotyping-by-sequencing highlights original diversity patterns within a European collection of 1191 maize flint lines, as compared to the maize USDA genebank

Genotyping by sequencing is suitable for analysis of global diversity in maize. We showed the distinctiveness of flint maize inbred lines of interest to enrich the diversity of breeding programs. Genotyping-by-sequencing (GBS) is a highly cost-effective procedure that permits the analysis of large collections of inbred lines. We used it to characterize diversity in 1191 maize flint inbred lines from the INRA collection, the European Cornfed association panel, and lines recently derived from landraces. We analyzed the properties of GBS data obtained with different imputation methods, through comparison with a 50 K SNP array. We identified seven ancestral groups within the Flint collection (dent, Northern flint, Italy, Pyrenees-Galicia, Argentina, Lacaune, Popcorn) in agreement with breeding knowledge. Analysis highlighted many crosses between different origins and the improvement of flint germplasm with dent germplasm. We performed association studies on different agronomic traits, revealing SNPs associated with cob color, kernel color, and male flowering time variation. We compared the diversity of both our collection and the USDA collection which has been previously analyzed by GBS. The population structure of the 4001 inbred lines confirmed the influence of the historical inbred lines (B73, A632, Oh43, Mo17, W182E, PH207, and Wf9) within the dent group. It showed distinctly different tropical and popcorn groups, a sweet-Northern flint group and a flint group sub-structured in Italian and European flint (Pyrenees-Galicia and Lacaune) groups. Interestingly, we identified several selective sweeps between dent, flint, and tropical inbred lines that co-localized with SNPs associated with flowering time variation. The joint analysis of collections by GBS offers opportunities for a global diversity analysis of maize inbred lines.

Leaf physico-chemical and physiological properties of maize (Zea mays L.) populations from different origins

In this study we evaluated the leaf surface properties of maize populations native to different water availability environments. Leaf surface topography, wettability and gas exchange performance of five maize populations from the Sahara desert, dry (south) and humid (north-western) areas of Spain were analysed. Differences in wettability, stomatal and trichome densities, surface free energy and solubility parameter values were recorded between populations and leaf sides. Leaves from the humid Spanish population with special regard to the abaxial side, were less wettable and less susceptible to polar interactions. The higher wettability and hydrophilicity of Sahara populations with emphasis on the abaxial leaf surfaces, may favour dew deposition and foliar water absorption, hence improving water use efficiency under extremely dry conditions. Compared to the other Saharan populations, the dwarf one had a higher photosynthesis rate suggesting that dwarfism may be a strategy for improving plant tolerance to arid conditions. The results obtained for different maize populations suggest that leaf surfaces may vary in response to drought, but further studies will be required to examine the potential relationship between leaf surface properties and plant stress tolerance.

Association mapping for cold tolerance in two large maize inbred panels

BACKGROUND

Breeding for cold tolerance in maize promises to allow increasing growth area and production in temperate zones. The objective of this research was to conduct genome-wide association analyses (GWAS) in temperate maize inbred lines and to find strategies for pyramiding genes for cold tolerance. Two panels of 306 dent and 292 European flint maize inbred lines were evaluated per se and in testcrosses under cold and control conditions in a growth chamber. We recorded indirect measures for cold tolerance as the traits number of days from sowing to emergence, relative leaf chlorophyll content or quantum efficiency of photosystem II. Association mapping for identifying genes associated to cold tolerance in both panels was based on genotyping with 49,585 genome-wide single nucleotide polymorphism (SNP) markers.

RESULTS

We found 275 significant associations, most of them in the inbreds evaluated per se, in the flint panel, and under control conditions. A few candidate genes coincided between the current research and previous reports. A total of 47 flint inbreds harbored the favorable alleles for six significant quantitative trait loci (QTL) detected for inbreds per se evaluated under cold conditions, four of them had also the favorable alleles for the main QTL detected from the testcrosses. Only four dent inbreds (EZ47, F924, NK807 and PHJ40) harbored the favorable alleles for three main QTL detected from the evaluation of the dent inbreds per se under cold conditions. There were more QTL in the flint panel and most of the QTL were associated with days to emergence and ΦPSII.

CONCLUSIONS

These results open new possibilities to genetically improve cold tolerance either with genome-wide selection or with marker assisted selection.

Dent and Flint maize diversity panels reveal important genetic potential for increasing biomass production

Genetic and phenotypic analysis of two complementary maize panels revealed an important variation for biomass yield. Flowering and biomass QTL were discovered by association mapping in both panels. The high whole plant biomass productivity of maize makes it a potential source of energy in animal feeding and biofuel production. The variability and the genetic determinism of traits related to biomass are poorly known. We analyzed two highly diverse panels of Dent and Flint lines representing complementary heterotic groups for Northern Europe. They were genotyped with the 50 k SNP-array and phenotyped as hybrids (crossed to a tester of the complementary pool) in a western European field trial network for traits related to flowering time, plant height, and biomass. The molecular information revealed to be a powerful tool for discovering different levels of structure and relatedness in both panels. This study revealed important variation and potential genetic progress for biomass production, even at constant precocity. Association mapping was run by combining genotypes and phenotypes in a mixed model with a random polygenic effect. This permitted the detection of significant associations, confirming height and flowering time quantitative trait loci (QTL) found in literature. Biomass yield QTL were detected in both panels but were unstable across the environments. Alternative kinship estimator only based on markers unlinked to the tested SNP increased the number of significant associations by around 40% with a satisfying control of the false positive rate. This study gave insights into the variability and the genetic architectures of biomass-related traits in Flint and Dent lines and suggests important potential of these two pools for breeding high biomass yielding hybrid varieties.

Genetic regulation of cold-induced albinism in the maize inbred line A661

In spite of multiple studies elucidating the regulatory pathways controlling chlorophyll biosynthesis and photosynthetic activity, little is known about the molecular mechanism regulating cold-induced chlorosis in higher plants. Herein the characterization of the maize inbred line A661 which shows a cold-induced albino phenotype is reported. The data show that exposure of seedlings to low temperatures during early leaf biogenesis led to chlorophyll losses in this inbred. A661 shows a high plasticity, recovering resting levels of photosynthesis activity when exposed to optimal temperatures. Biochemical and transcriptome data indicate that at suboptimal temperatures chlorophyll could not be fully accommodated in the photosynthetic antenna in A661, remaining free in the chloroplast. The accumulation of free chlorophyll activates the expression of an early light inducible protein (elip) gene which binds chlorophyll to avoid cross-reactions that could lead to the generation of harmful reactive oxygen species. Higher levels of the elip transcript were observed in plants showing a cold-induced albino phenotype. Forward genetic analysis reveals that a gene located on the short arm of chromosome 2 regulates this protective mechanism.

Relationship between time to flowering and stalk and ear damage by second generation corn borers

In the Mediterranean area, the main corn borer species are Sesamia nonagrioides Lefebvre (Mediterranean corn borer) and Ostrinia nubilalis Hübner (European corn borer). In the overall context of integrated pest control, it is possible to reduce the effect of a pest without having a negative effect on the environment by varying the sowing date. Benefits are possible if the most susceptible stages of the crop no longer coincide with the peak of the pest. We used different cycles of selection (0, 6, 8, 10, and 12) of two populations (Purdue A and Purdue B) of maize selected for early flowering to get a more precise estimation of the relationship between maturity of plant tissues and corn borer damage. We found a relationship between the damage produced by corn borers and the number of days from flowering to infestation. We conclude that, after flowering, a later stage of plant development at the moment of the infestation by corn borers reduces the damage caused by the larvae. Based on our results, we recommend to plant as early as possible so the tissues would be as mature as possible at the moment of insect attack.

Intraspecific variation of recombination rate in maize

BACKGROUND

In sexually reproducing organisms, meiotic crossovers ensure the proper segregation of chromosomes and contribute to genetic diversity by shuffling allelic combinations. Such genetic reassortment is exploited in breeding to combine favorable alleles, and in genetic research to identify genetic factors underlying traits of interest via linkage or association-based approaches. Crossover numbers and distributions along chromosomes vary between species, but little is known about their intraspecies variation.

RESULTS

Here, we report on the variation of recombination rates between 22 European maize inbred lines that belong to the Dent and Flint gene pools. We genotype 23 doubled-haploid populations derived from crosses between these lines with a 50 k-SNP array and construct high-density genetic maps, showing good correspondence with the maize B73 genome sequence assembly. By aligning each genetic map to the B73 sequence, we obtain the recombination rates along chromosomes specific to each population. We identify significant differences in recombination rates at the genome-wide, chromosome, and intrachromosomal levels between populations, as well as significant variation for genome-wide recombination rates among maize lines. Crossover interference analysis using a two-pathway modeling framework reveals a negative association between re combination rate and interference strength.

CONCLUSIONS

To our knowledge, the present work provides the most comprehensive study on intraspecific variation of recombination rates and crossover interference strength in eukaryotes. Differences found in recombination rates will allow for selection of high or low recombining lines in crossing programs. Our methodology should pave the way for precise identification of genes controlling recombination rates in maize and other organisms.

Comparison of two methods of reciprocal recurrent selection in maize (Zea mays L.)

Reciprocal recurrent selection (RRS) was proposed for simultaneously improving two populations and their cross. A modification of the classical full-sib RRS (FS-RRS) was proposed in which the performance of full-sibs and S(2) families is combined in a selection index (FS-S(2)-RRS). The Mediterranean corn borer (MCB) is the main corn borer species in the Mediterranean and adjacent areas and produces important yield losses. We started two RRS programs (FS-RRS and FS-S(2)-RRS) from the same maize population in which the selection criterion was grain yield under artificial infestation with MCB eggs. Original populations, two cycles of selection derived from them by both RRS methods, and population crosses were evaluated under MCB attack and under insecticide treatment in three different environments. The objective was to compare the efficiency of the FS-RRS and the FS-S(2)-RRS methods for improving grain yield. We found that the FS-S(2)-RRS method was successful for improving the yield of the population cross under optimum conditions (the regression coefficient over cycles was b = 0.87** Mg ha(-1) cycle(-1)) without losing yield under high pressure of MCB attack (b = 0.07). On the contrary, FS-RRS failed to improve the yield of the population cross under optimum conditions (b = 0.65) and tended to decrease the yield under high levels of MCB attack (b = -0.26). We conclude that for developing high yielding and stable varieties, FS-S(2)-RRS is more efficient than the classical FS-RRS method.

Is the basal area of maize internodes involved in borer resistance?

BACKGROUND

To elucidate the role of the length of the internode basal ring (LIBR) in resistance to the Mediterranean corn borer (MCB), we carried out a divergent selection program to modify the LIBR using two maize synthetic varieties (EPS20 and EPS21), each with a different genetic background. We investigated the biochemical mechanisms underlying the relationship between the LIBR and borer resistance. Selection to lengthen or shorten the LIBR was achieved for each synthetic variety. The resulting plants were analyzed to determine their LIBR response, growth, yield, and borer resistance.

RESULTS

In the synthetic variety EPS20 (Reid germplasm), reduction of the LIBR improved resistance against the MCB. The LIBR selection was also effective in the synthetic variety EPS21 (non-Reid germplasm), although there was no relationship detected between the LIBR and MCB resistance. The LIBR did not show correlations with agronomic traits such as plant height and yield. Compared with upper sections, the internode basal ring area contained lower concentrations of cell wall components such as acid detergent fiber (ADF), acid detergent lignin (ADL), and diferulates. In addition, some residual 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3-(4H)-one (DIMBOA), a natural antibiotic compound, was detected in the basal area at 30 days after silking.

CONCLUSION

We analyzed maize selections to determine whether the basal area of maize internodes is involved in borer resistance. The structural reinforcement of the cell walls was the most significant trait in the relationship between the LIBR and borer resistance. Lower contents of ADF and ADL in the rind of the basal section facilitated the entry of larvae in this area in both synthetic varieties, while lower concentrations of diferulates in the pith basal section of EPS20 facilitated larval feeding inside the stem. The higher concentrations of DIMBOA may have contributed to the lack of correlation between the LIBR and borer resistance in EPS21. This novel trait could be useful in maize breeding programs to improve borer resistance.

Yield and fruit quality of four sweet corn hybrids (Zea mays) under conventional and integrated fertilization with vermicompost

BACKGROUND

Vermicompost has been proposed as a valuable fertilizer for sustainable agriculture. The effects of vermicompost on yield and quality of sweet corn were evaluated in this study. In two field trials, sweet corn plants were grown under (i) a conventional fertilization regime with inorganic fertilizer, and integrated fertilization regimes in which 75% of the nutrients were supplied by the inorganic fertilizer and 25% of the nutrients were supplied by either (ii) rabbit manure, or (iii) vermicompost. All three types of fertilization regime were supplied at two doses. Two pairs of nearly isogenic sweet corn hybrids homozygous for sugary1 and shrunken2 mutants were included in the trials to explore fertilizer × genotype interactions. Growth, yield and ear quality of the plants were evaluated in relation to the three fertilization regimes.

RESULTS

In general, the integrated regimes yielded the same productivity levels as the conventional treatment. Moreover, both vermicompost and manure produced significant increases in plant growth and marketable yield, and also affected the chemical composition and quality of the marketable ear. Nevertheless, most of the observed effects of the organic fertilizers were genotype-dependent.

CONCLUSION

The results confirm that the use of organic fertilizers such as vermicompost has a positive effect on crop yield and quality. Nevertheless, these effects were not general, indicating the complexity of the organic amendment-plant interactions and the importance of controlling genetic variation when studying the effects of vermicompost on plant growth.

Identification of genes related to germination in aged maize seed by screening natural variability

Ageing reduces vigour and viability in maize inbred lines due to non-heritable degenerative changes. Besides non-heritable genetic changes due to chromosome aberrations and damage in the DNA sequence, heritable changes during maize conservation have been reported. Genetic variability among aged seeds of inbred lines could be used for association studies with seed germination. The objective of this study was to identify genes related to germination in aged seeds. The sweet corn inbred line P39 and the field corn inbred line EP44 were used as plant material. Bulks of living and dead seeds after 20 and 22 years of storage were compared by using simple sequence repeats (SSRs) and, when the bulks differed for a marker, the individual grains were genotyped. Differences between dead and living seeds could be explained by residual variability, spontaneous mutation, or ageing. Variability was larger for chromosome 7 than for other chromosomes, and for distal than for proximal markers, suggesting some relationships between position in the genome and viability in aged seed. Polymorphic SSRs between living and dead seeds were found in six known genes, including pathogenesis-related protein 2, superoxide dismutase 4, catalase 3, opaque endosperm 2, and metallothionein1 that were related to germination, along with golden plant 2. In addition, five novel candidate genes have been identified; three of them could be involved in resistance to diseases, one in detoxification of electrophillic compounds, and another in transcription regulation. Therefore, genetic variability among aged seeds of inbreds was useful for preliminary association analysis to identify candidate genes.

Corn borer (Lepidoptera: Noctuidae and Crambidae) resistance of main races of maize from North America

Resistance to corn borers, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) and Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae) in maize, Zea mays L., populations is partial, and more resistant populations are needed. The objective of this research was to compare resistance to corn borers of the main maize races from North America. Twenty open-pollinated maize populations belonging to the races Southern Dent, Corn Belt Dent, and Northern Flint, and three check populations, were evaluated under artificial infestation with S. nonagrioides and O. nubilalis. None of the populations had complete resistance. Northern Flint had the lowest yield under corn borer infestation, whereas Southern Dent had the highest yield but also the largest damage. Corn Belt Dent had a shorter growing cycle and similar yield of infected plants than Southern Dent. The checks had intermediate yield and resistance and were not significantly different from Corn Belt Dent for any trait. The Southern Dent populations Tennessee Red Cob and White Dent (PI221885 and PI311232) could be used as sources of tolerance to corn borers, although they are not expected to provide great gains compared with the levels of tolerance already present in some Corn Belt Dent and European Flint populations and would require adaptation to short growing cycle. The Corn Belt Dent synthetic BS17 had the highest yield and general agronomic performance under corn borer infestation, along with Rustler and Silver King, and the European Flint composite EPS13.

Evaluation of popcorn germplasm for resistance to Sesamia nonagrioides attack

Popcorn adapted to Spanish conditions could be an interesting and profitable alternative to field corn. However, little is known about breeding popcorn germplasm for adaptation to Spain. Sesamia nonagrioides Lefèvbre is the main insect pest affecting popcorn quality and yield under Spanish growing conditions. The objectives of the study were the search for sources of resistance to S. nonagrioides among popcorn germplasm and to study the genetics of the resistance to S. nonagrioides attack. Eight breeding populations along with a five-inbred line diallel and two popcorn commercial checks were evaluated under S. nonagrioides infestation in 2 yr. Significant differences were found among general combining ability (GCA) effects for days to silking, S. nonagrioides tunnel length, general appearance of the ear, kernel moisture, and yield. Specific combining ability (SCA) effects were found to be significant for yield and ear damage. Therefore, heterotic patterns among popcorn materials should be taken into account to generate new popcorn hybrids that are not only more productive but also have higher kernel quality. Breeding popcorn populations BSP4APC0 and PSPW1C1 could be base germplasms in a breeding program for obtaining parental inbreds of healthy kernel popcorn hybrids. New inbred lines could be generated from the cross BP1 x BP2 that would have improved GCA and SCA effects for S. nonagrioides resistance when crossed to South American inbreds.

Free phenols in maize pith and their relationship with resistance to Sesamia nonagrioides (Lepidoptera: Noctuidae) attack

The stem borer Sesamia nonagrioides (Lefèbvre) is the most important insect pest of maize, Zea mays L., in northwestern Spain. Among the metabolites present in maize, phenolic compounds could play an important role in resistance. The objective of this work was to determine whether a relationship between phenols and the amount of resistance exists. Amounts of free phenolic compounds in the pith of 13 inbred maize lines that differ in resistance were measured. The phenolic compounds identified were p-coumaric acid, cafeic acid, ferulic acid, vanillic acid, syringic acid, chorogenic acid, sinapic acid, p-hydroxybenzoic acid, and vanillin. The amount of free p-coumaric acid was correlated with the resistance level. Higher quantities of p-coumaric in the pith could contribute to general resistance to stem borer attack. Jointly with ferulic acid, p-coumaric could provide resistance mechanisms through cell wall fortification and lignification. The other compounds showed no or an unclear relationship with resistance. The vanillic acid showed a decreased tendency after silking, when maize is most attractive for S. nonagrioides, suggesting this acid could act as a chemoattractant for S. nonagrioides larvae or adults. Future studies that focus on these phenolic compounds could be useful in understanding S. nonagrioides resistance.

Effects of selection for the timing of vegetative phase transition on corn borer (Lepidoptera: Noctuidae and Crambidae) damage

In maize, Zea mays L., the timing of vegetative phase transition from juvenile to adult vegetative phases can be modified through selection. A reduction in the juvenile vegetative phase has been associated with resistance to diseases and pests. The major maize pest in temperate areas is Ostrinia nubilalis (Hübner) and in Europe Sesamia nonagrioides Lefebvre. The objective of our study was to determine the effects of divergent selection for the timing of vegetative phase transition in maize on resistance to corn borers. Three cycles of divergent selection for early and late phase transition in a field corn synthetic and in a sweet corn population were evaluated separately under S. nonagrioides and O. nubilalis artificial infestation. For the field corn experiment, yield and moisture improved with selection for phase transition in both directions, but improvement was due to artifacts of selection, rather than to the change in phase transition. There were no correlated responses for corn borer damage, yield, or grain moisture due to selection for the timing of vegetative phase transition. In the sweet corn experiment, selection for the timing of vegetative phase transition had no significant effects on corn borer damage in sweet corn harvested at the fresh stage. Our results do not support the use of phase transition as an indirect criterion for improving resistance to corn borers in maize. The relationship between phase transition and pest resistance reported by other studies could depend on the genotypes or could be too weak to be detected in a selection program with wild-type maize.

Molecular changes in the maize composite EPS12 during selection for resistance to pink stem borer

The pink stem borer (Sesamia nonagrioides Lefèvbre) is the most important pest of maize (Zea mays L.) throughout the Mediterranean area. The maize composite EPS12 has been chosen as the base population for a breeding program based on its resistance to pink stem borer, with the main selection criterion being resistance to stem tunneling. Yield was taken as a secondary selection criterion to avoid any unwanted negatively correlated response on this character. The aims of investigation were: (1) to monitor the effects of selection for resistance to pink stem borer on allele frequency at 70 simple sequence repeat (SSR) markers and their impact on the genetic structure of EPS12 and (2) to identify loci at which allelic frequencies changed significantly due to directional selection. Genetic diversity was reduced during the selection process (as expected since random genetic drift as well as selection could reduce genetic variability), but not significantly so. Although the loss of genetic variation was generally consistent with that expected in a model in which random genetic drift acts alone on neutral alleles, the changes observed in the frequency of five alleles were significantly greater than expected. Further, the linear trend of the departure from the random genetic drift model was significant for some allelic versions of two SSR markers, umc1329 and phi076; directional selection was therefore acting on these loci. The significant effect of directional selection on those markers suggests the presence of quantitative trait loci (QTLs) for tunnel length and/or for yield under artificial infestation with Sesamia nonagrioides on the long arm of chromosome 4.

Performance of crosses among flint maize populations under infestation by Sesamia nonagrioides (Lepidoptera: Noctuidae)

Flint maize, Zea mays L., varieties provide some interesting agronomic characteristics and kernels that possess a better ability than other kernels for developing high-quality flour. The pink stem borer, Sesamia nonagrioides Lefebvre, is an important constraint for the maize crop in Mediterranean regions. The objective of this work was to identify a "flint x flint" heterotic pattern that would perform well under artificial infestation by S. nonagrioides. A 10-population diallel was evaluated under infestation by S. nonagrioides in 2 yr. Variety effects were the only significant effects involved in stem and ear resistance to S. nonagrioides attack. Variety effects and average heterosis effects were the only significant effects for grain yield under artificial infestation conditions. Considering variety effects and cross-performance, the heterotic pattern Basto/Enano levantixo x Longfellow (BA/EL x LO) would be recommended for obtaining flint maize hybrids tolerant to S. nonagrioides attack because BA/EL had the most favorable variety effects for stem resistance, LO exhibited the most positive variety effects for grain yield, and the cross BA/EL x LO yielded significantly more than the remaining crosses.

Resistance of early maturing sweet corn varieties to damage caused by Sesamia nonagrioides (Lepidoptera: Noctuidae)

In Mediterranean countries, the principal pest of maize, Zea mays L., is Sesamia nonagrioides Lefebvre. The objective of this work was to study the resistance of the four early maturing varieties of sweet corn, Baby Orchard, Orchard Baby, Dorinny Sweet, and Golden Early Market, and to check the relationship among resistance to S. nonagrioides and infestation dates. In a previous study, these varieties had showed a confusing behavior, being the most resistant varieties to S. nonagrioides attack in some environments and the most susceptible in others. These varieties were again evaluated along with three medium maturing varieties, used as testers because they were more stable in both environments, by using two infestation dates. Plants were more damaged in the late infestation, but the effect of the infestation depends on the variety studied. Orchard Baby and Baby Orchard were the most resistant varieties under two infestation dates. These two varieties are closely related. Therefore, we can use either one as a source of precocity and resistance for modern sweet corn. Although many studies showed that early maturing inbreds had greater damage caused by S. nonagrioides and European corn borer than late maturing inbreds and hybrids, our results showed that the earliest maturing varieties were the most resistant.

Mean generation analysis of the damage caused by Sesamia nonagrioides (Lepidoptera: Noctuidae) and Ostrinia nubilalis (Lepidoptera: Crambidae) in sweet corn ears

Sesamia nonagrioides (Lefebvre) and Ostrinia nubilalis (Hübner) are the main maize (Zea mays L.) pests in Mediterranean countries. To develop insect-resistant cultivars, it is helpful to know the genetic control of the resistance. Our objective was to determine the genetic control of the resistance to both borers. For each of two crosses (EP59 x P51 and 15125 x EP61), six generations were evaluated (P1, P2, F1, F2, BC1, and BC2). Genetic effects x environment interactions were not significant. For the O. nubilalis resistance traits; both crosses fitted an additive-dominance model. EP59 x P51 had important dominance and additive effects, whereas for 15125 x EP61 we did not detect significant genetic effects, but significant year effects were detected. For S. nonagrioides infestation, both crosses fitted to an additive-dominance model. There were additive effects for most traits in EP59 x P51. The cross I5125 x EP61 showed significant dominance effects for several traits. Resistance to both corn borers fit an additive-dominance model, but genetic effects depend on the cross evaluated. In the resistance to S. nonagrioides, additive effects were important for shank resistance, which is a useful trait for avoiding S. nonagrioides damage on the ear. Early sowings are recommended to make good use of the resistance to both corn borers. In the late sowings, damage is so high that resistant plants are not able to control the pest.

Reactions of Open-Pollinated Sweet Corn Cultivars to Stewart's Wilt, Common Rust, Northern Leaf Blight, and Southern Leaf Blight

Over 800 open-pollinated (OP) varieties of sweet corn were grown and named in the century prior to the development of hybrids, but only a few of the historically important OP cultivars exist today. Alleles that could improve disease resistance of modern sweet corn may be present in the OP cultivars still in existence. The objectives of this research were to compare 36 OP sweet corn cultivars to modern commercial hybrids for reactions to Stewart's wilt, common rust, northern leaf blight (NLB), and southern leaf blight (SLB), and to classify the OP cultivars based on phenotypic reactions to these four diseases. Plants were inoculated in 1994, 1995, and 1996 with Erwinia stewartii, Puccinia sorghi, Exserohilum turcicum, or Bipolaris maydis. Symptoms were rated on a whole-plot basis, and ratings were analyzed by analysis of variance (ANOVA). Means were separated by Bayesian least significant difference values. Some of the OP cultivars had phenotypes that were intermediate to moderately resistant to Stewart's wilt, common rust, NLB, or SLB, but none of the cultivars were more resistant than the best commercial hybrids. Distributions of ratings for rust, NLB, and SLB were less disperse for the OP cultivars than for commercial hybrids. Hence, the resistance of modern sweet corn germ plasm to Stewart's wilt, rust, and NLB appears to be greater than that of the OP cultivars. OP cultivars and four standard hybrids were placed into groups based on a hierarchical cluster analysis of disease reactions. The seven groups formed from the cluster analysis of disease ratings were considerably different than those formed from isozyme variation and morphological characteristics. The partial resistance of some cultivars, e.g., Golden Sunshine, Country Gentleman, Stowell's Evergreen, and Red, may be relatively diverse since these cultivars were placed in different groups based on isozyme and morphological variation. OP cultivars with moderate levels of resistance may be sources of resistance alleles not present in commercial hybrids.