Crop Science 42:724-729 (2002)
© 2002 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Ear Damage of Sweet Corn Inbreds and Their Hybrids under Multiple Corn Borer Infestation
Pablo Velasco,
Pedro Revilla,
Ana Butrón,
Bernardo Ordás,
Amando Ordás and
Rosa A. Malvar*
Misión Biológica de Galicia, CSIC, Apartado 28, 36080 Pontevedra, Spain
* Corresponding author (rmalvar{at}mbg.cesga.es)
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ABSTRACT
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In Mediterranean countries, the principal corn (Zea mays L.) pest is the pink stem borer (PSB) (Sesamia nonagrioides Lef.), followed in importance by the European corn borer (ECB) [Ostrinia nubilalis (Hübner)]. Our objective was to quantify injury by these pests in a set of inbreds that showed different levels of ear damage by the PSB in a previous study and in their hybrids. A diallel among seven sweet corn inbreds that varied for ear damage by PSB was evaluated in three environments under PSB and ECB infestations. The importance of general combining ability (GCA) and specific combining ability (SCA) for ear damage by corn borer was determined. Inbred parents were also tested under manual infestation conditions with both borers. There was variability for ear damage traits among sweet corn inbred lines, but none of them were completely resistant. General combining ability effects were significant for the general appearance of the ear, ears with damaged grain, and ears with damaged shanks under PSB infestation. Under PSB infestation, SCA was not significant for any trait. Most traits showed significant GCA effects under ECB infestation, although SCA effects were also important for some damage traits. The inbred EP61 could supply some favorable alleles because it showed negative and significant GCA effects for ear damage under infestation by both species. Inbreds EP59 and V7726 showed negative and significant GCA effects for ear damage by ECB. These inbreds could be included in a sweet corn synthetic population that would be improved to decrease ear damage by corn borers.
Abbreviations: ECB, European corn borer • GCA, general combining ability • PSB, pink stem borer • SCA, specific combining ability
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INTRODUCTION
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IN SPAIN AND OTHER MEDITERRANEAN COUNTRIES, the principal corn pests are the PSB (Anglade, 1972) and the ECB. In Mediterranean countries, the PSB is more abundant than the ECB. The PSB prefers to attack stems, but there are similar numbers of larvae of both species in the ears (Cordero et al., 1998). In northwestern Spain, numbers of ECB and PSB larvae in the ears of sweet corn are similar (Velasco, 1997; Velasco et al., 1999b).
The first generation of both species infests young plants, feeding on whorl leaves, which may cause death of the plant (Anglade, 1961). In northwestern Spain, in the early season, the ECB is more common than the PSB. When planting is delayed, which often happens in northwestern Spain, the first generation of both insects causes little damage to the corn crop. Second generation larvae infest older corn plants, damaging stems and ears, and causing substantial yield losses due to lodging, dropped or malformed ears, and kernel damage. After the larvae are inside the plants, insecticides and biological controls offer little control. Insecticides are more effective when the larvae have just hatched or when they migrate to neighboring plants. Resistance to ECB in transgenic corn expressing a protein from Bacillus thuringiensis (Bt) has been reported (Armstrong et al., 1995; Jansens et al., 1997). At present, there are no data on the performance of this type of resistance under PSB attack.
In sweet corn, ear damage is more important than stem damage, because ears with minor damage to the husks or the kernels are unacceptable. Furthermore, sweet corn is harvested 18 to 22 d after flowering, and corn borer-induced lodging usually does not occur by that time. Sweet corn is now an important crop in Europe, justifying research to decrease damage by the PSB and ECB.
One of the most important methods for controlling insect pests in the context of integrated pest control is to grow insect-resistant cultivars (Ortega et al., 1980; Pathak, 1991). The first breeding step for resistance to insects is to identify sources of resistance. Partial resistance to PSB or ECB has been found among sweet corn inbreds (Andrew and Carlson, 1976; Davis et al., 1993; Velasco et al., 1999a) and populations (Velasco et al., 1999b). It is necessary to determine how plant behavior under insect attack is transmitted from the original varieties to the improved cultivars for designing an efficient breeding program (Pathak, 1991). There are no published studies on the inheritance of sweet corn resistance to PSB, and there are few studies about inheritance of the resistance of sweet corn to ECB (Joyce and Davis, 1995; Warnock et al., 1998).
The inbred cross diallel, using inbreds as a fixed factor, is a mating design often used to select lines with good GCA within a series of crosses, and good SCA for specific pairs of parents. This information is valuable, particularly if the selected set of parents represents an elite group of inbreds that are possible candidates for the production of single-cross hybrids (Hallauer and Miranda, 1988). Our objective was to study the level of ear damage by the PSBs and the ECBs in a set of inbreds and their hybrids.
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MATERIALS AND METHODS
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The corn genotypes used in this study (Table 1) were selected because they had shown different degrees of ear damage under PSB infestation in a previous study (Velasco et al., 1999a), where all sweet corn inbred lines adapted to northwestern Spain were evaluated in Pontevedra. We were aware that none of the genotypes evaluated could bring a major factor for resistance to ear-insect attack, since differences between susceptible and partially resistant inbred lines were small. In 1996, the inbreds were crossed in a diallel mating design. Crosses were made in both ways, using both inbred parents as males and females at the same time. The reciprocal crosses were pooled and the resulting 21 single crosses were evaluated in a split-plot design with three replicates. Main plots corresponded with infestation treatments. The two treatments were infestation with PSB and infestation with ECB. The 21 hybrids were the two-row subplots. Trials were carried out at Pontevedra (42°24' N, 8°38' W) on the Atlantic coast of northwestern Spain in 1997 and 1998. This location (20 m above sea level) has a humid, mild climate with an annual rainfall of 
1600 mm. Two planting dates were evaluated each year (29 April and 18 June in 1997, 19 May and 17 June in 1998). Rows consisted of nine two-kernel hills that were spaced 0.21 m apart. The distance between rows was 0.80 m. After thinning, one plant per hill was left for a density of 60 000 plants ha-1. The seven inbred parents were evaluated in adjacent trials with the same design and under the same infestation conditions.
Infestations were made at subplot silking. In the main plots assigned to PSB, five competitive and randomly distributed plants per subplot were manually infested with one egg mass of PSB per plant (50 eggs per plant) by placing the eggs between the uppermost ear and the stem. In the main plots assigned to ECB, five plants in each subplot were manually infested with two egg masses of ECB per plant (50 eggs per plant), placing the eggs on the uppermost ear. Eggs of the PSB were obtained following Eizaguirre's method (Eizaguirre, 1989). The ECB eggs were supplied by the Centre de Recherches de Poitou-Charentes (Institute National de la Recherche Agronomique, France).
The uppermost ears of the infested plants were harvested 20 d after infestation. Ears were examined for general appearance, damage rating, damaged husks, damaged grain, damaged cob, damaged shank, and percentage of ears completely damaged (all ear constituents damaged). General appearance of the ear was rated on a visual 5-point scale ranging from 1 (ear without injury) to 5 (more than 60% of kernels damaged). Damage rating was based on a 5-point scale, such that 1 = ear without injury, 2 = ear tip damaged with no damage on the grain, 3 = kernels damaged on 2.5 cm beyond the ear tip, 4 = kernels damaged on 5 cm beyond the ear tip, and 5 = kernels damaged on >5 cm beyond the ear tip (Pounders et al., 1975). The damage rating scale was developed to measure the damage produced by the ECB, which prefers to enter the ear through the silk channel. The damage rating scale would not be useful to differentiate the kernel damage produced by the PSB because those larvae generally enter the ear through the shank and damage is mainly on kernels located >5 cm from the ear tip. The numbers of PSB and ECB larvae per ear also were recorded. For each trait, two-row subplot means were used to perform statistical analysis.
In 1997, the early planting failed due to cold weather and poor growth, and that trial was not included in the analyses. The three remaining combinations of planting dates and years were considered as three environments. Individual and combined analyses of variance were made for each trait according to a split-plot design. Environments and replications were considered random factors, and inbreds, hybrids, and corn borer treatments were fixed factors. Comparisons of means were made using Waller–Duncan mean separation (Waller and Duncan, 1969). Analyses were made with the SAS package (SAS Institute, 1989). The source of variation due to hybrids was partitioned into GCA and SCA. Griffing's Method 4, Model I (fixed effects) (Griffing, 1956) was used to determine combining abilities. The diallel analyses were conducted using the program DIALLEL Analysis and Simulation (Burow and Coors, 1994). The standard errors for GCA, SCA, and differences among GCA and SCA effects were calculated according to Griffing (1956).
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RESULTS AND DISCUSSION
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In both inbred and hybrid trials, borer species x genotype x environment and borer species x genotype interactions were significant for most traits (data not shown). Therefore, PSB and ECB infestation blocks were analyzed separately because genotype performance under PSB infestation was different from that under ECB infestation and the difference depended on the genotype considered. The differential performance of many genotypes under PSB and ECB infestations could be conditioned by the different behavior of the larvae. The PSB moths lay eggs on the stem and the larvae go into the ear through the stem and the ear shank. The ECB moths lay the eggs at various positions (e.g., under leaves or on ears) and larvae go into the ear through the silks, the husks, or the shanks.
In the combined analyses of variance for inbreds under PSB infestation, genotypes did not interact with environments for most traits (Table 2). There were significant genotype x environment interactions for ears with damaged shank and with damaged cob among hybrids (Table 3). Under ECB infestation, the analysis of variance showed significant inbred x environment (Table 2) and hybrid x environment (Table 4) interactions for the number of ECB larvae and number of ears completely damaged, respectively. Other authors did not find significant genotype x environment interactions under manual infestation (Carlson and Andrew, 1976; Widstrom et al., 1993; Butrón et al., 1999).
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Table 2. Mean squares of the combined analysis of variance across environments for seven sweet corn inbreds evaluated under infestation with Sesamia nonagrioides and Ostrinia nubilalis eggs.
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Table 3. Mean squares of the combined analysis of variance across environments for 21 hybrids from a diallel of seven sweet corn inbreds, infested with Sesamia nonagrioides eggs.
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Table 4. Mean squares of the combined analysis of variance across environments for 21 hybrids from a diallel of seven sweet corn inbreds, infested with Ostrinia nubilalis eggs.
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Infestation with Pink Stem Borer
There were significant differences among inbreds under PSB infestation for general appearance of the ear, ears with damaged cob, and ears completely damaged (Table 2). Inbred C6 was the most damaged (Table 5). Among remaining inbreds, I5125, I453, and EP62 tended to show less damage. There were no significant differences among those inbreds, although Velasco et al. (1999a) identified different levels of damage by PSB in these inbreds when they were evaluated among 28 sweet corn inbreds.
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Table 5. Means of damage traits for the seven parental inbreds of a diallel infested with Sesamia nonagrioides eggs in three environments.
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In the hybrid trial, there were differences among environments for most damage traits (Table 3). There were differences among hybrids for general appearance of the ear, ears with damaged grain, and ears completely damaged. There were no differences among hybrids or lines infested with PSB eggs for damage rating, as it was expected, since PSB larvae do not enter the ear through the silk channel, and most of the damage was done on more than 5 cm beyond the ear tip. Hybrids with the lowest damage were EP59 x V7726, EP61 x I5125, EP61 x V7726, and EP61 x EP62 (Table 6).
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Table 6. Means of damage traits for 21 hybrids from a diallel of seven sweet corn inbreds infested with Sesamia nonagrioides and Ostrinia nubilalis eggs.
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The trait ears with damaged shank is useful for differentiating genotype performance because larvae of PSB generally enter the ear through the shanks. This trait was studied separately for each environment due to the significant hybrid x environment interaction. There were significant differences for this trait in both planting dates in 1998, but not in 1997. In the early planting, in 1998, I5125 x V7726 had the greatest shank damage, with 73% of ears with damaged shanks. In the late planting, EP59 x C6, EP59 x EP62, EP59 x I5125, EP59 x V7726, I5125 x V7726, I5125 x I453, I5125 x EP62, I5125 x EP61, EP62 x I453, EP62 x V7726, EP62 x C6, V7726 x I453, and EP61 x V7726 showed the least damage on shanks, with <13% of ears with damaged shanks.
Damage either in the husks or in the cobs is less important because PSB larvae generally penetrate the ear through the shank. Therefore, the most reliable traits for identifying resistant genotypes are those that involve grain or shank damage. In the present work, there were significant differences for those traits, indicating there is some variability in sweet corn for ear damage by PSB. Hence, inbreds that presented less damage by the PSB in this study could be the base material in a breeding program to decrease susceptibility under PSB infestation.
General combining ability was significant for general appearance of the ear and ears with damaged grain (Table 3), whereas SCA was not significant for any trait. Therefore, for this set of sweet corn inbreds, inheritance of ear damage by the PSB was mainly additive. Anglade and Bertin (1968) pointed out that field corn resistance to PSB was transmitted from inbreds to hybrids. Butrón et al. (1998) determined that the genetic control of resistance of field corn ears to PSB was additive, although they found a dominant component in the cob. EP61 showed significant negative GCA for general appearance of the ear, damage rating, and ears with damaged grain, while EP59 showed significant favorable GCA for ears completely damaged (Table 7). In this set of crosses, some inbreds transmitted their better performance under insect attack by PSB to their offspring, whereas C6 and I5125 transmitted susceptibility.
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Table 7. General combining ability (GCA) for damage traits of the seven parental inbreds of a diallel infested with Sesamis nonagrioides eggs.
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Infestation With European Corn Borer
There were significant differences among inbreds under ECB infestation for all characters except percentage of ears completely damaged and number of larvae of PSB (Table 2). Inbred C6 was the most damaged (Table 8), which parallels the results obtained in the infestation with PSB. This inbred had 100 and 96% of ears with husk and grain damage, respectively.
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Table 8. Means of damage traits for the seven parental inbreds of a diallel infested with Ostrinia nubilalis eggs in three environments.
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In the combined analysis for hybrid trials, there were differences among environments for most traits (Table 4). Ears completely damaged showed a significant genotype x environment interaction. There were differences among hybrids for all damage traits except for ears with damaged husks, ears completely damaged, and number of larvae of PSB (Table 4). The least damaged hybrids were EP61 x V7726, EP59 x EP61, EP59 x V7726, I5125 x V7726, EP61 x I5125, and I453 x V7726 (Table 6). Therefore, EP59 x V7726, EP61 x I5125, and EP61 x V7726 could carry minor resistance factors to both species. Traits general appearance of the ear and damage rating supplied complementary information. General appearance evaluated grain damage without paying attention to the ear area affected, while the damage rating was developed to distinguish among damage produced just on the ear tip or on different ear lengths measured from the ear tip.
General combining ability was significant for general appearance of the ear, damage rating, ears with grain, shank, and cob damage, ears without damage, and number of larvae of ECB, while SCA was significant for ears with grain, shank, and cob damage (Table 4). For the latter traits, the GCAs were larger than the SCA effects; therefore, additive effects were the most important in the inheritance of ear damage by ECB. Andrew and Mosley (1980) found that additive effects were the most important in the inheritance of ear resistance to ECB in sweet corn, although they detected SCA and genotype x environment interactions in some crosses. Lamb et al. (1994) found that only GCA was significant in the inheritance of field corn ear resistance. Joyce and Davis (1995) showed that resistance in a set of inbreds was transmitted partially to the offspring. Finally, Warnock et al. (1998) demonstrated that inheritance of sweet corn resistance to ECB depended on the parents and environment, varying from a dominant-additive model to an epistatic model. V7726 showed the most favorable GCA effect for general appearance of the ear, although it did not differ from EP59 and EP61 (Table 9). These inbreds could bring some favorable alleles to use in a recurrent selection program to improve sweet corn behavior against ECB attack.
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Table 9. General combining ability for damage traits of the seven parental inbreds of a diallel infested with Ostrinia nubilalis eggs.
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The evaluation of ear damage resistance among sweet corn inbred lines showed that C6 was clearly susceptible. The remaining inbreds, without being completely resistant to insect attack, could supply some resistance factors since their performance under insect infestation was significantly better than that of the inbred C6. The level of resistance is not at an acceptable level for growers; therefore, a breeding program for improving sweet corn resistance against corn borers must be conducted to release resistant inbreds and hybrids. For this set of inbreds, additive effects were more important than nonadditive effects in the inheritance of ear damage by corn borers. Ear damage by the PSB should be reduced by decreasing damage in the shank, while damage by the ECB must be reduced in all ear components together. Inbred EP61 showed significant and favorable GCA effects against both species, while EP59 and V7726 showed favorable GCA effects against the ECB. These inbreds could be included in a sweet corn synthetic population that would be improved to decrease ear damage by corn borers.
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ACKNOWLEDGMENTS
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P. Velasco acknowledges a fellowship from the Excma. Diputación Provincial de Pontevedra. The authors thank E. Muiños for her help in rearing insects. Research was supported by the Project XUGA 40301B98.
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NOTES
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Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at the Univ. of Santiago de Compostela, Spain.
Received for publication April 25, 2001.
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ABSTRACT
INTRODUCTION
