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Published online 1 January 2005
Published in Crop Sci 45:163-170 (2005)
© 2005 Crop Science Society of America
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Genetic Analysis of Resistance to Gray Leaf Spot of Midaltitude Maize Inbred Lines

Abebe Menkir* and Maria Ayodele

International Institute of Tropical Agriculture, c/o L.W. Lambourn Ltd., Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, UK

* Corresponding author (a.menkir{at}cgiar.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Gray leaf spot (GLS), caused by Cercospora zeae-maydis Tehon & E.Y. Daniels, poses a serious threat to maize (Zea mays L.) production in sub-Saharan Africa. The knowledge of inheritance of resistance to GLS in new inbred lines would be useful for efficient development of hybrids and synthetics. In this study, we determined (i) the mode of inheritance of resistance to GLS in midaltitude inbred lines, (ii) the effect of different doses of resistance to GLS in parents on the levels of resistance of their hybrids, and (iii) heterotic effects for GLS resistance. Ninety-six hybrids from 24 inbreds were produced using the Design II mating scheme. The parents and the hybrids were evaluated in separate trials in five environments in Nigeria. Both general (GCA) and specific (SCA) combining abilities were significant (P < 0.001), with GCA accounting for >70% of the variation for GLS scores, days to silking, plant height, ear height, ear aspect, and ear rot; 68% for grain yield; and 60% for plant aspect (visual phenotypic appeal) score. Predominantly, additive genetic effects influenced resistance to GLS and other traits in maize hybrids. Most of the crosses with one or more resistant parents produced resistant hybrids, whereas most crosses between susceptible lines generated susceptible hybrids. Prediction of GLS in hybrids using midparent values resulted in a R2 value of 0.53 for GLS disease score recorded 38 d after midsilking (GLS Score2). Negative heterosis observed in 75 hybrids for GLS Score2 suggested that resistance to GLS could be improved in midaltitude hybrids.

Abbreviations: GCA, general combining ability • GLS, gray leaf spot • IITA, International Institute of Tropical Agriculture • PC1, first principal component axis • PC2, second principal component axis • SCA, specific combining ability • Score1, gray leaf spot disease score recorded at 26 d after midsilking • Score2, gray leaf spot disease score recorded at 38 d after midsilking


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
GRAY LEAF SPOT has become one of the major yield-limiting diseases of maize in sub-Saharan Africa (Ward et al., 1999). It poses a serious threat to maize production in central, eastern, southern, and western Africa. This disease is most severe and damaging when extended periods of high relative humidity occurs, caused by slow-drying dews and prolonged late-season fogs (Beckman and Payne, 1983). Increased incidence of GLS in Africa has been associated with reduced tillage practices, continuous cultivation of maize, and use of susceptible maize cultivars (Gevers et al., 1994). Documented yield losses of maize attributed to GLS vary from 11 to 69% (Ward et al., 1999), with estimated losses as high as 100% when severe epidemics contributed to loss of photosynthetic area, increased stalk lodging, and premature plant death (Latterell and Rossi, 1983; Stromberg and Donahue, 1986; McGee, 1988). Gray leaf spot is particularly significant in Africa because maize is the main staple food crop for millions of people in the rural areas. Gray leaf spot has the potential to threaten food security in many countries (Ward et al., 1999).

Genetic resistance seems to be the best means to reduce yield loss from GLS in Africa, particularly for small-scale farmers that lack the financial means to use fungicides and other management options to control the disease in maize (Ward et al., 1999). Most of the sources of resistance to GLS identified and used in maize have genes for resistance inherited in a quantitative manner (Manh, 1977; Thompson et al., 1987; Huff et al., 1988; Ulrich et al., 1990; Donahue et al., 1991; Gevers et al., 1994). Thompson et al. (1987) indicated that quantitative resistance in two maize inbred lines, Va59 and T234, had not succumbed to C. zea-maydis pathotypes in the field for more than 10 yr. Quantitative resistance to GLS leads to prolonged latent and incubation periods and a reduction in infection rate, number of lesions, and sporulation capacity (Beckman and Payne, 1982, 1983; Ringer and Grybauskas, 1995). These resistance factors have been mapped to at least three different chromosomes, with some of the quantitative trait loci consistently expressed across environments and rating periods and having large effects on GLS resistance (Saghai Maroof et al., 1996; Clements et al., 2000). Considering the rapid expansion of GLS in Africa and its potential destructiveness, the International Institute of Tropical Agriculture (IITA) developed maize inbred lines with quantitative resistance to GLS from diverse sources of germplasm to combat this disease in West and Central Africa. However, the genetic basis of GLS resistance in these new maize inbred lines has not been defined.

Several studies have been conducted to determine the mode of inheritance of resistance to GLS in diverse sources of maize inbred lines. The genetic study of Manh (1977) using a generation mean analysis concluded that additive genetic effects accounted for 82 to 96% of the total variation in conditioning GLS resistance among generations, although dominance and epistasis provided some contribution. The results of other studies also show that additive genetic effects play a major role in the inheritance of resistance to GLS (Ulrich et al., 1990; Donahue et al., 1991; Gevers et al., 1994; Coates and White, 1994);. However, Elwinger et al. (1990) have argued that dominance should also be included in a model to fully explain the mode of inheritance of GLS resistance. It is important to note that each of these studies, except one, involved temperate inbred lines evaluated mainly in temperate maize-growing environments. Although these studies have provided useful information to maize breeders, the genetic information is usually applicable only to the specific germplasm and range of tested environments (Falconer, 1981). The occurrence of two sibling species of C. zea-maydis in the USA but only one in Africa (Dunkle and Levy, 2000; Okori et al., 2003) mirrors the differences in maize-growing environments. Thus, determining the genetic mechanism conditioning resistance to GLS in the new set of inbred lines developed at IITA in tropical test locations will be useful to efficiently develop resistant maize hybrids and synthetics.

Few studies have been conducted to determine dosage effects of the different levels of GLS resistance in parents on the degree of resistance of their hybrids. Ivanovic et al. (1992) found that crosses involving at least one parent resistant to the maize dwarf mosaic disease produced resistant hybrids, while those between susceptible parents generated susceptible hybrids. Coates and White (1994) reported that several maize inbred lines identified as resistant to GLS did not produce resistant hybrids in crosses with a susceptible tester line. The objectives of this study were to determine (i) the mode of inheritance of resistance to GLS in selected midaltitude maize inbred lines, (ii) the effects of zero, one, and two doses of resistance to GLS in parents on the degree of resistance in their hybrids, and (iii) heterotic effects for GLS resistance.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
In 1997, a large number of lines adapted to the midaltitude at S4 to S6 stage were evaluated for resistance to GLS under naturally occurring severe disease pressure at Tenti (9°48' N, 8°48' E, altitude 1350 m) and Vom (9°40' N, 8°50' E, altitude 1300 m). The GLS disease severity was rated on a scale of 1 to 5, which was similar to the one used by Saghai Maroof et al. (1993), where 1 = no visible infection, 2 = a few scattered lesions on leaves below the ear, 3 = many lesions on leaves below the ear, with a few lesions above the ear, 4 = severe lesions on all but uppermost leaves, which may have a few lesions, and 5 = abundant lesions on all leaves with most of the leaf tissue being necrotic. Lines with a GLS rating of 3 or less selected from this nursery were further evaluated in a nonreplicated test at the two locations in 1998 using the same rating scale. On the basis of the results obtained in 1998, 12 resistant lines with a GLS rating of 1.5 to 2.0, and 12 susceptible lines with a GLS rating of 4.5 to 5.0 were selected for this study (Table 1). The 24 inbred lines were divided into six sets each of four inbred lines. The four inbred lines in one set were used as females and crossed with the four inbred lines in another set used as males based on a Design II mating scheme (Hallauer and Miranda Fo, 1988). Each inbred line was used as a female parent in one set and as a male parent in a second set. The resulting 96 F1 hybrids (6 sets x 16 hybrids) along with four checks were arranged in a 10 by 10 triple lattice design and were evaluated at Tenti for 3 yr (1999–2001) and at Vom for 2 yr (1999 and 2001). The 24 parental inbred lines were also evaluated separately in a trial arranged in a randomized complete block design with three replications side by side, with the hybrid trial at the same locations and years.


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  		Table 1. Means for gray leaf spot (GLS) disease scores and other agronomic traits of parental lines used in Design II crosses tested at five environments in Nigeria between 1999 and 2001.

 
The two trials were planted in fields that had been planted continuously with maize for at least three previous years and had a history of severe GLS disease pressure. Each hybrid or parental line was planted at each location in a 5-m row plot spaced 75 cm apart with 50 cm between plants within each row. Within a row, two seeds were planted in a hill and thinned to one plant after emergence to attain a population density of 53000 plants ha–1 in each trial. A compound fertilizer was applied at the rates of 60 kg N, 60 kg P, and 60 kg K ha–1 at the time of sowing. An additional 60 kg N ha–1 was applied as top dressing 4 wk later. In the two trials, gramazone and atrazine were applied as preemergence herbicides at 5 L ha–1 each of paraquat (1,1'-dimethyl-4,4'-bipyridinium) and metolachlor [2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide]. Subsequent manual weeding was done to keep the trials weed-free. In each trial, the maize plants were inoculated at the four- to six-leaf stage by placing a pinch of ground infected leaf samples with GLS into the leaf whorls. The leaf samples were collected from susceptible plants infected with naturally occurring disease inoculum in the field in the previous years.

In each year, disease severity was rated at each location on a row using the scale previously described (Saghai Maroof et al., 1993) at 26 and 38 d after midsilking, which are referred to hereafter as GLS Score1 and GLS Score2, respectively. Days to silking was recorded in each plot as the number of days from planting to when 50% of the plants had emerged silks. Plant and ear heights were measured in centimeters as the distance from the base of the plant to the height of the first tassel branch and the node bearing the upper ear, respectively. Plant aspect was rated on a scale of 1 to 5, where 1 = excellent overall phenotypic appeal and 5 = poor overall phenotypic appeal. Ear aspect was scored on a 1 to 5 scale, where 1 = clean, uniform, large, and well-filled ears and 5 = rotten, variable, small, and partially filled ears. Ear rot was rated on a scale of 1 to 5, where 1 = little or no visible ear rot and 5 = extensive visible ear rot. All ears harvested from each plot were weighed, and representative samples of ears were shelled to determine percentage moisture. Grain yield adjusted to 15% moisture was computed from ear weight and grain moisture assuming a shelling percentage of 80%.

The ANOVA for both inbred and hybrid trials were performed with PROC GLM in SAS (SAS Institute, 1997) using a RANDOM statement with the TEST option. Each location–year combination within the inbred and hybrid trials was considered an environment. For the hybrid trial, ANOVAs were computed for each location–year combination to generate entry means adjusted for block effects according to the lattice design (Cochran and Cox, 1960). The pooled error mean squares was obtained by dividing the sum of the error sums of squares from all location–year combinations ANOVA with the corresponding sum of the error degrees of freedom. The adjusted means were used to conduct a combined ANOVA. The hybrid (sets) component of the variation was subdivided into variation due to female (sets), male (sets), and the female x male (sets) interaction. The F test for female (sets), male (sets), and female x male (sets) mean squares were made using the mean squares for their respective interaction with environments. The mean squares attributable to environment x female x male (sets) was tested using the pooled error mean squares. The main effects of female (sets) and male (sets) represent the GCA, and female x male (sets) interaction represents SCA (Hallauer and Miranda Fo, 1988). Line x tester analysis was calculated for GLS disease scores and agronomic traits using the adjusted means after the check entries were omitted based on a method described by Kempthorne (1957).

Spearman's rank correlation coefficients for GLS disease scores were calculated between each pair of environments means of the hybrids or parental inbred lines. For each trait, the midparent value for a cross was computed as the mean of the two parental line means averaged across environments. Hybrid means were then regressed on midparent values. Midparent heterosis for GLS disease score was calculated for each cross (Fehr, 1987). Simple correlation analysis between pairs of trait means averaged across environments was calculated for the parental inbred lines using PC-SAS (SAS Institute, 1997). To determine the relationship between GLS disease scores and a combination of agronomic traits, all trait means averaged across environments in the hybrid trial, except GLS Score1 and GLS Score2, were subjected to principal component analysis using PC-SAS (SAS Institute, 1997). The first principal component axis (PC1) scores for the 96 hybrids were regressed on hybrid means for GLS Score2.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Per Se Performance of Inbred Lines
The inbred lines selected for this study exhibited significant differences in GLS disease scores (Table 1). Means of these lines averaged across five environments varied from 1.4 to 4.6 for GLS Score1 and from 1.8 to 5.0 for GLS Score2, representing severe disease pressure at the test environments. The inbred lines selected as resistant parents had lower mean disease scores, whereas the susceptible ones had higher mean scores (Table 1). Significant (P < 0.0001) line x environment interaction was detected for the two GLS scores. Further assessment of line x environment interaction for GLS Score1 and GLS Score2 using Spearman's rank correlation analysis showed that the correlation between inbred means for all pairs of the five environments was positive and significant (r = 0.49 to 0.90, P < 0.05). The differences among lines and line x environment interaction were also significant for all agronomic traits (Table 1). Most of the resistant inbred lines had mean grain yields higher than the trial mean, while most of the susceptible lines had mean grain yields lower than the trial mean. The resistant lines also had better plant aspect scores than the susceptible lines. The differences between the two groups of inbred lines (resistant vs. susceptible) did not follow any consistent trend for other agronomic traits (Table 1).

Combining Ability Estimates
In the combined analysis, the environment and sets were significant sources of variation for GLS scores and other traits (Table 2). Environment had a small effect on GLS Score2 but had a strong effect on five of the eight agronomic traits. The effect of sets was strong on GLS scores but was weak on most other traits. Hybrids differed significantly in their GLS scores and all agronomic traits (Table 2). The mean rating of hybrids across environments ranged from 1.2 to 4.2 for GLS Score1 and from 1.3 to 4.9 for GLS Score2, representing a broad range of reactions to this disease (data not shown). Mean squares for both females (sets) and males (sets) were significant for GLS scores and all agronomic traits. Although mean squares for interaction of females (sets) and males (sets) with environments were also significant for almost all traits, their values were much smaller than the mean squares for females (sets) and males (sets). The females x males (sets) interaction was significant for all traits, whereas its interaction with environments was significant only for GLS Score1, days to silking, plant aspect, and ear aspect scores (Table 2). Further examination of environment x females x males (sets) interaction for GLS Score1 using rank correlation analysis showed that the correlation between hybrid means for all pairs of the five environments was positive and significant (r = 0.70 to 0.86, P < 0.001).


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  		Table 2. Sum of squares of selected sources of variation, expressed as percentages of the corrected total sum of squares, from the combined ANOVAs of crosses of 24 maize inbred lines tested across five environments in Nigeria between 1999 and 2001 for gray leaf spot (GLS).

 
Partitioning of the hybrid sums of squares showed that GCA accounted for >70% of the variation among hybrids for GLS scores and five other traits, 68% for grain yield, and 60% for plant aspect (Table 3). The SCA was sizable only for plant aspect and grain yield (>30%). The sum of squares for GCA of males was slightly higher than that of the females for the GLS scores and plant aspect score, whereas the reverse was the case for the remaining five traits (Table 3). All resistant parental lines except TZMI15 had negative GCA effects for GLS disease scores when used in crosses as females, and most of them also had negative GCA effects when used in crosses as male parents (Table 4). Among the susceptible lines, two had negative GCA effects for GLS scores when used in crosses as both female and male parents, and two had negative GCA effects when used in crosses only as male parents. In most instances, parental lines with negative GCA effects for GLS scores had positive GCA effects for grain yield and vice versa (Table 4). The total number of hybrids with significant and negative SCA effects was 26 for GLS Score1 and 29 for GLS Score2 (data not shown). Almost all hybrids had positive SCA effects for grain yield.


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  		Table 3. Percentages of the sums of squares for crosses attributable to general (GCA) and specific combining ability (SCA) for gray leaf spot (GLS) disease scores and other traits.

 

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  		Table 4. Estimates of general combining ability effects for gray leaf spot (GLS) disease scores and grain yield of 24 maize inbred lines evaluated in sets of factorial crosses at two locations in Nigeria between 1999 and 2001.

 
Effect of Different Doses of GLS Resistance in Parents on Resistance in their Hybrids
The mean performance of hybrids generated from crosses of inbred lines with different doses of resistance to GLS is presented in Table 5. Mean GLS scores of hybrids of resistant x resistant (R x R) crosses were lowest and those of susceptible x susceptible (S x S) crosses were highest. Hybrids of R x R crosses also had lower GLS scores than the hybrid checks. Mean GLS scores of hybrids of R x S crosses were significantly lower than those of S x R crosses (Table 5). Means for days to silking, plant height, and ear height of hybrids for the various combinations of lines did not follow any consistent trend. Mean plant aspect, ear aspect, and ear rot were the lowest for hybrids of R x R crosses and highest for hybrids of S x S crosses. Hybrids of the R x R crosses averaged 964 kg ha–1 more grain yield than those of the S x S crosses. The mean grain yield of hybrids of R x R crosses was similar to the mean yield of the hybrid checks. Mean grain yield of hybrids of the R x S crosses did not differ from that of the S x R crosses. Regression of GLS Score2 on corresponding GLS Score1 resulted in a significant (P < 0.0001) and positive regression coefficient with an R2 value of 0.98 (Fig. 1) . Most of the hybrids of R x R, R x S, and S x R crosses had low GLS Score1 and GLS Score2 (Fig. 1). Among the S x S crosses, four hybrids involving TZMI12 as a male parent and two hybrids involving TZMI23 either as a male or female parent had GLS Score2 varying from 1.6 to 3.0 (Fig. 1).


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  		Table 5. Means of gray leaf spot (GLS) and other traits along with their standard errors for different combinations of inbred lines evaluated at five test environments in Nigeria between 1999 and 2001.

 

Figure 1
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  		Fig. 1. Regression of gray leaf spot (GLS) diseases score recorded 38 d after midsilking (GLS Score2) on gray leaf spot (GLS) diseases score recorded at 26 d after midsilking (GLS Score1) of 96 hybrids involving lines with different levels of resistance to GLS (R = resistant and S = susceptible).

 
Regression of Hybrid Performance on Per Se Performance of the Lines
Regression of hybrid means on midparent values for the various traits is presented in Table 6. The regression of hybrid GLS scores on midparent GLS scores resulted in positive regression coefficients (P < 0.0001) with R2 values of 46% for GLS Score1 and 53% for GLS Score2. The regression coefficients for all other traits were also positive and significant (P < 0.01), with R2 values ranging from 10% for grain yield to 51% for plant aspect. The regression coefficients for GLS scores and five other traits were significant and varied from 0.70 to 1.05 (Table 6).


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  		Table 6. Regression of hybrid performance on midparent value and percentage minimum, maximum, and mean values for midparent heterosis of 96 crosses tested in five environments in Nigeria between 1999 and 2001 for gray leaf spot (GLS).

 
Estimates of Heterosis
The level of heterosis for GLS scores and other traits varied widely among hybrids (Table 6). Mean percentage heterosis of the hybrids was negative for all traits, except for plant height, ear height, and grain yield (Table 6). Among the 24 hybrids each of the R x R, R x S, and S x R crosses, a total of 22, 18, and 20 hybrids, respectively, had negative heterosis varying from –2% to –56% for GLS Score2 (Fig. 2) . Although 15 of the 24 S x S crosses also produced hybrids with negative heterosis, nine of them were still susceptible to GLS (Fig. 1), with GLS Score2 varying from 3.1 to 4.6. The remaining 21 hybrids had positive heterosis varying from 1% to 29% for GLS Score2. The R x S crosses registered the largest number of hybrids with high negative heterosis (–50%) for GLS Score2, while the R x R registered only one hybrid with high negative heterosis (Fig. 2).


Figure 2
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  		Fig. 2. Frequency distribution of percentage midparent heterosis of hybrids involving inbred lines with different levels of resistance to gray leaf spot (R = resistant and S = susceptible).

 
Relationship of GLS Disease Scores with Agronomic Traits
For the parental inbred lines, GLS Score1 was significantly correlated with GLS Score2 (r = 0.96, P < 0.0001). Also, GLS Score2 was significantly correlated with plant aspect (r = 0.92, P < 0.0001), ear aspect (r = 0.52, P = 0.006) and grain yield (r = 0.64, P < 0.0004) but not with other traits (r = –0.30 to 0.26) of the parental inbred lines. For hybrids, we computed principal component analysis using the correlation matrix of all agronomic traits except GLS scores to integrate these traits into an index. As shown in Table 7, the first two principal component axes (PC1 and PC2) accounted for 50 and 24% of the total variation in the data set. Principal component scores for the two axes were significantly correlated with GLS Score1 (P < 0.05) and GLS Score2 (P < 0.0001). Large PC1 scores were associated with early silking, shorter plant and ear heights, poor plant and ear aspect, ear rot, and low grain yield. Also, the same set of traits, except ear rot score, contributed significantly to PC2. As shown in Fig. 3 , the regression of GLS Score2 of hybrids on PC1 scores (Fig. 2) resulted in a positive and significant (P < 0.0001) regression coefficient. The regression model accounted for 66% of the variation in GLS Score2. Most of the hybrids of R x R, R x S, and S x R crosses combined low GLS Score2 with low PC1 scores (Fig. 3).


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  		Table 7. Eigenvectors of the first two principal component axes (PC1 and PC2) and the correlation of PC1 and PC2 scores with gray leaf spot (GLS) scores.

 

Figure 3
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  		Fig. 3. Regression of gray leaf spot (GLS) disease score recorded 38 d after midsilking (GLS Score2) on PC1 axis scores of 96 hybrids involving lines with different levels of resistance to GLS (R = resistant and S = susceptible).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Some studies propose that a single disease rating before the onset of leaf senescence is adequate to characterize maize genotypes for resistance to GLS (Thompson et al., 1987; Elwinger et al., 1990; Saghai Maroof et al., 1993). The disease scores recorded at 26 and 38 d after midsilking in our study were effective for assessing GLS resistance of the midaltitude maize inbred lines and their hybrids. The parental lines and their hybrids exhibited wide differences in GLS scores. Although the interaction of lines and female x male (sets) with environments were significant for at least one of the two GLS disease scores, the significant rank correlation between pairs of environments suggests that the parental lines and their hybrids had consistent reactions to GLS across environments. Thus, the differences in the magnitude of GLS scores between lines and their hybrid could be the main sources of the genotype x environment interactions observed in our study. Carson et al. (2002) found that the magnitude of differences for resistance to GLS between hybrids was the main source of hybrid x location interaction.

In a model with fixed effects, Kang (1994) suggests the use of the ratio of GCA to SCA sums of squares to determine their relative importance. In our study, the GCA of lines accounted for most of the variation in GLS scores among hybrids, suggesting that resistance to GLS was conditioned mainly by genes with additive effects. Similar conclusions were made in other studies (Manh, 1977; Thompson et al., 1987; Huff et al., 1988; Ulrich et al., 1990; Donahue et al., 1991; Gevers et al., 1994) using different sets of inbred lines. However, the significant SCA for GLS scores and the negative heterosis registered in most of the hybrids in our study suggest that hybrid development could be employed to exploit nonadditive gene action to improve resistance to GLS in maize.

The significant difference between R x S and S x R crosses for mean GLS disease scores as well as the observed difference of the sum of squares for female GCA and male GCA in the combined ANOVA suggested that cytoplasmic genes contributed significantly to the variation in GLS disease scores among hybrids. These results indicate that in single-cross hybrids involving a susceptible inbred parent, the resistant line could be used as a female to enhance the level of resistance to GLS. Seven resistant and two susceptible inbred lines in crosses with diverse inbred lines as both male and female parents had significant and negative GCA effects for GLS scores, suggesting that they may combine well with other maize inbred lines for resistance to GLS. These inbred lines also had positive GCA effects for grain yield, and thus could be excellent sources of resistance to GLS for use in other breeding programs for midaltitude environments.

Crosses of inbred lines with different doses of resistance to GLS resulted in hybrids with differential reactions to this disease. Most of the crosses with at least one resistant parental line produced resistant hybrids, whereas most crosses between susceptible lines generated susceptible hybrids. The significant regression coefficient of hybrid means on midparent values also indicated that inbred lines with high levels of GLS resistance generally produced hybrids with high levels of GLS resistance. The negative heterosis observed for most of the hybrids of R x R, R x S, and S x R crosses in our study implies that some genes exhibited major dominant effects as suggested by Gevers et al. (1994). Thus, the reaction of inbred lines to GLS under severe disease pressure could be used to eliminate the susceptible inbred lines in a preliminary screening nursery with minimal loss of potentially useful inbred lines. Since the regression of GLS Score2 on PC1 scores was significant, testing the selected GLS-resistant inbred lines in hybrid combinations could promote the selection of inbred lines that impart resistance to GLS with other desirable agronomic features. Such evaluation of inbred lines in hybrid combinations may also facilitate the identification of suitable donor lines with other desirable traits for introgression into elite maize germplasm.


   ACKNOWLEDGMENTS
 
This research was conducted at the International Institute of Tropical Agriculture (MS no. IITA 03/104/JA) and financed by IITA. The authors express their appreciation to all staff members that participated during planting, data recording, harvesting, and management of the trials at the two locations.

Received for publication April 17, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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