Published online 16 January 2008
Published in Crop Sci 48:93-98 (2008)
© 2008 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Gene Action Controlling Gray Leaf Spot Resistance in Southern African Maize Germplasm
John Dereraa,*,
Pangirayi Tongoonaa,
Kevin V. Pixleyb,
Bindiganavile Vivekc,
Mark D. Lainga and
Neil C. van Rijd
a African Centre for Crop Improvement, Univ. of KwaZulu-Natal, P. Bag X01, Scottsville 3209, Pietermaritzburg, Republic of South Africa
b CIMMYT Int., Apdo. Postal 6-641, Mexico, D.F. 06600, Mexico
c CIMMYT-Zimbabwe, P.O. Box MP163, MT. Pleasant, Harare, Zimbabwe
d Crop Protection, Cedara, KwaZulu-Natal Dep. of Agriculture and Environmental Affairs, P. Bag X9059, Pietermaritzburg, 3200, Republic of South Africa
* Corresponding author (Dereraj{at}ukzn.ac.za).
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ABSTRACT
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Gray leaf spot disease (GLS; caused by Cercospora zeae-maydis Tehon and Daniels) is among the major maize (Zea mays L.) production constraints in southern Africa. Maize is predominantly grown by small-scale farmers without fungicides; hence, there is need to develop GLS resistant hybrids. There is limited information about the mode of inheritance for GLS resistance in regionally adapted germplasm. This study was initiated to determine gene action controlling GLS resistance. Seventy-two hybrids were generated by mating 27 inbred lines in a North Carolina design II scheme. Experimental and check hybrids were evaluated in an 8 by 12 
-lattice design with two replications at three locations, during the 2004–2005 season. There was significant variation among the hybrids for GLS resistance and yield. Inbreds L13, L15, L18, L19, and L24, from A, N3, B, K, and SC heterotic groups, respectively, contributed high levels of resistance to hybrids. Both general combining ability (GCA) and specific combining ability (SCA) effects were highly significant (P < 0.01), but the predominance of GCA for GLS (86%) and yield (74%) indicated that additive effects were more important than nonadditive gene action in controlling both traits. Hybrids ranked similarly for GLS across environments, suggesting that few significant crossover genotype by environment interactions, which would cause problems in hybrid selection, were observed. Overall, results indicated that it would be readily possible to develop inbred lines with high GLS resistance from this germplasm.
Abbreviations: ART, Agricultural Research Trust; GCA, general combining ability • GLS, gray leaf spot • RARS, Rattray Arnold Research Station • SCA, specific combining ability.
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ACKNOWLEDGMENTS
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We are grateful to the Rockefeller Foundation for supporting this research. We also thank CIMMYT and Seed Co. Ltd. for providing maize germplasm for the study.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication April 3, 2007.
Gene Action Controlling Gray Leaf Spot Resistance in Southern African Maize Germplasm
John Dereraa,*,
Pangirayi Tongoonaa,
Kevin V. Pixleyb,
Bindiganavile Vivekc,
Mark D. Lainga and
Neil C. van Rijd
a African Centre for Crop Improvement, Univ. of KwaZulu-Natal, P. Bag X01, Scottsville 3209, Pietermaritzburg, Republic of South Africa
b CIMMYT Int., Apdo. Postal 6-641, Mexico, D.F. 06600, Mexico
c CIMMYT-Zimbabwe, P.O. Box MP163, MT. Pleasant, Harare, Zimbabwe
d Crop Protection, Cedara, KwaZulu-Natal Dep. of Agriculture and Environmental Affairs, P. Bag X9059, Pietermaritzburg, 3200, Republic of South Africa
* Corresponding author (Dereraj{at}ukzn.ac.za).
Gray leaf spot disease (GLS; caused by Cercospora zeae-maydis Tehon and Daniels) is among the major maize (Zea mays L.) production constraints in southern Africa. Maize is predominantly grown by small-scale farmers without fungicides; hence, there is need to develop GLS resistant hybrids. There is limited information about the mode of inheritance for GLS resistance in regionally adapted germplasm. This study was initiated to determine gene action controlling GLS resistance. Seventy-two hybrids were generated by mating 27 inbred lines in a North Carolina design II scheme. Experimental and check hybrids were evaluated in an 8 by 12 -lattice design with two replications at three locations, during the 2004–2005 season. There was significant variation among the hybrids for GLS resistance and yield. Inbreds L13, L15, L18, L19, and L24, from A, N3, B, K, and SC heterotic groups, respectively, contributed high levels of resistance to hybrids. Both general combining ability (GCA) and specific combining ability (SCA) effects were highly significant (P < 0.01), but the predominance of GCA for GLS (86%) and yield (74%) indicated that additive effects were more important than nonadditive gene action in controlling both traits. Hybrids ranked similarly for GLS across environments, suggesting that few significant crossover genotype by environment interactions, which would cause problems in hybrid selection, were observed. Overall, results indicated that it would be readily possible to develop inbred lines with high GLS resistance from this germplasm.
Abbreviations: ART, Agricultural Research Trust; GCA, general combining ability • GLS, gray leaf spot • RARS, Rattray Arnold Research Station • SCA, specific combining ability.
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INTRODUCTION
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GRAY LEAF SPOT (GLS; caused by Cercospora zeae-maydis Tehon and Daniels) disease is among the major maize (Zea mays L.) production constraints in the humid production ecologies in Africa. Gray leaf spot reduces yield by damaging photosynthetic tissue, especially in leaves of susceptible cultivars. Further yield reduction occurs through increased stem and root lodging. Previous studies have reported varying, but high levels of yield loss that is attributable to GLS. Munkvold et al. (2001) reported economic losses of $100 million in Iowa and 11 to 69% yield reduction in the USA and South Africa. Many previous studies have reported losses averaging ±20% in the USA (Huff et al., 1988; Elwinger et al., 1990; Donahue et al., 1991). Thus, GLS has the potential to pose a serious threat to maize production in the humid ecologies of southern Africa with implications for food security; in this region maize is the major staple, especially for the poor. Maize is predominantly grown by small-scale farmers without fungicides; hence there is need to breed for resistance in cultivars.
Genetic analysis of GLS resistance in southern African adapted maize germplasm would provide the basis for devising breeding strategies. Previous studies have mostly concluded that additive played a larger role than nonadditive gene action (Cromley et al., 2002; Menkir and Ayodele, 2005), and in some cases general combining ability (GCA) accounted for 100% of variation for GLS resistance (Thompson et al., 1987; Ulrich et al., 1990). A literature survey did not find situations where specific combining ability (SCA) was larger than GCA variance. Resistance was controlled by five to six additive genes (Bubeck et al., 1993; Clements et al., 2000), dominance and epistatic gene action in some temperate (Coates and White, 1998; Clements et al., 2000) and South African germplasm (Hohls et al., 1995). Based on restriction fragment length polymorphism analysis, three quantitative trait loci located on chromosomes 1, 4, and 8 have been reported to have large effects and accounted for about 56, 14 and 11% of variance for GLS resistance, respectively (Saghai Maroof et al., 1996). Another study using amplified fragment length polymorphisms also reported three quantitative trait loci on chromosomes 1, 3, and 5, which explained about 37, 10 and 11% of the variance, respectively (Lehmensiek et al., 2001). Selection and hybridization which utilize additive and nonadditive gene action can be used to improve GLS resistance.
Despite its regional significance there is little published information about inheritance of GLS resistance in African germplasm. Some early genetic studies in South Africa (Gevers et al., 1994; Hohls et al., 1995) used materials that might not be adapted to the more tropical areas of the subcontinent. Moreover the mode of inheritance of GLS resistance in germplasm developed over the past decade has not been established. The current study was conducted to determine gene action controlling GLS resistance in experimental maize hybrids derived from commonly used southern African germplasm.
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MATERIALS AND METHODS
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Germplasm
Inbred lines were selected from the major heterotic patterns that form the basis of hybrid-oriented programs in southern Africa (Table 1
). Gevers and Whythe (1987) and Mickelson et al. (2001) presented detailed accounts of these heterotic patterns. Before making crosses, 27 inbred lines were divided into subgroups of three each, according to their heterotic orientation and resistance status, based on their pedigree information. A North Carolina design II scheme was used to make hybrids in sets (Hallauer and Miranda, 1988). Three inbred lines from one subgroup were designated as female and crossed with three inbred lines from another subgroup used as male parents to form nine hybrids in each set. To achieve a balanced mating design, each subgroup of three lines was used once as female and once as male in different sets (Table 1). Eight out of the possible nine hybrid sets were successful and produced sufficient seed for evaluation in trials. One set of crosses failed to yield adequate seed due to severe damage of the lines L10, L11, and L12 by Maize streak virus disease (Set 9, Table 1). The inbred lines had different levels of GLS resistance, hence crosses fitted into four broad groups as (i) susceptible x susceptible (S x S); (ii) resistant x resistant (R x R), (iii) resistant x susceptible (R x S), and (iv) susceptible x resistant (S x R) lines. Overall 72 single crosses (eight sets x nine hybrids) were generated. Commercial hybrids of different maturity: Hybrid 1 (early), Hybrid 2 (medium), Hybrid 3 (late), and Hybrid 4 (late) were used as resistant checks; whereas Hybrid 5 (late), Hybrid 6 (late), Hybrid 7 (early), and Hybrid 8 (early) were used as susceptible checks. Sixteen promising and widely grown hybrids were also included.
Experimental Design
Hybrids were evaluated for GLS resistance at the Rattray Arnold Research Station (RARS) (1341 masl; 17°40' S, 31°13' E) and at the Agricultural Research Trust (ART) farm (1527 masl; 17°41' S, 31°04' E) in Zimbabwe; and at the Cedara Agricultural Institute (Cedara) (1076 masl; 29°31' S, 30°17' E) in South Africa, during the 2004–2005 season. Ninety-six hybrids (72 experimental and 24 checks) were evaluated in a 12 by 8 -lattice design with two replications of two rows by 3 to 5 m long plots. Plant population densities were 44,000 plants ha–1 at Cedara, and 53,000 plants ha–1 at the RARS and at the ART farm. Different fertilizer rates were applied as follows: 120 kg N–33 kg P–44 kg K ha–1 at Cedara; 208 kg N–35 kg P–21 kg K ha–1 at RARS; and 250 kg N–65 kg P–25 kg K ha–1 at the ART farm. Total rainfall amount ranged from 787 mm at the ART farm, 826 mm at RARS, and 885 mm at Cedara in the 2004–2005 season. Standard cultural practices were followed, but fields were left to natural disease inoculation. Gray leaf spot disease severity was assessed at mid silking (50% silk emergence) and at grain hard dough stages using a rating scale of 1 (resistant) to 9 (susceptible). Yield of shelled grain (adjusted to 12.5% H2O) was measured on a whole plot basis, and relative yield for each hybrid was calculated as percentage of mean yield for the trial.
Data Analyses
General analyses of variance for the lattice designs were performed for all hybrid data including check hybrids. Genetic analyses for GLS and yield data of experimental hybrids were performed in SAS using a fixed effects model for the experimental hybrids across locations for the individual sets and pooled over sets. Experimental hybrid data were analyzed as a randomized complete block design (RCBD) because the relative efficiency of the lattice designs over RCBD was not significant. Data were analyzed over sets and across locations using the following linear model (Hallauer and Miranda, 1988):
where i = 1, 2, 3; j = 1, 2, 3; k = 1, 2; p = 1, 2, 3, 4, 5, 6, 7, 8; q = 1, 2,3; and the terms of the model were defined as follows: Yijkpq denotes the GLS score or yield of the hybrid of a mating of the ith female line, the jth male line, in the kth replication, within set p and in the qth environment; µ = grand mean; Sp = the average effect of the pth set; gi(Sp) = the GCA effect common to all hybrids of the ith female line nested within pth set; gj(Sp) = the GCA effect common to all hybrids of the jth male line nested within pth set; hij(Sp) = the SCA effect specific to hybrid of the ith female and jth male line nested within pth set; Eq = average effect of qth environment; rk(SE)pq = the effect of the kth replication nested within the pth set and qth environment; (ES)pq = the interaction between set effects and the environment; (Eg)iq(Sp) and +(Eg)jq(Sp) = the interaction between environment and GCA nested within sets; (Eh)ijq(Sp) = the interaction between environment and SCA nested within sets, and eijkpq = random error.
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RESULTS AND DISCUSSION
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Gene Action
There was a highly significant and positive correlation (r = 0.78, P < 0.01) between the first GLS rating scores at the mid silking stage and the second rating at the grain hard dough stage, suggesting that either of the two could be used for hybrid selection. However, the second GLS rating scores had lower standard error (CV = 17%) than the first rating (CV = 23%); therefore, only the second GLS ratings were used for the genetic analyses. Menkir and Ayodele (2005) reported that rating at 26 to 38 d after 50% silk emergence was effective in discriminating hybrids.
Hybrids displayed significant variation for GLS scores and grain yield (Table 2
). Analyses pooled over sets showed highly significant (P 
0.01) variance among sets, GCA due to males within sets (GCAm), GCA due to females within sets (GCAf), and SCA within sets for both GLS and yield. Significant GCA and SCA mean squares indicated that both additive and nonadditive gene action controlled GLS resistance and yield. However, GCA effects were substantially greater than SCA effects, indicating a preponderance of additive over nonadditive gene action. In total, GCA effects (i.e., GCAm plus GCAf) accounted for 86 and 74% of the cross sum of squares for GLS and yield, respectively. Previous studies also concluded that additive was predominant over nonadditive gene action for GLS resistance. Menkir and Ayodele (2005) reported that GCA accounted for >70% of the variation for GLS resistance in tropical hybrids among 24 inbred lines. Elwinger et al. (1990) reported that GCA was 1.5 to 11.5 times larger than SCA effects in temperate germplasm. A few crosses (L9/L13; L9/L15; L7/L13) from R x S sets displayed high GLS resistance (data not shown) suggesting that alleles responsible for resistance exhibited dominance effects in these crosses.
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Table 2. Mean squares from the analysis of variance of gray leaf spot (GLS) scores and yield of hybrids pooled over sets across three locations during the 2004–2005 season.
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The GCAm effects were 1.5 times larger than GCAf effects and the GCAm sum of squares contributed more (53% of total variance) than the GCAf sum of squares (33% of total variance) for GLS scores. However, the ratio of their mean squares (i.e., the F-test or variance ratio as suggested by Kearsey and Pooni [1996]) was not statistically significant at P = 0.05, suggesting that maternal or cytoplasmic effects were not significant for GLS resistance. The trend of greater variation explained by male than female GCA effects for GLS effects was also found by Menkir and Ayodele (2005), who reported 44% for GCAm and 31% for GCAf in tropical maize germplasm. In contrast, for yield, the GCAf mean square was significantly (P < 0.05) larger (2.8 times) than GCAm (Table 2). GCAf contributed 54% and GCAm 20% for the cross sum of squares for yield. Based on this, we suggest that cytoplasmic or maternal effect genes probably influenced yield.
Analysis of the individual sets showed that GCAf effects were significantly larger than GCAm effects in three sets, whereas GCAm effects were significantly larger than GCAf effects in four other sets for GLS scores, depending on which subgroup of lines were used as the resistant sources in each set (Table 3
). Generally, resistant sources had larger GCA variance than the susceptible parents in five sets when used either as male or female (R x S or S x R) (Table 3). This probably reflects the fact that the resistant parents differed more for GLS resistance than the susceptible parents (i.e., the resistant parents had varying degrees of resistance whereas the susceptible parents were similarly susceptible). The most resistant sets were R x S and R x R (Sets 5 and 7, respectively), and as expected the S x S set (Set 2) had the largest GLS score (Table 3). All R x R and R x S sets had significantly (P < 0.001) lower GLS scores than the S x S set (Table 3), but when averaged over the four broad classes, resistance was highest in R x R (2.9), followed by S x R (3.2), R x S (3.4), and lowest in S x S (4.6) group. This trend is consistent with expectations for additive gene action, but does not provide clear evidence for maternal effects for GLS resistance as the R x S and S x R groups were not significantly different. However, the number of R x S and S x R sets was not balanced (two versus three, respectively). These results differ from the previous study by Menkir and Ayodele (2005), who reported large differences between R x S, and S x R groups and suggested the importance of cytoplasmic genes because resistance was higher when the resistant line was used as female.
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Table 3. Mean squares from the analysis of variance of gray leaf spot (GLS) scores in eight hybrid sets across three locations during 2004–2005 season.
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Analysis of the individual sets for grain yield indicates that, consistent with overall trends described previously, maternal effects were probably important for inheritance of yield in three sets. The GCAm effects were not significant (P 0.05) in three of the six sets in which hybrids differed significantly for yield (Table 4
). The ratio of GCAf/GCAm ranged from 4.6 (Set 2) to 10.2 (Set 1) and 17.2 (Set 6) in these sets, suggesting greater contribution by female sources. Predominance of GCAf over GCAm effects suggests importance of maternal effects for yield in these sets, but this result should be interpreted with caution because the model did not test reciprocal differences, which would be more reliable for estimating maternal effects. A survey of the literature indicated that except for some quality traits and seed size (Singh, 1993) and days to silk emergence (Khehra and Bhalla, 1976), maternal effects have generally not been reported to be important for yield in maize.
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Table 4. Mean squares from the analysis of variance of yield in eight hybrid sets across three locations during 2004–2005 season.
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Environmental Effects
There were significant (P 0.01) location main effects, and sets x location and hybrid x location interaction effects for both GLS and yield (Table 2), indicating the importance of environment and genotype x environment interaction effects for these traits. Location main effects were significant for GLS and yield in all sets (Tables 3 and 4). The GLS severity was greatest at Cedara (mean GLS score = 7), followed by RARS (mean GLS score = 6) and lowest at ART (mean GLS score = 4). Cedara (29° S) is a temperate environment, while ART and RARS (17° S) are subtropical. Cedara is also situated within the Natal mist-belt which makes it a favorable location for GLS development. Fields at Cedara were also under reduced tillage, which maintains a high soil-based disease inoculum, whereas deep plowing is practiced at ART and RARS. Large location main effects for GLS were a reflection of differences in the magnitude of disease severity at the three locations. Unlike the RARS (GLS scores 1–8) and ART farm (GLS scores 1–5), there were no hybrids with GLS score of 1 at Cedara (GLS scores 2–9); but hybrids were significantly (P < 0.01) different for GLS and yield at all locations, and disease pressure was adequate for selection. For example, susceptible check hybrids Hybrid 6 (late maturing), Hybrid 5 (late), and Hybrid 8 (early) had GLS scores of 9 at Cedara, 7 to 8 at the RARS, and 5 at the ART farm (data not shown). Location x GCAf effects were not significant, whereas location x GCAm effects were significant in four of the eight sets for GLS (Table 3). Location x SCA effects were significant only in one set for both GLS (Table 3) and yield (Table 4).
It appears that genotype by environment interaction effects would not cause any difficulty in selection for GLS resistance because the hybrids were ranked similarly for GLS resistance in the different locations. Despite the differences in disease severity, the rank correlations for GLS scores of the hybrids were positive and highly significant, ranging from 0.79 (P < 0.01) between Cedara and RARS, 0.80 (P < 0.01) between ART and RARS, and 0.84 (P < 0.01) between Cedara and ART. These results are in agreement with Lipps et al. (1998) who reported a similar hybrid ranking for GLS resistance over 22 environments with different GLS severity in the USA. Similar hybrid ranking suggested that few significant crossover interactions, which would cause problems in hybrid selection, were observed.
GLS Resistance and Grain Yield
Inbred lines L13, L15, L18, L19, and L24, from the A, N3, B, K, and SC heterotic groups, respectively (Table 1), contributed high levels of GLS resistance in hybrids in their respective sets and were constituents of the top 10 ranking hybrids (data not shown). The fact that resistance was found in at least five major heterotic groups suggests that it would be easy to develop resistant hybrids between complementary inbred lines. These lines were also parents of hybrids that combined high relative yield (109–119%) with high GLS resistance (GLS score of 1–2). Inbreds L13 and L15 were parents in the most resistant sets 5 and 7 as male and female, respectively (Table 3). The hybrids L9/L15, L15/L21, L24/L18, L5/L25, L5/L27, L8/L15, and L19/L5 displayed high yield and GLS resistance (GLS score = 1) superior to resistant checks. Resistant checks Hybrid 2, Hybrid 3, and Hybrid 4 were ranked 26 to 32 for yield (101–108%) and had average GLS scores of 2. Susceptible check hybrids had 77 to 96% relative yield and high GLS scores (7–9), but some susceptible experimental hybrids (GLS scores = 6–9) also displayed high relative yield (109–127%). For example, despite high GLS severity at Cedara, L1/L9 (GLS score = 9), L17/L2 (GLS score = 7), L4/L26 (GLS score = 7), L21/L4 (GLS = 6), and L3/L7 (GLS = 7) out-yielded most resistant hybrids. Possibly, these hybrids were tolerant to GLS, suggesting that resistance could be improved by selecting for yield under GLS. From the foregoing, L5 and L4, which are late maturing lines, played a significant role in enhancing yield as both male and female parents in Sets 2 and 4 (Table 4).
Although significant, the phenotypic correlations between yield and anthesis date (r = –0.158; P = 0.008), and between yield and mid silking date (r = –0.181; P < 0.0023) were too weak to explain the yield differences among hybrids. The correlations of GLS scores with anthesis and silking dates (r = –0.308 and –0.313, both at P < 0.001) were also too weak to suggest that some hybrids escaped GLS. In addition, there was no significant correlation between GLS resistance and yield in all locations (r = 0.1–0.3; P = 0.3009). This result contrasts with our experience during severe GLS epidemics and also contrasts with numerous reports in the literature of significant grain yield losses caused by GLS; but these findings suggest that there is some level of tolerance to GLS in this germplasm, because disease severity was particularly high at Cedara during the 2004–2005 season. Although not measured, it was observed that the plant stalks remained green with negligible stalk lodging after the leaves had been blighted by GLS, suggesting reduced infection of the stems. The stalks might remain green and continue to photosynthesize and contribute to grain yield by translocating assimilates to ears which could compensate for the losses in green leaf area under stress in susceptible hybrids (Lipps et al., 1998). The fact that there was a group of hybrids that combined high GLS resistance with high yield, and another group that exhibited high yield and GLS susceptibility partly explains the lack of the correlation of yield with GLS scores.
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CONCLUSIONS
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Significant GCA and SCA effects indicated importance of both additive and nonadditive gene action in controlling GLS resistance. Predominance of GCA effects, however, highlighted the greater role of additive relative to nonadditive gene action. Partial to almost complete dominance also conditioned GLS resistance, but only in a few hybrids. Similar hybrid ranking suggested that few significant crossover interactions, which would cause problems in hybrid selection, were observed. Overall, results indicated that it would be readily possible to breed for high GLS resistance from this set of germplasm.
We are grateful to the Rockefeller Foundation for supporting this research. We also thank CIMMYT and Seed Co. Ltd. for providing maize germplasm for the study.
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication April 3, 2007.
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ABSTRACT
INTRODUCTION
