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Crop Science 40:948-954 (2000)
© 2000 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Identification of Tropical and Temperate Maize Populations Having Favorable Alleles for Disease Resistance

Aldi Kraja, John W. Dudley and Donald G. White

Department of Crop Sciences, University of Illinois at Urbana-Champaign, 1102 S. Goodwin Ave., Urbana, IL 61801 USA

jdudley{at}uiuc.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
A possible use of nonelite germplasm is as a source of alleles for disease resistance. Our objective was to determine the value of tropical and temperate maize (Zea mays L.) germplasm accessions as sources of alleles to improve disease resistance of the Corn Belt hybrid FR1064 x LH185. A group of tropical populations and hybrids from the Germplasm Enhancement of Maize Project (GEM) crossed to either Mo17 or B73 (temperate inbreds), was studied. In addition, a group of temperate accessions was evaluated. Reaction to southern corn leaf blight (SCLB) [Bipolaris maydis (Nisikado & Miyake) Shoemaker = Helminthosporium maydis Nisikado & Miyake], northern corn leaf spot (NCLS) [Bipolaris zeicola (G. L. Stout) Shoemaker = Helminthosporium carbonum Ullstrup Races 2 and 3], gray leaf spot (GLS) (Cercospora zeae-maydis Tehon & E.Y. Daniels), northern corn leaf blight (NCLB) [Exserohilum turcicum (Pass.) K. J. Leonard & E. G. Suggs = Helminthosporium turcicum Pass., Races 1, 1, 2, 23N and three unidentified races] and common rust (Puccinia sorghi Schwein) was studied using Dudley's method for identifying populations with favorable alleles. All accessions had favorable dominant alleles for resistance not present in FR1064 x LH185 for common rust, GLS, and SCLB. Four accessions had a number of favorable alleles for resistance not significantly different from the best accession for all five diseases. Crosses of the accessions containing Mo17 to FR1064 and LH185 were less susceptible to SCLB, NCLB, and rust than crosses of accessions containing B73. At least seven of the best 10 populations were tropical x Mo17 crosses for SCLB, NCLB, and rust when populations were ranked for presence of favorable alleles for resistance not present in either LH185 or FR1064. Net value statistics for tropical x B73 and tropical x Mo17 accessions differed in indicating whether backcrossing or selfing from the F1 is more desirable. Therefore, if tropical populations are crossed to Corn Belt inbreds to increase adaptation, comparisons among tropical populations should only be made when all populations being compared have been crossed to the same Corn Belt inbred.

Abbreviations: GEM, germplasm enhancement of maize • GLS, gray leaf spot • NCLB, northern corn leaf blight • NCLS, northern corn leaf spot • SCLB, southern corn leaf blight


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
IN THE USA, commercially used dent corn hybrids are the result of intensive selection within only a small part of the total genetic resources of corn. Often the most improved inbreds have been intermated to extract better inbred lines. In the early 1970s a widespread outbreak of SCLB, race T in the USA increased awareness of the importance of finding different sources of disease resistance. To broaden the genetic base of corn germplasm many programs have evaluated exotic materials and used them in breeding programs (Wellhausen et al., 1957; Ullstrup, 1978; Hallauer, 1978; Eberhart, 1992).

Sources of resistance to the diseases studied are known. For SCLB, resistance has been identified in tropical corn (Holley and Goodman, 1989). Sources of resistance to NCLB, including single dominant genes, are available (Hooker and Perkins, 1980; Lipps et al., 1997; Lambert and White, 1997; Pataky et al., 1998). Northern corn leaf spot is usually only a problem on B73 type inbreds in seed production fields. A number of single gene sources of resistance to common rust have been identified (White, 1999). The lack of adequate resistance of commercial hybrids to GLS has made finding sources of resistance a high priority (Coates and White, 1998; Lipps et al., 1996). Even though sources of resistance for the diseases studied are known, new sources of resistance are valuable. Evaluation of the accessions from the GEM project for the presence of favorable alleles may be of value in identifying new sources of resistance.

Dudley's theory for the transfer of favorable alleles from donor populations to improve an elite hybrid (Dudley, 1987, 1988a) assumes that for any two lines there are four classes of alleles (i, j, k, and l). Elite Inbred 1 (I1) is homozygous for favorable alleles (++) in classes i and j. Elite Inbred 2 (I2), is homozygous for favorable alleles (++) in classes i and k. For class l both elite inbreds have alleles that are less favorable (--). In a population, the average frequency of favorable alleles in each class is i, j, k, and l, respectively. Complete dominance and negligible epistasis are assumed (Dudley, 1987, 1988a). Assuming µ is one-half the difference between homozygous genotypes, the statistics llµ', jjµ, and kkµ are estimated and interpreted. The symbols j, k, and l represent the number of loci in classes j, k, and l. Thus, llµ' represents the relative number of favorable dominant alleles in a population at class l loci. The statistics jjµ and kkµ represent the relative number of less favorable alleles at class j and k loci, respectively. From estimates of these statistics and the relationship of a population to I1 or I2, determination of which elite line to improve and whether to self or backcross can be made. This theory for assessing the value of populations as donors of favorable alleles has been shown to give results in agreement with known proportions of favorable alleles (Hogan and Dudley, 1991; Pfarr and Lamkey, 1992a, 1992b; Fabrizius and Openshaw, 1994; Stoj4.gif" BORDER="0">in and Kannenberg, 1995). In addition, the method has been used to identify populations having favorable alleles for a number of traits (Dudley et al., 1996; Dudley, 1988b; Pfarr and Lamkey, 1992a, 1992b; Cartea et al., 1996). These results suggest Dudley's method has been effective for identifying populations carrying favorable alleles for several quantitative traits. However, the method has not been used for identifying sources of disease resistance.

In the U.S. GEM project, tropical populations are being crossed to Corn Belt inbreds (Pollak, 1997; Salhuana et al., 1998). If tropical population x inbred crosses are to be evaluated, then the question of whether the populations should all be crossed to the same inbred prior to evaluation becomes important.

Dudley (1988a) suggested the appropriate hybrid to be improved was the best hybrid available in an area. To evaluate germplasm with unknown potential, an elite hybrid representative of a high percentage of the types of germplasm being grown in an area would provide information most useful to breeders. The hybrid FR1064 x LH185 is a hybrid representative of the Stiff-Stalk Synthetic x Lancaster heterotic pattern widely used in the U.S. Corn Belt.

The main objectives of this study were (i) to determine the value of GEM accessions as sources of favorable alleles to improve the hybrid FR1064 x LH185 for resistance to northern corn leaf blight, southern corn leaf blight, gray leaf spot, northern corn leaf spot, and rust; (ii) to compare the effect of crossing the same tropical populations to B73 or Mo17 on the value of the populations as sources of favorable alleles; (iii) to ascertain if a particular accession may be a source of genes for resistance to more than one disease; (iv) to determine, where favorable alleles are present, whether selfing should begin in a population x elite inbred cross or whether the F1 should be backcrossed to the elite line prior to selfing.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Genetic Materials
The hybrid to be improved was FR1064 (I1) x LH185 (I2). FR1064 was provided by Illinois Foundation Seeds and LH185 by Holden's Foundation Seeds. Seed of the tropical x B73 and tropical x Mo17 accessions used was provided by Linda Pollak, coordinator of the GEM project. Five accessions were crosses of tropical populations by B73. Ten were tropical population by Mo17 crosses. Seven tropical hybrids crossed to B73 were also crossed to Mo17. Thus, 15 different tropical populations and seven tropical hybrids were evaluated. Twelve temperate accessions were used directly. Thus, 34 different germplasm sources (15 tropical populations, seven tropical hybrids and 12 temperate accessions) were evaluated. The Ames accession numbers or PI numbers of these germplasm sources are shown in Table 1 of Kraja and Dudley (2000).


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  		Table 1 Mean leaf area blighted (averaged across 2 yr and all rating dates) for the elite hybrid and its parents.{dagger}

 
In 1996, the 41 accessions were crossed to both FR1064 and LH185 using the inbreds as pollinators. From each cross 100 to 150 ears were harvested and the seed bulked. At the same time, seed of the hybrid FR1064 x LH185 was produced.

Evaluation Trials
For disease resistance evaluation, each cross plus the hybrid FR1064 x LH185 was included in an {alpha}(0, 1) generalized lattice design with 85 entries (41 populations crossed to 2 elite inbreds, FR1064 x LH185, and 2 check hybrids), with 17 blocks, and two replications. To avoid competition with the taller crosses, the inbreds FR1064 and LH185 were planted in 10 replications of a randomized complete block design in a separate experiment in the same environments as the hybrid trials. Plots were single rows 5.3 m long with 0.76 m between rows. Standard cultural practices were used. Plots were thinned at the four- to five-leaf stage to a stand of 57100 plants ha-1. Separate experiments were grown for each of the five diseases. Each experiment was grown in 2 yr (1997 and 1998) on the Crop Sciences Research and Education Center of the University of Illinois at Urbana-Champaign.

Pathogens and Inoculation Methods
For disease resistance evaluation conidia suspensions of the causal organisms for NCLB, GLS, SCLB, NCLS, and common rust were used. The inoculum was prepared following standard procedures. (Lambert and White, 1997; Coates and White, 1998). The inoculum for rust, provided by J.K. Pataky, of Crop Sciences Department at the University of Illinois at Urbana-Champaign, was prepared following standard procedures (Pataky, 1987).

When plants were 20 to 50 cm tall they were sprayed in the leaf whirls and under the leaves with a spore suspension of the appropriate fungus. This procedure was repeated three times at weekly intervals. In 1997, plots of the NCLB, SCLB, and NCLS experiments were irrigated regularly with a mist irrigation system. The GLS plots were irrigated both years (Coates and White, 1998)

Data Collection
Percentage blighted leaf area for NCLB, SCLB, and GLS was rated every {approx}15 d. For GLS and SCLB, percentage blighted leaf area was recorded at the end of July and middle of August in 1997 and at the end of July, middle of August, and the end of August during 1998. For NCLB, plots were evaluated at the end of July, middle of August, and the end of August each year. For NCLS and rust disease development was slow in both years and differences were not detected until later in the season, thus only one evaluation, at the end of August, was recorded each year. For NCLB, SCLB, GLS, and NCLS all individual plants in a row were rated. For rust, five random plants from each row were evaluated. The data analyzed were the means on a plot basis expressed in percentage blighted leaf area.

Statistical Analysis
Proc GLM (SAS Institute, 1990, p. 891–996) and Proc Mixed (Littell et al., 1996) of SAS were used for calculating the components of variance and for F tests of effects. Years were treated as random effects. In the {alpha}(0,1) experiments, replications nested in environments and blocks nested in replications also were treated as random effects. Hybrids and time of evaluation were considered fixed effects. In the randomized block experiments, blocks were treated as random effects. Inbreds and time of evaluation were considered as fixed effects. Entry means over rating dates, reps, and environments, adjusted for design effects, were produced using Proc Mixed and lsmeans of SAS. For diseases with multiple rating dates, Spearman rank correlations among rating dates were calculated. Because these correlations were high (>0.9), means averaged across times of evaluation, reps, and environments were used to calculate statistics from Dudley's method of identifying parents.

For calculating llµ' and similar statistics (Dudley, 1987, 1988a), a program in C++ was written. The standard errors for these statistics were calculated as the square root of the variance of the linear functions associated with each statistic. Estimates of llµ' were considered different from zero if their absolute value was greater than twice their standard error. Differences between llµ' values were considered significant if they exceeded twice the standard error of the difference. Means were considered different if the difference exceeded twice the standard error.

The relative relationship of populations to FR1064 or LH185 was determined using relationship values based on grain yield data (Dudley, 1987, 1988a). The line to which a population was most closely related was considered the line to be improved. Decisions to backcross or self were made by comparing llµ' with either jjµ or kkµ as proposed by Dudley (1988a).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Means
Mean disease ratings for LH185 were significantly lower than for FR1064 for all diseases except common rust (Table 1). The hybrid, FR1064 x LH185, had significantly lower disease ratings than either parent for NCLB, SCLB, NCLS, and rust. For GLS, the hybrid mean was not significantly different from the mean of LH185.

Means of crosses of the GEM accessions to FR1064 and LH185 generally reflected the differences between the parents in that means of crosses to LH185 were significantly lower than means of crosses to FR1064 for GLS, SCLB, NCLB, and NCLS. For rust, tropical hybrids or populations x B73 had lower rust ratings when crossed to LH185 than when crossed to FR1064 (Table 2) . The reverse was true for the tropical x Mo17 crosses and temperate accessions crossed to LH185 or FR1064.


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  		Table 2 Mean leaf area blighted averaged across 2 yr and all rating dates for tropical x B73 and tropical x Mo17 accessions crossed to FR1064 (I1) and LH185 (I2)

 
For the seven tropical populations crossed to both B73 and Mo17, average disease ratings were significantly lower for crosses to Mo17 than for crosses to B73 for GLS, SCLB, NCLS, and NCLB (Table 2). The effect of Mo17 was greatest for SCLB, NCLS, and NCLB.

Breeding Value of Populations
llµ' Estimates
Because low values on the rating scale indicate less disease and dominance is for resistance, the largest negative values of llµ' indicate the presence of the largest number of favorable alleles. All the accessions studied (Tables 3, 4, and 5) , had llµ' values that were negative and significantly different from zero for GLS, SCLB, and rust (Table 6) . Therefore, all these accessions contain dominant favorable alleles not present in the elite inbreds FR1064 and LH185. Most accessions had significant numbers of favorable alleles for NCLB and NCLS.


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  		Table 3 Relative frequency of favorable alleles not present in FR 1064 x LH185 and net values for tropical populations crossed to both Mo17 and B73

 

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  		Table 4 Relative frequency of favorable alleles not present in FR1064 x LH185 and net values for tropical x B73 and tropical x Mo17 populations

 

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  		Table 5 Frequency of favorable alleles not present in FR1064 x LH185 and net values for temperate accessions

 

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  		Table 6 Summary of significant and nonsignificant estimates of llµ' and net values for five diseases

 
Correlation of llµ' Values among Diseases
Correlations between llµ' values for different diseases measure the degree to which accessions have favorable alleles for more than one disease. These correlations were all highly significant (P < 0.01) except for the correlation between NCLS and rust, where the significance was at the 0.05 level (Table 7) . The relatively high correlation between llµ' values for NCLB, SCLB, NCLS, and GLS suggest that it should be possible to select donors that would contribute favorable alleles for more than one disease. When populations having llµ' values not significantly different from the best population for particular diseases were compared, four populations (Brazil 051501 x Mo17, Dominican Republic 150 x Mo17, DK888 x B73, and XL380 x Mo17) were not significantly different from the best population for all five diseases. Five additional populations (B830 x Mo17, Pasco 14 x M017, Puerto Rico GP3 x Mo17, B844 x B73, and DK888 x Mo17) were not significantly different for at least four diseases, and five for at least three diseases. These findings could help in designing breeding strategies to improve corn for resistance to these diseases. Eberhart (1992), recommending strategies and methods to work with exotic germplasm, stressed that simultaneous selection for many desirable traits will result in very little gain from selection for any trait. In this study, the positive correlations among llµ' values for the different diseases suggest that populations can be selected that could improve an elite inbred for resistance to several diseases.


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  		Table 7 Correlations of llµ' values for 41 accessions and five diseases.{dagger}

 
Net Value Estimates
Net value estimates may be used to determine whether selfing directly from the F1 or backcrossing the F1 to the inbred parent will be most effective. If the net value estimate is negative and significant, backcrossing is indicated because the relative number of loci at which a loss in resistance might occur (jjµ or kkµ) is less than the number at which a gain might occur . If net values are nonsignificant or significantly positive, selfing in the F2 is indicated. For GLS, most of the populations had significant positive net value estimates indicating that llµ' was larger than jjµ or kkµ (Tables 3, 4, and 5). For rust, as for GLS, only one accession had a significant negative net value estimate. For GLS, this result suggests most accessions had more favorable alleles for resistance in class l than there were unfavorable alleles in classes j or k. However, for rust most of the populations had nonsignificant net values, indicating similar numbers of loci with favorable alleles in class l and of loci with unfavorable alleles in class j or k. Thus, for both GLS and rust, selfing directly from the accession x inbred cross would be suggested for obtaining a new inbred with increased disease resistance.

For NCLB and NCLS, most of the significant net value estimates were negative, indicating that backcrossing to the appropriate inbred prior to selfing would increase the probability of obtaining a new line with increased disease resistance (Tables 3, 4, and 5). For SCLB, approximately equal numbers of accessions had significant negative and significant positive net value estimates.

Effects of Crossing the Same Populations to B73 and Mo17
When tropical populations are crossed to Corn Belt germplasm to increase adaptation, the Corn Belt germplasm used will affect the performance of the exotic x Corn Belt cross. In general, germplasm with a Stiff-Stalk Synthetic background (B73 types) has less resistance to SCLB, NCLB, and NCLS than germplasm with a Lancaster background (Mo17 types). Both Lancaster and Stiff-Stalk germplasm are susceptible to GLS (D.G. White, 1999, unpublished). In this study, tropical x Mo17 crosses had consistently lower ratings (Table 2) for GLS, NCLB, SCLB, and NCLS than the same exotic populations crossed to B73 regardless of whether they were crossed to FR1064 or LH185.

The effect of crossing populations to Mo17 or B73 on estimates of llµ' varied with the disease. When accessions were ranked by magnitude of llµ', the top 10 accessions for SCLB were population x Mo17 crosses (Table 8) . For NCLB and rust, seven of the top 10 were Mo17 crosses. In contrast, seven of the top 10 accessions for NCLS were B73 crosses. Thus, the choice of B73 or Mo17 to provide genes for adaptation had a major effect on the ranking of the tropical population as a source of genes for disease resistance.


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  		Table 8 Top 10 accessions (in rank order for each disease) based on llµ' values{ddagger}

 
The line (B73 or Mo17) to which a population had been crossed influenced the need for backcrossing. For NCLB, all the populations crossed to Mo17 had significant negative net value estimates, indicating a need for backcrossing, while all the population x B73 crosses had significant positive or nonsignificant net value estimates (Tables 3 and 4). For SCLB, a similar pattern emerged, with all the crosses to B73 having significant positive net value estimates, while seven of the 17 populations crossed to Mo17 had significant negative net value estimates, indicating the need for backcrossing. For NCLS, 12 of the 17 populations crossed to Mo17 had significant negative net value estimates while none of the crosses to B73 had significant negative estimates. These results further emphasize the importance of using a common line as a parent when evaluating the genetic worth of tropical populations.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The choice of Mo17 or B73 as an inbred to cross to a tropical population for increasing adaptation had major effects on mean performance of crosses to elite inbreds, on ranking of accessions for presence of favorable alleles, and on the merit of selfing or backcrossing for developing resistant lines. These results suggest that if tropical populations are to be evaluated as sources of genes for disease resistance after being crossed to a Corn Belt inbred, all tropical lines being compared should be crossed to the same inbred.

Several of the GEM accessions studied could be sources of resistance to as many as three to five of the diseases studied. Nearly all of the accessions carried favorable alleles not present in FR1064 or LH185 for one or more diseases.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Supported in part by funds from The Illinois Agric. Exp. Stn. and by the GEM project.

Received for publication July 19, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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A. Kraja and J. W. Dudley
Identification of Tropical and Temperate Maize Populations Having Favorable Alleles for Yield and Other Phenotypic Traits
Crop Sci., July 1, 2000; 40(4): 941 - 947.
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