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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 8, Num. 4, 2000, pp. 345-353
African Crop Science Journal, Vol. 8. No. 4, pp. 345-353

African Crop Science Journal, Vol. 8. No. 4, pp. 345-353

Germplasm enhancement through cooperative research and breeding using elite tropical and U.S. Corn Belt maize germplasm

R.C. PRATT, P.E. LIPPS1, G. BIGIRWA2 and D.T. KYETERE2
Department of Horticulture and Crop Science, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, OH, USA 44691
1 Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, OH, USA 44691
2 Namulonge Agricultural and Animal Production Research Institute, P.O. Box 7084, Kampala, Uganda

(Received 10 August, 2000; accepted 15 October, 2000)

Code Number: CS00037

INTRODUCTION

The rationale for utilisation of exotic germplasm to enhance locally adapted cultivars is based on three principal arguments: 1) we may increase the genetic diversity of important crops and thereby reduce their genetic vulnerability to unpredictable biological and environmental hazards (Albrecht and Dudley, 1987; Goodman, 1999), 2) exploita-tion of exotic germplasm will reveal new heterotic associations that can enhance yield of elite cultivars (Crossa et al., 1987; Mungoma and Pollak, 1988; Parra and Hallauer, 1997), and 3) exotic germplasm can be a valuable donor of genes for special traits such as pest and disease resistance (Kramer and Ullstrup, 1959; Goodman, 1999), grain quality (Salhuana et al., 1994), tolerance of high plant population density (Hainzelin, 1998), and efficiency of nutrient uptake and distribution (Feil et al., 1992).

U.S. Corn Belt maize breeders have typically used introgression of exotic germplasm into locally adapted materials as a method of last resort to incorporate disease or insect resistance when none could be found in adapted material. Few U.S. hybrid maize cultivars contain any tropical or sub-tropical maize in their background (Gracen, 1986; Goodman, 1992; Parra and Hallauer, 1997). Temperate zone breeders have often found it difficult to use exotic open-pollinated accessions because the frequency of desirable alleles may be low, and they often will not tolerate inbreeding (Goodman, 1992). Late maturity and photoperiod sensitivity often must be addressed, and even when this has been accomplished, susceptibility to lodging and smut (incited by Ustilago maydis), poor germination and seedling vigour in cold, wet soils, excessive plant height, and slow grain drying make the use of tropical germplasm in temperate regions quite challenging.

Additionally, evaluation, pedigree, and heterotic association data have been slow forthcoming in the literature for the majority of tropical materials. Experience and research with utilisation of maize genetic resources is rather limited, and breeders may be uncertain about the best methodology for incorporation of desirable traits found in unadapted germplasm. Salhuana (1988) indicates that many of these problems can be overcome, and better utilisation of maize germplasm made, through cooperative efforts.

Opportunities now appear better than before, and breeders in temperate regions are increasing their efforts to utilise exotic germplasm (Goodman, 1999). Substantial progress in the selection of inbred parental lines from coordinated, cooperative efforts such as the Germplasm Enhancement of Maize Project (GEM) using 25-50% exotic (either temperate or tropical) crosses (Salhuana et al., 1994) has been realised. Goodman (1993) and Goodman (1999) also have demonstrated that competitive yield results are possible following selection of improved tropical germplasm (50-100% tropical) for adaptation and disease resistance in North Carolina (30 degrees North latitude).

Breeders from tropical regions may wish to consider whether or not temperate germplasm might play a role in improving locally adapted germplasm. It is generally considered unlikely that temperate germplasm can be readily exploited by tropical breeders because of its poor adaptation and susceptibility to pests and diseases (e.g. southern rust and downy mildew). However, it should be noted that an example of successful introgression of temperate germplasm into tropical maize has been reported (Kim, 1987). It is now considered that many of the newer commercial hybrids in the tropics contain at least some temperate germplasm in the background of one parent (Goodman, 1999). Germplasm from temperate regions is considered a promising source of variation for enhancing tropical germplasm yield and harvest index characteristics, and specifically for the ability to remain productive under high population densities (Gracen, 1986; Parra and Hallauer, 1997; Hainzelin, 1998). Breeders in tropical areas also have the option to evaluate and utilise temperate germplasm (with introgressed tropical germplasm) selected in temperate environments and materials selected in tropical environments that contain temperate germplasm. It may also be of interest to tropical breeders to examine the progress and pitfalls of exotic germplasm utilisation experienced by temperate region breeders before initiating an introgression programme.

The exchange and introgression of improved materials between tropical and temperate breeding programmes has been limited. Parra and Hallauer (1997) proposed that transfer of germplasm between programmes would be enhanced if the performance in the respective areas were known. Troyer (1990) proposed that introgression of exotic germplasm might be fostered by first sending one’s elite material to a foreign environment, crossing with that region’s superior germplasm, backcrossing to one’s elite material, and then selecting for better adapted progeny before returning them to their place of origin. Saluhana (1988) advocated crossing exotic germplasm to local testers because so doing would allow new favourable alleles not in the tester to be revealed. Testcrossing also gives some adaptation to the material being tested, and new sources of heterosis may be revealed. Once desirable material has been identified, the breeder can continue a recurrent selection or backcross programme to provide useable material. If testing in an environment to which the exotic germplasm is initially unadapted proves successful, then appropriate selection methods should enhance the value of that germplasm for future introgression.

Multi-stage evaluation programmes, such as those conducted in the Latin American Maize Project and at North Carolina State University (Salhuana et al., 1994; Holland et al., 1996), effectively eliminate poorly performing accessions and enable breeders to concentrate their resources on the agronomically superior accessions that possess favourable alleles not present in locally adapted material. The cost of large scale efforts may not be realistic for many programmes, and sharing of germplasm between collaborators in different regions often faces many barriers due to the increasing restrictions imposed by phytosanitary laws and intellectual property rights. All cooperative efforts need not be comprehensive, and it is entirely possible that targeted programmes may be successfully undertaken on a smaller scale.

Our objective was to fix desirable traits in partially inbred progenies derived from a cross between improved tropical inbreds using conventional pedigree selection methods in a temperate environment. We wished to determine if breeding lines with multiple disease resistance and good agronomic potential could be selected. Given success with this approach, we then wished to evaluate agronomic performance and combining ability of the breeding lines in testcross evaluations conducted in both tropical (mid-altitude) and temperate environments where gray leaf spot (GLS) (Cercospora zeae maydis causal agent) and northern leaf blight (NLB) (Exserohilum turcicum causal agent) are endemic. Acceptable performance across maro-environments would indicate suitability of the germplasm for utilisation by breeders from both locations.

MATERIALS AND METHODS

In 1990, a set of hybrids containing tropical x tropical and tropical x Corn Belt inbreds converted to maize mosaic virus (MMV) resistance at the University of Hawaii, were observed in replicated plots at two locations in Ohio. The hybrid Hi34 x Tzi17 appeared to mature early enough to produce viable seed, stood well, and displayed good plant health, so it was selected for this study. Hi34 is a mid-late maturity, tropical, yellow flint derived from ‘Antigua 2D’ developed at the University of Hawaii (22 degrees North latitude). It is resistant to southern rust (causal agent Puccinia polysora), MMV and NLB, but is highly susceptible to maize streak virus (MSV) (Brewbaker et al., 1991). Tzi17, developed at the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria (7 degrees North latitude), is a mid-maturity, tropical, white semi-flint derived from RppSR(TZ). It was selected for tolerance to MSV and multiple foliar diseases incited by fungal pathogens. It is considered to be tolerant to ear rot (causal pathogen Fusarium moniliforme) (Kim et al., 1987; Brewbaker et al., 1991). Neither of the inbreds had been characterised for GLS resistance prior to the initiation of the study.

Seed from controlled self pollination of the hybrid Hi34 x Tzi17 was sent to the winter nursery in Puerto Rico and several hundred controlled self pollinations were made on unselected F2 plants to produce F2:3 progenies. In 1994, 193 F2:3 progenies were planted in unreplicated short-row plots in Columbus, Ohio (40 degrees North latitude). Controlled self-pollinations were performed on two plants per row. Root and stalk lodging data were recorded just prior to harvest in late October. Progenies that did not mature adequate seed for planting, or which displayed high levels of lodging, were culled. In 1995, one F4 progeny from each of the remaining 148 progenies (selection intensity = 77%) was planted in separate GLS and NLB screening nurseries near Guerne, Ohio in unreplicated short-row plots. The respective plots were inoculated with oat kernel inoculum containing Cercospora zeae-maydis or Exserohilum turcicum, respectively. Irrigation was applied to the plots using an overhead sprinkler system for approximately 30 minutes at dusk and 30 minutes pre-dawn. Visual estimates of disease severity on leaves at mid-plant height were recorded during the middle of September (approximately 3 weeks after flowering).

Progenies were selected at 11% intensity on the basis of disease reaction to both pathogens, lodging, and maturity. F5 seed of sibling progenies derived from each of 16 selected F2:4 progenies (28 lines total), were planted in the Illinois Foundation Seed Inc. winter nursery in Hawaii. Nine lines were culled because of root lodging, and multiple controlled self-pollinations were made within the remaining progeny (68% selection intensity). In 1996, up to five sister lines representing each of 19 F5 families were planted at the King Farm NLB and GLS nurseries (72 entries total). Several plants within each row were self-pollinated using controlled methods. Plants pollinated after 23 August were considered to have a low probability of surviving freezing temperatures in the Fall, so when a frost advisory was issued, late-pollinated ears were removed with the shank, and placed shank-end down in a vessel containing enough water to wet the shanks. The ears were left at room temperature for approximately one week and water was refreshed as needed. After this time they were placed in a forced air drying oven at ambient temperature until dry.

F6 seed from 23 selected ears obtained from 6 disease resistant F5 progenies (30% selection intensity) were planted in the field in 1997. Cold, wet soils were experienced following planting and emergence in some progenies was poor. Several plants within each line were self-pollinated and testcrosses to inbred testers B73 and Mo17 were made. Nineteen F6 progenies were selected late in the season based on ear placement, root and stalk lodging resistance, and maturity (83% selection intensity).

In 1998, Mo17 and B73 testcrosses containing the 19 selected breeding lines were planted near Apple Creek, and near Guerne, Ohio (two locations) in May and near Namulonge, Uganda during October. Yield data were obtained by machine harvest in Ohio and by hand harvest in Uganda. Stalk penetrometer resistance data were obtained using a force gauge modified according to methods described by Sibale et al. (1992) to ensure selection pressure for stalk strength was obtained when natural stalk rotting organisms were not present. Rind penetrometer readings were taken two times on 5 plants per plot, i.e., one week before and one week after flowering. Final plant stands were recorded and prior to harvest the number of stalks broken below the ear was recorded. Stalk lodging as a percentage of broken plants was calculated. Average rind puncture resistance values were calculated for correlation analysis with lodging susceptibility values. Disease inoculation and severity assessment protocols were as described by Freppon et al. (1994). F4:6 progenies were planted in the 1998 nursery near Guerne and selected late in the season for lodging resistance, and low ear placement.

RESULTS AND DISCUSSION

The majority of the 193 F2:3 progenies in the 1994 Columbus nursery produced mature kernels before the first freezing temperatures. Root lodging was surprisingly low — 90% of the progenies displayed root lodging values below 10%. Both parental checks exhibited 11%, and the local check B73, 0% root lodging. Stalk lodging of the progenies ranged from 0-90%, with over half displaying values at or below 10%. B73 and Tzi17 had 0% stalk lodging and Hi34 displayed 14%. Evaluations of 148 progenies advanced to disease nurseries in 1995 showed the majority experienced half or less the plant leaf area affected (PLAA) by GLS or NLB apparent on the susceptible check inbred B73 (75% and 95% PLAA in the late season, respectively) (Fig.1 and Fig.2). In the 1996 NLB nursery, nearly all of the 70 selected progenies displayed less than 5% PLAA whereas the susceptible check inbred A619 displayed 37% PLAA. GLS severity was somewhat higher in 1996, but half of the selected progenies remained below 15% PLAA while the susceptible check displayed 42% PLAA (data not presented).

Gray leaf spot development at the 1998 Apple Creek site started before the onset of flowering and developed to high levels in the susceptible checks as the season progressed. Post flowering ratings showed 50% of the mid-leaves of susceptible hybrids were infected with gray leaf spot (Table 1). Many Mo17 testcrosses showed levels of resistance equivalent to that of resistant commercial hybrids. Severity of GLS in Uganda was not as high as in Ohio. At the late season rating period, testcrosses with Mo17 showed lower GLS mean severity values than did testcrosses with B73 (mean = 4.5% and 8.5%, respectively). Resistance levels were considered comparable to the local checks (Table 2).

Rind puncture resistance of the testcrosses at Apple Creek was comparable to that of the commercial checks. Stalk lodging was low in that plot so it was not possible to ascertain the differences in the degree of susceptibility (Table 1). The testcrosses also exhibited similar rind puncture resistance in comparison with the check hybrids at the site in Uganda. Stalk lodging was low in the Ugandan plots and no apparent differences between the entries could be established. Rind puncture resistance appeared to be of similar magnitude to that observed in the U.S. In Uganda, the testcrosses appeared to display somewhat lower puncture resistance than did the checks (Table 2).

TABLE 1. GLS resistance and stalk strength of tropical x temperate testcrosses in Ohio during 1998
Entry Rind Resist. Force (kg) Stalk lodging (%) GLSa PLAA (%)
7/24 9/02 Mean
Pioneer Hi-Bred Brand 3394 3.2 3.0 3.1 0 48
Pioneer Hi-Bred Brand 3352 4.7 4.0 4.3 2 21
Pioneer Hi-Bred Brand 33Y18 4.2 3.4 3.8 2 20
DeKalb Brand DK683 3.2 3.2 3.2 2 10
Comm. Check Mean 3.8 3.4 3.6 2 25
Mo17 x OSU:973001 4.4 3.5 3.9 0 6
Mo17 x OSU:973002 4.1 3.4 3.8 0 6
Mo17 x OSU:973004 4.6 3.2 3.9 5 5
Mo17 x OSU:973005 4.3 3.7 4.0 2 6
Mo17 x OSU:973008 3.4 2.8 3.1 0 6
Mo17 x OSU:973009 3.4 2.9 3.1 2 7
Mo17 x OSU:973010 3.8 3.2 3.5 3 9
Mo17 x OSU:973011 3.4 2.9 3.2 1 7
Mo17 x OSU:973012 3.7 2.6 3.1 4 13
Mo17 x OSU:973013 3.6 2.7 3.1 2 11
Mo17 x OSU:973014 3.2 3.4 3.3 2 7
Mo17 x OSU:973015 3.7 3.5 3.6 6 8
Mo17 x OSU:973019 3.9 3.5 3.7 3 4
Mo17 x OSU:973021 3.4 3.1 3.3 8 8
Mo17 x OSU:973022 2.6 3.1 2.9 2 8
Mo17 x OSU:973023 3.6 3.0 3.3 2 9
Testcross Mean 3.7 3.2 3.4 2 8
CV (%) 11 10 111 38  
LSD (0.05) 0.7 0.5 2.5 7  
aGLS PLAA = gray leaf spot plant leaf area affected by characteristic gray leaf spot lesions on September 1

Table 2. Rind puncture resistance, stalk lodging and gray leaf spot reaction of test crosses in Uganda during 1998
Entry Rind Resist. Force (kg) Mean Stalk lodging (%) GLS PLAA (%)
12/28 1/12
Mo17 x OSU:973004 4.0 4.2 4.1 1 5
Mo17 x OSU:973008 3.2 3.8 3.5 0 6
Mo17 x OSU:973009 2.6 3.7 3.2 1 6
Mo17 x OSU:973011 3.1 3.7 3.4 0 4
Mo17 x OSU:973012 3.2 3.5 3.4 1 4
Mo17 x OSU:973013 3.4 3.3 3.4 1 5
Mo17 x OSU:973014 3.3 3.5 3.4 0 5
Mo17 x OSU:973015 3.3 3.9 3.6 0 5
Mo17 x OSU:973021 3.4 3.6 3.5 1 6
Mo17 x OSU:973022 3.6 3.7 3.7 0 4
Testcross Mean 3.3 3.7 3.5 1 5
B73 x OSU:973004 3.4 3.8 3.6 2 10
B73 x OSU:973006 3.2 3.6 3.4 1 3
B73 x OSU:973007 3.4 3.7 3.6 3 9
B73 x OSU:973008 2.9 3.5 3.2 1 9
B73 x OSU:973009 3.7 3.5 3.6 1 7
B73 x OSU:973010 3.1 3.5 3.3 1 7
B73 x OSU:973011 3.1 3.7 3.4 0 7
B73 x OSU:973012 3.9 3.8 3.9 0 7
B73 x OSU:973013 3.6 3.4 3.5 0 7
B73 x OSU:973014 3.3 3.5 3.4 1 9
B73 x OSU:973015 3.4 3.5 3.5 1 9
B73 x OSU:973019 2.9 3.8 3.4 1 9
B73 x OSU:973021 3.4 3.8 3.6 0 8
B73 x OSU:973023 3.0 3.2 3.1 1 7
Testcross mean 3.3 3.6 3.5 1 8
NZ2 4.0 3.7 3.9 0 6
NZ3 3.6 4.4 4.0 0 6
Check mean 3.8 4.1 4.0 0 6
CV (%) 16.7 13.1 68.9 29.9  
LSD (0.05) 1.9 2.3 1.1 2.1  

The very low levels of lodging in Uganda also precluded meaningful correlation analysis between rind puncture resistance and lodging data. Correlation analysis of rind puncture resistance values obtained for the same set of testcrosses evaluated in both locations showed a fairly high positive correlation (r=0.68) indicating fairly consistent values were obtained across macroenvironments. Caution should, however, be used due to the small sample size (n=10).

Yield values of testcrosses and checks in Uganda were low due to drought. The range of yield in Uganda was from 2.1 to 3.1 Mg ha-1 and there were no significant differences among the entries and NZ (national maize programme experimental hybrid) checks (Table 3). In Ohio, the mean yield of the testcrosses was 9.5 Mg ha-1 and that of four commercial check hybrids was 11.9 Mg ha-1 (Table 3).

Table 3. Yield performance of Hi34 x Tzi17 testcrosses in Ohio and Uganda during 1998a
  Mean Mg ha-1
Tester Mean Check
Mo 17 B 73
Ohio Test (2 loc.)        
973007 10.4 10.9 10.6  
973002 10.5 9.6 10.0  
973003 8.3 10.3 9.3  
973023 7.2 10.3 8.7  
973014 7.8 10.2 9.0  
Mean 8.8 10.2 9.5  
Std. err. 1.5 0.4 0.8  
Pioneer Hi-Bred Brand 3394       12.3
B 73 x Mo 17       11.4
Mean       11.9
Std. err.       0.7
Uganda Test        
973008 3.1 2.7 2.9  
973012 2.6 2.9 2.7  
973019   2.9    
973015 2.8 2.7 2.7  
973014 2.8 2.5 2.7  
973010   2.7    
Mean 2.8 2.7 2.8  
Std. err. 0.2 0.1 0.1  
NZ2       2.6
NZ3       2.1
Mean       2.4
Std. err.       0.4
aOhio test results are the mean of two locations

One may conclude, based on testing across broad environments, that the best breeding selections, when testcrossed with Corn Belt maize inbreds were disease resistant and comparable with local check varieties in both Ohio and Uganda. The selected lines showed better combining ability with B73 than with Mo17 (Table 3). Selected F6 lines are viable and are now being used for further breeding purposes in crosses with temperate germplasm. Similar success in adapting synthetic lowland tropical populations to northern latitudes has been reported at Iowa State University (Hallauer, 1994).

We have not experienced sufficient disease severity or stem-borer infestation to fully evaluate the efficacy of using the rind penetrometer for measuring stalk strength. While conclusive results are lacking, it would appear that a potential use of the penetrometer would be to eliminate accessions with poor stalk strength from the breeding programme.

ACKNOWLEDGEMENT

We wish to express gratitude to Mr. Ssimwogerere of Kamenyamiggo, Uganda, for assisting in data taking, and Mark Casey and Stuart Gordon for their assistance with Ohio plots. The study was financed by IPM/CRSP USAID Grant No. LAG-4196-9-00-3053-00.

REFERENCES

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