Race Durango Research Articles

Coding Mutations in Vacuolar Protein-Sorting 4 AAA+ ATPase Endosomal Sorting Complexes Required for Transport Protein Homologs Underlie bc-2 and New bc-4 Gene Conferring Resistance to Bean Common Mosaic Virus in Common Bean.

Bean common mosaic virus (BCMV) is a major disease in common bean (Phaseolus vulgaris L.). Host plant resistance is the most effective strategy to minimize crop damage against BCMV and the related Bean common mosaic necrosis virus (BCMNV). To facilitate breeding for resistance, we sought to identify candidate genes and develop markers for the bc-2 gene and the unknown gene with which it interacts. Genome-wide association study (GWAS) of the Durango Diversity Panel (DDP) identified a peak region for bc-2 on chromosome Pv11. Haplotype mapping narrowed the bc-2 genomic interval and identified Phvul.011G092700, a vacuolar protein-sorting 4 (Vps4) AAA+ ATPase endosomal sorting complexes required for transport (ESCRT) protein, as the bc-2 candidate gene. The race Durango Phvul.011G092700 gene model, bc-2[UI 111], contains a 10-kb deletion, while the race Mesoamerican bc-2[Robust] consists of a single nucleotide polymorphism (SNP) deletion. Each mutation introduces a premature stop codon, and they exhibit the same interaction with the pathogroups (PGs) tested. Phvul.005G125100, another Vps4 AAA+ ATPase ESCRT protein, was identified as the candidate gene for the new recessive bc-4 gene, and the recessive allele is likely an amino acid substitution in the microtubule interacting and transport (MIT) domain. The two Vps4 AAA+ ATPase ESCRT proteins exhibit high similarity to the Zym Cucsa.385040 candidate gene associated with recessive resistance to Zucchini yellow mosaic virus in cucumber. bc-2 alone has no resistance effect but, when combined with bc-4, provides resistance to BCMV (except PG-V) but not BCMNV, and, when combined with bc-ud, provides resistance to BCMV (except BCMV PG-VII) and BCMNV. So instead of different resistance alleles (i.e., bc-2 and bc-22), there is only bc-2 with a differential reaction based on whether it is combined with bc-4 or bc-ud, which are tightly linked in repulsion. The new tools and enhanced understanding of this host-virus pathogen interaction will facilitate breeding common beans for resistance to BCMV and BCMNV.

Genome-Wide Association Mapping of bc-1 and bc-u Reveals Candidate Genes and New Adjustments to the Host-Pathogen Interaction for Resistance to Bean Common Mosaic Necrosis Virus in Common Bean.

Bean common mosaic necrosis virus (BCMNV) is a major disease in common bean (Phaseolus vulgaris L.). Host plant resistance is the primary disease control. We sought to identify candidate genes to better understand the host-pathogen interaction and develop tools for marker-assisted selection (MAS). A genome-wide association study (GWAS) approach using 182 lines from a race Durango Diversity Panel (DDP) challenged by BCMNV isolates NL-8 [Pathogroup (PG)-III] and NL-3 (PG-VI), and genotyped with 1.26 million single-nucleotide polymorphisms (SNPs), revealed significant peak regions on chromosomes Pv03 and Pv05, which correspond to bc-1 and bc-u resistance gene loci, respectively. Three candidate genes were identified for NL-3 and NL-8 resistance. Side-by-side receptor-like protein kinases (RLKs), Phvul.003G038700 and Phvul.003G038800 were candidate genes for bc-1. These RLKs were orthologous to linked RLKs associated with virus resistance in soybean (Glycine max). A basic Leucine Zipper (bZIP) transcription factor protein is the candidate gene for bc-u. bZIP protein gene Phvul.005G124100 carries a unique non-synonymous mutation at codon 14 in the first exon (Pv05: 36,114,516 bases), resulting in a premature termination codon that causes a nonfunctional protein. SNP markers for bc-1 and bc-u and new markers for I and bc-3 genes were used to genotype the resistance genes underpinning BCMNV phenotypes in the DDP, host group (HG) differentials, and segregating F3 families. Results revealed major adjustments to the current host-pathogen interaction model: (i) there is only one resistance allele bc-1 for the Bc-1 locus, and differential expression of the allele is based on presence vs. absence of bc-u; (ii) bc-1 exhibits dominance and incomplete dominance; (iii) bc-1 alone confers resistance to NL-8; (iv) bc-u was absent from HGs 2, 4, 5, and 7 necessitating a new gene symbol bc-ud to reflect this change; (v) bc-ud alone delays susceptible symptoms, and when combined with bc-1 enhanced resistance to NL-3; and (vi) bc-ud is on Pv05, not Pv03 as previously thought. These candidate genes, markers, and adjustments to the host-pathogen interaction will facilitate breeding for resistance to BCMNV and related Bean common mosaic virus (BCMV) in common bean.

Genetic Architecture of Dietary Fiber and Oligosaccharide Content in a Middle American Panel of Edible Dry Bean.

Common bean ( L.) is the most consumed edible grain legume worldwide and contains a wide range of nutrients for human health including dietary fiber. Diets high in beans are associated with lower rates of chronic diseases such as obesity and type 2 diabetes, and the content of dietary fibers varies among different market classes of dry bean. In this study, we evaluated the dietary fiber content in a Middle American diversity panel (MDP) of common bean and evaluated the genetic architecture of the various dietary fiber components. The dietary fiber components included insoluble and soluble dietary fibers as well as the antinutritional raffinose family of oligosaccharides (RFOs; raffinose, stachyose, and verbascose). All variables measured differed among market classes and entries. Colored bean seeds had higher levels of insoluble dietary fibers with the black market class showing also the highest raffinose and stachyose content. Cultivars and lines released since 1997 had higher insoluble dietary fibers and RFO content in race Durango. Higher levels of RFOs were also observed in cultivars with type II growth habit that was a recent breeding target in Durango race germplasm. Candidate genes for dietary fiber traits, especially homologs to two main genes in the RFO biosynthesis pathway, were identified. The knowledge of diversity of dietary fibers in the MDP accompanied with the identification of candidate genes could effectively improve dietary fiber components in common bean.

Ecogeography of teosinte.

Adaptation of crops to climate change has motivated an increasing interest in the potential value of novel traits from wild species; maize wild relatives, the teosintes, harbor traits that may be useful to maize breeding. To study the ecogeographic distribution of teosinte we constructed a robust database of 2363 teosinte occurrences from published sources for the period 1842–2016. A geographical information system integrating 216 environmental variables was created for Mexico and Central America and was used to characterize the environment of each teosinte occurrence site. The natural geographic distribution of teosinte extends from the Western Sierra Madre of the State of Chihuahua, Mexico to the Pacific coast of Nicaragua and Costa Rica, including practically the entire western part of Mesoamerica. The Mexican annuals Zea mays ssp. parviglumis and Zea mays ssp. mexicana show a wide distribution in Mexico, while Zea diploperennis, Zea luxurians, Zea perennis, Zea mays ssp. huehuetenangensis, Zea vespertilio and Zea nicaraguensis had more restricted and distinct ranges, representing less than 20% of the total occurrences. Only 11.2% of teosinte populations are found in Protected Natural Areas in Mexico and Central America. Ecogeographical analysis showed that teosinte can cope with extreme levels of precipitation and temperatures during growing season. Modelling teosinte geographic distribution demonstrated congruence between actual and potential distributions; however, some areas with no occurrences appear to be within the range of adaptation of teosintes. Field surveys should be prioritized to such regions to accelerate the discovery of unknown populations. Potential areas for teosintes Zea mays ssp. mexicana races Chalco, Nobogame, and Durango, Zea mays ssp. huehuetenangensis, Zea luxurians, Zea diploperennis and Zea nicaraguensis are geographically separated; however, partial overlapping occurs between Zea mays ssp. parviglumis and Zea perennis, between Zea mays ssp. parviglumis and Zea diploperennis, and between Zea mays ssp. mexicana race Chalco and Zea mays ssp. mexicana race Central Plateau. Assessing priority of collecting for conservation showed that permanent monitoring programs and in-situ conservation projects with participation of local farmer communities are critically needed; Zea mays ssp. mexicana (races Durango and Nobogame), Zea luxurians, Zea diploperennis, Zea perennis and Zea vespertilio should be considered as the highest priority taxa.

Phenotypic Diversity for Seed Mineral Concentration in North American Dry Bean Germplasm of Middle American Ancestry

Dry bean (Phaseolus vulgaris L.) seeds are a major protein, carbohydrate, and mineral source in the human diet of peoples in multiple regions of the world. Seed mineral biofortification is an ongoing objective to improve this important food source. The objective of this research was to assess the seed mineral concentration of five macroelements and eight microelements in a large panel (n = 277) of modern race Durango and race Mesoamerica genotypes to determine if variability existed that could be exploited for targeted seed biofortification. Varieties that derive from these races are found in many diets throughout the world. The panel was grown in replicated trials under typical production conditions in the major bean growing regions of the United States, and a subset of the panel was also grown in replicated trials at three locations under control and terminal drought conditions. Except for K, seed mineral concentrations were higher for race Mesoamerica genotypes. Significantly higher seed concentrations for the majority of the minerals were observed for white‐seeded genotypes and race Durango genotypes with the now preferred indeterminate, upright growth habit. Modern genotypes (since 1997) had equal or increased mineral concentrations compared with older genotypes. Drought affected mineral content differentially, having no effect on the microelement content but increased Co, Fe, and Ni concentrations. The correlation of Ca and Mn concentrations suggests that these elements may share seed deposition mechanisms. The high heritability for seed mineral concentration implies that breeding progress can be achieved by parental selection from this panel.

Genome-wide Association Analysis for Drought Tolerance and Associated Traits in Common Bean.

A genome-wide association study explored the genetic basis of variation for drought tolerance and related traits in a Middle American diversity panel comprising 96 common bean ( L.) genotypes. The panel was grown under irrigated and rainfed conditions and single nucleotide polymorphism (SNP) data were used to explore the genetic diversity and ancestry of the panel. Varying levels of admixtures and distinctly divergent individuals were observed. Estimations of genome-wide heterozygosity revealed that, on average, greater diversity is present in individuals with Mesoamerica (3.8%) ancestry, followed by admixed individuals (2.3%). The race Durango had the lowest level of heterozygosity (1.4%). We report 27 significant marker-trait associations based on best linear unbiased predictors. These associations include seven markers for shoot biomass at harvest under irrigation and five markers under rainfed conditions on () chromosome 11, two markers for shoot biomass at flowering under irrigation on 02 and 08, two markers for seed size under irrigated and rainfed conditions on 09, seven markers for lodging score under irrigation on 02 and 07, one marker for leaf elongation rate on 03 and one for wilting score on 11. Positional candidate genes, including on 11, associated with wilting, were identified. The SNP ss715639327 marker was located in the exon region of the gene, which codes for an aquaporin associated with water movement in beans. Significant quantitative trait loci identified in this study could be used in marker-assisted breeding to accelerate genetic improvement of drought tolerance in common bean.

Registration of Pinto Bean Germplasm Line USPT-WM-12 with Partial White Mold Resistance

Pinto bean (Phaseolus vulgaris L.), the most widely grown dry bean market class across the United States, is highly susceptible to white mold disease caused by the fungal pathogen Sclerotinia sclerotiorum Lib de Bary. The USDA Agricultural Research Service, Michigan State University AgBioResearch, and the University of Nebraska Agricultural Experiment Station announce the release of USPT-WM-12 (Reg. No. GP-294, PI 668537) pinto bean germplasm line with partial resistance to white mold. USPT-WM-12 was developed by modified pedigree breeding method from the cross G99750/USPT-WM-1 and was tested as PS02-037-2 or 37-2 in early generations. In later generations, USPT-WM-12 exhibited consistently high levels of partial resistance to white mold in field and straw tests conducted by the national Bean White Mold Nursery from 2008 to 2012. USPT-WM-12 also exhibited high yield potential under white mold disease pressure in Michigan, ranking the highest for yield of 64 lines tested for two consecutive years in 2010 and 2011. USPT-WM-12 has commercially acceptable agronomic characteristics, including upright architecture and moderate maturity, but seed appearance is too dark for commercial acceptance. USPT-WM-12 is intended for use by breeders to improve partial resistance to white mold in pinto and related great northern, small red, and pink dry bean market classes representative of Race Durango.

Genetic Diversity of the Canadian Dry Bean Varieties Released Since 1930: A Pedigree Analysis

ABSTRACTA pedigree database of bean (Phaseolus vulgaris L.) varieties of different market classes released in Canada since 1930 was developed. The coefficient of parentage was estimated for all pair‐wise combinations of varieties, parental and (or) ancestral genotypes. A cluster analysis was employed to study the genetic diversity of dry bean in the Canadian gene pool. The pedigree‐based estimate of genetic diversity (1 − coefficient of parentage) for all dry bean varieties released between 1930 and 2010 was 0.93. This value was smaller for varieties within each evolutionary race, estimated at 0.89 for the varieties of race Durango (pinto, Great Northern, and small red beans), 0.83 for varieties of race Mesoamerica (navy and black), and 0.76 for varieties of race Nueva Granada (kidney and cranberry beans). The results indicated a narrow genetic diversity among the Canadian beans of Andean origin (kidney and cranberry beans), suggesting that the breeding efforts for these market classes would benefit from the introduction of new genetic diversity. Among the small‐seeded beans of race Mesoamerica, varieties ‘Ex‐Rico 23’ (syn. ICA Bunsi) and ‘Seafarer’, the medium‐sized beans of race Durango varieties ‘UI‐111’ and ‘NW‐63’, and the large‐seeded beans of race Nueva Granada varieties, ‘Montcalm’ and ‘California Dark Red Kidney’, were identified as the major progenitors of Canadian dry beans.

Race Structure in the Mexican Collection of Common Bean Landraces

ABSTRACTMexico is a large bean‐producing country, the second most important in the Western Hemisphere, and the center of origin for common bean (Phaseolus vulgaris L.) as well as many wild relatives or cultigens. Given this long tradition with the crop, Mexican farmers have created a rich resource of native landraces over many millennia, and this germplasm is of value today for modern breeding and gene discovery. The objective of this research was to evaluate the core collection of the national Mexican germplasm repository (Instituto Nacional de Investigaciones Forestales, Agricolas y Pecuarias [INIFAP]) and to determine the diversity and population structure within the germplasm set based on fluorescent microsatellite marker genotyping. The results showed a good separation of races Durango, Jalisco, and Mesoamerica, which were the majority of the landraces, versus a small group of Andean landraces that were all well supported both by population structure analysis and coherent results with principal component analysis and a molecular analysis of variance. Genepool separation has been observed in many previous studies of diversity in common bean but this study was among the first with microsatellites to find separation of the Durango and Jalisco races. The utility of the INIFAP core collection for association mapping studies of traits important to Mexican agriculture is discussed.

Phenotyping common beans for adaptation to drought

Common beans (Phaseolus vulgaris L.) originated in the New World and are the grain legume of greatest production for direct human consumption. Common bean production is subject to frequent droughts in highland Mexico, in the Pacific coast of Central America, in northeast Brazil, and in eastern and southern Africa from Ethiopia to South Africa. This article reviews efforts to improve common bean for drought tolerance, referring to genetic diversity for drought response, the physiology of drought tolerance mechanisms, and breeding strategies. Different races of common bean respond differently to drought, with race Durango of highland Mexico being a major source of genes. Sister species of P. vulgaris likewise have unique traits, especially P. acutifolius which is well adapted to dryland conditions. Diverse sources of tolerance may have different mechanisms of plant response, implying the need for different methods of phenotyping to recognize the relevant traits. Practical considerations of field management are discussed including: trial planning; water management; and field preparation.

Response of Cultivars of Race Durango to Continual Dry Bean Versus Rotational Production Systems

Continual dry bean (CB, Phaseolus vulgaris L.) production in the same field for 2 or more consecutive years is common worldwide. However, the effects on and performance of dry bean cultivars in the CB production system is not known. The objectives of this research were to (i) determine effects of the CB production system on seed yield, seed weight, and days to maturity of race Durango cultivars; (ii) determine the correlations for those three traits between the CB and rotational production (RP) systems; and (iii) identify superior cultivars within and across both production systems. Medium‐sized (25–40 g 100 seed−1) cultivars of great northern (8), pink (3), pinto (9), and red (5) of race Durango released between 1932 and 1998 were evaluated from 1999 to 2001 in a field that had been under 50 yr of CB production at the University of Idaho, Kimberly Research and Extension Center. Concurrently, they were evaluated under the standard RP system. The cultivar effects and cultivar × year interaction effects were significant (P < 0.01) for seed yield, seed weight, and days to maturity in both production systems. Continual dry bean production reduced mean seed yield by 68%, seed weight by 11%, and days to maturity by 6 d. Large differences among cultivars existed within and between market classes in both production systems. On average, great northern and pinto cultivars had the greatest reduction (71%) and pink (56%) followed by red (66%) cultivars had the least reduction in seed yield due to CB production. Cultivars UI 537, UI 239, Viva, and Harold had the highest levels of resistance to CB production. Despite significant (P < 0.01) positive phenotypic correlation between the two production systems for seed yield (r2 = 0.70), seed weight (r2 = 0.90), and days to maturity (r2 = 0.67), testing of dry bean cultivars using both production systems is suggested for identification of superior cultivars to be grown either within or across production systems.

Drought Resistance in the Race Durango Dry Bean Landraces and Cultivars

Increasing water deficit or drought is a problem for dry bean (Phaseolus vulgaris L.) production in the western USA. Identification of drought‐resistant cultivars is crucial for sustainable production in the region. The objectives of this study were to (i) measure the effects of drought on seed yield, seed weight, and days to maturity; (ii) determine correlation among the three traits in drought‐stressed (DS) and nonstressed (NS) environments; and (iii) identify drought‐resistant dry bean landraces and cultivars. Three landraces and 17 cultivars of race Durango were evaluated in DS and NS environments. Drought intensity index ranged from 0.32 to 0.88. There was a mean reduction in yield of 60% due to drought stress. Also, seed weight was reduced by 14% and maturity was reduced by 4 d in DS. Yield, seed weight, and maturity in NS and DS were positively correlated, and so were seed weight and maturity in NS and DS. But, the DS yield was negatively correlated with seed weight in both DS and NS. The ability to detect yield differences among 20 genotypes decreased with increasing drought stress. Cultivars Viva, NW 63, UI 239, and Common Red Mexican landrace had high yield in both DS and NS and below average reduction in yield due to DS, thus, exhibiting drought resistance. Buster, UI 126, and UI 465 were most susceptible. The frequency and timing of irrigation for the four drought‐resistant genotypes should be determined and breeding, genetics, and physiology research intensified to maximize yield and water use efficiency in the western USA, which is facing increasing water shortage.

Race structure within the Mesoamerican gene pool of common bean (Phaseolus vulgaris L.) as determined by microsatellite markers

Common bean (Phaseolus vulgaris L.) cultivars are distinguished morphologically, agronomically and ecologically into specific races within each of the two gene pools found for the species (Andean and Mesoamerican). The objective of this study was to describe the race structure of the Mesoamerican gene pool using microsatellite markers. A total of 60 genotypes previously described as pertaining to specific Mesoamerican races as well as two Andean control genotypes were analyzed with 52 markers. A total of 267 bands were generated with an average of 5.1 alleles per marker and 0.297 heterozygosity across all microsatellites. Correspondence analysis identified two major groups equivalent to the Mesoamerica race and a group containing both Durango and Jalisco race genotypes. Two outlying individuals were classified as potentially of the Guatemala race although this race does not have a defined structure and previously classified members of this race were classified with other races. Population structure analysis with K = 1-4 agreed with this classification. The genetic diversity based on Nei's index for the entire set of genotypes was 0.468 while this was highest for the Durango-Jalisco group (0.414), intermediate for race Mesoamerica (0.340) and low for race Guatemala (0.262). Genetic differentiation (G (ST)) between the Mesoamerican races was 0.27 while genetic distance and identity showed race Durango and Jalisco individuals to be closely related with high gene flow (N (m)) both between these two races (1.67) and between races Durango and Mesoamerica (1.58). Observed heterozygosity was low in all the races as would be expected for an inbreeding species. The analysis with microsatellite markers identified subgroups, which agreed well with commercial class divisions, and seed size was the main distinguishing factor between the two major groups identified.

Microsatellite marker diversity in common bean (Phaseolus vulgaris L.)

A diversity survey was used to estimate allelic diversity and heterozygosity of 129 microsatellite markers in a panel of 44 common bean (Phaseolus vulgaris L.) genotypes that have been used as parents of mapping populations. Two types of microsatellites were evaluated, based respectively on gene coding and genomic sequences. Genetic diversity was evaluated by estimating the polymorphism information content (PIC), as well as the distribution and range of alleles sizes. Gene-based microsatellites proved to be less polymorphic than genomic microsatellites in terms of both number of alleles (6.0 vs. 9.2) and PIC values (0.446 vs. 0.594) while greater size differences between the largest and the smallest allele were observed for the genomic microsatellites than for the gene-based microsatellites (31.4 vs. 19.1 bp). Markers that showed a high number of alleles were identified with a maximum of 28 alleles for the marker BMd1. The microsatellites were useful for distinguishing Andean and Mesoamerican genotypes, for uncovering the races within each genepool and for separating wild accessions from cultivars. Greater polymorphism and race structure was found within the Andean gene pool than within the Mesoamerican gene pool and polymorphism rate between genotypes was consistent with genepool and race identity. Comparisons between Andean genotypes had higher polymorphism (53.0%) on average than comparisons among Mesoamerican genotypes (33.4%). Within the Mesoamerican parental combinations, the intra-racial combinations between Mesoamerica and Durango or Jalisco race genotypes showed higher average rates of polymorphism (37.5%) than the within-race combinations between Mesoamerica race genotypes (31.7%). In multiple correspondance analysis we found two principal clusters of genotypes corresponding to the Mesoamerican and Andean gene pools and subgroups representing specific races especially for the Nueva Granada and Peru races of the Andean gene pool. Intra population diversity was higher within the Andean genepool than within the Mesoamerican genepool and this pattern was observed for both gene-based and genomic microsatellites. Furthermore, intra-population diversity within the Andean races (0.356 on average) was higher than within the Mesoamerican races (0.302). Within the Andean gene pool, race Peru had higher diversity compared to race Nueva Granada, while within the Mesoamerican gene pool, the races Durango, Guatemala and Jalisco had comparable levels of diversity which were below that of race Mesoamerica.

Comparison of Sources and Lines Selected for Drought Resistance in Common Bean.

Drought is a major constraint to common bean (Phaseolus vulgaris L.) production worldwide. Our objectives were to (i) identify sources of drought resistant germplasm in common bean cultivars and (ii) compare drought resistant germplasm with lines selected from interracial and intergene pool populations. We included in this study 12 of the most promising drought resistant cultivars from race Durango and 11 from race Jalisco, nine drought resistant lines selected from interracial or intergene pool populations, and two drought resistant and two susceptible checks. The 36 genotypes were evaluated in drought-stressed (DS) and nonstressed (NS) environments in four cropping seasons between 1996 and 1998 at the International Center for Tropical Agriculture (CIAT), Palmira, Colombia. Drought stress reduced seed yield by 53%, 100-seed weight by 13%, and days to maturity by 3%. Race Durango cultivars had higher yield, larger seed weight, and earlier maturity than race Jalisco cultivars in DS and NS environments. Large variations within the two races were found for the three traits. Drought resistant selected lines out-yielded drought resistant checks by 44% in DS and 15% in NS and cultivars from race Durango by 48% in DS and 30% in NS and race Jalisco by 96% in DS and 46% in NS environments. Seed yield in DS was correlated negatively with the percent reduction (PR) because of drought stress and drought susceptibility index (DSI), whereas a positive correlation existed between PR and DSI. Drought resistant selected lines and race Durango cultivars had similar maturity. Mean 100-seed weight of selected lines (23 g) was less than race Durango (34 g) and race Jalisco cultivars (29 g). While new sources of drought resistance could be identified in races Durango and Jalisco, these drought resistant germplasm and selected lines derived from interracial and intergene pool populations should be utilized for improvement of drought resistance in common bean.

Detection and Mapping of a Major Locus for Fusarium Wilt Resistance in Common Bean

Fusarium oxysporum Schlectend. Fr. f. sp. phaseoli J.B. Kendrick and W.C. Snyder (FOP) is a vascular pathogen that causes Fusarium wilt in common bean (Phaseolus vulgaris L.). This disease is an increasing problem in the western U.S., and exploitation of genetic resistance is considered the most feasible control method. The objective of this study was to detect quantitative trait loci (QTL) for Fusarium wilt resistance in a population derived from an interracial cross between FOP‐susceptible Belneb RR‐1 (race Durango) × FOP‐resistant A55 (race Mesoamerica). Seventy‐six F6‐derived recombinant inbred lines (RILs) were screened for disease severity in greenhouse inoculations and rated on a scale of 1 (resistant) to 9 (susceptible). The phenotypic data were compared to existing random amplified polymorphic DNA (RAPD) marker data using single‐factor analysis of variance. Marker U20.750 on linkage group (LG) 10 accounted for 63.5% of the phenotypic variance for this trait. Lines exhibiting the A55 banding pattern at this locus had disease severity scores that averaged 3.6 points lower than lines with the Belneb RR‐1 pattern. Two additional markers, AD4.450 on LG 3 and K10.700 on LG 11, were significant (P < 0.01) in single‐factor analysis of variance, but only marker U20.750 on LG 10 remained significant when composite interval mapping (CIM) was conducted. The tight linkage between the putative QTL and U20.750, as indicated by CIM, makes this marker a promising candidate for conversion to a sequence‐characterized amplified region (SCAR) for use in marker‐assisted selection in Fusarium wilt resistant common bean cultivar development.

Inheritance of Resistance to Fusarium Wilt in Two Common Bean Races

Fusarium wilt in common bean (Phaseolus vulgaris L.) caused by Fusarium oxysporum Schlechtend. Fr. forma specialis phaseoli Kendrick and Snyder (Fop) occurs worldwide and can result in severe yield loss. Because cultural methods to control disease loss are not completely effective, cultivars with genetic resistance are needed. The objectives of this study were to determine whether genetic control of resistance to Fop race 4 differs between germplasm of races Durango and Mesoamerica of common bean and to estimate heritability of resistance found in race Mesoamerica. Resistant and susceptible lines of races Durango and Mesoamerica were crossed within races to produce F2 and F3 progeny. Reaction to Fop was evaluated using a root‐dip inoculation method and scored using a CIAT disease severity scale from one to nine. F2 populations derived from race Durango parents showed a 3:1 (resistant/susceptible) plant segregation ratio, and F3 progeny tests confirmed that resistance was controlled by a single dominant gene. F2 data from crosses between parents of race Mesoamerica had continuous distributions for reaction to Fop race 4, suggesting polygenic control of resistance. The narrow‐sense heritability estimate derived from midparent–offspring regression of 10 F2 populations derived from Mesoamerican parents was 0.85 ± 0.34, and realized heritability estimates ranged from 0.25 ± 0.19 to 0.60 ± 0.16 among five populations. The heritability estimates as well as the continuous variation in disease severity observed support the hypothesis that resistance to Fop race 4 among parents of race Mesoamerican is polygenic.

Characterization of agronomic traits and markers of recombinant inbred lines from intra- and interracial populations of Phaseolus vulgaris L.

The value of intra- and interracial populations in common bean (Phaseolus vulgaris L.) needs to be determined in order to create useful genetic variation for maximizing gains from selection, broadening the genetic base of commercial cultivars, and making efficient use of available resources. Five large-seeded parents of race Nueva Granada (N), two small-seeded race Mesoamerica (M), and one medium-seeded race Durango (D) were hybridized to produce one intraracial (N x N) and three interracial (two N x M and one N x D) populations. Seventy-nine F2-derived F6 lines randomly taken from each population along with their parents were evaluated for agronomic traits and markers at Palmira and Popayán, Colombia, in 1990 and 1991. Variation for agronomic traits and for morphological, protein, and isozyme markers was larger in interracial populations than in the intraracial population. Mean seed yield of all lines as well as yield of the highest yielding line from two interracial populations were significantly higher than that of the intraracial population. The highest (≥ 0.80±0.15) heritability was recorded for 100-seed weight. Values for seed yield varied from 0.19±0.17 to 0.50±0.16. Gains from selection (at 20% selection pressure) for seed yield ranged from 3.9% to 11.4%. Seed yield was positively associated with biomass yield, pods/m(2), and days to maturity, but harvest index showed negative correlations with these traits and a positive value with 100-seed weight. Polymorphism was recorded for phaseolin, lectins, protein Group-1 and protein Group-2 fractions, and six isozyme loci. Lines with indeterminate growth habit had significantly (P < 0.01) higher seed yield than lines with determinate growth habit in a Redkloud x MAM 4 population. Also, 23 other associations of markers with agronomic traits other than seed yield were recorded. Of these associations, lines with T phaseolin, the Diap1 (2) allele, and lilac flower color tended to possess greater seed weight.

Selection for Water‐Stress Tolerance in Interracial Populations of Common Bean

Crop cultivars that tolerate water stress would stabilize and expand production in drought endemic environments. They would require less irrigation and help conserve water, an essential natural resource. My objectives were to (i) compare, in water‐stressed and nonstressed environments, the performance of inbred lines from interracial populations of common bean (Phaseolus vulgaris L.), involving water‐stresstolerant parents, (ii) identify lines that out‐yielded the parents, and (iii) determine heritability and expected gains from selection. Eighty‐five Fs‐derived Fs random lines, a population bulk, and four parents from each of two populations (TR 7790 and TR 7791) were evaluated between 1991 and 1993 at Palmira, Colombia. The TR 7790 population involved crosses between common bean races Mesoamerica and Durango, and TR 7791 involved races Mesoamerica and Nueva Granada. Water stress reduced seed yield and 100‐seed weight and accelerated maturity. The mean seed yield for population bulk, inbred lines, and parents of TR 7790 in both water‐stressed and nonstressed environments were higher than those of TR 7791. In population TR 7790, eight lines yielded higher (by an average of 52%) than the mean of parents and six lines yielded higher (by an average of 41%) than the highest yielding parent under water stress. In population TR 7791, eight lines yielded higher than the mean of parents and one line yielded higher than the best parent under water stress. For seed yield, heritability was low for TR 7791 (0.26 ± 0.18 to 0.38 ± 0.17) and high for TR 7790 (0.66 ± 0.15 to 0.80 ± 0.15). Expected gains from selection ranged from 10 to 48%. These results justify searching for and using more germplasm highly tolerant to water stress from races Durango and Mesoamerica.

Comparison of recurrent and congruity backcrossing for interracial hybridization in common bean

Two common bean (Phaseolus vulgaris L.) cultivars were used for recurrent backcrossing (i.e., repeated backcrossing to one of the parents) to both parents independently and for congruity backcrossing (i.e., backcrossing alternately to both parents). One cultivar, ‘ICA Pijao’ of race Mesoamerica, was late maturing, with erect type II growth habit and small seeds. The other was ‘Pinto UI 114’ of race Durango, an early maturing cultivar with prostrate type III growth habit and medium seeds. One to three recurrent backcrosses (RBC) to both parents, and two rounds of congruity backcrosses (CBC), were made. Thirty-two random lines from each method along with two parents were compared in a reps-in-set design at two locations (Popayan and Quilichao) in Colombia in 1992–1993.