Inheritance of angular leaf spot resistance in common bean line BAT 332 and identification of RAPD markers linked to the resistance gene

The existence of genetic variability for angular leaf spot (ALS) resistance in the common bean germplasm allows the development of breeding lines resistant to this disease. The BAT 332 line is an important resistance source to common bean ALS. In this work we determined the inheritance pattern and identified RAPD markers linked to a resistance gene present in BAT 332. Populations F₁, F₂,BCs and BCr derived from crosses between BAT 332 and cultivar Rudá were used. Rudá is a commercial cultivar with carioca type grains and susceptible to ALS. The resistance of BAT 332 to race 61.41 of the pathogen was confirmed. Segregation analysis of the plants indicated that a single dominant gene confers resistance. For identification of RAPD markers linked to the resistance gene, bulk segregant analysis (BSA) was used. Two RAPD markers,OPAA07₉₅₀ and OPAO12₉₅₀, linked in coupling phase at 5.10 and 5.83 cM of this gene, respectively, were identified. These molecular markers are important for common bean breeders and geneticists as source of genetic information and for marker assisted selection in breeding programs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetics of angular leaf spot resistance in the Andean common bean accession G5686 and identification of markers linked to the resistance genes

Angular leaf spot (ALS), caused by the fungus Phaeoisariopsis griseola is an economically important and widely distributed disease of common bean. Due to the co-evolution of P. griseola with the large and small seeded bean gene pools, stacking Andean and Mesoamerican resistance genes is a strategy most likely to provide lasting resistance to ALS disease. This strategy requires identification and characterization of effective Andean and Mesoamerican resistance genes, and the development of molecular markers linked to these genes. This study was conducted to elucidate the genetics of ALS resistance in the Andean accession G5686 using an F₂ population derived from a G5686 × Sprite cross. Segregation analysis revealed that three dominant and complementary genes conditioned resistance of G5686 to P. griseola pathotype 31-0. Three microsatellite markers, Pv-ag004, Pv-at007 and Pv-ctt001 segregated in coupling phase with the resistance genes in G5686. Microsatellites Pv-ag004 and Pv-ctt001, located on opposite ends of linkage group B04 segregated with resistance genes Phg _G5686A , Phg _G5686B at 0.0 and 17.1 cM, respectively, while marker Pv-at007, localized on linkage group B09 segregated with resistance gene Phg _G5686C at 12.1 cM. Parental surveys showed that these markers were polymorphic in Andean and Mesoamerican backgrounds. The usefulness of G5686 ALS resistance genes in managing the ALS disease, and the potential utility of identified molecular markers for marker assisted breeding are discussed.     

Inheritance of resistance to angular leaf spot (Isariopsis griseola Sacc.) in french bean (Phaseolus vulgaris L.)

Summary Angular leaf spot (Isariopsis griseola Sacc.) is a serious disease of French bean in the hills of India and 40 to 70 per cent of the green pods are damaged and rendered unmarketable. Crosses were made between PLB 257, (Phaseolus coccineus L.), a red flowering pole tope, resistant to angular leaf spot, and Contender (Phaseolus vulgaris L.), a highly susceptible commercial cultivar. Studies of the F₁, F₂, and F₃ progenies indicated that PLB 257, carries a recessive gene imparting resistance to angular leaf spot.     

Molecular markers and allelic relationships of anthracnose resistance gene cluster B4 in common bean

Angular leaf spot is one of the major diseases of the common bean. The extensive genetic variability of this pathogen requires the constant development of new resistant cultivars. Different sources of resistance have been identified and characterized. For the State of Minas Gerais, Brazil, four main resistance sources were found: Mexico 54, AND 277, MAR 2 and Cornell 49-242. Independent characterization of these genotypes demonstrates that resistance in all four sources is dominant and monogenic. However, there are no studies on the relationship and independence of these genes. In the present work, allelism tests were carried out to understand the relationship among the resistance genes present in these four resistance sources. The data revealed a much higher complexity in the resistance inheritance of these genes than previously reported. It was demonstrated that Cornell 49-242 possesses a dominant gene (Phg-3); Mexico 54 possesses three genes, denominated Phg-2, Phg-5 and Phg-6. In MAR 2, two genes were found, one independent designated Phg-4 and the other, an allelic form of Phg-5, denominated of Phg-5². Allelic forms were also found in AND 277, Phg-2², Phg-3² and Phg-4². These results have special importance for breeding programs aiming to pyramid resistance genes.     

Andean beans (Phaseolus vulgaris L.) with resistance to the angular leaf spot pathogen (Phaeoisariopsis griseola) in southern and eastern Africa

Common beans (Phaseolus vulgaris) are separated into two distinct groups: Andean and Middle American. We identified CAL 143 as the first Andean bean with resistance to angular leaf spot disease caused by Phaeoisariopsis griseola. Angular leaf spot is the most widespread and economically important bean disease in southern and eastern Africa, and it is especially severe on the extensively grown Andean beans. Cal 143 was resistant in Malawi, South Africa, Tanzania, and Zambia, but it was susceptible in Uganda. This was attributed to the presence of races of P. griseola in Uganda not present in the other countries. We identified two additional Andean bean lines, AND 277 and AND 279, with resistance to angular leaf spot in Malawi. We also characterized the virulence diversity of 15 isolates of P. griseola from southern and eastern Africa into nine different races. Five of six isolates from Malawi and two of seven from Uganda, obtained from large-seeded Andean beans, were characterized into four different races considered Andean. These were compatible only or mostly with large-seeded Andean cultivars. The other eight isolates from Uganda, Malawi, and the Democratic Republic of Congo, obtained from a small- or medium-seeded Middle American beans, were characterized into five different Middle American races. These were compatible with Middle American and Andean cultivars. CAL 143 was resistant or intermediate under greenhouse conditions to all but one of the same 15 isolates from southern and eastern Africa, but it was susceptible to an isolate from Uganda obtained from a medium-seeded Middle American bean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular marker-assisted selection for development of common bean lines resistant to angular leaf spot

The objective of this work was to develop homozygous common bean lines carrying angular leaf spot resistance genes derived from the cultivars ‘Mexico 54’, ‘MAR 2’ and ‘BAT 332’ through marker‐assisted selection. Molecular markers SCAR OPN02₈₉₀, RAPD OPE04₅₀₀ and OPAO12₉₅₀ linked to the resistance genes of ‘Mexico 54’, ‘MAR 2’ and ‘BAT 332’, respectively, were used in segregating backcross‐derived populations to selection. DNA fingerprinting was used to select homozygous BC₂F₃ and BC₁F₃ resistant plants genetically closer to the recurrent parent. Two homozygous BC₂F_2:3 and two and five BC₁F_2:3 families derived from ‘Ruda’ vs. ‘Mexico 54’ (RM), ‘MAR 2’ (RMA) and ‘BAT 332’ (RB) crosses were selected, respectively. After only one (RMA, RB) or two backcrosses (RM), five and eight BC₁F₃ lines derived from RMA and RB, respectively, and seven BC₂F₃ lines derived from RM, with 14.9–16.6, 16.9–18.6 and 9.3–11.1% of relative genetic distances to the recurrent parent were selected. This is the first report of lines resistant to angular leaf spot carrying genes of the cultivars ‘Mexico 54’, ‘MAR 2’ and ‘BAT 332’ developed with the aid of molecular markers.     

Sources of resistance to angular leaf spot (Phaeoisariopsis griseola) in common bean core collection, wild Phaseolus vulgaris and secondary gene pool

Angular leaf spot (ALS) is one of the most devastating diseases of common bean (Phaseolus vulgaris L.) in tropical and subtropical countries. The causal fungus, Phaeoisariopsis griseola(Sacc.) Ferr. is highly variable and a diverse source of resistance genes is required to manage this disease. We evaluated a common bean core collection,primary and secondary gene pools and lines derived from inter-specific crosses of P. vulgaris and P. coccineus or P. polyanthus (secondary gene pool) for resistance to angular leaf spot. Of the 1441 accessiones in the core collection, only 2.2% were resistant to both Andean and Mesoamerican races of P. griseola, 28% were resistant only to Andean and 9% to Mesoamerican races. Of the 32 resistant accessions, 68%originated from Bolivia, Colombia,Guatemala and Mexico. More accessions from these countries should be examined for P. griseola reaction. Very few wild P. vulgaris accessions (4%), were resistant to ALS. In contrast, high levels of resistance (62%) were found in the secondary gene pool. Among the 1010 lines from inter-specific crosses, 109 lines were highly resistant. These genotypes from the primary and secondary common bean gene pools resistant to Andean and Mesoamerican races of P. griseola offer a potential for developing broad and durable ALS resistance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of anthracnose resistance in the common bean cultivar Widusa

Snap bean (Phaseolus vulgaris L.) cultivar, Widusa, was crossed to Michigan Dark Red Kidney (MDRK), Michelite, BAT 93, Mexico 222, Cornell 49–242, and TO cultivars to study the inheritance of resistance to anthracnose in Widusa. The segregation patterns observed in six F₂ populations supported an expected 3R:1S ratio suggesting that Widusa carries a single dominant gene conditioning resistance to races 7, 65, 73, and 453 of Colletotrichum lindemuthianum, the causal organism of bean anthracnose. Allelism tests conducted with F₂ populations derived from crosses between Widusa and Cornell 49–242 (Co-2), Mexico 222 (Co-3), TO (Co-4), TU (Co-5), AB 136 (Co-6), BAT 93 (Co-9), and Ouro Negro (Co-10), inoculated with races 7, 9, 65 and 73, showed a segregation ratio of 15R:1S. These results suggest that the anthracnose resistance gene in Widusa is independent from the Co-2, Co-3, Co-4,Co-5, Co-6, Co-9, and Co-10 genes. A lack of segregation was observed among 200 F₂ individuals from the cross Widusa/MDRK, and among 138 F₂ individuals from the cross Widusa/Kaboon inoculated with race 65, suggesting that Widusa carries an allele at the Co-1 locus. We propose that the anthracnose resistance allele in Widusa be named Co-1 ⁵ as Widusa exhibits a unique reaction to race 89 compared to other alleles at the Co-1 locus. RAPD marker A18₁₅₀₀ co-segregated in repulsion-phase linkage with the Co-1 ⁵ gene at a distance of 1.2 cM and will provide bean breeders with a ready tool to enhance the use of the Co-1 ⁵ gene in future bean cultivars.     

Development and validation of DNA markers linked to Sdvy-1, a common bean gene conferring resistance to the yellowing strain of Soybean dwarf virus

The yellowing strain of Soybean dwarf virus (SbDV-YS) causes yellowing and yield loss in common bean (Phaseolus vulgaris). The most effective control is achieved through breeding for resistance. An indeterminate climbing cultivar with a white seed coat, ‘Oofuku’, is resistant to SbDV-YS in inoculation tests. We crossed ‘Oofuku’ with an elite cultivar, ‘Taisho-Kintoki’, which is SbDV-YS-susceptible, determinate dwarf with a red-purple seed coat, and performed amplified-fragment-length polymorphism analysis of F₃ lines. From nucleotide sequences of the resistant-specific fragments and their flanking regions, we developed five DNA markers, of which DV86, DV386, and DV398 were closely linked to Sdvy-1, a resistance gene. Using the markers, we developed ‘Toiku-B79’ and ‘Toiku-B80’, the near-isogenic lines (NILs) incorporating Sdvy-1 in the background of ‘Taisho-Kintoki’. The NILs had similar growth habit, maturity date and seed shape to those of ‘Taisho-Kintoki’. The quality of boiled beans was also similar, except that the NILs had more seed coat cracking than ‘Taisho-Kintoki’. The NILs showed no SbDV-YS infection in inoculation tests. We suggest that Sdvy-1 is a useful source of SbDV-YS resistance in common bean.     

Gamete selection for improving physiological resistance to white mold in common bean

White mold (WM), caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a widespread disease of dry and green bean (Phaseolus vulgaris L.) in North America. Gamete selection (GS) was effective to combine and pyramide resistant genes and quantitative trait loci (QTL) for common bacterial blight. Our objective was to determine the effectiveness of GS to introgress physiological resistance to white mold. Two inter-gene-pool double-cross populations were developed. Selection for WM resistance was practiced from F₁ to F₄. Thirteen selected F_1:5 breeding lines of each population and their four parents were evaluated. Two separate inoculations were made on each plant 1 week apart using a cut-stem method. The WM reaction was scored at 16, 23, and 33 days post inoculation (DPI) using a scale from 1 (no disease) to 9 (severely diseased or dead). In F₁, 52% of Pop I (USPT-WM-1/CORN 601//USPT-CBB-1/92BG-7) and 67% of Pop II (Chase/I9365-25//ABL 15/A 195) susceptible plants were discarded. In F_4, only 1.2% of families from Pop I, and 0.9% for Pop II, survived the selection process. An average of 20.5% gain in WM resistance was obtained for both populations in F₄. Four breeding lines of Pop I had significantly (P = 0.05) lower WM score (4.1–4.6) and four were equal (4.7–4.9) to the best WM-resistant parent 92BG-7 (4.9), while ten breeding lines of Pop II were equal (4.5–4.8) to the best WM-resistant parent A 195 (4.6). Thus, GS was effective for improving WM resistance in common bean.     

Pathogenic variation in, sources of, and breeding for resistance to Phaeoisariopsis griseola causing angular leaf spot in common bean

If we are to breed common bean (Phaseolus vulgaris L.) for durable resistance to diseases, we must understand pathogenic variation and find sources of resistance. Our first objective was to determine the patterns of pathogenic variation found among isolates of Phaeoisariopsis griseola (PG), the fungus that causes angular leaf spot (ALS) in common bean. We characterized 433 PG isolates from 11 Latin American and 10 African countries, using differential cultivars, isozymes, and/or random amplified polymorphic DNA (RAPD) markers. We also systematically screened, for ALS resistance, common bean accessions from the world collection held at CIAT, and assessed the progress so far made in breeding for resistance to ALS. Despite their great diversity within and between countries on both continents, the PG isolates were classified into two major groups: Andean, and Middle American. Although each group had internal differences for virulence, and biochemical and molecular characteristics, the ‘Andean’ PG isolates were more virulent on common beans of Andean origin, than on those of Middle American origin, thus, suggesting a host-pathogen co-evolution. The ‘Middle American’ PG isolates, although more virulent on common beans from Middle America, also attacked Andean beans, thus, exhibiting a much broader virulence spectrum. To find sources of resistance, we tested 22,832 common bean accessions against naturally occurring PG isolates in the field at CIAT's Experiment Station, Quilichao, Colombia, between 1985 and 1992. The resulting 123 intermediate (scores of 4 to 6) and resistant (scores of 1 to 3) accessions were then tested in the greenhouse against selected 14 PG isolates of diverse origins. Nineteen accessions were intermediate or resistant to at least 13 of 14 PG isolates. Similarly, of 13,219 bred lines tested in the field between 1978 and 1996, 89 were intermediate or resistant. Of these, 33 bred lines proved intermediate or resistant to at least eight of nine PG isolates to which they were challenged in the greenhouse. We suggest that, to breed for durable resistance to ALS, common bean populations should be developed from crosses between Andean and Middle American gene pools. The populations should then be systematically evaluated and selected against the broadest range of the most virulent PG isolates of diverse evolutionary origins. This revised version was published online in August 2006 with corrections to the Cover Date.     

New sources of resistance to anthracnose and angular leaf spot of beans (Phaseolus vulgaris L.)

Summary over 13000 CIAT bean accessions were evaluated for their reactions to the anthracnose (Colletotrichum lindemuthianum) and angular leaf spot (Isariopsis griseola) pathogens over a 3 yr period. Among these accessions, 156 were resistant to all races of the anthracnose pathogen collected from Popayán, Colombia. Thirty were resistant to numerous races obtained from other parts of the world, including Europe. Although many of these new resistant sources originated in Mexico and Central America, they are quite diverse for geographic origin, plant type, seed color and seed size. In addition, more than 50 of the 156 lines were also resistant to isolates of I. griseola with diverse sources of origin throughout Colombia.     

Inheritance of resistance to Acanthoscelides obtectus in a wild common bean accession crossed to commercial bean cultivars

Summary The bean weevil Acanthoscelides obtectus, is an important storage pest of common beans in Latin America and Africa. A few wild bean accessions from Mexico have been identified as highly resistant to the weevil. One accession, G 12952, was crossed to two susceptible bean cultivars differing in seed size. Reciprocal F1 and F2 individual seed were evaluated for days to adult emergence (DAE) and emerged adult weight. Maternally inherited seed size affected resistance measurements only in the F1 reciprocal crosses, however, the overall resistance level of the F1 was more similar to that of the susceptible cultivars. The F2 showed a continuous, but skewed distribution from low to high DAE. Very few F2 individuals had the resistance level of G 12952. When the frequency distributions were divided into discrete categories based on parental response, resistance was found to be inherited as two recessive complementary genes. The F3 generation showed an overall lowering of resistance levels compared to their original F2 evaluations. However, none of the lines classified as resistant (50 DAE) in the F2, fell into the susceptible category in the F3, indicating that the resistant genotypes were relatively stable as expected with recessively inherited traits. Modifying genes from the commercial parents may be responsible for general lowering of resistance. Seed size was negatively correlated with adult weight but not with DAE. The unique resistance of the wild bean accessions is discussed in relation to its inheritance. The results and obstacles encountered in the A. obtectus breeding program at CIAT are described.     

Genetic diversity of Phaeoisariopsis griseola in the State of Minas Gerais, Brazil

Due to the importance of common bean angular leaf spotin the state of Minas Gerais-Brazil and to the greatvariability of the pathogen, Phaeoisariopsisgriseola, monitoring races becomes an important toolfor breeding programs aiming at genetic resistance.The pathogenic variability of 30 isolates of the P. griseola, collected from various locations in thestate of Minas Gerais, was studied using the followingcommon bean differential series (Don Timóteo,Bolón Bayo, Montcalm, G 5686, Amendoin, G 11796,BAT 332, PAN 72, Cornell 49-242, México 54, Florde Mayo and G 2858). The first trifoliate leaf wasinoculated with a 2 × 10⁴ conidia/mL. Plants weremaintained at 20–22 ^°C and 95% relativehumidity for 48 hours. Symptom evaluation wasperformed 15 days after inoculation. Thirteen raceswere identified demonstrating the wide geneticvariability of the pathogen in the state of MinasGerais. Race 63.63 was the most virulent, whereas race63.23 was the most frequent (10 of 30 isolates), beingwidely distributed among the regions studied. Thevirulence phenotype indicated that the races studiedbelonged to the Mesoamerican group, which wasconfirmed when the 30 isolates were compared to Andeanand Mesoamerican standards using RAPD markers.     

Use of molecular markers to accelerate the breeding of common bean lines resistant to rust and anthracnose

The main goal of this work was to introduce resistance genes for rust, caused by Uromyces appendiculatus, and anthracnose, caused by Colletotrichum lindemuthianum, in an adapted common bean cultivar through marker-assisted backcrossing. DNA fingerprinting was used to select plants genetically closer to the recurrent parent which were also resistant to rust and to race 89 of C. lindemuthianum. DNA samples extracted from the resistant parent (cv. Ouro Negro), the recurrent parent (cv. Rudá), and from BC₁, BC₂ and BC₃ resistant plants were amplified by the RAPD technique. The relative genetic distances in relation to the recurrent parent varied between 9 and 59% for BC₁, 7 and 33% for BC₂, and 0 and 7% for BC₃ resistant plants. After only three backcrosses, five lines resistant to rust and anthracnose with, approximately, 0% genetic distance in relation to the recurrent parent were obtained. These lines underwent field yield tests in two consecutive growing seasons and three of them presented a good yield performance, surpassing in that sense their parents and most of the reference cultivars tested.     

Identification of a novel source of resistance to angular leaf spot disease of common bean within the secondary gene pool

Phaeoisariopsis griseola (Sacc.) Ferr., the agent of angular leaf spot disease of common bean, is a highly variable pathogen for which resistance gene diversification is required. This study analysed genetic resistance to this disease within genotypes of three Phaseolus species. Twenty-nine genotypes of Phaseolus vulgaris, Phaseolus coccineus and Phaseolus polyanthus were inoculated with 54 isolates of Phaeoisariopsis griseola. The genetic resistance was estimated according to the symptom intensity observed for each plant genotype-pathogen isolate combination. Globally, genotypes of the common bean secondary gene pool were resistant to a higher number of isolates than common bean varieties. Interactions between plant genotypes and pathogen isolates suggested vertical resistance genes within P. vulgaris, as well as within P. coccineus and P. polyanthus. The ‘NI666’accession (P. coccineus) showed resistance to all the fungal isolates inoculated while the variety ‘Aroana’(P. vulgaris) was susceptible to most of the isolates. Interspecific hybridization between these two genotypes gave F₁ hybrid plants which showed resistance to angular leaf spot disease.     

Genetic mapping for resistance to gray leaf spot in maize

The molecular marker technology has been used on mapping of quantitative trait loci (QTLs) associated with plant resistance. The objectives of this research were to estimate genetic parameters and to map genomic regions involved in the resistance to gray leaf spot in maize. Ninety F₃ families from the BS03 (susceptible) and BS04 (resistant) cross were used. Field trials were performed using a 10 × 10 square lattice design with three replications. Data from 62 SSR markers were used for linkage analysis. The locations of the QTLs on the linkage groups were determined by composite interval mapping method and the phenotypic variance explained by each marker was determined by regression analysis. Several QTLs associated to disease resistance were identified in the population BS03 × BS04. Some QTLs showed significant effects over the different environments studied. The existence of significant QTLs in common among different environments indicates these genomic regions as possible new tools for marker-assisted selection in maize breeding programs.     

Breeding the common bean (Phaseolus vulgaris L.) for resistance to bean common mosaic virus: alternatives to backcrossing

A series of field experiments was undertaken in order to determine whether resistance to bean common mosaic virus (BCMV) could be incorporated into genotypes of the common bean (Phaseolus vulgaris L.) suitable for cultivation in Zimbabwe without recourse to backcrossing. Six inbred genotypes carrying the resistance-conferring alleles at the loci I and Bc-3 were crossed with five locally-adapted inbred genotypes. The first experiment comprised F₃ progeny rows, each derived from a single unselected F₂ plant, the second, F₃ bulks selected for resistance, and the third, a comparison of selected and unselected F₂-derived F₄ lines. The number of days to flowering and to maturity, the incidence of mosaic and necrosis symptoms, seed yield and seed size were recorded. There was evidence that late flowering and maturity were associated with BCMV resistance in some crosses, though not strongly enough to present an obstacle to plant breeding. The incidence of virus symptoms and seed yield were influenced by genetic factors additional to the major resistance genes, and variation in seed yield was present not only between bulk populations of crosses, but also between single-row plots of lines within crosses. This indicates that early-generation selection for yield in the presence of BCMV, even among progeny selected for BCMV-resistace, is likely to be effective. However, the variation in yield among F₄ lines was least in the highest-yielding crosses, which may represent a limit to successful selection for yield. Seed size was partly under additive genetic control, but there was also evidence of non-allelic interactions. There was no association between large seed size, preferred by consumers, and susceptibility to BCMV in the progeny, indicating that the association between these characters in the parent lines is fortuitous and will not present an obstacle to plant breeding. It is noted that a considerable amount of useful genetic information can be obtained without recourse to elaborate crossing schemes, provided that unselected progeny are included in experiments as controls. The evidence presented indicates that resistance to BCMV can be combined with appropriate values of maturity date, yield and seed size without the need for backcrossing.     

Inheritance of resistance to wildfire and angular leaf spot derived from Nicotiana rustica var. Brasilea

Summary The introgression of wildfire (races 0 and 1) and angular leaf spot (ALS) resistance from N. rustica var. Brasilea into N. tabacum has proved economically useful in Zimbabwe although the mode of inheritance of, and genetic relationships between the resistance are unknown. This study was undertaken to (1) examine the mode of inheritance of the resistance to races 0 and 1 of wildfire, and ALS, (2) determine the genetic relationship between the resistances and (3) establish whether the N. rustica-derived wildfire race 0 resistance is allelic to that obtained from N. longiflora. Inheritance was examined under greenhouse and field conditions by studying disease reactions in the parental, F1, F2 and backcross generations derived from crosses of three susceptible lines to a resistant line Nr-7. Three-point backcrosses to the susceptible parent were examined for linkage and segregating generations from a cross of Nr-7 to Burley 21 which carries the N. longiflora race 0 resistance were used to test for allelism. In general, we observed that all resistances are determined by a single dominant gene although some incosistent ratios were obtained likely due to misclassification of disease reactions and erratic transmission. All resistances showed linkage although pleiotropism cannot be ruled out. Allelism tests demonstrated that the N. rustica race 0 resistance is not allelic to that obtained from N. longiflora. Our findings are examined in relation to the efficacy of indirect selection for resistance.     

Genetic progress after four cycles of recurrent selection for yield and grain traits in common bean

The objective of this study was to evaluate the genetic progress after four cycles of recurrent selection in common bean. The base segregating population was obtained from 10 parents, and derived the S_0:1 and S_0:2 families that were evaluated. The S_0:3 families with higher grain yield and grain color, like the standard carioca were selected, and were intercrossed to generate the population of the following cycle. This process was repeated for four cycles. The best families were evaluated in each cycle by many generations and locations, and the five best lines of each cycle were identified. The 20 lines thus obtained were evaluated in two growing seasons, sown in July and November 2002. The grain yield (kg/ha) and grain type (scale of scores) were evaluated. Genetic progress was confirmed for both traits. The mean annual gain with selection for the grain type was 10.5% and 5.7% for grain yield, with no evidence of variability reduction in the population. These results show that recurrent selection is a good alternative for improving common bean quantitative traits.