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.     

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.     

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.     

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.     

Identification of RAPD and SCAR markers linked to northern leaf blight resistance in waxy corn (Zea mays var. ceratina)

Exserohilum turcicum causes northern corn leaf blight (NCLB), an important disease occurring in maize producing areas throughout the world. Currently, the development of cultivars resistant to E. turcicum seems to be the most efficient method to control NCLB damage. Marker-assisted selection (MAS) enables breeders to improve selection efficiency. The objective of this work was to identify random amplified polymorphic DNA (RAPD) and sequence characterized amplified region (SCAR) markers associated with NCLB resistance. Bulked segregant analysis (BSA) was used to search for RAPD markers linked to NCLB resistance genes, using F₂ segregating population obtained by crossing a susceptible inbred ‘209W’ line with a resistant inbred ‘241W’ line. Two hundred and twenty-two decamer primers were screened to identify four RAPD markers: OPA07₅₂₁, OPA16₄₅₇, OPB09₅₂₀, and OPE20₅₃₆ linked to NCLB resistance phenotype. These markers were converted into dominant SCAR markers: SCA07₄₉₆, SCA16₄₂₀, SCB09₄₆₄, and SCE20₄₂₉, respectively. The RAPD and SCAR markers were developed successfully to identify NCLB resistant genotypes in segregating progenies carrying NCLB resistant traits. Thus, the markers identified in this study should be applicable for MAS for the NCLB resistance in waxy corn breeding programs.     

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.     

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.     

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 SCAR Markers Co-Segregating with an Agropyron Elongatum Derived Leaf Rust Resistance Gene Lr24 in Wheat

Summary An Agropyron elongatum-derived leaf rust resistance gene Lr24 located on chromosome 3DL of wheat was tagged with six random amplified polymorphic DNA (RAPD) markers which co-segregated with the gene. The markers were identified in homozygous resistant F₂ plants taken from a population segregating for leaf rust resistance generated from a cross between two near-isogenic lines (NILs) differing only for Lr24. Phenotyping was done by inoculating the plants with pathotype 77-5 of Puccinia triticina. To enable gene-specific selection, three RAPD markers (S1302₆₀₉, S1326₆₁₅ and OPAB-1₃₈₈) were successfully converted to polymorphic sequence characterized amplified region (SCAR) markers, amplifying only the critical DNA fragments co-segregating with Lr24. The SCAR markers were validated for specificity to the gene Lr24 in wheat NILs possessing Lr24 in 10 additional genetic backgrounds including the Thatcher NIL, but not to 43 Thatcher NILs possessing designated leaf rust resistance genes other than Lr24. This indicated the potential usefulness of these SCAR markers in marker assisted selection (MAS) and for pyramiding leaf rust resistance genes in wheat.     

The role of resistance breeding in the intergrated control of downy mildew (Bremia lactucae) in protected lettuce

Summary Over the last 30 years, six resistance alleles (Dm2, Dm3, Dm6, Dm7, Dm11 andDm16) located in two linkage groups, have contributed to the control of downy mildew in lettuce crops grown under protection (glass or polythene) in northern Europe. More recently, an as yet genetically uncharacterised resistance factor, R18, has also begun to assume importance. The occurrence of the various combinations of these resistance alleles that exist in commercial cultivars has been dictated by the pathotypes ofBremia lactucae used in their selection but also restricted by linkage in repulsion. In the UK, a pathotype ofB. lactucae insensitive to phenylamide fungicides, such as metalaxyl, emerged in 1978 and became prevalent throughout lettuce production areas in subsequent years. The specific virulence of this pathotype was identical to the previously described phenylamide sensitive pathotype NL10 and cultivars carryingDm11, Dm16 or R18 were resistant. Consequently, an integrated control strategy based on the utilisation of metalaxyl on cultivars carryingDm11 provided effective control in UK until 1987 when a new phenylamide insensitive pathotype began to cause problems. The specific virulence of this second pathotype, which was first reported in the Netherlands and France, was identical to the previously described phenylamide sensitive pathotype NL15. Cultivars carryingDm6, Dm16 or R18, but notDm11, were resistant to NL15; consequently an appropriate change in the cultivar recommendations for use in the integrated control strategy was successfully promulgated. It is predicted that variations of this integrated control strategy involving the use of appropriately selectedDm gene combinations may prove effective for some time. This prediction is based on studies of the status of the avirulence loci in the two phenylamide insensitive pathotypes and of the specific virulence characteristics of phenylamide sensitive components of the pathogen population.     

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 temperature-sensitive leaf rust resistance and adult plant stripe rust resistance in common wheat cultivar PBW343

Genetic analysis of common wheat cultivar PBW343 confirmed temperature-sensitive leaf rust resistance and adult plant stripe rust resistance. At low temperatures, PBW343 was resistant to P. triticina (Ptr) pathotype (pt.) 121R63-1, and at high temperature it was resistant to Ptr pt. 121R127. The low temperature resistance to pt. 121R63-1 was attributed to interaction between dominant and recessive genes. The dominant gene involved in low-temperature resistance to pt. 121R63-1 also conferred resistance to pt. 45R35. The high-temperature resistance to Ptr pt. 121R127 was governed by a different single partially dominant gene. Agra Local (a commonly used susceptible check) and IWP94 (a leaf rust differential used in India) are also resistant to pt. 121R127 at high temperatures. An allelism test indicated that PBW343 and IWP94 possessed a common gene for high temperature resistance to this pathotype. The adult plant stripe rust resistance against P. striiformis (Pst) was possibly conferred by one gene in addition to Yr27.     

RAPD and SCAR markers for resistance to acochyta blight in lentil

Resistance to ascochyta blight of lentil (Lens culinaris Medikus),caused by the fungus Ascochyta lentis, is determined by a single recessive gene, ral ₂, in the lentil cultivar Indian head. Sixty F₂ individuals from a cross between Eston (susceptible) and Indian head (resistant) lentil were analyzed for the presence of random amplified polymorphic DNA (RAPD) markers linked to the ral ₂gene, using bulked segregant analysis (BSA). Out of 800 decanucleotide primers screened, two produced polymorphic markers that co-segregated with the resistance locus. These two RAPD markers, UBC227₁₂₉₀and OPD-10₈₇₀, flanked and were linked in repulsion phase to the gene ral ₂ at 12 cm and 16 cm, respectively. The RAPD fragments were converted to SCAR markers. The SCAR marker developed from UBC227₁₂₉₀ could not detect any polymorphism between the two parents or in the F₂. The SCAR marker developed from OPD-10₈₇₀ retained its polymorphism. The polymorphic RAPD marker UBC227₁₂₉₀ and the SCAR marker developed from OPD-10₈₇₀ can be used together in a marker assisted selection program for ascochyta blight resistance in lentil. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of durable resistance to stem rust in barley

Summary Since the mid-1940's, barley cultivars grown in the northern Great Plains of the USA and Canada have been resistant to stem rust caused byPuccinia graminis f. sp.tritici. This durable resistance is largely conferred by a single gene,Rpg1, derived from a single plant selection of the cultivar Wisconsin 37 and an unimproved Swiss cultivar. At the seedling stage, barley genotypes withRpg1 generally exhibit low mesothetic reactions at 16–20° C and slightly higher mesothetic reactions at 24–28° C to many stem rust pathotypes. This resistance is manifested by a low level of rust infection and mostly incompatible type uredia on adult plants.Rpg1 reacts in a pathotype-specific manner since some genotypes ofP. g. f. sp.tritici are virulent on cultivars carrying this gene in the field. Several factors may have contributed to the longevity of stem rust resistance in barley, a) since barley is planted early and matures early, it can sometimes escape damage from stem rust inoculum carried from the south; b) one or more minor genes may augment the level of resistance already provided byRpg1; c) the cultivation of resistant wheat cultivars and eradication of barberry have reduced the effective population size and number of potential new pathotypes ofP. g. f. sp.tritici, respectively; and d) virulent pathotypes ofP. g. f. sp.tritici andP. g. f. sp.secalis have not become established. This situation changed in 1989 when a virulent pathotype (Pgt-QCC) ofP. g. f. sp.tritici became widely distributed over the Great Plains. However,Rpg1 may still confer some degree of resistance to pathotype QCC because stem rust severities have been low to moderate and yield losses light on barley cultivars carrying the gene during the last four seasons (1989–1992). Several sources of incomplete resistance to pathotype QCC have been identified in barley. To facilitate the transfer of resistance genes from these sources into advanced breeding lines, molecular marker assisted selection is being employed.     

An allelic series at the Co-1 locus conditioning resistance to anthracnose in common bean of Andean origin

In this study, we characterized the genetic resistance of the Andean bean cultivars Kaboon and Perry Marrow and their relation to other sources of anthracnose resistance in common bean. Based on the segregation ratio (3R:1S) observed in two F₂ populations we demonstrated that Kaboon carries one major dominant gene conferring resistance to races 7 and 73 of Colletotrichum lindemuthianum. This gene in Kaboon is independent from the Co-2 gene and is an allele of the Co-1 gene present in Michigan Dark Red Kidney (MDRK) cultivar. Therefore, we propose the symbol CO-1 ² for the major dominant gene in Kaboon. The Co-1 is the only gene of Andean origin among the Co anthracnose resistance genes characterized in common bean. When inoculated with the less virulent Andean race 5, the segregation ratio in the F₂ progeny of Cardinal and Kaboon was 57R:7S (p = 0.38). These data indicate that Kaboon must possess other weaker dominant resistance genes with a complementary mode of action, since Cardinal is not known to possess genes for anthracnose resistance. Perry Marrow, a second Andean cultivar with resistance to a different group of races, was shown to possess another resistant allele at the Co-1 locus and the gene symbol Co-1 ³ was assigned. In R × R crosses between Perry Marrow and MDRK or Kaboon, no susceptible F₂ plants were found when inoculated with race 73. These findings support the presence of a multiple allelic series at the Andean Co-1 locus, and have major implications in breeding for durable anthracnose resistance in common bean. This revised version was published online in July 2006 with corrections to the Cover Date.     

A major QTL for common bacterial blight resistance derives from the common bean great northern landrace cultivar Montana No.5

Knowledge of the evolutionary origin and sources of pest resistance genes will facilitate gene deployment and development of crop cultivars with durable resistance. Our objective was to determine the source of common bacterial blight (CBB) resistance in the common bean Great Northern Nebraska #1 (GN#1) and GN#1 Selection 27 (GN#1 Sel 27). Several great northern cultivars including GN#1, GN#1 Sel 27, and Montana No.5 (the female parent of the common x tepary bean interspecific population from which GN #1 and GN # 1 Sel 27 were derived) and known susceptible checks were evaluated for CBB reaction in field and greenhouse environments. These genotypes and CBB resistant and susceptible tepary bean including Tepary #4, the male parent and presumed contributor of CBB resistance toGN#1 and GN#1 Sel 27, were assayed for presence or absence of three SCAR markers tightly linked with independent QTLs conditioning CBB resistance. The parents and F₂ of Montana No. 5/GN #1 Sel 27 and Montana No.5/Othello(CBB susceptible) were screened for CBB reaction and SCAR markers. CBB resistance in Montana No.5 was comparable to that of GN#1 and GN#1 Sel27. The SAP6 SCAR marker present in GN#1 and GN#1 Sel 27 was also present in Montana No.5, and it co-segregated (R ² =35%) with the CBB resistance in the Montana No.5/Othello F₂ population. Although a few CBB resistant and susceptible transgressive segregants were found in the F₂ of MontanaNo.5/GN #1 Sel 27 and later confirmed by F₃ progeny tests, SAP6 SCAR marker was present in all progenies. None of the tepary bean specific CBB resistance-linked SCAR markers were present in GN#1, GN#1 Sel 27, or Montana No.5. A cluster analysis of 169 polymorphic PCR-based markers across three common bean and Tepary #4 indicated that GN#1, GN#1 Sel 27, and Montana No.5 were closely related, and not related at all with Tepary #4.Thus, these results clearly indicate Montana No.5, not Tepary #4, as the source of CBB resistance in GN#1 and GN#1 Sel 27. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sources of resistance to Pyrenophora tritici-repentis in bread wheats

Summary No complete resistance to Pyrenophora tritici-repentis has been located in more than 1400 bread wheats examined. Incomplete resistance, however, occurs at different levels in many spring and winter types and data are presented for the strongest sources of resistance detected. In particular, there is a high frequency of Brazilian spring wheats with appreciable levels of resistance to this pathogen. Recent international nurseries from CIMMYT, Mexico, also contain numerous potentially valuable sources of resistance and these wheats may be shorter and higher yielding in Australia than the Brazilian wheats. The resistances in many Brazilian cultivars may be largely common because the cultivars are often strongly interrelated. Some of the Brazilian wheats resistant to P. tritici-repentis are also resistant to one or more of the septoria diseases and/or possess tolerance to aluminium toxicity.     

Breeding beans for resistance to terminal drought in the Lowland tropics

In the lowland regions of Latin America, a large proportion of beans are sown at the beginning of a dry season where a guaranteed terminal (end-of-season) drought will reduce yields. This study was undertaken to identify lines within two black bean recombinant inbred line (RIL) populations with resistance to terminal drought. The two RIL populations were developed from crosses between a drought resistant line, B98311 from Michigan, with TLP 19 and VAX 5, two lines from CIAT with improved disease resistance and adaptation to growing conditions in Latin America. The RIL populations were evaluated in experiments conducted in Zamorano, Honduras and Veracruz, Mexico under drought stress and well-watered (non-stress) treatments. Yields were reduced in each experiment by drought and the fungal pathogen, Macrophomina phaseolina. Drought stress, disease pressure and low yields contributed to high coefficients of variation (CV), which made it difficult to select superior lines. Selection was based on rank of geometric mean (GM) yield calculated from the yield in the stress and non-stress treatments. One RIL, L88-63 ranked first in GM yield at both locations. Subsequent testing in Honduras and Michigan confirmed the high yield potential and broad adaptation of L88-63. Breeding beans for drought resistance in lowland tropical environments should also include breeding for resistance to M. phaseolina. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Inter and intra-population variability of <Emphasis Type="Italic">Jatropha curcas</Emphasis> (L.) characterized by RAPD and ISSR markers and development of population-specific SCAR markers

Jatropha curcas (Euphorbiaceae) is an oil-bearing species with multiple uses and considerable potential as a bioenergy crop. The present investigation has been undertaken to assess the extent of genetic diversity in a representative set of 42 accessions of J. curcas encompassing different crop growing regions in India along with a non-toxic genotype from Mexico as a prelude for utilization of promising and genetically divergent materials in the breeding programmes. Molecular polymorphism was 42.0% with 400 RAPD primers and 33.5% with 100 ISSR primers between accessions indicating modest levels of genetic variation in the Indian germplasm. The within-population variation based on RAPD polymorphism was 64.0% and was on par with the inter-population variation. Polymorphic ISSR markers have been identified that could differentiate the Indian accessions from the Mexican genotype and two of them were converted to SCAR markers. The SCAR primer pair ISPJ1 amplified a 543 bp fragment in all the Indian populations, while ISPJ2 with a specific amplicon of 1,096 bp was specific to the Mexican genotype. Population-specific bands have been identified for the accession from Kerala (2 RAPD markers), Neemuch-1 from Rajasthan (1 each of RAPD and ISSR markers) and the non-toxic genotype from Mexico (17 RAPD and 4 ISSR markers), which serve as diagnostic markers in genotyping. The study indicates an immediate need for widening the genetic base of J. curcas germplasm through introduction of accessions with broader geographical background.