Sources Of Rust Resistance Research Articles

New sources of rust resistance in a panel of foxtail millet genotypes

New sources of rust resistance in a panel of foxtail millet genotypes

Characterization and identification of sources of rust resistance in Triticum militinae derivatives

Triticum militinae (2n = 4X = 28, AtAtGG), belonging to the secondary gene pool of wheat, is known to carry resistance to many diseases. Though some disease resistance genes were reported from T. timopheevii, the closest wild relative of T. militinae, there are no reports from T. militinae. Twenty-one T. militinae Derivatives (TMD lines) developed at the Division of Genetics, IARI, New Delhi, were evaluated for leaf and stripe rusts at seedling and adult plant stages. Eight TMD lines (6–4, 6–5, 11–6, 12–4, 12–8, 12–12, 13–7 and 13–9) showed seedling resistance to both leaf and stripe rusts while six TMD lines (7–5, 7–6, 11–5, 13–1, 13–3 and 13–4) showed seedling resistance to leaf rust but adult plant resistance to stripe rust and three TMD lines (9–1, 9–2 and 15) showed seedling resistance to leaf rust but susceptibility to stripe rust. Three TMD lines (2–7, 2–8 and 6–1) with adult plant resistance to leaf and stripe rusts were found to carry the known gene Lr34/Yr18. Ten TMD lines (7–5, 7–6, 9–1, 9–2, 11–5, 11–6, 12–12, 12–4, 12–8, and 15) with seedling resistance to leaf rust, showing absence of known genes Lr18 and Lr50 with linked markers requires further confirmation by the test of allelism studies. As not a single stripe rust resistance gene has been reported from T. militinae or its close relative T. timpopheevii, all the 8 TMD lines (6–4, 6–5, 11–6,12–4, 12–8, 12–12, 13–7 and 13–9) identified of carrying seedling resistance to stripe rust and 3 TMD lines (13–1, 13–3 and 13–4) identified of carrying adult plant resistance to stripe rust are expected to carry unknown genes. Also, all the TMD lines were found to be cytologically stable and thus can be used in inheritance and mapping studies.

Identification of novel sources of partial and incomplete hypersensitive resistance to rust and associated genomic regions in common bean

Common bean (Phaseolus vulgaris) is one of the legume crops most consumed worldwide and bean rust is one of the most severe foliar biotrophic fungal diseases impacting its production. In this work, we searched for new sources of rust resistance (Uromyces appendiculatus) in a representative collection of the Portuguese germplasm, known to have accessions with an admixed genetic background between Mesoamerican and Andean gene pools. We identified six accessions with incomplete hypersensitive resistance and 20 partially resistant accessions of Andean, Mesoamerican, and admixed origin. We detected 11 disease severity-associated single-nucleotide polymorphisms (SNPs) using a genome-wide association approach. Six of the associations were related to partial (incomplete non-hypersensitive) resistance and five to incomplete hypersensitive resistance, and the proportion of variance explained by each association varied from 4.7 to 25.2%. Bean rust severity values ranged from 0.2 to 49.1% and all the infection types were identified, reflecting the diversity of resistance mechanisms deployed by the Portuguese germplasm.The associations with U. appendiculatus partial resistance were located in chromosome Pv08, and with incomplete hypersensitive resistance in chromosomes Pv06, Pv07, and Pv08, suggesting an oligogenic inheritance of both types of resistance. A resolution to the gene level was achieved for eight of the associations. The candidate genes proposed included several resistance-associated enzymes, namely β-amylase 7, acyl-CoA thioesterase, protein kinase, and aspartyl protease. Both SNPs and candidate genes here identified constitute promising genomics targets to develop functional molecular tools to support bean rust resistance precision breeding.

Genomic regions associated with resistance to soybean rust (Phakopsora pachyrhizi) under field conditions in soybean germplasm accessions from Japan, Indonesia and Vietnam

Key messageEight soybean genomic regions, including six never before reported, were found to be associated with resistance to soybean rust (Phakopsora pachyrhizi) in the southeastern USA.Soybean rust caused by Phakopsora pachyrhizi is one of the most important foliar diseases of soybean [Glycine max (L.) Merr.]. Although seven Rpp resistance gene loci have been reported, extensive pathotype variation in and among fungal populations increases the importance of identifying additional genes and loci associated with rust resistance. One hundred and ninety-one soybean plant introductions from Japan, Indonesia and Vietnam, and 65 plant introductions from other countries were screened for resistance to P. pachyrhizi under field conditions in the southeastern USA between 2008 and 2015. The results indicated that 84, 69, and 49% of the accessions from southern Japan, Vietnam or central Indonesia, respectively, had negative BLUP values, indicating less disease than the panel mean. A genome-wide association analysis using SoySNP50K Infinium BeadChip data identified eight genomic regions on seven chromosomes associated with SBR resistance, including previously unreported regions of Chromosomes 1, 4, 6, 9, 13, and 15, in addition to the locations of the Rpp3 and Rpp6 loci. The six unreported genomic regions might contain novel Rpp loci. The identification of additional sources of rust resistance and associated genomic regions will further efforts to develop soybean cultivars with broad and durable resistance to soybean rust in the southern USA.

Registration of two oilseed sunflower germplasms, HA‐R18 and HA‐R19, resistant to sunflower rust

AbstractSunflower rust, incited by the fungus Puccinia helianthi Schwein., is a serious fungal disease in the sunflower (Helianthus annuus L.) growing areas worldwide, with an increasing importance in North America in recent years due to the frequent evolution of new pathogen races. HA‐R18 (Reg. no. GP‐377, PI 698197) and HA‐R19 (Reg. no. GP‐378, PI 698198) are rust‐resistant germplasm lines that were developed and released cooperatively in 2020 by the USDA‐ARS, Sunflower and Plant Biology Research Unit and the Agricultural Experiment Station of North Dakota State University. The rust resistance in HA‐R18 and HA‐R19 originated from KP193 and KP199, respectively, which were identified segregating for rust resistance after screening of 58 sunflower lines introduced from South Africa. Rust evaluation of HA‐R18 and HA‐R19 with 11 P. helianthi races indicated that both lines exhibited resistance to the most rust races tested including the most predominant and the most virulent races currently identified in the United States. HA‐R18 and HA‐R19 possess the new rust resistance genes R17 and R18, respectively, both of which have been mapped to sunflower chromosome 13. The two new lines will diversify the pool of rust resistance genes and should serve as useful sources of rust resistance in sunflower.

Comparative transcriptome analysis of two contrasting resistant and susceptible Aegilops tauschii accessions to wheat leaf rust (Puccinia triticina) using RNA-sequencing

Leaf rust, caused by Puccinia triticina Eriks., is the most common rust disease of wheat (Triticum aestivum L.) worldwide. Owing to the rapid evolution of virulent pathotypes, new and effective leaf rust resistance sources must be found. Aegilops tauschii, an excellent source of resistance genes to a wide range of diseases and pests, may provide novel routes for resistance to this disease. In this study, we aimed to elucidate the transcriptome of leaf rust resistance in two contrasting resistant and susceptible Ae. tauschii accessions using RNA-sequencing. Gene ontology, analysis of pathway enrichment and transcription factors provided an apprehensible review of differentially expressed genes and highlighted biological mechanisms behind the Aegilops–P. triticina interaction. The results showed the resistant accession could uniquely recognize pathogen invasion and respond precisely via reducing galactosyltransferase and overexpressing chromatin remodeling, signaling pathways, cellular homeostasis regulation, alkaloid biosynthesis pathway and alpha-linolenic acid metabolism. However, the suppression of photosynthetic pathway and external stimulus responses were observed upon rust infection in the susceptible genotype. In particular, this first report of comparative transcriptome analysis offers an insight into the strength and weakness of Aegilops against leaf rust and exhibits a pipeline for future wheat breeding programs.

Identification of genomic regions conferring rust resistance and enhanced mineral accumulation in a HarvestPlus Association Mapping Panel ofwheat.

New genomic regions for high accumulation of 10 minerals were identified. The 1B:1R and 2NS translocations enhanced concentrations of four and two minerals, respectively, in addition to disease resistance. Puccinia species, the causal agents of rust diseases of wheat, have the potential to cause total crop failures due their high evolutionary ability to acquire virulence for resistance genes deployed in commercial cultivars. Hence, the discovery of genetically diverse sources of rust resistance is essential. On the other hand, biofortification of wheat for essential nutrients, such as zinc (Zn) and iron (Fe), is also an objective in wheat improvement programs to tackle micronutrient deficiency. The development of rust-resistant and nutrient-concentrated wheat cultivars would be important for sustainable production and the fight against malnutrition. The HarvestPlus association mapping panel (HPAMP) that included nutrient-dense sources from diverse genetic backgrounds was genotyped using a 90K Infinium SNP array and 13 markers linked with rust resistance genes. The HPAMP was used for genome-wide association mapping to identify genomic regions underpinning rust resistance and mineral accumulation. Twelve QTL for rust resistance and 53 for concentrations of 10 minerals were identified. Comparison of results from this study with the published QTL information revealed the detection of already known and some putatively new genes/QTL underpinning stripe rust and leaf rust resistance in this panel. Thirty-six new QTL for mineral concentration were identified on 17 chromosomes. Accessions carrying the 1B:1R translocation accumulated higher concentrations of Zn, Fe, Copper (Cu) and sulphur (S). The 2NS segment showed enhanced accumulation of grain Fe and Cu. Fifteen rust-resistant and biofortified accessions were identified for use as donor sources in breeding programs.

Finding new sources of rust resistance from Watkins wheat landraces and wild relatives in Egypt through resistance gene enrichment breeding

Finding new sources of rust resistance from Watkins wheat landraces and wild relatives in Egypt through resistance gene enrichment breeding

Evaluations of Genomic Prediction and Identification of New Loci for Resistance to Stripe Rust Disease in Wheat (Triticum aestivum L.).

Stripe rust is one of the most destructive diseases of wheat (Triticum aestivum L.), caused by Puccinia striiformis f. sp. tritici (Pst), and responsible for significant yield losses worldwide. Single-nucleotide polymorphism (SNP) diagnostic markers were used to identify new sources of resistance at adult plant stage to wheat stripe rust (YR) in 141 CIMMYT advanced bread wheat lines over 3 years in replicated trials at Borlaug Institute for South Asia (BISA), Ludhiana. We performed a genome-wide association study and genomic prediction to aid the genetic gain by accumulating disease resistance alleles. The responses to YR in 141 advanced wheat breeding lines at adult plant stage were used to generate G × E (genotype × environment)-dependent rust scores for prediction and genome-wide association study (GWAS), eliminating variation due to climate and disease pressure changes. The lowest mean prediction accuracies were 0.59 for genomic best linear unbiased prediction (GBLUP) and ridge-regression BLUP (RRBLUP), while the highest mean was 0.63 for extended GBLUP (EGBLUP) and random forest (RF), using 14,563 SNPs and the G × E rust score results. RF and EGBLUP predicted higher accuracies (∼3%) than did GBLUP and RRBLUP. Promising genomic prediction demonstrates the viability and efficacy of improving quantitative rust tolerance. The resistance to YR in these lines was attributed to eight quantitative trait loci (QTLs) using the FarmCPU algorithm. Four (Q.Yr.bisa-2A.1, Q.Yr.bisa-2D, Q.Yr.bisa-5B.2, and Q.Yr.bisa-7A) of eight QTLs linked to the diagnostic markers were mapped at unique loci (previously unidentified for Pst resistance) and possibly new loci. The statistical evidence of effectiveness and distribution of the new diagnostic markers for the resistance loci would help to develop new stripe rust resistance sources. These diagnostic markers along with previously established markers would be used to create novel DNA biosensor-based microarrays for rapid detection of the resistance loci on large panels upon functional validation of the candidate genes identified in the present study to aid in rapid genetic gain in the future breeding programs.

Molecular and phenotypic evaluation of wheat (Triticum aestivum) containing different combinations of rust resistance genes

AbstractLeaf, stem and stripe rust caused by Puccinia triticina, P. graminis f. sp. tritici and P. striiformis f. sp. tritici, respectively, are common wheat diseases. The deployment of complex rust resistance sources is likely to contribute to their durability. In this study, wheat varieties and experimental plants (F2), with different rust resistance gene combinations, were characterised using molecular markers, phenotypic evaluation and quantification of fungal biomass accumulation. The experimental wheat plants were developed from six donor parental varieties that in combination carry seven known rust resistance genes/quantitative trait loci (QTL). A timeline study revealed that quantification of fungal biomass accumulation for the rusts, 10‐day postinoculation (dpi), can successfully distinguish between resistant and susceptible wheat varieties. Phenotypic evaluation included seedling infection types and adult‐plant response types that were determined 15 dpi. Data obtained from the three methods were in support of each other and in combination successfully distinguished between rust‐resistant and susceptible experimental wheat plants. Resistance genes/QTL evaluated in this study, when deployed in combination, may lower the risk of rust outbreaks.

Preliminary screening of Avena sterilis L. for resistance to crown rust

The best source of crown rust resistance genes (Pc) in genus Avena is a wild hexaploid A. sterilis L. In this study, accessions of A. sterilis gathered from European and North American gene banks, originated from 21 countries were evaluated at the seedling stage for crown rust reaction using the host–pathogen test and two Puccinia coronata f. sp. avenae isolates. Of the 45 oat accessions analyzed, 12 were resistant to one crown rust race (3.2). Resistance to both pathotypes used in the study was observed in two of the accessions, first of which was collected in Libya (AVE 2532) and second in Portugal (CN 26036). Further research is required to evaluate the genetic background of the discovered resistance, however, obtained results provide a valuable first step in the identification of new promising crown rust resistance sources.

Identification of yellow rust resistance sources in advanced breeding lines of barley (Hordeum vulgare L.)

Yellow rust of barley caused by Puccinia striiformis f. sp. hordei (Psh), is an important disease in barley growing regions of India and worldwide. For identifying adult plant stage resistance to yellow rust, a set of 29 barley advanced breeding lines was tested at five locations, Durgapura, Jammu, Ludhiana, Bajaura and Karnal during 2016-17 and 2017-18. The seedling resistance test was conducted on 27 barley advanced breeding lines against seven Psh pathotypes separately under controlled conditions at ICAR-IIWBR, Shimla during 2017-18. As consequences, seven lines (DWRFB10, DWRFB12, DWRFB14, DWRFB15, DWRFB19, DWRFB20, and DWRFB28) were found immune to highly resistant against yellow rust at adult plant stage across the locations. Similarly, seven advanced breeding lines viz., DWRFB11, DWRFB12, DWRFB13, DWRFB14, DWRFB19, DWRFB20, and DWRFB27, were found resistant to yellow rust at the seedling stage. On the basis of APR and SRT, four advanced barley breeding lines, DWRFB12, DWRFB14, DWRFB19 and DWRFB20 were highly resistant to yellow rust (nearly immune) both at seedling and adult-plant stages. DOI.org10.25174/2249-4065/2019/85076

Genetic analysis of stripe rust resistance in a set of European winter wheat genotypes

Stripe rust disease caused by Puccinia striiformis f. sp. tritici is of much importance in North-Western Plain Zone of India (NWPZ) since the resistance breakdown of the widely grown wheat cultivar PBW 343. With the rapid emergence and spread of aggressive rust pathogens, robust and durable resistant crop cultivar development becomes the urgent need for any wheat breeding program to sustain. In order to identify new stripe rust resistance sources, a panel of European winter wheat germplasm screened against prevalent pathotypes of stripe rust pathogens of NWPZ under artificial epiphytotic conditions. Crosses were attempted between fourteen selected resistant European winter wheat genotypes and four highly susceptible spring wheat varieties. Backcross populations were developed and screened for stripe rust reaction. Genetic analysis was performed in BC1F2 populations and confirmed with single plant derived BC1F3 and BC1F4 progenies. Among fourteen European parents used for introgressing stripe rust resistance, two lines namely Acienda and Aubusson showed single dominant gene, four lines viz. Aciento, Bagatelle_007, Brilliant and Nuage displayed digenic segregation ratio. Overall genetic analysis gave indication of the major genes for stripe rust resistance.

Screening of Cowpea ( Vigna unguiculata L. Walp) Genotypes for Rust ( Uromyces phaseoli var. vignae ) Resistance in Ghana

The demand for cowpea (Vigna unguiculata L. Walp) is higher than supply in Ghana due to low yields caused by pathogenic diseases, predominantly rust disease. The use of rust resistant cultivars is the most effective method to control cowpea rust. Genetic variations among cowpea genotypes may be potential sources of rust resistance to control cowpea rust and increase cowpea yield and production in Ghana. The study assessed rust disease incidence and severity among cowpea genotypes and determined resistance to cowpea rust under field conditions. Twenty-four cowpea genotypes were sowed in four agro-ecological zones in two cropping seasons in Ghana. Cowpea rust incidence, severity, area under disease progress curve (AUDPC) and relative area under disease progress curve (rAUDPC) were significantly (p < 0.05) higher in the semi-deciduous forest and minor cropping season compared with deciduous forest, coastal savannah, Sudan savannah and major cropping season. The cowpea genotypes also showed significant differences (p < 0.05) in response to rust infection. Positive and negative correlations existed in rust incidence, severity, AUDPC and rAUDPC within the agro-ecological zones and cropping seasons. The differences observed were due to variations in climatic conditions and genetic composition of the cowpea genotypes. Five cowpea genotypes were better slow rusting, eleven cowpea genotypes were slow rusting and eight cowpea genotypes were fast rusting. Interestingly, eleven cowpea genotypes showed resistance and eight cowpea genotypes showed moderate resistance to cowpea rust. The rust resistant cowpea genotypes identified in this work can be recommended for farmers to cultivate and used in breeding programmes to further improve the crop. This will maximize yields and increase cowpea production particularly in rust prone areas.

Field evaluation and components of peanut rust resistance of newly developed breeding lines

ABSTRACT Field, greenhouse, and growth chamber experiments were conducted to determine the level of resistance to Puccinia arachidis Speg. in newly developed breeding lines of peanut (Arachis hypogaea L.). These lines were developed in the UF150 project of the Peanut Collaborative Research and Support Program (Peanut CRSP) as part of the United States Agency for International Development (USAID). Field experiments were carried out in Citra, FL and Tifton, GA from 2010 to 2013. Five genotypes Tifrust-10 and Tifrust-13, and CRSP breeding lines PTBOL3-3, 97x36-HO2-1-B2G-3-1-2-2, and BOL3-7 had the lowest standardized area under the disease progress curve and final disease severity score for rust. The CRSP breeding lines 97x36-HO2-1-B2G-3-1-2-2 and BOL3-7 also appeared to be highly resistant to late leaf spot, caused by Cercosporidium personatum (Berk &amp; M. A. Curtis Deighton). In growth chamber studies, genotypes with longer latent periods generally had lower infection frequencies at 7, 11, and 16 d after inoculation, and smaller percent diseased areas. Latent period and percent diseased area were significantly correlated with stAUDPC. CRSP breeding lines 97x36-HO2-1-B2G-3-1-2-2 and BOL3-7, and plant introductions PI562530, PI568164, and PI298115, were among the genotypes with the lower scores for these components. Several genotypes with multiple disease resistance in different environments and under high disease pressure were identified in these studies. These results indicate sources of rust resistance in the CRSP breeding lines, including several genotypes that could be used as parents in peanut germplasm enhancement programs, and indicate that latent period, percent diseased area, and lesion diameter may be used as indicators for rust resistance in growth chamber studies.

Characterization and genome-wide association mapping of resistance to leaf rust, stem rust and stripe rust in a geographically diverse collection of spring wheat landraces

The challenge posed by rapidly changing wheat rust pathogens, both in virulence and in environmental adaptation, calls for the development and application of new techniques to accelerate the process of breeding for durable resistance. To expand the resistance gene pool available for germplasm improvement, a panel of 159 landraces plus old cultivars was evaluated for seedling and adult plant resistance (APR) to over 35 Australian pathotypes of Puccinia triticina, Puccinia graminis f. sp. tritici, and Puccinia striiformis f. sp. tritici. Known seedling resistance (SR) genes for leaf rust (Lr2a, Lr3a, Lr13, Lr23, Lr16, and Lr20), stem rust (Sr12, Sr13, Sr23, Sr30, and Sr36), and stripe rust (Yr3, Yr4, Yr5, Yr9, Yr10, Yr17, and Yr27) were postulated. The APR genes identified via field assessments and marker analyses included the pleiotropic genes (Lr34/Yr18/Sr57, Lr46/Yr29/Sr58, Lr67/Yr46/Sr55, and Sr2/Lr27/Yr30), Lr68, Lr74, and uncharacterized APR. A genome-wide association analysis using linear mixed models detected 79 single nucleotide polymorphism (SNP) markers significantly associated with rust resistance, which were mapped on chromosomes 1A, 1B, 1D, 2A, 2B, 3A, 3B, 3D, 4A, 5A, 5B, 6A, 6B, 6D, 7A, 7B and 7D. SNPs associated with multiple rust resistances probably indicate the presence of new pleiotropic or closely linked genes. SNPs were mapped on chromosome positions (1AL, 1DS, 2AL, 4AS, 5BS, 6DL, and 7AL) that have not been known to carry APR genes. This study revealed the presence of a range of possibly unidentified effective seedling and APRs among the landraces, which might represent new sources of rust resistance for the ongoing effort to develop improved wheat cultivars.

Phenotypic and genotypic screening for rust resistance in common bean germplasm in Uganda

Rust caused by Uromyces appendiculatus (Pers., Pers.) Unger is one of the major foliar diseases of common bean (Phaseolus vulgaris) in Uganda. The use of host resistance remains the best option in managing this disease. The objective of this study was to identify sources of broad-spectrum rust resistance in common bean germplasm including landraces, commercial cultivars and introduced genotypes using a combination of phenotypic and genotypic screening with 22 simple sequence repeat (SSR) markers located on chromosome Pv04. A total of 138 genotypes were field screened from 2014 and 2015 using an alpha lattice design. The variance and correlation of disease incidence, area under the disease progression curve (AUDPC) and total grain yield were computed using GenStat. The polymorphism information content of the genotypes was determined, and the association of the markers and the disease resistance traits were analyzed using PowerMarker and TASSEL respectively. Resistance of each genotype was compared to the presence and absence of amplified markers. There were highly significant differences (P < 0.001) among the genotypes for disease incidence, AUDPC and total grain yield and a strong correlation (P < 0.001) between disease incidence and AUDPC in both years. The SSR markers, BARC_PV_SSR04725, bean_ssr_0778 and bean_ssr_2892 were associated (P ≤ 0.05) with rust resistance. Fifteen 15 genotypes which included the landraces Nabufumbo, and Kapchorwa white, and the commercial cultivar NABE 2 were identified as new sources of rust resistance that would be useful in future bean breeding programmes in Uganda.

Stem and leaf rust resistance in wild relatives of wheat with D genome (Aegilops spp.)

Resistance to stem rust and leaf rust in five D genome species of wheat viz., 267 accessions of Aegilops tauschii Coss., 39 of Ae. cylindrica Host, 17 of Ae. ventricosa Tausch, 4 of Ae. crassa Boiss. and 8 of Ae. juvenalis (Thell.) Eig were evaluated at adult plant stage. Two hundred and thirty nine (90 %) accessions of Ae. tauschii, 30 (77 %) of Ae. cylindrica, 16 (94 %) of Ae. ventricosa, 3 (75 %) of Ae. crassa Boiss. and 5 (62.5 %) of Ae. juvenalis were resistant to stem rust pathotypes prevalent in South India at Wellington under field condition. Invariably, all the accessions of the five species were resistant to leaf rust pathotypes. Quantitative measurement of disease using area under the disease progress curve revealed the slow progress of disease in the resistant accessions compared to susceptible check (Agra Local). Since all the five species have D genome, it could be concluded that the genes present in D genome might play a vital role in leaf rust resistance, but in case of stem rust resistance wide range of differential response was noticed. Among the species evaluated, Ae. tauschii was exploited to a larger extent, followed by Ae. ventricosa and Ae. cylindrica for leaf and stem rust resistance because of the homology of D genome with hexaploid bread wheat. While, Ae. crassa and Ae. juvenalis could not be utilized so far, possibly due to partial homology which makes the transfer of traits difficult. So, these species have considerable potential as a source of rust resistance and may enhance the existing gene pool of resistance to stem and leaf rusts.

Soybean rust resistance sources and inheritance in the common bean (Phaseolus vulgaris L.).

Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, has been reported in common bean (Phaseolus vulgaris L.) cultivars and elite lines that were infected under controlled and natural field conditions in South Africa, the United States, Argentina, and Brazil. Although SBR is currently not a top priority problem for the common bean crop, many bean breeders are concerned about this disease because of the high severity and virulence diversity of P. pachyrhizi and its broad host range. In this study, a set of 44 P. vulgaris genotypes were tested for resistance to P. pachyrhizi; these genotypes included resistance sources to several fungal common bean diseases, carioca-, black- and red-seeded Brazilian cultivars, and elite lines that were developed by the main common bean breeding programs in Brazil. Twenty-four SBR resistance sources were identified. They presented the reddish-brown (RB) lesion type, characterizing resistance reactions. In addition to the RB lesion type, the PI181996 line presented the lowest disease severity mean score, considering its associated standard error value. For this reason, it was crossed with susceptible lines to study the inheritance of resistance. The results support the hypothesis that resistance to SBR in PI181996 is monogenic and dominant. We propose that this SBR resistance gene, the first to be identified and characterized in common bean, might be designated as Pkp-1.

Wild populations as a source of germplasm for black poplar (Populus nigra L.) breeding programmes

The aims were to explore the use of wild populations of Populus nigra for breeding and to compare crosses made between genotypes from different areas with different climates. Genetic distances ranged from 0.175 (populations growing under optimal conditions) up to 0.875 (populations growing under marginal conditions). A total of 39 trees were chosen from these populations, and in four different years 58 different parental crosses were made, from which a total of 2,277 individuals were produced for assessment as potential biomass producers. The progenies from each cross were rated according to a selection index (SI) based on three characteristics (height, leading shoot diameter and resistance to the rust Melampsora larici-populina). Sixteen of such crosses had significantly higher SI compared to the lowest SI values observed. In 19 of the crosses, it was shown that certain individual trees had made a significant contribution to lower susceptibility to rust, and in particular, two male trees used in the parental crosses were implicated as being the source of rust resistance. Selected individuals were cloned and used in a field trial with NE-42, as a control. It was found that six clones had significantly higher SI than NE-42. In crosses between trees originating from contrasting conditions, there was a higher average SI than in crosses between trees from the same conditions. However, these differences were not statistically significant. The successful clones are recommended for use in those areas where the native populations of pure black poplar are threatened by cross-pollination with allochthonous species.