Fungus Uromyces Appendiculatus Research Articles

Host Resistance to Uromyces appendiculatus in Common Bean Genotypes.

Rust, induced by the fungus Uromyces appendiculatus, is one of the most serious bean diseases. The involved mechanisms in rust resistance were evaluated in 10 common bean genotypes during the 2019/2020 and 2020/2021 growing seasons. The disease parameters such as final rust severity (FRS%), area under the disease progress curve (AUDPC) and disease increase rate (r-value) were lower in the resistant genotypes than in highly susceptible genotypes. Biochemical compounds such as total phenols and the activity of antioxidant enzymes such as catalase, peroxidase and polyphenol oxidase were increased in the resistant genotypes compared to susceptible genotypes. In the resistance genotypes, the levels of oxidative stress markers such as hydrogen peroxide (H2O2) and superoxide (O2•−) increased dramatically after infection. The electrolyte leakage percentage (EL%), was found to be much greater in susceptible genotypes than resistant genotypes. The resistant gene SA14, which was found in genotypes Nebraska and Calypso at 800 bp, had an adequate level of resistance to bean rust with high grain yield potential. After infection, the transcriptions levels of 1,3-D-glucanases and phenylalanine ammonia lyase) were higher in the resistant genotypes than susceptible genotypes. In conclusion, the resistant genotypes successfully displayed desirable agronomic traits and promising expectations in breeding programs for improving management strategies of common bean rust disease. The resistance was mediated by antioxidant enzymes, phenolic compounds, and defense gene expressions, as well as the resistant gene SA14.

Protection Against Common Bean Rust Conferred by a Gene-Silencing Method

Rust disease of the dry bean plant, Phaseolus vulgaris, is caused by the fungus Uromyces appendiculatus. The fungus acquires its nutrients and energy from bean leaves using a specialized cell structure, the haustorium, through which it secretes effector proteins that contribute to pathogenicity by defeating the plant immune system. Candidate effectors have been identified by DNA sequencing and motif analysis, and some candidates have been observed in infected leaves by mass spectrometry. To assess their roles in pathogenicity, we have inserted small fragments of genes for five candidates into Bean pod mottle virus. Plants were infected with recombinant virus and then challenged with U. appendiculatus. Virus-infected plants expressing gene fragments for four of five candidate effectors accumulated lower amounts of rust and had dramatically less rust disease. By contrast, controls that included a fungal gene fragment for a septin protein not expressed in the haustorium died from a synergistic reaction between the virus and the fungus. The results imply that RNA generated in the plant moved across the fungal haustorium to silence effector genes important to fungal pathogenicity. This study shows that four bean rust fungal genes encode pathogenicity determinants and that the expression of fungal RNA in the plant can be an effective method for protecting bean plants from rust.

Breeding for common bean (Phaseolus vulgaris L.) rust resistance in Brazil

Common bean is an economically, nutritionally, and socially important crop. It is grown in distinct regions and different seasons around the world by subsistence level farmers with low-technology input as well as by farmers that use high input technologies. One important factor that can limit the bean growing and drastically affect grain yields is the high number of destructive pathogens that attack and cause serious damage to the crop. Among them is bean rust, incited by the fungus Uromyces appendiculatus. This disease is distributed throughout the world, but it effectively causes major production problems in humid tropical and subtropical regions. In Brazil, rust causes major losses in south, southeast, and central regions of the country. Bean rust control by resistant cultivars is an easy and economical strategy to be used in association to other rust management practices. Pyramiding of different race-specific resistance genes in association with other genes conferring adult plant resistance, slow rusting, and reduced pustule size can prolong the lifespan of a common bean cultivar by creating a more durable resistance complex against the rust pathogen. This review manuscript presents an overview on bean rust and reports some breeding efforts aiming to develop rust resistant cultivars in Brazil.

Uromyces appendiculatus in Honduras: Pathogen Diversity and Host Resistance Screening.

Bean rust, caused by the fungus Uromyces appendiculatus, is a major constraint for common bean production worldwide. Virulence of U. appendiculatus collected from wild and cultivated Phaseolus spp. was examined in 28 locations across Honduras. Host accessions representing wild and domesticated Phaseolus spp. collected at the same sampling locations were evaluated for resistance against U. appendiculatus. In total, 91 pathotypes were identified from 385 U. appendiculatus isolates according to their virulence on each of the 12 host differentials. No significant difference in pathogen total virulence, measured as the mean disease score, was found between locations. However, significant differences were found in pathotype virulence among isolates collected from different Phaseolus spp. within a location. Moreover, when locations were compared on the basis of pathotype occurrence and frequency, differences among locations were evident. No two locations had the same pathotype composition. The most common pathotype was virulent on 9 of the 12 differential lines. A high number of resistant accessions were identified in Phaseolus coccineus and P. lunatus. Although most wild P. vulgaris accessions were highly susceptible, rust resistance was observed in P. vulgaris landraces collected from farmer's fields. Thirty-two (52%) of the accessions screened showed intermediate to high levels of resistance and, of those, 16% were P. coccineus accessions. Our findings support the hypothesis that interaction of U. appendiculatus in host populations composed of diverse Phaseolus spp. and genotypes has favored highly diverse and virulent pathotypes, creating a center for virulence diversity of the pathogen in Honduras. The high percentage of intermediate and highly resistant accessions identified in the present study supports the strategy of collecting plants from the center of diversity of a pathogen or in locations with high incidence of disease and pathogen diversity to maximize the probability of identifying new sources of resistance.

Pathogenic Diversity in and Sources of Resistance to Uromyces appendiculatus in Southern Africa

The reaction of the first (1983) common bean international differential set and other germplasm to 248 single pustule isolates of the rust fungus Uromyces appendiculatus, collected from various southern African countries, was evaluated. Eleven of the most important isolates were re-purified and re-inoculated, this time also on the second (2002) revised and smaller international differential set. The 248 isolates could be grouped into 44 race-groups. These were subjected to principal coordinates analysis (PCoA). A second PCoA was carried out using 25 of the most important of the 44, together with 34 African races reported by previous authors. Isolates were generally avirulent on accessions with the resistance genes Ur-3+, -5 or -11, as well as Compuesto Negro Chimaltenango (CNC) and A 286, all small seeded, and the most useful sources were accessions carrying both Ur-3 and Ur-11, for instance BelMiNeb-RMR-7, BelDakMi-RMR-14 and -18. Isolates were generally virulent on large seeded accessions (with, among others Ur-4, -6 or -9), reflecting the preference for large seeded beans in southern Africa and co-evolution of host and pathogen. No large seeded accessions showed broad resistance. The least susceptible was Plant Introduction 260418, which rated resistant to moderately susceptible to the 11 races. These observations were confirmed by field ratings on the same accessions over multiple seasons. According to the PCoA, which proved useful for the identification of differentiating accessions, southern African isolates fell into three main clusters, for which Redlands Pioneer and the South Africa cultivar Teebus were the most discriminating differentials. Other accessions that showed particularly useful differentiating ability were Olathe and 51051. Of these, only Redlands Pioneer has been included in the 2002 differential set. The PCoA grouping of the African races was similar to that of the southern African race-groups.

Control of the bean rust fungus Uromyces appendiculatus by means of Trichoderma harzianum: leaf disc assays on the antibiotic effect of spore suspensions and culture filtrates

Several species of the fungal genus Trichoderma act as antagonists of other fungi. A number of strains from the Trichoderma species T. harzianum Rifai are used as biological control agents for the control of soilborne as well as foliar plant pathogens. Six T. harzianum strains, five of them isolated from commercial preparations, were evaluated for their capability to control the bean rust fungus Uromyces appendiculatus (Pers. ex Pers.) Unger. Different kinds of leaf disc assays were performed with conidial spore suspensions and sterile culture filtrates of the T. harzianum strains. Great differences were observed concerning the efficacy of the Trichoderma strains to reduce the number of the uredial pustules developing after rust inoculation which followed the application of the particular Trichoderma strains. Efficacy values ranged from 1 to over 50%. Increasing spore or culture filtrate concentrations of the two most effective isolates T12 and TU led to decreases in the number of developing uredial pustules. Culture filtrate applications had a protective but no curative effect. T12 spore suspensions maintained their disease reducing activity even when autoclaved. This and some other evidence for an antibiotic interaction between T. harzianum and U. appendiculatus are discussed.

Allelism Tests between the Rust Resistance Gene Present in Common Bean Cultivar Ouro Negro and Genes Ur‐5 and Ur‐11

AbstractThe pathogenic variability of the fungus Uromyces appendiculatus is an obstacle for the creation of rust‐resistant common bean (Phaseolus vulgaris L.) varieties. Gene pyramiding is an alternative strategy for the development of varieties with durable resistance. However, to reach this goal it is important to identify different genes with ample resistance spectra. Cultivars Ouro Negro, Mexico 309 and Belmidak RR‐3 have been shown to be resistant to several rust races identified in the state of Minas Gerais, Brazil. Ouro Negro is the only rust resistance source being used in the BIOAGRO/Universidade Federal de Viçosa (UFV) breeding programme, which aims at pyramiding resistance genes in the ‘carioca‐type’ cultivar Rudá. It would be also interesting to use Mexico 309 (Ur‐5) and Belmidak RR‐3 (Ur‐11) in the breeding programme. However, there is no available information on the possible allelic relationships between the Ouro Negro resistance gene and Ur‐5 and Ur‐11. This work aimed at: (1) determining the allelic relationship between the Ouro Negro resistance gene and Ur‐5 and Ur‐11; and (2) evaluating a random amplified polymorphic DNA (RAPD) marker previously reported as being linked to Ur‐11, in populations from crosses between Belmidak RR‐3 and Rudá. The allelism tests confirmed that the Ouro Negro rust resistance gene is distinct from Ur‐5 and Ur‐11 and the molecular analyses confirmed that the RAPD marker can be used in our breeding programme to develop ‘carioca‐type’ cultivars with the Ur‐11 gene.

`BeltGlade RR-1', `BeltGlade RR-2', and `BeltGlade RR-3': Rust-resistant Snap Bean Breeding Lines with the Ur-4 and Ur-11 Genes

Received for publication 21 Sept. 1999. Accepted for publication 18 Feb. 2000. Florida Agricultural Experiment Station Journal Series No. R-07072. This research was supported in part by the Florida Foundation Seed Service. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Associate Professor, Plant Breeding and Genetics. Biological Scientist. Extension Agent. Research Plant Pathologist. To whom reprint requests should be addressed. Bean rust, incited by the fungus Uromyces appendiculatus (Pers.) Unger var. appendiculatus [formerly U. phaseoli (Ruben) Wint.] is a common disease of Phaseolus vulgaris L. It has the potential to cause serious crop losses, and is particularly troublesome in warm, humid production regions such as Florida (Stavely, 1999a, 1999b; Stavely and Pastor-Corrales, 1989). Currently, the Florida snap bean industry is valued at over $100 million annually, and generally relies on just over a half-dozen cultivars. The major source of rust resistance in these popular cultivars is the Ur-4 gene (Kelly et al., 1996), previously identified in ‘Early Gallatin’ as Up-2 (Christ and Groth, 1982). Unfortunately, this gene is not completely effective against the races that now infect snap beans in Florida, and growers must routinely apply fungicides to control bean rust and a number of other foliar diseases. In the past few years, 90 of the estimated 150 worldwide bean rust races have been identified and are curated at Beltsville, Md. (Stavely, 1998a, 1999b). To date, 11 dominant resistant genes have been identified to control these pathogenic races. They confer various host plant resistance reactions, including immunity, hypersensitivity, and/or reduced uredina size (Stavely, 1998a). The Ur-4 gene confers a hypersensitve type resistance, with necrotic lesions of 0.3–1.0 mm in diameter, to 30 of these races (Stavely, 1998a). Another gene, the dominant Ur-11 allele, confers hypersensitive or reduced uredinium (≤0.3 mm) type resistance to 89 of these races (Stavely, 1998a). The Ur-11 gene was previously identified as Ur-3 and later found to be linked in repulsion phase with the Ur-3 locus (Stavely, 1998b). The Ur-11 allele is not protective against Race 108 from Honduras (= H951, Steadman, pers. comm.), but Ur-4 is effective. When combined, the Ur-4 and Ur-11 genes concurrently provide either immunity, chlorotic or necrotic hypersensitive, or tiny uredinium resistance to all of the 90 races, with duplicate resistance to 29 of these races (Stavely, 1998b). Given the dynamic epidemiology of bean rust, more races will likley arise in the United States, Central America, and the Caribbean basin. A prudent strategy for the management of this disease is the deployment of different genes that confer resistance to a wide array of races, thereby diversifing the genetic basis of resistance. The primary objective of this breeding effort was to generate broad-spectrum bean rust resistance in subtropically adapted, fresh market, snap bean germplasm. To accomplish this objective, the Ur-11 gene was introgressed into snap bean germplasm with existing resistance based on the Ur-4 gene. Additional emphasis was placed on the retention of resistance to bean common mosaic virus as conferred by the I gene (Drijfhout, 1978 ) and presently deployed in many of the important cultivars grown in Florida. These breeding lines are intended for use as a combined source of rust resistance based on Ur-11 and Ur-4 genes.

RAPD markers linked to a block of genes conferring rust resistance to the common bean

Rust, caused by the fungus Uromyces appendiculatus, may cause a significant loss to common bean (Phaseolus vulgaris L.) yield. RAPD markers tightly linked to the resistance genes may be used in breeding programs to aid the development of rust-resistant bean cultivars. In this sense, the objective of the present work was to identify RAPD markers linked to a rust resistance gene block present in the cultivar Ouro Negro. Two hundred and fourteen F2 individuals from a cross between the resistant cultivar Ouro Negro and the susceptible cultivar US Pinto 111 were inoculated with a mixture of eight races of U. appendiculatus. The segregation ratio obtained suggested that resistance is monogenic and dominant. Bulked segregant analysis was used in conjunction with the RAPD technique to search for markers linked to rust resistance genes. Two molecular markers flanking the rust resistance gene block were identified, one at 5.8 ± 1.6 cM (OX11(630)) and the other at 7.7 ± 1.7 cM (OF10(1,050)) of the gene. Simulated indirect selection efficiency in the F2 population using the two markers was 100%. The molecular markers identified in this work are currently being used for the selection of disease-resistant plants in the commom bean breeding program of the Federal University of Viçosa.

Transient transformation of the rust fungus Puccinia graminis f. sp. tritici.

The biotrophic rust fungus Puccinia graminis f. sp. tritici (Pgt) was transformed by particle bombardment. The promoter from the Pgt translation elongation factor 1alpha (EF-1alpha) gene was fused to the bacterial marker genes hygromycin B phosphotransferase (hpt) and beta-glucuronidase (GUS). Transformation constructs were introduced into uredospores of Pgt, an obligate pathogen of wheat, by biolistic bombardment. Uredospores transformed with the construct containing the hpt gene germinated and initiated branching on selective medium, indicating that they had acquired resistance to hygromycin B. However, transformants stopped growing 5 days after bombardment. GUS activity in uredospores and germlings was histochemically detected 4-16 h after bombardment. GUS expression was also obtained using the INF24 promoter from the bean rust fungus Uromyces appendiculatus, demonstrating that heterologous genes can be expressed in P. graminis under the control of regulatory sequences from closely related organisms.

Isolates of Uromyces appendiculatus with Specific Virulence to Landraces of Phaseolus vulgaris of Andean Origin.

Five isolates of the bean rust fungus Uromyces appendiculatus were shown to be specifically virulent on bean genotypes of Andean origin. This specificity was demonstrated by the virulence of five pairs of isolates on a differential set of 30 Phaseolus vulgaris landraces. Each isolate pair was from a different country in the Americas and consisted of one Andean-specific isolate and one nonspecific isolate. Of the differential P. vulgaris landraces, 15 were of Middle American origin and 15 were of Andean origin. The Andean-specific rust isolates were highly virulent on Andean landraces but not on landraces of Middle American origin. Rust isolates with virulence to Middle American landraces were also generally virulent on Andean material; no truly Middle American-specific isolates were found. Random amplified polymorphic DNA (RAPD) analysis of the rust isolates also distinguished the two groups. Four of the Andean-specific rust isolates formed a distinct group compared to four of the nonspecific isolates. Two of the isolates, one from each of the two virulence groups, had intermediate RAPD banding patterns, suggesting that plasmagomy but not karyogamy occurred between isolates of the two groups.

Changes in Metabolite Pools in Host and Pathogen during the Uredinio and Teliospore Development of the Bean Rust Fungus Uromyces appendiculatus

AbstractMetabolite pool sizes as well as indications for changes in the translocation of metabolites from the host to the parasite were investigated on primary leaves of decapitated bean plants infected with the bean rust fungus Uromyces appendiculatus var. appendiculatus (Pers.) Link with respect to changes in the spore type from uredinioto teliospores. Noninfected control leaves were compared to the directly influenced sorus region of infected leaves and the mycelium‐free areas of these leaves (‘leaf‐remainder’).The data show evidence for an intensive change in the metabolite pools, indicating translocation from the metabolically active leaf‐remainder to the sori, expressing characteristic changes according to the spore type produced. The importance of single metabolites and changes in their pool sizes is discussed and correlated to the respective pools within the uredinio‐ and teliospores. The data indicate that vegetative urediniospores contain mainly preformed metabolites and precursors, whereas teliospores need to undergo metabolite syntheses before germination.

Incomplete Dominance of Rust Resistance in Tepary Bean

Foliar diseases are a major constraint to cultivated tepary bean (Phaseolus acutifolius A. Gray var. latifolius Freeman) production in some environments. The reactions of 12 cultivated teparies to eight individual races (41, 47, 49, 51, 53, 58, 67, and 73) of the bean rust fungus Uromyces appendiculatus (Pers.) Unger var. appendiculatus maintained at Beltsville, Md., were examined under greenhouse conditions. These diverse races, used together, overcome all of the major rust-resistance genes present within the 19 host differential cultivars of common bean (Phaseolus vulgaris L.). Seven lines (GN-605-s, GN-610-s, PI 321638-s, PI 502217-s, Neb-T-6-s, Neb-T-8a-s, and Neb-T-15-s) exhibited similarly high levels of resistance (immunity or necrotic spots without sporulation) to all eight races. Inheritance of resistance was examined across five susceptible × resistant (S × R) and three resistant × resistant (R × R) populations. The rust reactions in the F1, F2, and F3 generations derived from S × R crosses revealed that the immune or necrotic resistance response was conditioned by a single locus exhibiting incomplete dominance. The rust resistance of four lines tested for allelism in R × R crosses was found to be derived from the same gene. This apparent lack of variability for rust resistance suggests that a single introgression event may realize the full potential for cultivated tepary bean to contribute rust resistance to common bean through interspecific hybridization. In addition, the limited variability for resistance to the highly variable rust pathogen in cultivated tepary bean supports the occurrence of a “bottleneck effect” during domestication of this species, as observed in germplasm diversity studies.

Incomplete Dominance of Rust Resistance in Tepary Bean

Foliar diseases are a major constraint to cultivated tepary bean (Phaseolus acutifolius A. Gray var. latifolius Freeman) production in some environments. The reactions of 12 cultivated teparies to eight individual races (41, 47, 49, 51, 53, 58, 67, and 73) of the bean rust fungus Uromyces appendiculatus (Pers.) Unger var. appendiculatus maintained at Beltsville, Md., were examined under greenhouse conditions. These diverse races, used together, overcome all of the major rust-resistance genes present within the 19 host differential cultivars of common bean (Phaseolus vulgaris L.). Seven lines (GN-605-s, GN-610-s, PI 321638-s, PI 502217-s, Neb-T-6-s, Neb-T-8a-s, and Neb-T-15-s) exhibited similarly high levels of resistance (immunity or necrotic spots without sporulation) to all eight races. Inheritance of resistance was examined across five susceptible × resistant (S × R) and three resistant × resistant (R × R) populations. The rust reactions in the F1, F2, and F3 generations derived from S × R crosses revealed that the immune or necrotic resistance response was conditioned by a single locus exhibiting incomplete dominance. The rust resistance of four lines tested for allelism in R × R crosses was found to be derived from the same gene. This apparent lack of variability for rust resistance suggests that a single introgression event may realize the full potential for cultivated tepary bean to contribute rust resistance to common bean through interspecific hybridization. In addition, the limited variability for resistance to the highly variable rust pathogen in cultivated tepary bean supports the occurrence of a “bottleneck effect” during domestication of this species, as observed in germplasm diversity studies.

Superoxide Dismutase: A Differentiation Protein Expressed in Uromyces Germlings during Early Appressorium Development

Lamboy, J. S., Staples, R. C., and Hoch H. C. 1995. Superoxide dismutase: A differentiation protein expressed in Uromyces germlings during early appressorium development. Experimental Mycology 19, 284-296. Germlings of the bean rust fungus Uromyces appendiculatus detect penetration sites on the surface of the host leaf by thigmosensing topographical features. Within 2-4 min after the apex of a urediospore germ tube encounters the cuticular lip of a stomate, the germling ceases polarized growth and begins to swell over the aperture. The mechanism by which the cells detect topographical signals is not understood; however, previous experiments indicated that the initiation process does not involve de novo gene expression. In order to detect posttranslational modifications, the protein profiles of induced and noninduced germlings were compared at the earliest stages of appressorium formation, and a 21-kDa differentiation protein was identified by a shift in isoelectric point. The N-terminal amino acid sequence exhibited homology with superoxide dismutase (SOD), and antibodies to a synthetic peptide fragment of the respective sequence recognized cooper/zinc isozymes of SOD in electroblots of native gels. Electroelution of the active enzyme bands and separation by SDS-PAGE indicated that the 21-kDa protein is a component of a tetrameric 85-kDa SOD.

Changes in Carbohydrate, Protein and Chlorophyll Content, and Enzyme Activity During the Switch from Uredinio‐ to Teliospore Sporulation in the Bean‐rust Fungus Uromyces appendiculatus (Pers.) Link

AbstractChanges in carbohydrate metabolism and senescence during the development of the bean‐rust fungus Uromyces appendiculatus (Pers.) Link were investigated. At different stages of uredinio‐ and teliospore production, samples were taken from infected and non‐infected bean leaves, beginnig with the flecking stage. The unsoluble‐carbohydrate, sucrose, glucose and fructose content, as well as fluctuations in soluble acidinvertase activity and parameters of leaf senescence were determined.While fructose remained constant without significant differences between infected leaf sections and the control, differences in the other carbohydrates and in soluble‐invertase activity could be detected. Parameters indicating leaf senescence were retarded and decreased with age, except for peroxidase activity, which was enhanced and increasd with age in infected tissue to a higher degree than in the control. The results are discussed in relation to their significance for fungal development.

Chlorophyll Fluorescence Induction in Leaves of Phaseolus vulgaris Infected with Bean Rust (Uromyces appendiculatus).

To our knowledge, this report describes the first application of video imaging of Chl fluorescence to the study of light utilization in photosystem II of attached leaves of Phaseolus vulgaris infected with the obligate biotrophic fungus Uromyces appendiculatus (race 38). The video-based detection system produced a spatially resolved, quantifiable signal that was highly specific for chlorophyll fluorescence. Video images of spatial variation in the initial stage of the fluorescence induction (dark-light) transient revealed discreet regions of intense emission coinciding with centers of subsequent lesion development and accompanying chlorosis. Incipient lesions were visible by this procedure 3 d following inoculation, fully 3 to 4 d prior to visible symptoms. Fluorescence emission patterns in infected areas during the induction transient were heterogeneous with radial distance from the point of invasion and varied with the length of the time delay between re-illumination and image capture. During later ([greater than or equal to]1 min) stages of the induction transient, fluorescence emission in incipient lesions was quenched compared to surrounding tissue. These essential features of the induction transient observed in video images were also noted when individual lesions were examined using pulse modulation fluorimetry.

Coupling- and repulsion-phase RAPDs for marker-assisted selection of PI 181996 rust resistance in common bean

The Guatemalan black bean (Phaseolus vulgaris L.) plant introduction (PI) 181996 is resistant to all known US races of the bean rust fungus Uromyces appendiculatus (Pers. ex Pers.) Unger var. appendiculatus [syn. U. phaseoli (Reben) Wint.]. We report on two random amplified polymorphic DNA (RAPD) markers OAC20490 tightly linked (no recombinants) in coupling phase and OAE19890 linked in repulsion phase (at 6.2±2.8 cM) to PI 181996 rust resistance. These RAPDs, generated by single decamer primers in the polymerase chain reaction, were identified in near-isogenic bulks of non-segregating resistant and susceptible BC4F2 (NX-040*4/PI 181996) lines. Linkage of the RAPD markers was confirmed by screening 19 BC4F2 and 57 BC4F3 individuals segregating for PI 181996 resistance. Utility of the RAPDs OAC20490 and OAE19890 was investigated in a diverse group of common bean cultivars and lines. All cultivars into which the PI 181996 resistance was introgressed had the RAPD OAC20490. A RAPD similar in size to OAC20490, observed in some susceptible common bean lines, was confirmed by Southern blotting to be homologous to the RAPD OAC20490. Use of the RAPDs OAC20490 and OAE19890 in marker-assisted selection (MAS) is proposed. The coupling-phase RAPD is most useful for MAS of resistant BCnF1individuals during traditional backcross breeding. The repulsion-phase RAPD has greatest utility in MAS of homozygous-resistant individuals in F2 or later-segregating generations.

Random Amplified Polymorphic DNA Reveals Relationships Among Diverse Genotypes in Australian and American Collections ofUromyces appendiculatus

Random amplified polymorphic DNA (RAPD) markers were used to assess genetic diversity among 42 Australian and five American field isolates of the rust fungus Uromyces appendiculatus var. appendiculatus, pathogenic on cultivated bean Phaseolus vulgaris. Phenetic analysis of polymorphisms generated by 10 arbitrary decanucleotide primers indicated that the Australian isolates represented two background genotypes, A and B (17% band dissimilarity), with a third group (genotype AB) representing putative hybrids and recombinants between A and B. This confirms a previous restriction fragment length polymorphism and RAPD study carried out on a subset of 12 Australian isolates. The American isolates, of unknown homology to the Australian isolates prior to this investigation, were selected to represent the two major genetic groups determined previously using isozymic markers. RAPDs confirmed the isozymic groupings and revealed that one group, represented by one American isolate, clustered with the Australian isolates of genotype B. However, the other four American isolates formed a divergent cluster, designated genotype C, that showed an average band dissimilarity of 45% compared with genotypes A and B. A graphic display of polymorphic RAPD bands was used to compare genomic components common to different genotype clusters. This revealed a group of RAPD markers common to genotypes C and A but absent from B. The data were consistent with A being derived by recombination between progenitors of genotypes B and C. Although genotype C is commonly found in North America, its absence from the 42 Australian isolates suggests that it is either uncommon or has not yet been introduced to Australia. Results are discussed in relation to the evolution and possible mechanisms of generation of current genotypes of U. appendiculatus.

DNA Markers Reveal Hybrids Between Two Diverse Background Genotypes in Australian Collections of the Bean Rust Fungus Uromyces appendiculatus

Two classes of molecular markers, RFLPs (restriction fragment length polymorphisms) and RAPDs (random amplified polymorphic DNA) were used to assess genetic diversity among Australian bean rust isolates (Uromyces appendiculatus (Pers.) Unger var. appendiculatus) collected from diverse geographic locations spanning the period 1973-1990. Initially we screened 22 isolates using WLPs from five DNA probes. This was followed by analysis of a subset of 12 of these isolates using RFLPs from 10 cDNA probes and RAPDs from 10 arbitrary primers for a comprehensive evaluation of background genotype. Polymorphic bands revealed the existence of two divergent clusters of isolates, representing genotypes designated A and B. The RFLP markers showed 30% band dissimilarity between A and B, and RAPDs 16% dissimilarity. Isolates in a third cluster (genotype AB) exhibited most of the polymorphic bands present in A and B, but no unique polymorphic bands of their own, indicating that they had most probably arisen from recent hybridisation between isolates of genotype A and B. The subset of 12 isolates included 10 race phenotypes, but no clear correlation between background genotype (as assessed by RFLP and RAPD markers) and race phenotype was evident. We suggest that Australian races of bean rust have most probably evolved by a combination of mutation to virulence from common A and B background genotypes, and genetic recombination.