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Bean rust, caused by Uromyces appendiculatus (Pets.: Pets.) Unger, is a major disease of common bean (Phaseolus vulgaris L.). A recommended strategy to obtain durable rust resistance is to use molecular markers linked to rust resistance genes for pyramiding monogenic resistance genes into a single bean cultivar. However, markers linked to the Ur-7 gene for specific resistance (SR) to rust present in the cultivar Great Northern (GN) 1140 have not been reported. Our objectives were to identify random amplified polymorphic DNA (RAPD) markers linked to the Ur-7 gene for SR to rust race 59 using bulked segregant analysis in an [F.sub.2] population from the Middle American (MA) common bean cross 'GN1140' (resistant) x GN Nebr. #1 (susceptible) and to map the Ur-7 gene on an existing RAPD marker-based linkage map constructed by means of recombinant inbred lines (RILs) from the MA cross GN BelNeb-RR-1 x A 55. A single dominant gene controlling SR to race 59 was found in the [F.sub.2] and confirmed in the [F.sub.3]. Six RAPD markers were detected in a coupling phase linkage with the Ur-7 gene. Cosegregating coupling-phase markers OAD12.550 and OAF17.900 were found. These were also present in pinto US-5 from which the rust resistance of GN1140 was derived. Among the three repulsion-phase markers, marker OAB18.650 was the most closely linked to the Ur-7 gene at a distance of 7.6 centimorgans (cM). All linked markers detected in the [F.sub.2] population also segregated in the RILs and were located on linkage group 11 of the existing linkage map. These markers linked to the Ur-7 gene of MA origin identified here, along with other independent rust resistance genes from other germplasm, could be utilized to pyramid multiple genes into a bean cultivar for more durable rust resistance., BEAN RUST is an important disease resulting in reduced bean yield and quality in many parts of the world (Stavely and Pastor-Corrales, 1989). It consistently causes yield reductions ranging from [...]

Leaf rust, caused by the fungus Puccinia triticina Eriks., is a major disease nearly wherever wheat (Triticum spp.) is grown in the USA. Soft red winter wheat (T. aestivum L.) [...]

Leaf rust caused by Puccinia hordei G. Otth is an important disease of barley (Hordeum vulgare L.) in many regions of the world. Yield losses up to 32% have been reported in susceptible cultivars. The Rph5 gene confers resistance to the most prevalent races (8 and 30) of barley leaf rust in the USA. Therefore, the molecular mapping of Rph5 is of great interest. The objectives of this study were to map Rph5 and identify closely linked molecular markers. Genetic studies were performed by analysis of 93 and 91 [F.sub.2] plants derived from the crosses `Bowman' (rph5) x `Magnif 102' (Rph5) and `Moore' (rph5) x Virginia 92-42-46 (Rph5), respectively. Bulk segregent analysis (BSA) using amplified fragment length polymorphism (AFLP), restriction fragment length polymorphism (RFLP), and simple sequence repeat (SSR) markers was conducted. Linkage analysis positioned the Rph5 locus to the extreme telomeric region of the short arm of barley chromosome 3H at 0.2 centimorgans (cM) proximal to RFLP marker VT1 and 0.5 cM distal from RFLP marker C970 in the Bowman x Magnif 102 population. Map positions and the relative order of the markers were confirmed in the Moore x Virginia 92-42-46 population. RFLP analysis of the near isogenic line (NIL) Magnif 102/*8Bowman, the susceptible recurrent parent Bowman, and RpH5 donor Magnif 102, confirmed the close linkage of the markers VT1, BCD907, and CD0549 to Rph5. Results from this study will be useful for marker-assisted selection and gene pyramiding in programs breeding for leaf rust resistance and provide the basis for physical mapping and further cloning activities., LEAF RUST caused by P. hordei is generally considered the most important rust disease of barley on a worldwide basis. Severe yield losses have been observed in Australia (31%) (Coterill [...]

Stem rust resistance in wheat (Triticum aestivum L.) conferred by the Sr2 gene has remained effective against Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn. worldwide for more than 50 yr. Sr2 resistance is associated with variable levels of disease symptoms; it shows recessive inheritance and is expressed primarily during the adult-plant stage. A genetic and physical interval for the Sr2 gene was determined on the short arm of chromosome 3B of wheat. Flanking microsatellite markers were identified and one tightly linked marker (gwm533) was shown to be associated with presence of Sr2. This marker was validated in a wide range of germplasm and can now be implemented in marker-assisted breeding to facilitate selection for this durable, broad-spectrum but difficult to score rust resistance gene., RESISTANCE TO STEM RUST was transferred from Triticum turgidum L. ssp. dicoccum Schrank ex Schubler (cv Yaroslav) into hexaploid wheat by McFadden (1930) producing the variety Hope. Hare and McIntosh [...]

Leaf rust, caused by Puccinia recondita Roberge ex Desmaz., is an important disease of wheat (Triticum aestivum L.) worldwide. To estimate the genetic progress in reducing grain yield losses through breeding for resistance to leaf rust, replicated trials including 15 popular CIMMYT germplasm-derived wheat cultivars released between 1966 and 1988 in northwestern Mexico were sown in that same area; normal and late planting dates were used for four and two seasons, respectively. Leaf rust epidemics were established by inoculating spreader rows planted adjacent to plots of the cultivars which were not protected by fungicide. Average losses in grain yields of the cultivars due to leaf rust ranged between 6.6 and 62.7% and were highly correlated with final disease severity (r = 0.898, P [is less than] 0.01) and relative area under the disease progress curve (r = 0.917, P [is less than] 0.01). The losses in grain yield were mostly due to reductions in kernel weight, kernels per square meter, and grain fill rate. Grain yield losses (7.710.4%) in slow rusting cultivars Cocoraque 75, Nacozari 76, Opata 85, and Bacanora 88 were similar to those observed in the immune `Oasis 86' (6.6%) or resistant `Ciano 79' (10.2%). The average annual progress in grain yield potential achieved through breeding averaged over the six trials was estimated to be 0.48% ([r.sup.2] 0.38, P [is less than] 0.01) for fungicide protected and 2.21% ([r.sup.2] = 0.47, P [is less than] 0.01) when not protected by fungicide. We conclude that while the grain yield potential of CIMMYT-derived cultivars has increased significantly over the years, progress in protecting this yield potential through the incorporation of genes that confer slow rusting resistance has been more dramatic., SPRING BREAD WHEAT CULTIVARS derived from germplasm developed by the International Maize and Wheat Improvement Center (CIMMYT) are currently grown on [is greater than] 50.7 million hectares worldwide (Dalrymple, 1986). [...]

The oat (Avena sativa L.) cultivars, Steele and Dumont, differ by a chromosome interchange. Two tightly linked or allelic crown rust resistance genes (Pc-63 and Pc-38) are located on one of the segments involved in the interchange. Recombinant chromosomes can result from crossing-over in the region between the crown rust resistance locus and the interchange breakpoint. Our objectives were to use a diallelic duplicate locus to (i) identify recombinant [BC.sub.i][F.sub.1] genotypes with Pc-63 in the interchanged position, (ii) estimate the linkage relationship between the crown rust resistance locus and the interchange breakpoint, and (iii) recover duplication genotypes that possess four copies of Pc-63. Segregation of [BC.sub.1][F.sub.2] families was used to detect recombination within an interchanged chromosome segment in [F.sub.1] interchange heterozygotes. Only 3/7 of the single crossover recombinant gametes could be recognized by duplicate dominant segregation in [BC.sub.1][F.sub.2] families. After inoculation of 145 [BC.sub.1][F.sub.2] families with a crown rust (Puccinia coronata Corda var. avenae W.P. Fraser & Ledingham) isolate avirulent on Pc-63 and virulent on Pc-38, we identified 11 recombinant families segregating 15R:1S. A 15:1 segregation ratio indicated Pc-63 recombined to an interchanged position as a result of crossing-over between the crown rust resistance locus and the interchange breakpoint. Our data indicated Pc-63 was linked in repulsion and distal to the interchange breakpoint with 17.7% recombination observed between the crown rust resistance locus and the breakpoint. Testcross segregation ratios confirmed that recombination occurred and allowed identification of a true breeding duplication genotype that possessed four copies of Pc-63. Increasing the dosage of Pc-63 did not alter the expected infection type conferred by a one or two copies of Pc-63., Many hexaploid oat (Avena sativa and A. sterilis L.) genotypes differ by chromosome interchange differences (McMullen et al., 1982; Leach, 1989; Singh and Kolb, 1991). Although cytologically identifiable, the genetic [...]

Combining crown rust resistance (CRR) genes is an objective of many oat (A vena sativa L.) breeding programs. Genetic suppressors of CRR have been reported, but the genetic nature of such suppressors has not been investigated. An excess of susceptible seedlings in segregating progeny, of crosses involving genotypes with Pc-38 and Pc-62 suggested the Pc-38 parent may have a factor that inhibits the expression of Pc-62. Our objectives were to (i) examine a putative genetic interaction between oat CRR gene Pc-62 and a dominant suppressor, (ii) utilize chromosome interchange stocks to define genetically the location of the suppressor, (iii) evaluate the effect of gene dosage combinations on the resistance-suppression relationship, and (iv) examine alternative explanations of distorted segregation (genetic background and differential gamete transmission). The use of duplication-deficiency (Dp-Df) genotypes placed a putative suppressor on an interchanged chromosome segment that contains Pc-38. Resistance conferred by, Pc-62 was suppressed in [F.sub.1] trisomic CRR locus combinations (1 Pc-62:2 Pc-38) in comparison to the expression of resistance in [F.sub.1] seedlings hemizygous for Pc-62. The Pc-38 segment contributed by two independently derived cultivars; produced equivalent suppression in [F.sub.1] combinations with Pc-62. The presence of Pc38 in an interchanged position in the cultivar Dumont allowed production of mono- and diallelic combinations of resistance genes and evaluation of mono-, di-, and trisomic segregation of the CRR locus. Diallelic trisomic segregation for the CRR locus indicated that an increasing dosage of Pc-62, relative to the suppressor and Pc-38, decreased the degree of suppression. Male transmission of Dp-Df gametes was not reduced relative to the frequency of gametes with a complete chromosome complement., Genetic and cytogenetic analysis of oat hybrids indicate chromosome interchange differences are present among oat cultivars (Leach, 1989; Singh and Kolb, 1991) and the wild hexaploid species A. sterilis L. [...]