The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Variation in pathogenicity of the potato root eelworm (Heterodera rostochiensis woll.,) and its significance in potato breeding

Summary Breeding for resistance to Heterodera rostochiensis was started in Scotland in 1952, using certain clones of Solanum tuberosum subsp. andigena as the source of resistance. Resistance was manifest by the almost complete failure of larvae to mature in the roots of resistant seedlings, particularly those derived from the clone C.P.C.1673.In 1955 an eelworm population was found which overcame this resistance and was, for this reason, designated as aggressive.Resistance was also overcome in selfed and hybrid seedlings derived from subsp. andigena C.P.C. 1685 and C.P.C.1690. Resistance in Solanum vernei was apparently maintained against the aggressive eelworm population which was studied. This aggressive eelworm population multiplied normally on commercial potato varieties.In a first survey of Potato Root eelworm infested soils in Britain about 10 per cent of the eelworm populations sampled were found to differentiate little or no resistance in a test plant which was at least simplex for a resistance factor H, derived from C.P.C. 1673.

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Hiermede wordt hetzelfde bedoeld als met S. andigenum.     

Genetics of multiple disease resistance in a doubled-haploid population of barley

The barley accession Q21861 possesses resistance to the stem-rust (Puccinia graminis f.sp. tritici), leaf-rust (P. hordei), and powdery-mildew (Blumeria graminis f.sp. hordei) pathogens. An anther-culture-derived doubled-haploid population was produced from F₁ plants from a cross of this accession and the susceptible breeding line SM89010 as a means of rapidly and efficiently determining the genetics of multiple disease resistance. The doubled-haploid population segregated 1:1 (resistant:susceptible) for resistance to the stem rust pathotype QCC indicating the involvement of a single resistance gene, rpg4. Two-gene (3:1) and one-gene (1:1) segregation ratios were observed for resistance to the stem-rust pathotype MCC at low (23–25°c) and high (27–29°C) temperature, respectively. These different segregation patterns were due to a pathotype × temperature interaction exhibited by rpg4 and Rpg1. another stem-rust-resistance gene present in Q21861. One-gene and two-gene segregation ratios were observed in reaction to the leaf rust and powdery mildew pathogens. These data demonstrate the utility of doubled haploid populations for determining the genetics of multiple disease resistance in barley.     

Sources of resistance to Mayetiola Destructor in breadwheat and durum wheat in Morocco

Summary Grain yield reductions of both breadwheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. durum) caused by attacks of Hessian fly (Mayetiola destructor Say) are second perhaps only to those caused by inadequate soil moisture in Morocco. To identify effective sources of resistance, 817 entries of common wheat and durum wheat reported to be resistant to Hessian fly were evaluated under natural infestations in Morocco. A large number of genes conferring virulence are present in populations of Moroccan Mayetiola. The genes H₁, H₂, H₃, h₄, H₆, H₇, H₈, H₉, H₁₀, H₁₁, H₁₄, H₁₅, and H₁₆ as well as the Marquillo, Kawvale and PI 94587 resistance sources are not useful for cereal improvement in North Africa. Luso, which has the gene H₁₂, also appeared susceptible in limited testing. Genotypes having the genes H₅ and H₁₃ were identified as significantly reducing larval survial in natural populations of Mayetiola. Of 11 resistant breadwheats identified with unknown genes, seven were from Portugal and three were from the Soviet Union. Although none of the durums tested had high levels of reistance, the two most promising durums were from Portugal. It is proposed that initially H₅ be deployed in durum wheats and H₁₃ be used in common wheat improvement. Leaf pubescence appears of little use in reducing the larval survival of Mayetiola.     

Inheritance of seed resistance to bruchids in cultivated mungbean (Vigna radiata, L. Wilczek)

Bruchid beetles or seed weevils are the most devastating stored pests of grain legumes causing considerable loss to mungbean (Vigna radiata (L.) Wilczek). Breeding for bruchid resistance is a major goal in mungbean improvement. Few sources of resistance in cultivated genepool were identified and characterized, however, there has been no study on the genetic control of the resistance. In this study, we investigated the inheritance of seed resistance to Callosobruchus chinensis (L.) and C. maculatus (F.) in two landrace mungbean accessions, V2709BG and V2802BG. The F₁, F₂ and BC generations were developed from crosses between the resistant and susceptible accessions and evaluated for resistance to the insects. It was found that resistance to bruchids in seeds is controlled by maternal plant genotype. All F₁ plants derived from both direct and reciprocal crosses exhibited resistance to the bruchids. Segregation pattern of reaction to the beetles in the F₂ and backcross populations showed that the resistance is controlled by a major gene, with resistance is dominant at varying degrees of expressivity. Although the presence of modifiers was also observed. The gene is likely the same locus in both V2709BG and V2802BG. The resistant gene is considered very useful in breeding for seed resistance to bruchids in mungbean.     

Novel pathotypes of lettuce mosaic virus — breakdown of a durable resistance?

Summary Resistance to lettuce mosaic virus (LMV) is derived either from cv. Gallega (g gene) or the wild accession PI251245 (mo gene). Previous studies indicated that these two genes were identical. Breeders in Europe produced numerous resistant cultivars utilisingg while in the USAmo was used. The resistance has been effective for over 20 years. However, recently there have been reports of LMV isolates causing unusually severe and sometimes necrotic symptoms on cultivars with these resistance genes. Investigations of these severe isolates have distinguished three new pathotypes in addition to the common pathotype (II) and identified a novel dominant gene for resistance. Themo/g genes confer resistance to pathotypes I and II but pathotype III possesses virulence for cultivars withg but not for those withmo. These two genes are therefore not identical but are probably either closely linked genes or alleles. Pathotype IV possesses virulence for all lettuce lines so far tested. Some isolates of this pathotype are seed transmitted in cultivars possessingmo org and have caused severe crop losses in southern France. The durability of the resistance conditioned by these two genes is discussed.     

Inheritance of resistance to Puccinia coronata var. avenae in six selections of Avena sterilis

Summary Six selections of Avena sterilis, introduced from Israel as sources of resistance to oat crown rust, were crossed with susceptible A. byzantina Frazier. The number of genes conditioning resistance to culture H-14 of race 326 of Puccinia coronata var. avenae in each of the six selections was determined from studies of F₁, F₂ and F₃ populations from the crosses. P.I. 287211, P.I. 295919 and P.I. 296244 each appeared to have a single dominant gene for resistance, and P.I. 296265 and P.I. 296266 each two dominant ones. C.I. 8295 had a single partially dominant gene for resistance. Crosses among the A. sterilis parents indicated that at least four different genes conditioned resistance to culture H-14.Association between F₂ reaction to crown rust and morphological characters of the spikelet was determined with the % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaaiaacaqabeaadaqaaqaaaOqaaiabeE8aJnaaCa% aaleqabaGaaGOmaaaaaaa!39FC!\[\chi ^2 \] test for independence. In four crosses, all spikelet characters appeared to be independent of rust reaction. In Frazier x P.I. 296244, basal pubescence of the lemma on the secondary floret appeared to be associated with reaction to crown rust. Strong association between reaction to crown rust and lemma pubescence on the primary floret was evident in Frazier x P.I. 296265.     

Detection and inheritance of leaf rust resistance in common wheat lines Agra Local and IWP94

Genetic studies were undertaken to determine the number and identities of leaf rust resistance genes in common wheat lines Agra Local and IWP94. The infection type arrays of the two lines with eight pathotypes (pt.) of P. triticina were different from those of lines possessing known leaf rust resistance (Lr) genes. Agra Local possessed two recessive resistance genes, one conditioning resistance to pathotype 4R9-7, and the other, a temperature-sensitive factor, gave resistance to pt. 121R127 at high temperature (27°C). IWP94 was previously demonstrated to carry Lr23. From the present study IWP94 was determined to have at least four leaf rust resistance genes. The first of these was the same recessive gene conferring resistance to pathotype 4R9-7 which was found in Agra Local. A second partially dominant gene conferred resistance to pathotype 121R127 at high temperature and two additional recessive genes governed resistance to pathotype 93R15. When present together, these two recessive genes complemented each other and provided resistance to pathotype 69R13 as well. One of the two recessive genes conferring resistance to pathotypes 93R15 and 69R13 was Lr23.     

Inheritance of resistance to angular leaf spot in common bean and validation of the utility of resistance linked markers for marker assisted selection out side the mapping population

Inheritance of resistance to angular leaf spot (ALS) disease caused by Phaeoisariopsis griseola (Sacc.) Ferr was investigated in two common bean cultivars, Mexico 54 and BAT 332. Both Andean and Mesoamerican backgrounds were used to determine the stability of the resistance gene in each of the two cultivars. Resistance to P. griseola was phenotypically evaluated by artificial inoculation with one of the most widely distributed pathotypes, 63–39. Evaluation of the parental genotypes, F₁, F₂ and backcross populations revealed that the resistance to angular leaf spot in the cultivars Mexico 54 and BAT 332 to pathotype 63–39 is controlled by a single dominant gene, when both the Andean and Mesoamerican backgrounds were used. Allelism test showed that ALS resistance in Mexico 54 and BAT 332 to pathotype 63–39 was conditioned by the same resistance locus. Resistant and susceptible segregating populations generated using Mexico 54 resistant parent were selected for DNA extraction and amplification to check for the presence /absence of the SCAR OPN02 and RAPD OPE04 markers linked to the Phg-2 resistance gene. The results indicated that the SCAR OPN02 was not polymorphic in the study populations and therefore of limited application in selecting resistant genotypes in such populations. On the other hand, the RAPD OPE04 marker was observed in all resistant individuals and was absent in those scored susceptible based on virulence data. Use of the RAPD OPE04 marker in marker-assisted selection is underway.     

Resistance to potato virus Y (pathotype 1–2) in Capsicum annuum and Capsicum chinense is controlled by two independent major genes

Summary The genetic basis of the resistance to a pepper strain of potato virus Y (PVY pathotype 1–2) was investigated in two lines: Capsicum annuum Criollo de Morellos 334 and Capsicum chinense PI 159236. F1, backcrosses, and F2 populations were produced using C. annuum Magda as the susceptible parent. Segregation ratios indicated that the resistance in C. annuum Criollo de Morellos 334 is governed by a nuclear, single dominant gene with a very high but incomplete penetrance. On the other hand, the resistant response of C. chinense PI 159236 is associated with the presence of a major recessive gene with an apparently complete penetrance. The failure in recovering or detecting PVY from/in the inoculated leaves of both resistant genotypes is strongly suggestive that both genes are acting by preventing the multiplication of the virus in the leaf tissue. Both mechanisms of resistance closely resemble either an operational immunity or a single-cell hypersensitive reaction to PVY pathotype 1–2. The name Ry 1–2 is suggested for the resistance gene from Criollo de Morellos 334. The gene for resistance to PVY pathotype 1–2 identified in C. chinense PI 159236 may be the same designated et ^c1, which is mentioned to be allelic to the et ^av locus. The phenotypic expression of the et ^av gene was characterized in C. annuum Avelar as a tolerant but not immune or hypersensitive reaction against some potyviruses in Florida. These data suggest that a critical reexamination of the allelic relationships at the locus et should be performed.     

Inheritance of zucchini yellow mosaic virus resistance in Cucumis melo L.

Summary Resistance to zucchini yellow mosaic virus has been found in the muskmelon line PI 414723 from India. This resistance is effective against the ZYMV strains E15 and 1318 belonging respectively to the NF and F pathotypes. Resistance to E15 (no vein clearing and yellowing symptoms) is governed by one dominant gene (symbol Zym) according to segregations observed in F₁, F₂ and BC₁ progenies. This gene is epistatic dominant over Fn, which induces wilting and necrosis after inoculation with F pathotype. Linkage studies suggest that Zym inherits independently from Fom-1, Fom-2, Vat, Wmv and Fn but is linked with a (13.1 ±2.4 units).     

Inheritance of resistance to Verticillium wilt (Verticillium dahliae) in cotton (Gossypium hirsutum L.)

Verticillium wilt, caused by Verticillium dahliae Kleb., is a major constraint to cotton production in almost all countries where cotton is cultivated. Developing new cotton cultivars resistant to Verticillium wilt is the most effective and feasible way to combat the problem. Little is known about the inheritance of resistance to Verticillium wilt of cotton, especially that caused by the defoliating (D) and nondefoliating (ND) pathotypes of the soil‐borne fungus V. dahliae. The objective of this study was to determine the inheritance of resistance in cotton against both pathotypes of V. dahliae. Crosses were made between the susceptible parent ‘Cukurova 1518’ and each of four resistant parents PAUM 401, PAUM 403, PAUM 405 and PAUM 406 to produce F₂ generations in 2002 and F_2:3 families in 2003. Disease responses of parent and progeny populations to the D and ND pathotypes were scored based on a scale of 0‐4 (0, resistant; 4, susceptible). F₂ populations inoculated with the D pathotype showed a 3 : 1 (resistant : susceptible) plant segregation ratio. Tests of F_2:3 families confirmed that resistance was controlled by a single dominant gene. In contrast, analysis of data from F₂‐ and F₂‐derived F₃ families suggested that resistance to the ND pathotype is controlled by dominant alleles at two loci.     

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.     

Inheritance of a resistance specific to tomato spotted wilt tospovirus in Capsicum chinense ‘PI 159236’

Summary Inheritance studies were conducted to determine the genetic basis of resistance in pepper against one Tospovirus isolate classified as tomato spotted wilt virus (TSWV). F₁, backcrosses and F₂ populations were developed using the resistant parent Capsicum chinense PI 159236 (CNPH 679) and the susceptible parent C. annuum Magda (CNPH 192). Segregation ratios strongly indicated that the resistant response (a localization, hypersensitive-like reaction) to TSWV fits a single-dominant gene model. Under our experimental conditions, the penetrance of this gene was very high. This gene (tentatively named Tsw) is highly effective only against TSWV isolates. The resistance governed by the Tsw gene was not effective against isolates belonging to tomato chlorotic spot virus (TCSV) and groundnut ring spot virus (GRSV), two other previously described Tospovirus species.     

Generation means analysis of <Emphasis Type="Italic">wheat streak mosaic</Emphasis> virus resistance in winter wheat

Wheat streak mosaic virus (WSMV; Family: potyviridae; Genus: Tritimovirus) is a major threat to winter wheat (Triticum aestivum L. em. Thell) production worldwide, yet little is known about the genetic control of resistance. Our objective was to determine the mode of inheritance and type of gene action of WSMV resistance in two winter wheat crosses involving a resistant line, OK65C93-8, and two susceptible cultivars, Tandem and Vista. For each cross, parents, F₁, F₂, and backcross plants were inoculated and evaluated for WSMV resistance in two replicated greenhouse experiments. Generation means analysis indicated that additive, dominance, and epistatic effects were all involved in the inheritance of WSMV resistance. Broad-sense heritability estimates for visual symptom rating and ELISA values were high for both crosses (0.84–0.91). Narrow-sense heritability estimates were low in the Tandem/OK65C93-8 cross (0.43–0.45) and moderate in the Vista/OK65C93-8 cross (0.71–0.74). Due to the presence of greater non-additive gene effects combined with low narrow-sense heritability in the Tandem/OK65C93-8 cross, selecting for WSMV resistance in this cross would be complex if using conventional methods. On the other hand, the significant contribution of additive gene effects combined with moderate narrow-sense heritability in the Vista/OK65C93-8 cross suggested that it could be exploited to select for WSMV resistance. Progress from selection for WSMV resistance in early generations of winter wheat may vary among populations as indicated in this study. Therefore, evaluating genetic control of parental combinations may be warranted prior to selecting for WSMV resistance from this source.     

Persistence over time,overlapping distribution and molecular indications of interspecific hybridization in wild potato populations of Northwest Argentina

Solanum gourlayi and Solanum spegazzinii, wild potatoes endemic to Argentina, possess desirable traits for breeding. In periodical regenerations of accessions, variability was detected for morphology and breeding barriers. The persistence of these populations in nature was evaluated after more than 20 years. Both species were observed in all visited sites, along with other wild and cultivated potatoes. Chromosome numbers coincided with the originally reported, except for one population of Solanum gourlayi, with diploid and tetraploid cytotypes. The accompanying flora and environmental conditions revealed important alterations as the result of road construction, excessive stocking rates and overgrazing. Principal coordinate and cluster analyses and an AMOVA using AFLP data of three original accessions and the corresponding new accessions revealed high molecular variability and extensive overlapping. Plant grouping of accessions occurred at a distance of 0.58 for S. gourlayi, 0.62 for S. spegazzinii and 0.67 for both species. The role of natural hybridization and sexual polyploidization in the evolution of sympatric populations of wild potatoes is discussed.

The Importance of the Bolivian Wild Potato Species in Breeding for Globodera pallida Resistance

Screening for resistance to the potato cyst nematode, Globodera pallida, in potatoes from. Bolivia, was carried out in 1983 and 1984, using a mixture of four nematode populations representing pathotypes Pa₁, Pa₂ and Pa₃ From the 66 accessions of 17 species and subspecies evaluated, highly resistant genotypes were identified in 21 accessions from seven species. All had Pf/Pi values of 2 or less, whereas the susceptible control, Solanum tuberosum cv. ‘Disiree’ had Pf/Pi values of more than 2G in both tests. Two diploid wild species, S. brevicaule and S. leptophyes, showed the best resistant. The geographical distributional of resistant populations and the evolution of resistance in wild potato populations are discussed.     

Heritability of β-glucan, groat percentage, and crown rust resistance in two oat crosses

Summary Parent-offspring regression was used to estimate heritability for three traits (-glucan content, groat percentage, and resistance to crown rust (Puccinia coronata Cda. f. sp. avenae Eriks.) in oat (Avena sativa L.). The populations used were derived from two crosses, Nova x Marion QC and Sylva x Marion QC. Marion QC was used as a parent because other research had shown that it is relatively high in -glucan, a trait for which heritability had not previously been estimated. Nova and Sylva are similar in adaptation to Marion QC, and Sylva may be a source of general resistance to crown rust. Random F₅ plants were grown in a greenhouse, and their F₆ and F₇ progeny were grown in replicated field trials. Heritability estimates for the two grain quality traits were based on regression of F₆ values on F₅ values, F₇ values on F₆ values, and F₇ values on F₅ values. Heritability estimates for -glucan content were between 0.27 and 0.45. The highest estimate was the one based on the F₆ and F₇ generations of Sylva x Marion QC: 0.45, compared to estimates of 0.32 or less for earlier generations of the same cross, and to estimates of 0.35 or less for all generations of Nova x Marion QC. Heritability estimates for groat percentage were all between 0.23 and 0.32. The F₆ and F₇ generations were evaluated for resistance to crown rust resistance. The Sylva x Marion QC cross seemed to segregate for heritable resistance (h ²=0.31) but the Nova x Marion QC cross did not (h ²=0.07). Several lines from the Sylva x Marion QC cross had low symptoms in both the F₆ and F₇ generations. There were no strong genetic correlations among the traits.     

Identification and characterization of seedling and adult plant resistance to <Emphasis Type="Italic">Puccinia hordei</Emphasis> in selected African barley germplasm

A collection of 112 African barley accessions were assessed for response to Puccinia hordei in seedling greenhouse tests using 10 pathotypes and in adult plant field tests over three successive field seasons in Australia. One of the 10 pathotypes (viz. 5457P+) used in seedling tests was also used in field tests to allow assessment of the presence of adult plant resistance (APR) in lines that were seedling susceptible to this pathotype. The seedling resistance genes Rph1, Rph2, Rph3, Rph9.am and Rph9.z were postulated in a number of accessions, singly and in various combinations, with Rph2 and Rph9.z being the most common. Twenty-six accessions carried seedling resistance that was either uncharacterized or could not be determined using the 10 P. hordei pathotypes. One accession carried high levels of APR and 11 accessions showed moderate levels of APR, all of which were susceptible to all P. hordei pathotypes at the seedling stage. All barley accessions were genotyped for the presence of marker alleles that are closely linked to the APR genes Rph20 and Rph23 (bPb-0837 and Ebmac0603, respectively). No accession was positive for bPb-0837, suggesting that Rph20 is not frequent in African germplasm. Thirteen accessions were postulated to carry Rph23 based on the presence of the marker allele Ebmac0603 found in Yerong (Rph23), and 10 out of the 11 accessions with moderate APR lacked the bPb-0837 and Ebmac0603 marker alleles, indicating that they likely carry new uncharacterized APR genes. Inheritance studies were performed using populations derived from four of the accessions that carried APR (Clho 9776, Clho 11958, Mecknes Maroc and Sinai) by crossing with the susceptible barley genotype Gus. Chi squared analysis of the phenotypic data from F₃ populations suggested that CIho9776 carried a single APR gene and CIho11958, Mecknes Maroc and Sinai each carried two genes for APR to leaf rust.     

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.