The common wheat cultivar Parula possesses a high level of slow rusting, adult plant resistance (APR) to all three rust diseases of wheat. Previous mapping studies using an Avocet-YrA/Parula recombinant inbred line (RIL) population showed that APR to leaf rust (Puccinia triticina) in Parula is governed by at least three independent slow rusting resistance genes: Lr34 on 7DS, Lr46 on 1BL, and a previously unknown gene on 7BL. The use of field rust reaction and flanking markers identified two F6 RILs, Arula1 and Arula2, from the above population that lacked Lr34 and Lr46 but carried the leaf rust resistance gene in 7BL, hereby designated Lr68. Arula1 and Arula2 were crossed with Apav, a highly susceptible line from the cross Avocet-YrA/Pavon 76, and 396 F4-derived F5 RILs were developed for mapping Lr68. The RILs were phenotyped for leaf rust resistance for over 2 years in Ciudad Obregon, Mexico, with a mixture of P. triticina races MBJ/SP and MCJ/SP. Close genetic linkages with several DNA markers on 7BL were established using 367 RILs; Psy1-1 and gwm146 flanked Lr68 and were estimated at 0.5 and 0.6 cM, respectively. The relationship between Lr68 and the race-specific seedling resistance gene Lr14b, located in the same region and present in Parula, Arula1 and Arula2, was investigated by evaluating the RILs with Lr14b-avirulent P. triticina race TCT/QB in the greenhouse. Although Lr14b and Lr68 homozygous recombinants in repulsion were not identified in RILs, γ-irradiation-induced deletion stocks that lacked Lr68 but possessed Lr14b showed that Lr68 and Lr14b are different loci. Flanking DNA markers that are tightly linked to Lr68 in a wide array of genotypes can be utilized for selection of APR to leaf rust.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.1998.88.9.890

Lr68: a new gene conferring slow rusting resistance to leaf rust in wheat

The common wheat cultivar Parula possesses a high level of slow rusting, adult plant resistance (APR) to all three rust diseases of wheat. Previous mapping studies using an Avocet-YrA/Parula recombinant inbred line (RIL) population showed that APR to leaf rust (Puccinia tritici- na) in Parula is governed by at least three independent slow rusting resistance genes: Lr34 on 7DS, Lr46 on 1BL, and a previously unknown gene on 7BL. The use of field rust reaction and flanking markers identified two F6 RILs, Arula1 and Arula2, from the above population that lacked Lr34 and Lr46 but carried the leaf rust resistance gene in 7BL, hereby designated Lr68. Arula1 and Arula2 were crossed with Apav, a highly susceptible line from the cross Avocet-YrA/Pavon 76, and 396 F4-derived F5 RILs were developed for mapping Lr68. The RILs were phenotyped for leaf rust resistance for over 2 years in Ciudad Obregon, Mexico, with a mixture of P. triticina races MBJ/SP and MCJ/SP. Close genetic linkages with several DNA markers on 7BL were established using 367 RILs; Psy1-1 and gwm146 flanked Lr68 and were estimated at 0.5 and 0.6 cM, respectively. The relationship between Lr68 and the race-specific seedling resistance gene Lr14b, located in the same region and present in Parula, Arula1 and Arula2, was investigated by evaluating the RILs with Lr14b-avir- ulent P. triticina race TCT/QB in the greenhouse. Although Lr14b and Lr68 homozygous recombinants in repulsion were not identified in RILs, c-irradiation-induced deletion stocks that lacked Lr68 but possessed Lr14b showed that Lr68 and Lr14b are different loci. Flanking DNA markers that are tightly linked to Lr68 in a wide array of genotypes can be utilized for selection of APR to leaf rust.

Lr68: a new gene conferring slow rusting resistance to leaf rust in wheat Sybil A. Herrera-FoesselRavi P. SinghJulio Huerta-Espino • Garry M. RosewarneSambasivam K. PeriyannanLibby Viccars • Violeta Calvo-SalazarCaixia LanEvans S. Lagudah

Nine Mexican spring bread wheat (Triticum aestivum L.) cultivars derived from CIMMYT germplasm and the U.S. spring wheat cultivar Wheaton were susceptible to the Mexican Puccinia striiformis pathotype 14E14 in seedling growth stage, but displayed different levels of adult plant resistances to the same pathotype when tested in the field. One hundred and eighteen random F2 plant derived F3 and F5 lines from the crosses of these ten adult plant resistant wheats and susceptible cultivar Jupateco 73S were evaluated in the field. The moderate adult plant resistance of Penjamo 62, Lerma Rojo 64, Nacozari 76, Tesia 79, and Wheaton was under monogenic genetic control and was attributed to the adult plant stripe rust resistance gene Yr18. The moderate resistances of Cleopatra 74, Zaragoza 75, and Apache 81 were also monogenic, but gene Yr18 was absent. Pavon 76 carried two partially effective additive genes; and the adult plant resistance of Tonichi 81 was based on additive interaction involving Yr18 and two additional partially effective genes. Tonichi 81 does not carry any seedling resistance gene, however, the adult plant resistance is highly effective worldwide. This resistance, designated as the Yr18 complex, is of a durable nature. The partial adult plant resistance of Pavon 76 has also remained durable in Mexico and other countries where it is grown.

Genetics of adult plant resistance to stripe rust in ten spring bread wheats

Approximately 10 million hectares of wheat (Triticum aestivum L.) globally experience medium toserious waterlogging. The inheritance of waterloggingtolerance was determined in reciprocal crosses ofthree tolerant (Prl/Sara, Ducula and Vee/Myna), andtwo sensitive (Seri-82 and Kite/Glen) spring breadwheat lines. Parents, F1, F2, F3, and backcrossgenerations were studied under field conditions in Cd.Obregon, State of Sonora, Mexico. Flooding was appliedwhen plants were at the three-leaf and first-internodestages. Basins were drained after 40 days of flooding.Leaf chlorosis was used as a measure of waterloggingtolerance. The sensitive by sensitive cross, Seri-82 × Kite/Glen, showed the highest mean values forpercentage leaf chlorosis and area under chlorosisprogress curve (AUCPC), and the lowest mean values forplant height, biomass, grain yield, and kernel weight.The F3 of the cross between the two tolerant parentsDucula and Vee/Myna had the lowest mean values forpercentage leaf chlorosis and AUCPC, and the highestmean values for plant height, biomass, and grainyield. The expression of waterlogging tolerance wasnot influenced by a maternal effect. The F1 hybridswere intermediate for leaf chlorosis, indicating thattolerance was additive. Quantitative analysis alsoindicated that additive gene effects mainly controlledwaterlogging tolerance in these crosses. Segregationratios of F3 lines indicated that up to four genescontrolled waterlogging tolerance in these crosses,with two genes adequate to provide significanttolerance. Early-generation selection for tolerancewould be effective in these populations.

Expression and inheritance of tolerance to waterlogging stress in wheat

Summary

(1) Horizontal resistance (HR) to diseases of crops has the following leading features: it is polygenically controlled and must be studied by biometrical-genetic methods; it is pathotype-non-specific and essentially ‘durable’; it has several components (e.g. infection rate, latency period, sporulation potential) which tend to be correlated; its use is relevant to the control of all classes of pathogen (fungi, bacteria, viruses, insects, nematodes); though often unrecognized, it is very common and generally the means by which ‘minor’ diseases remain minor. HR may be contrasted with ‘vertical resistance’ (VR) which is procured by major genes and is pathotype-specific. VR has uses in particular circumstances but has often failed against airborne epidemic pathogens such as many rusts; hence the importance of understanding HR.



(2) In this review the genetic evidence of HR is reviewed and examples are summarized in appendices. Very diverse crops, places and pathogens are represented. So far as may be judged, HR is indeed universal, and found or constructable wherever sought. The most important genetic evidence is based on generation mean and variance analysis, general and specific combining ability, offspring-parent regression and response to selection. Useful supplementary evidence comes from historical observations, continuity and repeatability of resistance and yield-related effects.



(3) The main conclusions are that HR is universally available, usually highly heritable and responsive to selection, already keeps a host of crop diseases down to acceptably low levels and has socio-economically attractive features that are likely to increase its use in the future. In particular, it offers long-term stability of performance that must be valuable to small farmers in the Third World and is environmentally attractive because successful breeding programmes minimize the need for environmentally damaging chemical protectants. ‘Green’ pressures are likely to favour HR and some relevance to public policy is thereby implied.

Genetics of horizontal resistance to diseases of crops

Quantitative determination of the gene action of leaf rust resistance in four cultivars of wheat, Triticum aestivum.

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Quantitative determination of the gene action of leaf rust resistance in four cultivars of wheat, Triticum aestivum.