Exotic barley germplasms in breeding for resistance to soil-borne viruses

Summary Soil-borne mosaic inducing viruses, i.e., barley mild mosaic virus (BaMMV), barley yellow mosaic virus (BaYMV), and BaYMV-2, cause one of the most important diseases of winter barley in Western Europe. Since resistance of all commercial European barley cultivars is due to a single recessive gene (ym4) which is not effective against BaYMV-2, exotic barley germplasms (Hordeum vulgare L., H. spontaneum Koch) were screened for resistance to the different viruses and analyzed for genetic diversity concerning BaMMV resistance. In these studies it turned out that resistance to BaMMV is entirely inherited recessively and that a high degree of genetic diversity concerning resistance is present within the barley gene pool at least to BaMMV. Therefore, exotic barley germplasms are a very useful source for the incorporation of different resistance genes into barley breeding lines, thereby enabling the breeder to create cultivars adapted to cultivation in the growing area of fields infested by soil-borne viruses. Furthermore, in order to obtain more information on these germplasms they were evaluated for agronomic traits and isozyme, RFLP and RAPD analyses were carried out on these varieties to detect markers linked to the respective resistance genes and to obtain information on the genetic similarity between yellow mosaic resistant barley accessions derived from different parts of the world. Actual results of these studies are briefly reviewed.     

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.     

Conference on breeding for disease resistance

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Conference on breeding for disease resistance     

The perspectives of polygenic resistance in breeding for durable disease resistance

Polygenic resistance is generally quantitative without clear race specific effects. With the onset of molecular markers technologies, the identification of chromosome regions that are involved in quantitative resistance has become feasible. These regions are designated quantitative trait loci (QTLs). The mapping of `major' QTLs can be independent of environment, season, year or race of the challenging pathogen. However, the detection of minor QTLs may be dependent on the `environment'. As QTLs are defined by the position on the genome and the quantitative effect on resistance, they are not informative about the mechanism of resistance. By comparing QTL with the loci that are involved in race specific resistance the coincidence of these loci may suggest a common mechanism. However, the histological characterisation of the resistance is more informative about the resistance mechanism. Estimations about the durability of polygenic, quantitative resistance are still academic as there is hardly any experience with large-scale usage of quantitative resistance over a longer period. The clearest example of non-durable resistance is race specific monogenic resistance that is associated with a hypersensitive response (HR). Hence, there is a great chance that polygenic resistance that is not associated with HR is more durable. In some pathosystems with a long experience with non-durable race specific HR genes, quantitative resistance offers a good alternative and marker-assisted breeding will facilitate the exploitation of these resistance for commercial purposes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluating resistance of barley breeding lines to scald in field plots

Summary A 0–4 scoring system to quantify scald (Rhynchosporium secalis) infection is suggested. Scores 1, 2, 3 and 4 allocated to represent 1/4, 1/2, 3/4 and 4/4 of the crop canopy scalded are easy to comprehend and intermediate scores e.g. 0.5, 1.5, 2.5 and 3.5 give it the breadth of a quantitative scale. Scores on a large number of lines showed a high degree of repeatability and were found to be highly correlated with the log transformed values of the actual leaf area damage. Although it was suggested that predictions of leaf area damage at scores 3–4 should be applied with caution, broad generalization of the scores in discriminating the amount of disease were shown to be soundly based and offered plant breeders a tool to standardize the evaluation of scald resistance in field plots on a large scale with this quick and reliable scoring system.     

Molecular breeding for the BaMMV/BaYMV resistance gene ym4 in winter barley

The aim of this investigation was to develop a procedure for the largescale molecular breeding for ym4, allowing resistance to BaMMV/BaYMV to be fixed in early breeding generations of winter barley. A codominant STS marker derived from the restriction fragment length polymorphism marker MWG838 for the ym4 resistance gene was combined with a new and easy procedure for preparing leaf samples for polymerase chain reaction (PCR), theoretically allowing one person to extract DNA from 5000 samples in a single day. In the procedure for molecular breeding for ym4, all steps, including leaf sampling, DNA extraction, PCR amplification and digestion with restriction enzyme were assembled in microtitre plates allowing multipipetting throughout the procedure, including the loading of gels. The method is amenable to further automation with the aid of a robot arm. Double haploid (DH) lines, as well as F₂ and F₄ breeding lines were analysed and, based on markers, homozygous and heterozygous BaMMV/BaYMV resistant plants were identified for further breeding. The winter barley breeding programmes were modified to include marker-based selection for BaMMV/BaYMV resistance on DH or on F₂ individuals, which had been preselected for mildew and leaf rust resistance.     

Assessment of varietal reaction and breeding for resistance to the yellow-dwarf virus in barley

A series of barley varieties of practical interest in Britain and Western Europe were assessed for reaction to the barley yellow-dwarf virus (BYDV). All proved to be highly susceptible. Fourteen barley selections found to be tolerant to BYDV in the United States were also tested. Although all were tolerant to BYDV when tested in the glasshouse, only two were tolerant under British field conditions. One selection was slightly less tolerant of one virus isolate than it was of another isolate.In progeny from crosses of tolerant and susceptible genotypes, the inheritance of BYDV tolerance was conditioned by a single major gene which showed little or no dominance. Results of preliminary work to introduce BYDV tolerance into the new high yielding European commercial barley varieties, using the backcross method, are reported. Following two or three successive generations of backcrossing, the selection of plants containing the tolerance gene in the heterozygous condition became impossible and the homozygous tolerant genotypes had to be reconstituted to facilitate the selection of plants for further breeding.     

Approaches for breeding apples with durable disease resistance

Summary In most apple breeding programmes resistance to diseases has a high priority. The main emphasis is put on scab and mildew resistance. Recent evidence about the risk of breakdown of the Vf scab resistance demonstrates the need to search for new approaches in disease resistance breeding. The following options might be considered: discover and exploit new sources of resistance to enlarge the genetic basis of resistance; accumulate different functional resistances in one genotype; develop orchard designs in order to increase diversity in the host-pathogen interactions and establish a resistance management system. To implement these strategies the genetic structure of the resistance must be examined by using modern methods of molecular biology. Resistance donors and advanced selections should be examined under high natural inoculum pressure at several locations around the world. We examined the possibility of identifying polygenic or partial scab resistance in apple. Nineteen apple varieties and selections, among them the differential hosts for scab races 1 to 5, were artificially inoculated in the glasshouse with scab conidia. The leaf symptoms were analysed macroscopically and microscopically There were significant differences which are actually exploited in our breeding programme. Conclusions are drawn towards new approaches for breeding and growing apples with durable disease resistance.     

Sources of resistance to mildew and stripe rust in breeding spring barley

From 1960 onwards the Cereal Section of the Foundation for Agricultural Plant Breeding has paid much attention to seeking sources of resistance against yellow rust and mildew in barley.So far no variety has been found with a complete resistance to yellow rust. A number of varieties with at most a very slight degree of attack are mentioned in the tables.Mildew resistance in barley is discussed on the basis of German and Swedish data on the physiological specialization on barley.     

Possibility of exploiting the Yd2 resistance to BYDV in spring barley breeding

The effects of the Yd2 gene on tolerance to barley yellow dwarf virus (BYDV) and other agronomically important characters in spring barley were evaluated in a set of randomly selected doubled haploid (DH) lines of an‘Igri’/‘Atlas 68’ cross and three crosses between CIMMYT Yd2 materials and the Czech malting barley ‘Akcent’. The cleaved amplified polymorphic site (CAPS) diagnostic marker Yd2 was used for identification of the Yd2 gene and this analysis showed high agreement with the results of field infection tests. Yd2 lines exhibited significantly lower symptom scores and lower reductions of some grain yield characters, but their resistance level was not consistent over the years. The presence of secondary stresses (high temperature/drought) in 2000 led to relatively higher sensitivity to BYDV infection, strengthened by the long life cycle of genotypes. In cases where secondary stresses were mild (in 2002), the longer life cycle significantly increased sensitivity to BYDV infection only in the absence of the Yd2 gene (in susceptible genotypes). The examination of different vegetative, grain yield and malting quality characters separately for groups of Yd2 and non‐ Yd2 lines did not show any evidence of adverse effect of the Yd2 gene on any character.     

Winter barley breeding in the Netherlands

Summary Various Dutch breeders have devoted themselves to winter barley breeding; the local variety Groningen Winter Barley used always to be the foundation material for breeding work. In the course of years 8 new varieties have been derived from it by line selection which demonstrates clearly the degree of variation.The varietal range (fig. 1) shows the peculiarity that the leading variety Vindicat has almost completely been superseded by Urania within some years. If ever, the slogan the Dutch farmer readily accepts new varieties applies here.Its sister variety Vinesco makes a favourable impression.The varieties of winter barley are now for 97 percent based on homebred material.     

Postulation of leaf-rust resistance genes in Czech and Slovak barley cultivars and breeding lines

Leaf‐rust resistance (Rph) genes in 61 Czech and Slovak barley cultivars and 32 breeding lines from registration trials of the Czech Republic were postulated based on their reaction to 12 isolates of Puccinia hordei with different combinations of virulence genes. Five known Rph genes (Rph2, Rph3, Rph4, Rph7, and Rph12) and one unknown Rph gene were postulated to be present in this germplasm. To corroborate this result, the pedigree of the barley accessions was analysed. Gene Rph2, as well as Rph4, originated from old European cultivars. The donor of Rph3, which has been mainly used by Czech and Slovak breeders, is ‘Ribari’ (‘Baladi 16’). Rph12 originates from barley cultivars developed in the former East Germany. Rph7 in the registered cultivar ‘Heris’ originates from ‘Forrajera’. A combination of two genes was found in 10 cultivars. Nine heterogeneous cultivars were identified; they were composed of one component with an identified Rph gene and a second component without any resistance gene. No gene for leaf rust resistance was found in 17 of the accessions tested. This study demonstrates the utility of using selected pathotypes of P. hordei for postulating Rph genes in barley.     

Problems involved in breeding for resistance

At the start of a breeding programme mostly a cross is made between the resistant plant and a plant belonging to a commercial variety of the crop in question. If both belong to the same species the exchange of genes can be easily brought about. If, however, the resistance is present only in other more distantly related species, difficulties mostly arise.In consequence of successful breeding work races of the parasite able to break down the resistance can come to the fore. It is of the utmost importance to combine the genes for resistance from all available sources so as to produce barriers that cannot be broken down or at least only with great difficulty. Therefore not only the easy line of intraspecific crossing should be followed but also the much more difficult one of interspecific and even of intergeneric crossing.As in most cases resistance is dominant, the method of repeated backcrossing can be used with great profit. In each step of the backcross programme large numbers of seedlings should be raised, selecting very carefully the parent material that will be used for the next step, even if this selection takes several years. As a consequence the number of subsequent backcrosses can be restricted and the danger of loss of valuable genes for resistance and for other characters is very much reduced.If resistance is recessive the backcross method cannot be applied to its full extent. The F₁ is susceptible and by inbreeding more or less resistant seedlings can be obtained. If the resistant source is a wild species this program of outbreeding followed by inbreeding has to be repeated and progress will be very slow.A point of major importance is the availability of quick methods for mass-testing. These methods need not be more reliable than is demanded by the kind of material that has to be tested.

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Samenvatting Bij de uitvoering van een kweekprogramma wordt altijd begonnen met het kruisen van de resistence plant met een plant uit het ras dat men wil verbeteren.Als beide planten tot dezelfde soort behoren kan de uitwisseling van genen gemakkelijk worden bewerkstelligd. Indien de resistentie echter alleen in andere meer of minder verwante soorten voorkomt, dan treden gewoonlijk complicaties op.Als gevolg van de verbouw van resistente rassen zijn fysio's van de parasiet, welke de resistentie kunnen doorbreken, in staat zich naar verhouding sterk te vermeerderen. Het is daarom van het grootste belang de resistentie-genen uit alle beschikbare bronnen te combineren, teneinde een barrière op te kunnen werpen die niet of slechts met grote moeite te doorbreken is. Men moet derhalve niet alleen de gemakkelijke weg van de raskruising volgen maar ook de veel moeilijker weg van de soortskruising en zelfs die van de geslachtskruising, teneinde ook die genen in het werk te betrekken die buiten de soort voorkomen.Daar de resistentie in de meeste gevallen dominant is kan de methode van de herhaalde terugkruising met voordeel worden toegepast.Bij iedere volgende stap in het terugkruisingsschema moeten grote aantallen zaailingen worden opgekweekt, terwijl het toekomstige oudermateriaal met grote zorg moet worden uitgezocht. Daardoor kan het aantal terugkruisingen worden beperkt en tevens het gevaar voor verlies van waardevolle genen.Als de resistentie recessief overerft kan de methode van de herhaalde terugkruising niet in volle omvang worden toegepast. De F₁ is vatbaar en door inteelt kunnen meer of minder resistente zaailingen worden verkregen. Als de bron van resistentie een wilde soort is dan moet dit programma van kruisen gevolgd door inteelt worden herhaald en zal er slechts langzaam vooruitgang optreden.Van veel belang is de mogelijkheid snelle methoden voor massale toetsing te ontwikkelen. Deze methoden behoeven niet betrouwbaarder te zijn dan door de aard van het te toetsen materiaal wordt geëist.

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Lecture read at the A-course Resistance in agriculture, 12–14 Jan. 1959, organised by the Royal Society of Agricultural Science and the Netherlands Society of Graduates in Agriculture at Wageningen.     

Choice of selection strategy in breeding barley for stress environments

To determine the optimum selection environment for barley (Hordeum vulgare L.) targeted at low-input, stress environment, barley lines were selected for high yield under stress (YS), high yield under non-stress (YNS), or average yield in stress and non-stress conditions (YA) during three breeding cycles (cohorts) of three years each. The lines were then tested in a total of 21 year-location combinations with average grain yields ranging from 0.35 to 4.86 t ha^-1. Yield under stress of the YS lines was between 27% and 54% higher than that of the YNS lines, with the top YS lines yielding under stress between 16% and 30% more than the top YNS lines. Realized heritability was between 0.35 and 0.67 when selection was conducted under stress and was significant in all three cohorts. By contrast, selection under non-stress gave a significant response in only one cohort, and its efficiency in improving yield under stress was significantly lower than selection under stress. The best YNS line ranked only 19th for yield under stress. The highest-yielding lines under stress were not only selected under stress, but were also landraces collected in very dry areas (< 250 mm total annual rainfall). This confirms earlier findings and supports the idea that the most effective way to improve productivity of crops grown in less-favored areas is to use locally adapted germplasm and select in the target environment(s). This revised version was published online in July 2006 with corrections to the Cover Date.     

Farmers’ participation and breeding for durable disease resistance in the Andean region

In the Andean region, the Preduza project and its partners combined breeding for durable disease resistance using locally adapted cultivars and farmer participatory methods. The approach taken resembles participatory variety selection (PVS). Farmers participated in the selection of advanced materials, rather than finished cultivars. This paper describes this approach and reports experiences with farmers–breeders collaboration. As breeders involved farmers as participants, they learned more about the most important criteria of male and female farmers for preferred cultivars in the marginal environments of Andean cropping systems. This approach encouraged the use of locally adapted cultivars (often landraces), made the breeders less dependent on foreign materials, and has resulted in selection and development of new wheat, barley, common bean, quinoa, potato and maize cultivars. Breeding programmes based on crossing locally adapted cultivars followed by selection by the breeders in the early phases of the breeding programmes and by participatory selection with the farmers in the more advanced stages of the breeding programmes appeared successful. It became clear that breeders must be well acquainted with the farmer preferences such as the requirements for specific agronomic, storage, processing and marketing traits. Over a period of five years the centralized formal breeding approach predominantly based on material produced by the international institutes was replaced by decentralized breeding approaches based largely on local germplasm with extensive farmer participation.     

The cost of participatory barley breeding

Farmer participation in agricultural research is increasingly seen as a powerful methodology to increase the relevance of technologies developed to benefit farmers' communities. In plant breeding, farmer participation is considered as a way to increase the probability of adoption of new varieties. However, the higher expected cost of participatory plant breeding (PPB) is seen as one of the main obstacles to its wider adoption.This paper addresses the issue of the different costs to an Institution of running a PPB program or a non-participatory program and uses the barley-breeding program at the International Center for Agricultural Research in the Dry Areas (ICARDA) as a case study.Observations and data collection were carried out during one full cropping season on the cost of the three main components of the breeding program, i.e. the management of the field trials (land and seedbed preparation, planting, fertilizer application, weed control, harvesting, and seed threshing, cleaning, treating and packaging), the travel to farmers' fields or to the research sites, and the human resources (scientists, technical staff, local workers and farmers) involved in breeding activities.We compared two options for the centralized–non-participatory breeding program, differing in the number of sites (8 and 16) used for the on-farm trials, with 160 options for the decentralized-participatory breeding program, differing in the combination of number of sites (from 4 to 16) and number of trials per site (from 1 to 10).The results show that in both decentralized-participatory and centralized–non-participatory plant breeding the cost of managing the field trials is the highest followed by the cost of human resources and that of travel: the contribution of each component to the total cost varies with the various options and the various combinations of the number of sites and of farmers.The comparison of the aggregated costs indicates that in the case of the ICARDA' barley-breeding program there are no relevant differences between the participatory and the non-participatory plant breeding programs. This is largely associated with the fact that the decentralized-participatory breeding program reaches the same level of development of the breeding material 3 years earlier than the centralized–non-participatory breeding program. Depending on the type of centralized-breeding program and on the combination of number of sites and number of farmers per site in the participatory program, the aggregated costs of the participatory program are lower than those of the centralized-breeding program by between 5 and 28%. At the same level of cost of the centralized program, the model of participatory program used in this study generates more information due to the use of more trials at each site. This improves selection efficiency and provides an analytical tool to optimize the number of sites and of farmers per site.     

Selection for multiple disease resistance reduces cucumber yield potential

Summary Experiments were conducted in Wisconsin and Michigan to determine whether selection for multiple disease resistance adversely affects yielding ability of commercial cucumber (Cucumis sativus var. sativus L.; CS) lines. Inbred F₄ and F₅ lines were developed from C. sativus var. hardwickii (R.) Alef. (CH) x CS which had been either selected or not selected for resistance to the causal organisms of scab, anthracnose, and downy mildew. The exotic CH germplasms possesses a multiple fruiting habit with high yield potential and the CS genotypes include several disease resistance. In each comparison between selected and unselected progeny, the unselected families either significantly outyielded, or were not significantly different than their selected counterparts. In no case did the selected progeny outyield the unselected progeny. Since this was observed for both fruit number per plant and total fruit weight per plant, and it occurred despite differences in fruit size, we conclude that selection for disease resistance led to a reduction in yield potential in these populations.Received for publication-. 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.     

An alternative approach to breeding for high diastatic power in barley

Summary Several lines from the cross Akka x Feebar were observed to have -amylase activity considerably in excess of either parent. It is suggested that, from crosses between two-rowed and six-rowed varieties, two-rowed genotypes with enhanced levels of grain nitrogen and -amylase activity may be obtained. These can be successfully exploited in a breeding programme to produce barley varities with high diastic power.