Showing papers on "Plant breeding published in 1997"

Exploitation of wild Cicer species for yield improvement in chickpea

Abstract: Chickpea (Cicer arietinum L.) ranks third in the world, and first in the Mediterranean basin, for production among pulses. Despite its importance as a crop and considerable research effort, traditional breeding methods have so far been unable to produce cultivars with a large impact on chickpea production. Interspecific hybridization is known to improve yield in many crops. Therefore, an attempt was made to increase the seed yield in chickpea through the introgression of genes from wild relatives at the International Center for Agricultural Research in the Dry Areas (ICARDA), Syria, from 1987 to 1995. Four crosses, ILC 482 (C. arietinum)×ILWC 179 (C. echinospermum) and ILC 482×ILWC 124 (C. reticulatum) and their reciprocals, were made. Pedigree selection was used to advance the material. Heterosis was recorded visually in F1s, and single plant measurements for seed yield were recorded in F2 populations. Promising and uniform progenies were bulked in the F5 generation. Out of 96 F6 lines, 22 were selected on the basis of seed yield and other agronomic characters, and evaluated in a replicated trial for seed yield and 14 agronomical, morphological and quality characters. A high level of heterosis was observed in F1s. Several F2 plants produced two to three times more seed yield than the best plant from the cultigen. Nine F7 lines out-yielded the cultigen parent by up to 39%. Over 2 years, 12 lines had a higher yield than the cultigen parent. These lines were not only high yielding but also free of any known undesirable traits from the wild species, such as spreading growth habit, pod dehiscence, and non-uniform maturity. Quality traits, such as seed shape, type, colour, weight, and testa texture, protein content, cooking time and an organoleptic test of a Middle East dish, Homos Bi-Tehineh, were also similar to the cultigen parent. Both C. echinospermum and C. reticulatum contributed towards the increased yield.

Genotypic variation in plant response to low boron and implications for plant breeding

Abstract: Plant response to low B in the soil varies widely among species, and among genotypes within a species. Boron efficient genotypes are those that are able to grow well in soils in which other genotypes are adversely affected by B deficiency. This review considers the extent of variation in B efficiency in plant species and genotypes, the physiological nature of the efficiency mechanisms, what is known of the genetic basis for inheritance, screening techniques and the practical implications of the genotypic variations. Frequently, B efficiency is the sole reason for a difference between an average yield and complete crop failure. Severe yield losses can be effectively prevented by the inclusion of B efficiency as a selection criterion in crop breeding and improvement programmes for regions with low B soils. In addition, the expression of B deficiency primarily through male sterility, which is common in many species, creates opportunities for outcrossing in normally self-fertilised species. This, in turn, leads to two possibilities. Firstly, self fertilisation, and therefore maintenance of pure lines, cannot always be assumed in self pollinated species where B efficient and inefficient genotypes are grown side by side on low B soils. Secondly, B deficiency, in soil or artificial media, may be used as a fertility selective medium in which the male sterile B inefficient genotypes and the male fertile B efficient genotypes could hybridise naturally. This would be useful as a simple and economical method for creating heterozygous populations in breeding programmes as well as for producing hybrid seeds. Now that the roles of B in plant growth and development are beginning to be clarified, the efficiency mechanisms as well as the governing genetics can be explained. Practical benefits from the genetic diversity of B efficiency will be enhanced by a better understanding of B efficiency mechanisms and the molecular bases for their genetic control.

Quantitative trait loci associated with stomatal conductance, leaf rolling and heading date mapped in upland rice (Oryza sativa)

Abstract: summary Improving the drought resistance of high yielding rice (Oryza sativa L.) varieties for upland areas is a goal of rice breeders since upland rice relies exclusively on rainfall for water and is generally low yielding. Identifying quantitative trait loci (QTLs) which confer drought resistance promises to speed this goal. We have previously described the use of 82 restriction fragment length polymorphism (RFLP) markers in the genetic mapping of root growth characteristics that are potentially involved in drought resistance in a population of 178 F2 plants derived from a cross of two drought resistant upland varieties, Azucena and Bala. Here we report the characterization and QTL mapping of three other mechanisms of drought resistance, leaf rolling, stomatal behaviour and early flowering, in this population. When the youngest fully expanded leaf was excised from 12 rice varieties,‘IR20’and‘Bala’ were identified as slow at rolling. Although ‘Azucena’ rolled its leaves much faster than ‘Bala’ upon excision, stomatal resistance rose more rapidly in‘Bala’, resulting in a lower rate of water loss. By assessing leaf rolling and stomatal resistance in the F2 population, it was possible to identify QTLs for these traits. A QTL for slow leaf rolling from ‘Bala’ was found on chromosome 1. Two QTLs associated with resting stomatal conductance were found on chromosomes 3 and 12. QTLs for the rate of stomatal closure were found on chromosomes 3 and 7. In addition, it was possible to map QTLs associated with the days to heading which were found on chromosomes 3, 8 and 10. These results identify markers of potential value to breeders and are discussed in the context of previously reported data for heading date QTLs in rice and for stomatal conductance QTLs in maize.

Phosphorus nutrition of spring wheat (Triticum aestivum L.) 1. Effects of phosphorus supply on plant symptoms, yield, components of yield, and plant phosphorus uptake.

Abstract: The effects of phosphorus (P) deficiency on plant symptoms, yield, and components of yield of wheat (Triticum aestivum L. cv. Halberd), P uptake, and the distribution of dry weight within plants of variable P status were examined in 2 glasshouse and 5 field experiments. Apart from stunted growth and depressed tillering, the symptoms of acute P deficiency, most noticeable on older leaf blades, were equivocal; they were not always observed on acutely deficient plants and were absent on moderately deficient plants. In glasshouse experiments, the leaves of acutely deficient plants were spindly, erect, and dark green, whereas in field experiments, the leaves were pale green. In acutely P-stressed plants, leaf senescence, phasic development, and anthesis were delayed. The disorder restricted tiller development and therefore the rate of appearance and the number of leaves per plant. It depressed grain yield principally by reducing the number of fertile tillers. Severe P deficiency depressed shoot growth within 15 days of sowing and ultimately reduced plant height, root mass, and grain yield. In all experiments, shoot yield responses to applied P increased progressively until stem elongation (Zadoks Scale 30) and changed little thereafter. As a result, the external requirement for P (i.e. P level required for 90% maximum growth) increased with time during vegetative development in most experiments. Severe P deficiency also affected the distribution of dry matter between the roots and shoots and between the leaf blades and conducting tissues (sheaths and stems). Both of these responses intensified with advancing plant age. Treatment differences in P uptake in shoots also occurred early in growth and persisted until grain maturity. The partitioning of P between roots and shoots favoured P uptake or retention in the roots of P-deficient plants. Under conditions of acute and moderate P stress, the resources of the wheat plant appear to be directed towards maintaining root growth (at least initially), limiting and delaying shoot proliferation, and maximising the leaf : stem ratio. These regulations appear circumstantially to be adaptive mechanisms for conserving suffiient P to ensure the survival of at least 1 weak, but fertile, tiller on each plant.

Correlation between Molecular Marker Distance and Hybrid Performance in U.S. Southern Long Grain Rice

Abstract: Improving grain and milling yields of long-grain rice (Oryza saliva L.) continues to be a major objective of southern U.S. breeding programs. The objective of this study was to determine the relationship between molecular marker polymorphism of the parents and performance of the F 1 hybrids. A diallel cross was made by intermating eight lines commonly used in U.S. southern long-grain rice breeding programs. The 28 F 1 hybrids and the parents were evaluated for rough rice yield, head rice percentage, and head rice yield in replicated field trials. Very little heterosis was detected among hybrids for head rice percentage, whereas heterosis was high for rough rice yield and head rice yield. A survey for DNA polymorphisms with 82 RFLPs and 26 microsatellites revealed high levels of variation among the eight parents. A cluster analysis resolved these eight lines into indica (one line) and tropical japonica (seven lines) rice subgroups. A majority of the markers detected statistically significant effects on one or more traits by an analysis of variance. Marker F 1 ,heterozygosity was highly correlated with rough rice yield (0.79 ** ) and head rice yield (0.82 ** ), and was also significantly correlated with heterosis of these two traits (0,47 * , 0.58 ** , respectively). However, the correlations were largely attributed to the concurrence between high levels of heterozygosity and high performance in crosses between the indica variety Jasmine 85 and the remaining seven lines. Compared with the data published in previous studies, it was concluded that the level of correlations between marker distance and hybrid performance is dependent on the germplasm used.

Grain weight in wheat cultivars released from 1920 to 1990 as affected by post-anthesis defoliation

Abstract: Although it has been generally recognized that the difference in yield potential amongst wheat cultivars released in different eras is related to differences in their reproductive sink strength, there have been few investigations about changes in source-sink ratios as a consequence of wheat breeding. In the present study, two field experiments, in which plots were fertilized and irrigated and lodging and diseases were prevented, were carried out with seven cultivars (including a commercial hybrid) representing different periods of plant breeding in Argentina from 1920 to 1990. The cultivars were defoliated during post-anthesis to analyse the response of grain weight at particular positions within the spike (which have intrinsic differences in potential size). Individual grain weight was virtually unaffected by defoliation in the old cultivars, but modern cultivars exhibited a significant reduction in individual grain weight for several positions within the spike, although this reduction was small (c. 15 %) and many grains were unaffected. In addition, no relationship was found between individual grain weight in the controls and its reduction due to defoliation. We concluded that if the source-sink ratio is further reduced, the grain yield of modern wheats will be simultaneously limited by the source and the sink. Future breeding should therefore attempt to improve simultaneously both sink and source strengths.

Gene flow among maize landraces, improved maize varieties, and teosinte: Implications for transgenic maize

Abstract: Proceedings of a forum organized by the Mexican National Institute of Forestry, Agriculture and Livestock Research (INIFAP), the Mexican National Agricultural Biosafety Committee (CNBA), and the International Maize and Wheat Improvement Center (CIMMYT) and held at CIMMYT headquarters, El Batán, Mexico, in September 1995, the publication carries presentations and discussions involving some 20 biotech and genetic resource specialists — including Mexican and invited foreign experts — on the distribution of teosinte in Mexico, the frequency and intensity of gene flow between maize and teosinte, the flow of genes from improved maize to landraces, and regulations and risk assessment for the release of transgenic maize in the center of origin of this crop and teosinte. The publication is targeted to researchers in genetic engineering, private seed company representatives, policymakers in Mexico and other countries wherein are found centers of genetic origin for crop species, and farmers who may eventually wish to sow transgenic seed. AGROVOC Descriptors: Zea mays; Maize; Euchlaena mexicana; Plant breeding; Transgenic plants; Gene transfer; Genetic engineering; Mexico AGRIS Category Codes: F30 Dewey Decimal Classification: 633.1523 ISBN: 968-6923-53-5

Induction of small-segment-translocation between wheat and rye chromosomes.

Abstract: A new approach to produce wheat-rye translocation, based on the genetic instability caused by monosomic addition of rye chromosome in wheat, is described. 1 283 plants from the selfed progenies of monosomic addition lines with single chromosome of inbred rye line R12 and complete chromosome complement of wheat cultivar Mianyang 11 were cytologically analyzed on a plant-by-plant basis by the improved C-banding technique. 63 of the plants, with 2n = 42, were found containing wheat-rye translocation or substitution, with a frequency of 4.91%. Compared with the wheat parent, other 32 plants with 2n = 42 exhibited obvious phenotypic variation, but their component of rye chromosome could not be detected using the C-banding technique.In situ hybridization with a biotin-la-beled DNA probe was used to detect rye chromatin and to determine the insertion sites of rye segments in the wheat chromosomes. In 20 out of the 32 variant wheat plants, small segments of rye chromosomes were found being inserted into different wheat chromosomes and form small-segment-translocation (SS translocation). The physical mapping of the translocated small segments of rye chromosomes indicated that alien insertion could occur in both the terminal and intermediate regions of wheat chromosomes. The technique described appeared to be an effective means to induce SS translocation. The wide application of SS translocation in the study of molecular cytogenetics and plant breeding is also discussed.

Improving Phaseolus genotypes for multiple cropping systems

Abstract: Common bean, Phaseolus vulgaris L. is intercropped or relay cropped with maize in many Andean highlands of Colombia and Peru. Breeding beans for the target multiple cropping systems is essential for the development of productive and sustainable agriculture for the Andean smallholders. Outline of the breeding programme should follow the farming system approach with the establishment of on-farm trials and early farmers involvement. Bean breeding is oriented to minimize intercrop competition and to stabilize complementarity with maize. Genetic traits needed for improved varieties are divided as follows : traits not interacting with the cropping systems, traits specific to intercrops and traits related with socioeconomic and seed quality aspects. Screening, prebreeding and recombination nursery are better made under sole cropping while varietal improvement and on-farm trials are conducted under the target multiple cropping systems. Breeding schemes may involve recurrent, pedigree and bulk hybrid selection. The given application concerns the genetic improvement of P. coccineus, P. polyanthus and interspecific hybrids of P. vulgaris for both simultaneous and relay intercropping in Colombia and Peru. Earliness, cold tolerance, resistance to fungus diseases (mainly Ascochyta leaf blight and anthracnosis) and seed yield potential were the major objectives of the bean improvement programme. Priority has been given to the exploitation of the large diversity available in the secondary gene pool of common bean.

Haploid production of Australian wheat (Triticum aestivum L.) cultivars through wheat£maize (Zea mays L.) crosses

Abstract: In order to reduce the labour for wheat haploid production through wheat maize crosses, several emasculation methods were investigated in combination with the ‘spike culture method’. Although the standard method whereby wheat spikes were hand-emasculated and pollinated on the day of anthesis gave a higher efficiency, the ‘non-emasculation method’ gave a comparable response. The use of the non-emasculation method and spike culture could eliminate much of the labour required for emasculation and treatment with 2,4-D, which is normally applied by injection into wheat internodes or by dropping onto florets after pollination with maize. Most of the selfed seeds were easily identified by the presence of endosperm, and the probability of contamination by the embryos originating from selfing among the presumptive haploid embryos in the non-emasculation method was very low. Twenty-seven Australian wheat cultivars were investigated for haploid production through wheat × maize crosses using the non-emasculation and spike culture methods. All of the 27 cultivars produced embryos after crossing with maize, with a mean efficiency of 33·1% (embryos/florets). Except for one cultivar, Tincurrin, plants were recovered from all of the cultivars (average of 61·5% including Tinccurin). This innovation of haploid production through wheat maize crosses is discussed.

A cytogenetic approach to the improvement of aluminium tolerance in wheat

Abstract: SUMMARY High levels of soil aluminium place serious constraints on wheat production on acidic soils, especially in the tropical areas of Africa and South America. Conventional plant breeding has improved the tolerance of the wheat crop, but available genetic variation is limited. The wild relatives of wheat provide a valuable gene pool for the introduction of further genetic variation. One wild species, Aegilops utniaristata Vis. (2n = 2x = 14,NN), is being utilized as a new source of tolerance. Of the addition lines of individual N genome chromosomes of A. utniaristata to wheat (Triticutm aestivutm L.) which have been established and characterized, chromosome 3N has been shown to confer tolerance to wheat. The three substitution lines in which 3N replaces the homoeologous wheat chromosomes, 3A, 3B or 3D, have also been produced. Growing plants to maturity in a low pH/high Al hydroponics system confirmed that chromosome 3N conferred tolerance to the substitution lines as well as to the addition line. By manipulating the genetic control of homoeologous chromosome pairing, chromosome 3N is being recombined with its wheat homoeologues in order to introduce a smaller alien segment which carries the gene(s) for tolerance but not the agronomically unacceptable brittle rachis gene also carried on chromosome 3N.

Triticale and barley for grain and for dual-purpose (forage+grain) in a Mediterranean-type environment. I. Growth analyses

Abstract: Field experiments were conducted for 2 growing seasons (1992 and 1993) at 2 sites in north-eastern Spain under irrigated conditions and high soil fertility. Two 6-rowed barley varieties, 3 spring triticales, and 2 winter triticales were evaluated for grain yield and for forage and grain production in the same cropping season. Forage was cut when the first node was detectable, and grain was harvested at ripening in both cut and uncut plots. Barley, spring triticale, and winter triticale did not differ in biomass at cutting. The number of tillers per plant at the beginning of jointing was about 3·2 in both barley and winter triticale, and 0·7 in spring triticale. Almost all of the biomass components at cutting were positively and significantly correlated with forage yield. Changes in dry matter accumulation and leaf area index and its components in the uncut treatment fitted accurately to the same logistic curve. The maximum number of living leaves per plant was reached between the beginning of jointing and booting in barley and spring triticale, and around jointing in winter triticale. The number of living tillers per plant at anthesis was significantly higher in barley than in triticale. The number of spikes per plant at anthesis was significantly lower in spring triticale than in barley and winter triticale. The efficiency of the plant to accumulate dry matter was greater in triticale than in barley. In barley, grain filling in both cut and uncut harvesting treatments was mainly dependent on current photosynthesis after anthesis. In triticale, which was more affected by terminal abiotic stresses, both photosynthesis and translocation of assimilates contributed to grain filling, independent of the harvesting treatment.

Diallel analysis for seed-cotton yield and its contributing traits in upland cotton (Gossypium hirsutum)

Abstract: Four upland cotton (Gossypium hirsutum L.) varieties were genetically analysed for gene action controlling the phenotypic expression of seed-cotton yield and its related traits. Additive type of gene action with partial dominance was observed for boll number/plant, boll weight, seed-cotton yield and seed index. Epistatic effects were also Involved in the expression of all the characters except boll weight.

Genetic analysis of variation for grain yield and protein concentration in two wheat crosses

Abstract: Grain yield and protein concentration are two of the more important criteria for wheat breeding in Queensland. Three aspects of the inheritance of both of these traits can have an impact on achieving genetic progress: (i) the magnitude and form of the genetic correlation between the traits, (ii) the magnitude of genetic variation and genotype × environment interactions, and (iii) the importance of epistasis in genetic variation. These 3 factors were examined for 2 crosses in a multi- environment trial conducted in Queensland in 1989. Negative genetic correlations were found between grain yield and protein concentration in both crosses. Genetic variation and genotype × environment interactions were found to be important for both traits. There was little evidence for the existence of significant additive × additive epistasis for either trait and the genotype × environment interactions were predominantly additive × environment in nature. From both crosses, progeny combining the high yield and high protein levels of the parents were identified. This suggests that there was a degree of independent segregation of the genes controlling grain yield and protein concentration in both crosses. Therefore, simultaneous genetic progress for yield and protein concentration is possible in Queensland environments.

Genetic engineering of crop plants : From genome to gene

Abstract: Genetic engineering of crop plants has been in progress since the dawn of agriculture, about 10 000 years ago For millennia the genetic make-up of our crop plants has been changed by mankind's selection of naturally occurring variants As the trade routes were developed, novel plant types were introduced into new environments and provided more variation from which to choose At the end of the nineteenth century an understanding of the laws of heredity was gained and plant breeding protocols were devised whereby selection became accompanied by deliberate crossing As the knowledge of the genetic structure of crop plants improved, new ways of manipulation were invented and exploited Indeed plant breeding became a testing bed for new ideas in genetics For the plant breeder the techniques which were most widely employed in the past were those which aided breeding, for example techniques which speeded up the production of new varieties, but still used traditional routes of crossing and selection This was a transitional phase between plant breeding as an art and plant breeding as a science

Gene-for-gene relationships between strawberry and the causal agent of red stele root rot, Phytophthora fragariae var. fragariae

Abstract: Red stele (red core) root rot is the major soil-borne disease of strawberries (Fragaria spp) in many areas with cool, moist soil conditions It is caused by the soil-borne fungus Phytophthora fragariae var fragariae Red stele is a quarantine disease in Europe with a zero tolerance for commercial stock plants Any lot of such plants with even just a trace of the disease has to be destroyed The plot on which the plants were harvested has to be abandoned for commercial strawberry propagation forever Consequently, this disease is not only a menace to growers, but also to nurseries Red stele has been successfully combatted by resistant cultivars in the USA and Canada In Western Europe, no cultivars with an effective resistance are available The Centre for Plant Breeding and Reproduction Research (CPRO-DLO) in The Netherlands initiated the breeding for resistance in 1968 and a number of advanced resistant selections has been produced Their horticultural value remained behind that of the successful CPRO-DLO cultivar Elsanta, so none of them was as yet released as cultivar The breeding for resistance has thus far been complicated by a general lack of insight in the genetics of resistance as well as the absence of a reliable resistance test The research described in this thesis aimed to increase opportunities to create elite cultivars with red stele resistance by overcoming these complications Tests for resistance Resistance tests were developed for use under controlled environmental conditions Their main innovation is their systematic account for incomplete as well as for complete resistance This is achieved by comparing the level of disease of the tested genotype with that of a universally susceptible reference cultivar The tested genotype is supposed to possess resistance when it is significantly less diseased than the reference cultivar This approach thus includes the introduction of a flexible disease threshold to distinguish resistance from susceptibility An other innovation deals with the assessment of segregation ratios for resistance in inheritance studies The percentage of genetically resistant descendants is estimated by adjusting the proportion of relatively healthy seedlings for susceptible descendants which escaped from (severe) infection These two innovations led to the identification of some individual resistance genes, including one for moderate resistance of the cultivar Cambridge Favourite Genetics of resistance A genetic model was developed It explains the resistance of strawberry cultivars to isolates of Pfragariae by the interaction of five resistance and five avirulence factors This so called gene-for-gene (GFG) model is similar to the one developed by Flor for the flax - Melampsora l ini relationship Inheritance studies on two of the five resistance factors revealed that each of them was based on a single gene, which were designated Rpf1 and Rpf2 This study thus showed that race specific resistance in strawberry for P fragariae is single-gene based The model is the first GFG-model for a soil-borne fungus, while its genes are the first identified resistance genes in strawberryThe establishment of a gene-for-gene relationship is a major step in the elucidation of the genetics of resistance to P fragariae The proposed GFG-model can improve the efficiency of breeding programs since it allows the combining of consciously chosen resistance genes into cultivars The model makes it also possible to develop a universally applicable differential series of strawberry genotypes, which series is essential for the identification of fungal racesThe tests for resistance and the genetic model as proposed in this thesis can be a positive contribution in the breeding of red stele resistant cultivars, due to which they are fully incorporated into the strawberry breeding program at CPRO-DLO The results of this research can thereby ultimately lead to a more reliable, paying, and environmentally acceptable culture of strawberry in red stele infested areas

Producing Genetic Diversity in Crop Plants: The Case of Canadian Rapeseed, 1954–1991

Abstract: The paper develops a theoretical framework to explain changes in crop genetic resources. For this purpose the genetic diversity of crop plants is treated as a social/natural co-construct of human beings in particular historical settings. To illustrate the potential of such an approach, the history of rapeseed (Brassica rapa L. and Brassica napus L.) breeding in Canada from 1954 to 1991 is examined. Pedigree and cluster analyses are used to evaluate the genetic diversity of rapeseed cultivars as well as its change through time. Relationships between the genetic diversity of crop plants and the ecological sustainability of agriculture are discussed.

Dna markers for disease resistance breeding in peas (pisum sativum l.)

Abstract: Introducing resistance genes through plant breeding remains an important and effective means of protecting plants from diseases. Plant species commonly carry genes for disease resistance within their collective germplasm base. Disease resistance can either be monogenic (ie. encoded by a single gene) or quantitative (ie. encoded by a number of genes). The first step in disease resistance breeding is to identify accessions carrying the resistance phenotype. The conventional process of breeding for resistance involves making controlled crosses and selecting sexual progeny for improved disease resistance. While relatively straight-forward for dominantly-inherited single gene resistance, this process becomes progressively more difficult and time-comsuming for resistance encoded by single genes with recessive inheritance and for quantitatively inherited resistance. Through the process of genetic linkage mapping, molecular markers which are linked to disease resistance genes can be identified, and these can then be applied in plant breeding programmes to assist in resistance gene introgression. Our research group has identified DNA tags for a number of genes for resistance to diseases affecting peas (Pisum sativum L.). For example, DNA markers linked to recessive genes for resistance to pea seed-borne mosaic virus (PSbMV) pathotype P-1 (Timmerman et al. 1993) and to powdery mildew fungus (Timmerman et al. 1994) have been identified. These genes are termed sbm-1 and er-1, respectively. In more recent research, we have identified markers linked to the dominantly inherited gene for resistance to pea enation mosaic virus (PEMV). This gene is termed En. The molecular markers linked to these three monogenic disease resistance genes have been applied in a field pea breeding programme to develop germplasm containing multiple disease resistance phenotypes. DNA tags linked to sbm-1 and er-1 have been used in conjunction with limited direct testing for disease resistance. Although widely distributed throughout the world, PEMV does not occur in New Zealand; therefore breeding for resistance to PEMV requires the use of overseas disease testing or DNA tags. The DNA tags linked to En have been used in the early stages of cultivar development without direct testing for disease resistance. The genes contributing to quantitatively inherited disease resistance can also be characterised using linkage maps and DNA markers and DNA tags can be developed (Michelmore 1995). Experiments to map the genes for resistance to Ascochyta blight of peas are currently underway, using a QTL mapping experimental design. Similar experiments have been carried out by our research group to map genes for two other quantitatively inherited traits, seed weight (Timmerman-Vaughan et al. 1996) and green seed colour (McCallum et al. 1997). Ascochyta blight is a serious disease of peas which is caused by a trio of fungal pathogens: Mycosphaerella pinodes, Ascochyta pisi and Phoma medicaginis. Accessions with improved resistance to Ascochyta blight have been identified among peas bred by the Crop & Food Research field pea breeding programme. Using these accessions as the resistant parents, large populations of F 2 progeny and their descendants have been developed. The genotypes of these segregating