Breeding for resistance to rhizomania in sugar beet: A review

Currently rhizomania is the most important disease in sugar beet worldwide, and attack can lead to serious yield losses. The disease is caused by beet necrotic yellow vein virus (BNYVV) that is transmitted by the soil-borne fungus Polymyxa betae. Breeding sugar beet cultivars with resistance to rhizomania is regarded as the most appropriate way to enable continued production of this crop in BNYVV-infested fields and also to slow the spread of the disease. Breeding for resistance started with selection by scoring disease symptoms in field experiments. The development of non-destructive greenhouse tests, with determination of the virus concentration in rootlets using ELISA, has greatly improved the efficiency of selection. In this paper the impact of scientific research on the progress in breeding cultivars with resistance to rhizomania is reviewed. This includes the distribution, composition, and pathogenicity of the virus, the sources of resistance to virus and vector, the genetics of virus resistance, progress with breeding methods, and the use of molecular markers and pathogen-derived resistance. The yields and quality characteristics of recently introduced resistant cultivars now equal those of the commercial susceptible cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Potato germplasm enhancement for resistance to biotic stresses at CIP. Conventional and biotechnology-assisted approaches using a wide range of Solanum species

Summary Potato genetic improvement has been facilitated using new knowledge of potato reproductive biology and new techniques. Many wild diploid species as well as landrace cultivars have been used in breeding at the diploid level, a strategy which is supported by 1) 2n gametes and 2) haploids from tetraploid cultivars. Different categories of wild species which have been under-utilized are now being exploited further in more systematic enhancement programmes using semi-conventional and biotechnological methods. Molecular maps of the potato genome are used actively to achieve marker-assisted introgression and improved selection among the germplasm collections to facilitate the use of valuable wild genetic resources. As an alternative method to incorporate a high level of fesistance, genetic engineering has been employed to facilitate the initial breeding process using various gene constructs for controlling major biotic stresses in the world.     

Genetic resources in apple,pear and vineyard peach populations in former Yugoslavia

Summary During the period 1988–90, several germplasm collecting trips were made to all republics of former Yugoslavia. A total of 56 old apple cultivars, many of which are represented in up to 5 types, 38 old pear cultivars and 367 genotypes of vineyard peaches were collected. The availability of so much genetic and genotypic wealth made it possible to start apple disease resistance breeding and peach cultivar and rootstock breeding programmes.     

Exploitable genetic variation in a composite bulk population of barley

Summary Advanced agricultures are characterized by a great increase in the adoptation of uniform crop cultivars associated with a drastic reduction in locally adapted variation, the plant breeders should also examine means of creating and conserving genetic resources. Composite cross breeding is a technique that creates and preserves genetic variation in an exploitable form. The merits and problems of this method were discussed. using data obtained from barley Composite Cross XXI.     

Screening fruit tree genetic resources in Belgium for disease resistance and other desirable characters

Summary The wide diversity of old fruit-tree cultivars originating or introduced into Belgium during the 18 ^th and 19 ^th centuries was collected as far as feasible over the last fifteen years at the State Plant Pathology Station in Gembloux. Out of the 2400 accessions now collected, one quarter was recovered from old public collections, and three quarters came from farms or gardens. The initial intention was to screen the material for disease resistance and other characters of agronomic interest with a view to using the best cultivars as breeding parents. However, as the collection developed, genetic resources conservation also became an objectiveper se. The collection presently contains 1150 apple, 850 pear and 300 plum accessions, and smaller numbers of other fruit species. Each accession is evaluated in an experimental orchard for at least ten years. In view of the growing public interest in old fruit-tree cultivars, the Plant Pathology Station has for several years been releasing to the nursery trade the better cultivars emerging from the evaluation, namely nine apple and four plum cultivars, and one peach cultivar. The principal features of the apple cultivars are presented in this paper. Since 1988, old apple and plum cultivars have been being used at the Station as parents in a breeding programme, with both controlled and open pollination. In some instances, old apple cultivars have also been crossed with a modern parent carrying the Vf gene for scab resistance. The preliminary observations on some of these seedlings are presented.     

The role of selection on the genetic structure of pathogen populations: Evidence from field experiments with Mycosphaerella graminicola on wheat

Summary Coevolution refers to reciprocal genetic changes that occur in two or more ecologically interacting species. In agricultural ecosystems, we are especially concerned with the genetic response of pathogen populations to resistant cultivars produced by plant breeding programs. It would be useful to be able to predict whether disease resistance is likely to be durable or ephemeral before a cultivar is widely grown. Though it may not be possible to predict durability in advance, knowledge of the genetic structure of pathogen populations may prove useful for making predictions about the rate at which pathogens adapt to resistant varieties. Much has been learned about the genetic structure of populations of obligate fungal pathogens such as rusts and mildews, which have become paradigms for plant pathology. We have focused our effort on the population genetics of the less known, non-specialized, necrotrophic pathogens, such as the Septorias of small grains. Our approach has been to use DNA fingerprinting and RFLP analysis to conduct field experiments that elucidate how populations of fungal pathogens adapt in agroecosystems. Our results suggest that mating system may have a greater impact than natural selection on the genetic structure of populations of Mycosphaerella graminicola (anamorph Septoria tritici).     

Genetic improvement and breeding of tea plant (Camellia sinensis) in China: from individual selection to hybridization and molecular breeding

Tea is an important revenue source for the tea producing countries in the world, including China. China is the place of origin of tea plants, it has the broadest genetic variations in the world. It also has bred more than 200 improved cultivars. The improved cultivars have made important contribution in the tea industry. In this paper the achievements of tea genetic improvement and breeding, the current situation of collection, conservation, appraisal and evaluation of tea germplasms, the establishment and development of tea breeding system were reviewed. The main research emphases for the genetics and breeding of tea plant in the near future in China are proposed.     

Tissue culture-derived variation in crop improvement

Tissue culture generates a wide range of genetic variation in plant species which can be incorporated in plant breeding programmes. By in vitro selection, mutants with useful agronomic traits, e.g. salt or drought tolerance or disease resistance, can be isolated in a short duration. The successful use of somaclonal variation is very much dependent on its genetic stability in the subsequent generations for which molecular markers such as RAPDs, AFLPs, SSRs and others can be helpful. The potential of somaclonal variation has yet to be fully exploited by breeders, even though a few cultivars have been developed in crops such as Brassica juncea, rice and others. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of T-DNA on the agronomic performance of transgenic flax plants

Summary Registration and commercial release of crop cultivars improved using recombinant DNA technologies will require extensive field testing of the transgenic lines. In addition to testing the efficacy of the transferred gene(s), regulatory bodies and farmers need data from large scale agronomic studies to provide assurance that the foreign DNA does not adversely affect normal agronomic productivity or quality of the crop. Here, five transgenic sulfonylurea resistant flax (Linum usitatissimum) lines were included in a multiple location cultivar registration trial, evaluated and compared with four standard commercial cultivars and 24 other (conventionally produced) breeding lines. The transgenic lines did not appear to be affected by T-DNA for any quality or agronomic parameter tested, including yield.     

Genetic variation in Zantedeschia spp. (Araceae) for resistance to soft rot caused by Erwinia carotovora subsp. carotovora

Bacterial soft rot caused by Erwiniacarotovora subsp. carotovora is amajor disease in Zantedeschia spp.,particularly in cultivars from the sectionAestivae. The disease can partiallybe controlled by cultivation measures, so acombination with resistant cultivars couldeffectively protect the crop. However,resistant commercial Aestivaecultivars are not available yet. By meansof a recently developed non-destructiveresistance test, variation inaggressiveness was observed among fiveisolates of Erwinia carotovora subsp.carotovora without interactionsbetween the isolates and three Zantedeschia accessions. Within elevenaccessions of Z. aethiopica,variation was observed from almost completeto moderate resistance, while theZ. odorata accession was susceptible.All 21 Aestivae cultivars weresusceptible. Within the Aestivaespecies, Z. elliotiana and Z.pentlandii were also susceptible, butwithin twelve accessions of Z.albomaculata, as well as in six accessionsof Z. rehmannii, variation was foundfrom susceptible to moderately resistant.Hence, new sources of resistance wereidentified that show good potentials forresistance breeding.     

Current status and future strategies of in vitro culture techniques for genetic improvement of mungbean (Vigna radiata (L.) Wilczek)

Summary Mungbean is an important source of vegetable protein for the growing population in many developing countries of South East Asia. Its production is limited due to its susceptibility to diseases and insect pests besides many other undesirable agronomic traits. Strategies for increasing and stabilising its production have been to develop varieties resistant to diseases, pests and with other desirable agronomic traits. Genetic improvement of this crop by classical breeding has met with limited success due to the lack of sufficient and satisfactory level of genetic variability within germplasm. Recent advances in biotechnology have offered the opportunity to develop new germplasms. The development of such technologies largely depends on efficient regeneration of sexually mature plants from organs, tissues and protoplasts. An overview of plant regeneration by direct or indirect organogenesis and embryogenesis is presented. The use of in vitro and molecular techniques such as somaclonal variation, screening for various desirable traits, interspecific crossing and genetic transformation to supplement conventional breeding, for genetic improvement of this crop is described. The advantages and limitations of these techniques along with directions for future research are discussed.     

作物驯化和品种改良所选择的关键基因及其特点

近15~20年作物基因组学迅速发展,特别是第2代测序技术的普及,显著降低了测序成本,使单核苷酸多态性(SNP)分析和单元型区段(也称单倍型区段)分析渗透到生命科学的各个领域,对系统生物学、遗传学、种质资源学和育种学影响最为深刻,使其进入基因组学的全新时代。一批驯化选择基因的克隆,特别是对一些控制复杂性状形成的遗传基础及其调控机制的解析,更清晰地揭示了作物驯化和品种改良的历史,提升了人们对育种的认知,推动育种方法的改进。驯化和育种既有相似之处,也存在明显的差异。驯化选择常常发生在少数关键基因或位点,对基因的选择几乎是一步到位;而现代作物育种虽然只有100年左右的历史,但其对基因组影响更为强烈,是一些重要代谢途径不断优化的过程。随着生态环境或栽培条件的变化,育种选择目标基因(等位变异)会发生相应的变化或调整,因此对基因(等位变异)的选择是逐步的。此外,强烈的定向选择重塑了多倍体物种的基因组,使其亚基因组与供体种基因组明显不同。在群体水平上,系统分析驯化和育种在作物基因组和基因中留下的踪迹,凝炼其中的规律,将为品种改良和育种提供科学理论和指导,本文也简要介绍了"十三五"国家重点研发计划专项"主要农作物优异种质资源形成与演化规律"的基本研究思路。     

Screening techniques and sources of resistance to foliar diseases caused by major necrotrophic fungi in grain legumes

Summary Necrotrophic pathogens of the cool season food legumes (pea, lentil, chickpea, faba bean and lupin) cause wide spread disease and severe crop losses throughout the world. Environmental conditions play an important role in the development and spread of these diseases. Form of inoculum, inoculum concentration and physiological plant growth stage all affect the degree of infection and the amount of crop loss. Measures to control these diseases have relied on identification of resistant germplasm and development of resistant varieties through screening in the field and in controlled environments. Procedures for screening and scoring germplasm and breeding lines for resistance have lacked uniformity among the various programs worldwide. However, this review highlights the most consistent screening and scoring procedures that are simple to use and provide reliable results. Sources of resistance to the major necrotrophic fungi are summarized for each of the cool season food legumes. Marker-assisted selection is underway for Ascochyta blight of pea, lentil and chickpea, and Phomopsis blight of lupin. Other measures such as fungicidal control and cultural control are also reviewed. The emerging genomic information on the model legume, Medicago truncatula, which has various degrees of genetic synteny with the cool season food legumes, has promise for identification of closely linked markers for resistance genes and possibly for eventual map-based cloning of resistance genes. Durable resistance to the necrotrophic pathogens is a common goal of cool season food legume breeders.     

Disease resistance in protected crops and mushrooms

Summary Cultivars of tomatoes, cucumbers, lettuce and peppers have been bred for resistance to one or more pathogens. Some tomato and cucumber cultivars have resistance to a wide range of diseases. Resistance has been transient in many cases and a succession of cultivars with new genes or new combinations of resistance genes has been necessary to maintain control. There has been a number of notable exceptions and these have included durable resistance to such pathogens asFulvia fulva and tomato mosaic virus. With lettuce the resistance situation is complicated by the occurrence of fungicide resistant pathotypes. There are no strains ofAgaricus bisporus purposely bred for disease resistance.In protected flower crops only resistance to Fusarium wilt in carnations has been purposely bred but differences in disease resistance are apparent in cultivars of many ornamental crops. This is particularly so in chrysanthemums where there are cultivars with resistance to many of the major pathogens. Similar situations occur with other flower crops and pot plants. Cultivars of some species have not been systematically investigated for resistance.The need for genetic resistance will increase with the further reduction, in the limits on pesticide use and an increasing public awareness and importance of pesticide pollution.ADAS is an executive agency of the Ministry of Agiculture, Fisheries and Food and the Welsh Office.     

Specific adaptation and breeding for marginal conditions

Summary Breeding has been very successful in generating cultivars that in favorable environments, and together with large use of fertilizer and chemical control of weeds, pest and diseases, have increased agricultural production several fold. Today the environmental impact of high input agriculture in more favorable environments causes growing concern. By contrast, the impact of breeding in marginal environments has been elusive. The paper discusses evidence showing that the use of breeding principles developed for, and successfully applied, in favorable environments may be the main reason for the lack of breeding progress in marginal environments. Very little breeding work has actually been done in marginal environments, although the theory of correlated responses to selection indicates that selection conducted in good environments or in well-managed experiment stations is not expected to be very efficient when genotype by environment interactions of a cross-over type exist. The assumptions that heritability is higher under good conditions and that there is a carry-over effect of high yield potential are not supported by experimental evidence. If the target environment is below the cross-over point, selection has to be conducted for specific adaptation to that environment. The concept of wide adaptation has more a geographical than an environmental meaning, and it reduces genetic diversity and increases genetic vulnerability. Eventually the issue of genetic heterogeneity versus genetic uniformity is discussed in relation to specific adaptation to marginal environments.     

Wheat grain yield and stability assessed through regional trials in the Eastern Gangetic Plains of South Asia

Improving the level and stability of grain yield is the primary objective of wheat breeding programs in the Eastern Gangetic Plains (EGP) of South Asia. A regional wheat trial, the Eastern Gangetic Plains Yield Trial (EGPYT), was initiated by CIMMYT in collaboration with national wheat research programs in Bangladesh, Nepal, and India in 1999–2000 to identify wheat genotypes with high and stable grain yield, disease resistance, and superior agronomic traits for the EGP region. A set of 21 wheat experimental genotypes selected from a regional wheat screening nursery in South Asia, three improved widely grown cultivars (Kanchan, PBW343 and Bhrikuti), and one long-term cultivar (Sonalika) were tested at 9–11 sites in six wheat growing seasons (2000–2005) in the EGP. The 21 experimental genotypes were different in each year, whereas the four check cultivars were common. In each year, one or more of the experimental genotypes showed high and stable grain yield and acceptable maturity, plant height, and disease resistance compared to the check cultivars. Three improved cultivars have already been commercially released in the region through EGPYT and many germplasm lines have been used in the breeding programs as parents. Identification of wheat genotypes with high-grain yield in individual sites and high and stable yield across the EGP region underlines their value for regional wheat breeding programs attempting to improve grain yield and agronomic performance.     

Variation in virulence and resistance in the bean-bean rust pathosystem in Ecuador

Twenty two Uromyces appendiculatus isolates were tested on 20 differential and 25 Ecuadorian Phaseolus vulgaris cultivars in the seedling stage. Based on the infection types 20 races could be discerned. The Ecuadorian cultivars differed greatly in their reaction to the isolates, from resistant to only one isolate (`Red Small Garden') to resistant to all isolates (’G2333‘). The isolates showed a wide range of virulence to the Ecuadorian cultivars, from virulent to only two cultivars (isolate 13) to virulent to 21 cultivars (isolates 5 and 23). Seven cultivars with a basically susceptible infection type appeared to differ greatly in quantitative resistance when tested in three consecutive crop cycles. The disease severities in percentage leaf area affected averaged over the three cycles ranged between 83.9% for ‘Red Small Garden’ and 13.1% for ‘INIAP-414’. Race-specific resistance does not seem an advisable breeding strategy, but the quantitative resistance offers a good alternative.     

Partial resistance to a Fusarium root disease in Egyptian white lupin landraces

A Fusarium sp. root pathogen of lupin is the causal agent of the most important disease that affects the cultivation of white lupin (Lupinus albus L.) in Egypt. The aim of our research was to investigate whether host resistance to Fusariumroot disease was available in Egyptian landraces ofLupinus albus. Five Fusarium isolates collected from white lupin samples in Egypt were tested with an Egyptian landrace and a French cultivar. The most aggressive isolate was used to screen an additional 15 Egyptian landraces, two cultivars released in Egypt after selection among landraces, one Polish cultivar, and two French cultivars. The assessment of host response to Fusarium was performed in a field, and under controlled conditions in a greenhouse pot experiment. Most landraces and the two Egyptian cultivars showed better resistance with good accordance between field and pot experiment. This experiment showed that Egyptian genetic resources of white lupin possess partial resistance to Fusarium root rot. Egyptian germplasm may be an alternative genetic source for incorporating partial resistance to Fusarium root rot into the breeding pools. This revised version was published online in July 2006 with corrections to the Cover Date.     

Past,present and future opportunities in breeding for disease resistance,with examples from wheat

Summary This introductory chapter contains some general comments about plant breeding and breeding for disease resistance. The use of disease resistant crop plants is an environmentally favourable method of controlling disease but the process of breeding for disease resistance is subject to several constraints. Among them is the variability of pathogens in relation to host resistance. Some parts of this variation can be resolved into gene-for-gene interactions, but the boundaries within which such interactions can be detected are not sharp. The discussion of this variation is illustrated by reference to some important diseases of wheat, especially yellow rust, septoria and eyespot. The objective of obtaining durable resistance is discussed and some contributions of new genetical and molecular techniques to breeding for resistance are considered. It is suggested that new technology will enhance breeding for disease resistance but that established techniques of plant breeding will remain relevant and important.