Breeding crop plants for disease resistance

Breeding Oat for Resistance to Diseases

Oat (Avena sativa L.) is an important cereal crop that is grown worldwide, whose productivity is often affected by fungal diseases. The major fungal diseases limiting oat production are crown rust, stem rust, Fusarium head blight, and powdery mildew. Control of these diseases has been achieved mainly via host resistance genes, but frequent changes in pathogen virulence provide a continuing threat to oat production. The identification of molecular markers, mainly diversity array (DArT) and single nucleotide polymorphism (SNP) markers, has enabled genetic mapping of disease resistance genes and quantitative trait loci (QTL). Markers linked to disease resistance genes have been used successfully in marker-assisted selection of disease resistant genotypes. In this chapter, we review genetic approaches to controlling the major fungal diseases of oat, and the role of molecular markers associated with genes important in crop improvement.

Oat Fungal Diseases and the Application of Molecular Marker Technology for Their Control

In breeding for resistance, specific resistance is not the ideal to aim at. Generalized resistance to biotypes of the pathogen is the ultimate goal but difficult to attain. The concept of evolving multiline hybrids to combat the ever increasing threat from bioraces, especially in the cereal rusts, now engages the attention of plant breeders. Locating gene centers as the repositories of greatest wealth of hereditary variants, displaying naturally inbred characteristics, are indispensible as gene pool for ongoing breeding programmes and development of “Population resistance” or “Synthetic horizontal resistance”. Two concepts of breeding for Vertical Resistance (VR) or for Horizontal Resistance (HR), the former conditioned by oligogenes and the latter by polygenes, are invaluable in evolving resistant varieties that can slow down epidemics and offer protection. VR confers complete but impermanent protection, whereas HR confers incomplete but permanent protection. A point of interest is that differentially, pathogen races, differ in virulence and other races differ in aggressiveness. A race with unnecessary virulence is less fit to survive on a strong resistance gene host than a weakly virulent one. With the discovery of ‘Phytoalexins’ producedin vivo in host under pathogenesis, which are essentially phenolics, with a possible role in initiating defence mechanism in plants, the search is on for similar substances in resistant cultivars. Enhanced synthesis of lignins are also considered as a means of building up resistance in plants to pathogens. Plant pathologists have been looking for genetic markers in the many end-products of biosynthesis that could resist pathogenic invasion of tissues. Alterations in respiratory rates and oxidase activity, in general under pathogenesis, have received much attention. More recently, germination of a fungus spore has been shown to be accompanied by mRNA synthesis with subsequent changes in ribosomal activity. It has been suggested that host-parasite interaction could be between host genes and infective RNA. One of the major roles of activated oxidative processes could be that of detoxificationin vivo of the pathogen’s toxin, a characteristic of resistant cultivars so far examined. The present trend may lead on to a better understanding of the much debated question as to whether natural immunity exists in plants against specific pathogens.

Breeding for disease resistance in plants

Development of resistant varieties has been receiving more attention recently than in earlier times. It has been reported that 15% of crop losses is due to attack by insect-pests, despite the use of huge amounts of pesticide. In the 19th century, the French wine industry suffered a major setback when its vines were attacked by grape Phylloxera and being forced to resort to the use of a resistant root stock imported from America, which still maintains resistance levels against the aphid-like pest. The increase in insect injury that has occurred recently may be due to the more extensive cultivation of crop varieties over wider areas. Although many of the established field crop insects are well known, their prevalence is now felt because of the increased acreage of crops, vegetables, fruits, etc. This has led to a greater interest in developing resistant varieties and this is the best method of insect control in crop plants. Such varieties possess the ability to give a higher yield of good quality than susceptible varieties at the same level of insect population. The chapter deals with various aspects of breeding for disease and insect-pest resistance.

Breeding crop plants for disease resistance

Citations

Breeding Oat for Resistance to Diseases

Oat Fungal Diseases and the Application of Molecular Marker Technology for Their Control

Breeding for disease resistance in plants

Breeding for Disease and Insect-Pest Resistance