Disease Resistance in Wheat: Present Status and Future Prospects

Abstract

Wheat is one of the most important cereal crops in the world. It is the main source of calories and protein for human consumption, especially in developing countries. Over 700 million tons of wheat is produced annually from around 215 million hectares of land worldwide. However, there is a continuous increase in demand for wheat owing to the rise in human population and changes in dietary habits resulting from rising economies in developing countries. Wheat has become an integral part of the daily diet in many developing countries, accounting for 20–60% of daily calorie intake; therefore, to meet the challenge of increasing demand, there is a continuous need to increase wheat yield and production. Recent reports have indicated that at least a 60% increase in wheat production will be required to meet the global demand for wheat by 2050. One of the major limiting factors in achieving increased yields is the yield losses caused by pests and diseases. Out of a documented list of 200 diseases and pests of wheat, about 50 have been considered significant in major wheat-growing regions of the world. Among these, fungal diseases rank the highest in causing considerable damage to wheat, thus hindering wheat production. Pathogenic fungi can be broadly classified as biotrophic or necrotrophic on the basis of their lifestyles. Rust pathogens and powdery mildews are classified as biotrophic, as they survive on living plants, whereas necrotrophs such as Fusarium head blight, Septoria tritici blotch, tan spot, spot blotch, Stagonospora nodorum blotch, and wheat blast can derive nutrition from dead tissue when live plants are not available. Overall, diseases and pests cause 13% actual losses in wheat. In the case of rust pathogens and powdery mildews, new races of these pathogens with new degrees of virulence continue to evolve, thus rendering resistance genes ineffective. Although populations of the pathogens that cause other nonrust diseases remain somewhat constant, there seems to be rather frequent evolution of rust races, largely due to their asexual or sexual modes of reproduction. Although race-specific genes have been deployed widely in breeding, this has resulted in frequent “boom and bust” cycles; therefore, deployment of combinations of race non-specific resistance genes is considered an alternative to enhance the durability of resistance. Overall, significant achievements in breeding resistant wheat cultivars have been made since the onset of the Green Revolution. However, there is still a need to explore and mobilize genetic diversity for disease resistance in modern wheat cultivars through use of advanced biotechnological interventions. Marker-assisted selection, genomic selection, and genome editing are promising tools. Continual emergence of new virulent races of pathogens should be monitored. A combination of molecular breeding and phenotypic evaluations is required for development of high-yielding wheat cultivars with resistance to multiple diseases. This chapter describes wheat diseases caused by biotrophic and necrotrophic fungi, the losses incurred, the current situation, and potential options for resistance breeding.