Improving plant resistance to biotic stresses

Breeding for disease and pest resistance is one of the primary objectives of breeding programmes. It requires an understanding of parasite biology and ecology, disease cycles and drivers influencing the evolution of plant-pathogen interactions because, unlike other traits, pest resistance is influenced by genetic variability in the pest population, especially in diseases. With evolving pathogen populations and changes in fitness favouring new pathotypes, as a result of climate change or not, the continuous improvement of resistance to biotic stresses is paramount in maintaining yield potential and genetic gains. Resistance is essential for food security in economies where farmers cannot afford to use chemical control, and increasingly in advanced countries where the reduction in authorized active ingredients on the market, due to environmental concerns of the public and policy makers, has meant that farmers have to rely more on host resistance. There are numerous examples documenting the progress in host resistance in many crops. Sayre et al. (1998) demonstrated the impact of breeding for leaf rust resistance over time using a set of Mexican wheat cultivars released between 1966 and 1988. Data showed that while yield potential (yield with fungicide applied) had increased significantly (0.52%/year), progress protecting the yield potential due to incorporation of leaf rust resistance genes (yield without fungicide) was higher (2.1%/year). Progress in biotechnology, particularly marker assisted selection, will contribute to making breeding for resistance against difficult traits more efficient. Tactics and methods might change, depending on the pathosystems, but breeding for durable resistance is perhaps the major objective of plant breeders. Although durable resistance can be confirmed only after a cultivar has been grown on a large scale for a relatively long time, it is generally accepted that it is more likely to be achieved by breeding for non-race-specific resistance and the accumulation of minor genes conferring partial resistance. There has been a major genetics and breeding emphasis in recent decades on slow-rusting, minor-resistance genes with additive effects against leaf and yellow rust pathogens in wheat. The use of Sr2 and Lr34 and minor genes in controlling stem rust and leaf rust in wheat illustrates this approach (Singh et al., 2000). A study with Lr34 isolines showed yield losses of approximately 15% associated with leaf rust infection in the presence of the genes, while when Lr34 was absent losses were 40-85%, depending on planting date (Singh and Huerta-Espino, 1997). Gene pyramiding and the deployment of major genes could offer an option for a rapid response against a new threat. However, these remain controversial because although the selection of new complex races is possible, the effect might not last in the case of a rapid race evolution. The development of resistant material against the Sr24 virulent variant of Ug99 confirms the value of breeding strategies based on minor genes, as demonstrated by the development of new genotypes (Singh et al., 2009).

In the context of climate change, breeding for resistance against several pathogens should not be disconnected from improving resistance to abiotic stresses, particularly for water-use efficiency and heat tolerance, because abiotic stress factors could enhance the disease effect. Spot blotch of wheat is more severe under heat stress, and therefore improving yield potential and heat tolerance, particularly to night heat during grain filling, should contribute to lower disease losses (Sharma et al., 2007). It is also likely that improving root systems and drought tolerance could increase resistance to soil-borne foot rot diseases. Breeding priorities in a given geographical region might be evolving as new crops and systems are introduced. Stubble-borne diseases such as tan spot (P. tritici-repentis), Fusarium ear rot or Septoria leaf blotch in wheat receive more attention in areas where reduced tillage is being adopted.

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