Resistance to biotic stress in fruit trees
Resistance to pests and diseases is a highly desirable objective in fruit breeding but is a long-term endeavor. It requires appropriate tests for early screening and an analysis of host-parasite interactions for durability of resistance. Host resistance is considered durable when it remains effective for a long period despite an intensive exposure to the parasite. Durability has to be the key-point of any fruit breeding programme for resistance. Non-durable resistance is usually associated with a hypersensitive reaction (HR) and major gene resistance which often occur in multiple allelic series and/or complex loci. HR resistance or race-specific resistance is often rendered ineffective because the pathogen population shifts genetically to avoid the resistance gene. Resistance of a more durable nature against fungi and bacteria is often polygenic and based upon the additive or interaction effects of a few to several genes not promoting a hypersensitive reaction. The more complex the genetics of the resistance, the more difficult it is for the pathogen to evolve to a form that can overcome the resistance. Polygenic resistance is usually associated with a quantitative expression that is characterized by a continuous variation between susceptibility and resistance. To approach the number of genes involved in polygenic resistance (QTL) and the relative effect of each of these genes, they have to be mapped on the genome. Marker-assisted selection (MAS) gives the possibility to recombine QTL for resistance; major genes could be associated with QTL. The combining of resistance genes is desired to: acquire a higher level of resistance against a particular pathotype, obtain resistance against a broader range of pathotypes, and combine different mechanisms of resistance. The current situation of fruit crops - monocultivar orchards established for several years - enhances the selection pressure on the pathogen population and results in population shifts which could be dramatic. Guidelines have to be proposed to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure; the most important parameters to consider are reproductive/mating systems and gene/genotype flow. Pathogens that have the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. Several examples of different strategies used by fruit breeders will be reported mainly on Coffea (coffee rust), Musa (black leaf streak disease), Prunus (root knot nematodes) and Malus (scab, mildew, fire blight). Fruit breeders can use the lessons learned from deploying resistance genes in annual crops and selecting resistance in forest trees and apply these to their strategies for fruit tree improvement