What is Durable Resistance, A General Outline

Summary

Resistance to pathogens may be durable for several reasons. i) The farming system prevents the building up of inoculum thus reducing the chances of adaptation in the pathogen population. ii) Pathogens differ in their versatility; especially biotrophic and hemi-biotrophic airborne fungi adapt easily to introduced resistances. iii) Non-durable, race-specific genes may last longer if certain strategies are applied, such as multilines or cultivar mixtures, gene deployment and multiple gene barriers. Neither of these possibilities are considered to be of much practical value. iv) Resistance genes themselves are durable; the pathogen cannot adapt to these genes at least not within short time spans. The genetically durable resistance is often of a quantitative nature. To discern small differences in resistance, good screening and assessment methods are necessary. Provided small differences in resistance can be measured free from confounding effects (interplot interference, earliness, tallness, irregular distribution of inoculum), one should select against susceptibility and against complete resistance, i.e. removal of the most susceptible entries and those which are virtually free of the pathogen (assumed to carry a major gene). Only against pathogens belonging to groups not known to form races easily, any resistance observed can be selected and used.