Genetic diversity and disease resistance: some considerations for research, breeding, and deployment

Fungal pathogens present a complex challenge for genetic management of forest trees. The need is for disease resistance that withstands mutations and genetic shifts in pathogens. Also desirable are defences against new and dangerous pathogens. An understanding of how hosts and pathogens can continue to coexist should help. Experience from agriculture has allowed modelling of pathosystems, the genetic variations within hosts and pathogens that permit coexistence. While it is impractical to construct a comprehensive model, two phenomena seem generally conducive to stability: a cost of virulence in pathogen fitness and a multiplicity of host resistance mechanisms. However, other factors, notably indirect costs of resistance, are very difficult to model. Overall, the diversity of behaviour of models, of the nature of resistance and virulence genes, and of biology of both hosts and pathogens precludes any unique formula for stability. For current crops, genetic diversity offers risk spread for susceptibility to a new and serious pathogen or pathotype. For longer-term breeding, relatively rare resistance may be useful, but pedigreed breeding populations typically entail very finite population sizes. Providing for selecting within improved production populations may therefore be needed. This would give breeders technical challenges, and give forest managers opportunity costs and major logistical challenges.