APPLE AND SCAB: GENE EMPLOYMENT STRATEGY FOR THE TWO PARTNERS IN NATURE -- IMPLICATIONS FOR CONTROL
C GESSLER and T KOCH
Institute of Plant Sciences -- Pathology, Swiss Federal Institute of Technology, 8092 ETH-Zurich, Switzerland
Background and objectives
Early this century researchers observed that particular cultivar were susceptible in one site and resistant at an other site and the fungus could adapt to cvs which were resistant. By the middle of this century genetic analysis become popular and genes for pathogenicity (what we would call today virulence/avirulence genes) were identified which were able to selectively overcome resistance genes in crab apple and particular selection of American domestic apple. However the notion that all "modern" domestic apple cv. are susceptible to scab and therefore lack resistance genes, found wide consensus. In the last decade researchers rediscovered the ephemeral resistances present in domestic cv. In this paper we try to assign a role in nature to these resistances and to the corresponding virulences.
Material and methods
Results and conclusions
Apple is an obligate outbreeder (self-incompatible) and therefore in natural ecosystems resistance genes are recombined differently in each tree. This natural variation of resistance gene pattern will give the offspring a better chance to survive scab attack as the pathogen would need the maching combination of virulence genes. The pathogen inoculum at the beginning of the season comes predominantly from ascospores, product of the sexual recombination, which are randomly distributed. Since they cannot choose to land on the tree to which they are best adapted, the best strategy for the pathogen is to create a large array of various pattern of virulences. Even if a large part of the ascospores will not have the maching virulence pattern, some ascospores will have it and guarantee the survival of the pathogen. In monoculture, only one specific virulence is needed for successful reproduction. As all genotypes present will have it, also all ascospores will be capable of infection. If nature used variation in resistance gene composition in each individual to reduce scab incidence, this concept could be used also in designing orchards.