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Abstract
We hypothesize that the evolutionary potential of a pathogen population is reflected
in its population genetic structure. Pathogen populations with a high evolutionary
potential are more likely to overcome genetic resistance than pathogen populations
with a low evolutionary potential. We propose a flexible framework to predict the
evolutionary potential of pathogen populations based on analysis of their genetic
structure. According to this framework, pathogens that pose the greatest risk of breaking
down resistance genes have a mixed reproduction system, a high potential for genotype
flow, large effective population sizes, and high mutation rates. The lowest risk pathogens
are those with strict asexual reproduction, low potential for gene flow, small effective
population sizes, and low mutation rates. We present examples of high-risk and low-risk
pathogens. We propose general guidelines for a rational approach to breed durable
resistance according to the evolutionary potential of the pathogen.
Fusarium oxysporum is an anamorphic species that includes both pathogenic and nonpathogenic strains. Plant pathogenic forms cause a wilt disease and are grouped into formae speciales based on their host range; some are further subdivided into pathogenic races. Many formae speciales are comprised of multiple clonal lineages and, in some cases, a pathogenic race is associated with more than one clonal lineage, suggesting independent origins. Although some evidence suggests one pathogenic race may give rise to another, recent derivation of a pathogen from a nonpathogen has not been documented. Most new occurrences of Fusarium wilt appear to be the result of a recent introduction rather than an independent local origin of the pathotype. Asexual propagation is the dominant influence on population structure in F. oxysporum and the absence of sexual reproduction is not likely to prevent this pathogen from continuing to inflict significant damage on susceptible crop hosts.