The dynamics of virulence evolution in vector-born plant pathogens can
be complex. Here we use individual-based simulations to investigate how
virulence evolution depends on genetic trade-offs and population
structure in pathogen populations. Although quite generic, the model is
inspired by the ecology of the plant-pathogenic bacterium Xylella
fastidiosa, and we use it to gain insights into possible modes of
evolution of virulence in that group. In particular, we aim to sharpen
our intuition about how virulence may evolve over short time scales in
response to decreases in vector efficacy. We find that even when
pathogens find themselves much more often in hosts than vectors,
selection in the vector environment can cause correlational and
potentially non-adaptive changes in virulence in the host. The extent on
such correlational virulence evolution depends on many system
parameters, including the pathogen transmission rate, the relative
proportions of the pathogen population occurring in hosts versus
vectors, the strengths of selection in host and vector environments, and
the extent of virulence per se. But there is a statistical interaction
between the strength of selection in vectors and the predominance of
pathogens in hosts, such that if within-vector selection is strong
enough, the predominance of pathogens within hosts has little effect on
the evolution of virulence.