• adaptation;
  • fitness;
  • late blight;
  • life history traits;
  • potato;
  • survival

The strict dependency of biotrophic pathogens upon living host tissue for multiplication and survival makes them particularly exposed to evolutionary trade-offs. Such trade-offs can occur both between life history traits directly involved in pathogenicity (e.g. fitness costs due to unnecessary virulence factors), or between traits involved in either pathogenicity (within-season fitness) or survival (between-season fitness). Both types of trade-offs should result in the limitation of maximum pathogenicity, and shape the invasive potential of pathogen genotypes. While strong theoretical developments have been made on evolutionary consequences, including recent work taking seasonality into account (i.e. periodic host absence and/or periodic sexual/asexual reproduction), experimental evidence to confirm theoretical predictions is still scarce. This paper will therefore attempt to illustrate the different kinds of trade-offs that can be measured, and their likely consequences, taking Phytophthora infestans (the cause of potato and tomato late blight) as a case study.