How will plant pathogens adapt to host plant resistance at elevated CO under a changing climate?

•   To better understand evolution we have studied aggressiveness of the anthracnose pathogen, Colletotrichum gloeosporioides, collected from Stylosanthes scabra pastures between 1978 and 2000 and by inoculating two isolates onto two cultivars over 25 sequential infection cycles at ambient (350 ppm) and twice-ambient atmospheric CO in controlled environments. •   Regression analysis of the field population showed that aggressiveness increased towards a resistant cultivar, but not towards a susceptible cultivar, that is no longer grown commercially. •   Here we report for the first time that aggressiveness increased on both cultivars after a few initial infection cycles at twice-ambient CO as isolates adapted to combat enhanced host resistance, while at ambient CO this increased steadily for most cycles as both cultivars selected for increased aggressiveness. Genetic fingerprint and karyotype of isolates changed for some CO -cultivar combinations, but these were not related to changed aggressiveness. •   At 700 ppm fecundity increased for both isolates, and this increased population size, in combination with a conducive microclimate for anthracnose from an enlarged plant canopy under elevated CO , could accelerate pathogen evolution.