Local adaptation of a fungal pathogen to temperature along a latitudinal gradient

Whether climate warming will increase or decrease prevalence of an infectious disease partly depends on the potential for pathogens to adapt to higher temperatures. This potential can be assessed by investigating regional variation in pathogen thermal performance and testing for local adaptation to current temperature regimes and host populations. We collected seeds of a host plant ( Plantago rugelii , a perennial herb) and isolated strains of its specialist fungal pathogen ( Golovinomyces sordidus , a powdery mildew) from five locations along a latitudinal transect from southern Mississippi to northern Wisconsin, USA. In a laboratory experiment, we placed sympatric and allopatric host–pathogen pairings into seven temperature treatments from 7 to 33 °C. We fitted thermal performance curves to pathogen growth data for each strain. Pathogen strains were locally adapted to temperature, with estimated thermal optima ranging from 20.6 °C (southernmost strain) to 16.7 °C (second-northernmost strain) and generally decreasing 0.26 °C for each degree increase in latitude of origin. However, there was no evidence of pathogen local adaptation to sympatric hosts. Given that powdery mildew spores can disperse long distances via wind, our results suggest that northward spread of warm-adapted strains could facilitate pathogen adaptation to warming climates in this and similar systems.