Many plant pathogens go through extreme bottlenecks in population size during winter. This includes pathogens specializing on foliar tissue of hosts that die back to rootstock each winter. In the absence of suitable host tissue, powdery mildews of herbaceous perennials survive by producing resting structures called chasmothecia, which overwinter on dead plant material and release spores to reinitiate epidemics in spring. In a metapopulation of the powdery mildew Podosphaera plantaginis infecting Plantago lanceolata in the Åland Islands of southwest Finland, overwinter pathogen survival is highly variable through space and time. Previously, we found that over a 13-year period, years with more days that were freezing and snowless, measured at a single meteorological station, had lower overwinter pathogen survival across the metapopulation. Thus, we hypothesize that snow protects chasmothecia from freezing damage. In the present study, we investigated how spatial variation in climate during winters 2014-2015 and 2015-2016 affected pathogen survival at hundreds of populations surveyed for disease in both fall and spring. We deployed temperature and humidity dataloggers to more than 30 sites across the metapopulation, and used kriging to interpolate climate variables between sites. Snow cover is inferred from temperature and humidity profiles, following calibration to observed snow depths.
Results/Conclusions
Daily average temperature and relative humidity varied greatly among sites and across the winter months in the Åland Islands. A preliminary analysis of data from the first year of our study revealed a striking pattern of average December 2014 and January 2015 temperatures below freezing in the interior of the metapopulation, and slightly above freezing along the exposed coastlines. This spatial variation in temperature was significantly associated with powdery mildew overwintering success, consistent with our hypothesis that freezing temperatures damage the pathogen’s chasmothecia. Ongoing analyses will evaluate the consistency in this pattern between the two winters of our study. Moreover, we are currently mapping snow cover through space and time, to test whether longer duration of snow-free frozen ground increases pathogen local extinction rates. Finally, we plan to compare our maps of temperature and snow cover to data obtained from satellite remote sensing to assess whether remotely sensed data could be used to expand the temporal scope of our study relating spatial variation in winter climate to pathogen persistence. These results will provide important insight into abiotic factors governing pathogen survival during the seldom-studied winter offseason. Such insight is critically needed as climate change is dramatically altering winter conditions worldwide.