Niche differentiation within a cryptic pathogen complex: climatic drivers and hyperparasitism at multiple spatial scales

Pathogens are embedded in multi-trophic food webs, which often include
co-occurring cryptic species within the same pathogen complex.
Nonetheless, we still lack an understanding of what dimensions of the
ecological niche might allow these cryptic species to coexist. We explored
the role of climate, host characteristics (tree autumn phenology) and
attack by the fungal hyperparasite Ampelomyces (a group of fungi attacking
plant pathogens) in defining the niches of three powdery mildew species
(Erysiphe alphitoides, E. hypophylla and E. quercicola) within a cryptic
pathogen complex on the pedunculate oak Quercus robur at the continental
(Europe), national (Sweden and France) and landscape scales (a 5 km2
island in southwestern Finland). Previous studies have shown that climate
separated the niches of three powdery mildew species (E. alphitoides, E.
hypophylla and E. quercicola) in Europe and two species (E.
alphitoides and E. quercicola) in France. In our study, we did
not detect a significant relationship between temperature or precipitation
and the distribution of E. alphitoides and E. hypophylla present in
Sweden, while at the landscape scale, temperature, but not relative
humidity, negatively affected disease incidence of E. alphitoides in an
exceptionally warm year. Tree variation in autumn phenology did not
influence disease incidence of powdery mildew species, and hyperparasite
presence did not differ among powdery mildew species at the continental,
national and landscape scale. Climate did not affect the distribution of
the hyperparasite at the continental scale and at the national scale in
Sweden. However, climate affected the hyperparasite distribution in
France, with a negative relationship between non-growing season
temperature and presence of the hyperparasite. Overall, our findings, in
combination with earlier evidence, suggest that climatic factors are more
important than species interactions in defining the niches of cryptic
species within a pathogen complex on oak.