Oak powdery mildew changes growth patterns in its host tree: host tolerance response and potential manipulation of host physiology by the parasite

Parasites can induce strong effects on their host's growth, not only as a result of host resource exploitation (growth loss) but also with a potential adaptive value for host (tolerance response) and themselves (increased transmission). We assessed these three types of phenotypic changes in oak seedlings infected by powdery mildew. A manipulative field experiment with three levels of parasite inoculum was designed in order to tease apart infection from genetic effects on oak growth. Seedlings were monitored during 3 years for height growth, phenology and infection. Powdery mildew infection induced both significant growth loss and qualitative changes in plant architecture. The most striking and unexpected change was increased growth polycyclism in infected seedlings. This benefitted both the host as a form of compensation for infection-caused height loss, and the pathogen, by increasing sporulation. The study highlights the effect of parasites in the expression of plant phenotypic traits, such as phenology and ultimately tree architecture. Both host tolerance and parasitic manipulation may be involved in the observed changes in growth patterns. These results suggest a complex interplay between development and defence in trees and emphasize the need to better assess tolerance mechanisms when considering the defence strategies of trees against pathogens.