Variation in traits associated with parasitism and saprotrophism in a fungal root-rot pathogen invading intensive pine plantations

Armillaria ostoyae, the causal agent of root- and butt-rot in several forest trees, has a lifecycle consisting of alternating parasitic and saprotrophic stages. It causes high levels of mortality in the intensively managed monospecific plantations of maritime pine (Pinus pinaster) in south-western France (Landes forest). In this region, the pathogen was native to the forests, pre-dating the large plantations of the 19th century. The first objective of this study was to estimate the variation in aggressiveness on maritime pine, determined as rate of host mortality caused by the infection process. The second objective was to characterize relationships between aggressiveness and traits likely to be involved in parasitism (i.e. rhizomorph production and colonization of host tissues) and saprotrophism (ability to decompose wood). The A. ostoyae isolates studied caused high rates of mortality in maritime pine, with significant differences between isolates. However, there was no variation of aggressiveness between A. ostoyae isolates from ancient forested and from more recently afforested areas, and did not support the hypothesis of a higher aggressiveness linked to a recent range expansion and the intensification of silviculture in this area. Rhizomorph production and aggressiveness were significantly correlated. In addition, we did not detect any trade-off between components of parasitism and saprotrophism, suggesting no significant evolutionary constraint driving these traits.