Collybia fusipes is a common root rot fungus in mature pedunculate oak forest, that causes drastic destruction of the tree root systems, especially in dry or mildly waterlogged soils. We wanted to check, under controlled conditions or in forest ecosystems, whether reduced O2 during saturation of the soil by water could interact with disease evolution. Susceptibility of waterlogged oak seedlings to C. fusipes was tested in a greenhouse and the survival of the pathogen in woody substrates was assessed in hydromorphic soils in a forest. A direct and detrimental effect of soil waterlogging on C. fusipes survival was evidenced both under controlled conditions and in forest stands. Growth of C. fusipes mycelium on agar media was monitored under low O2 mole fraction and compared to that of Armillaria mellea and Heterobasidion annosum. A drastic reduction in mycelial growth was evidenced in C. fusipes and H. annosum but not in A. mellea.