Authors: Demeter, László; Molnár, Ábel Péter;Kiš, Alen;Vadász, Csaba;Horváth, Ferenc;Csóka, György;Öllerer, Kinga;Molnár, Zsolt;

Pedunculate oak (Quercus robur) is a European keystone tree species, hosting a great amount of biodiversity, but its future role in (near-)natural forests is uncertain due to the lack of natural regeneration. The scientific discourse on the main factors impeding its natural regeneration is biased because knowledge from historical and non-English literatures is unassessed and the real impacts of an introduced pathogen fungus remain unknown. We reviewed historical and recent ecological, pathological and forestry literature on topics related to the impact of non-native oak powdery mildew (PPM) on the success of advanced natural regeneration of pedunculate oak in (semi-)natural forests. We propose the novel ‘oak powdery mildew’ hypothesis, to explain the failure of natural regeneration of pedunculate oak throughout its distribution range. PPM reduces shade tolerance and vertical growth in pedunculate oak seedlings and saplings, so sapling vitality and competitiveness have diminished considerably since PPM was unintentionally introduced to Europe in the early 20th century. We found that forest ecologists and conservationists often overlook the impacts of this ‘recent’ driver. Our study suggests that nature conservation and forest ecology should pay greater attention to the impact of introduced microorganisms and to the integration of knowledge from historical and non-English literature.