Native fungal endophytes provide effective biocontrol against Phytophthora plurivora associated with alder decline in northern Spain.

Phytophthora plurivora has become a major driver of alder decline across Europe, posing a critical threat to Alnus lusitanica populations in Northern Spain. Effective and sustainable control measures for this oomycete pathogen remain lacking, particularly in riparian ecosystems where chemical options are unsuitable. This study evaluated the biocontrol potential of native endophytic fungi associated with symptomatic A. lusitanica. P. plurivora and 43 endophytic fungi were isolated from declining trees across six river systems. Nine (9) out of the 43 endophytic fungi were promising and used for dual-antagonism assays. Dual antagonism assays revealed 8 out of the 9 inhibited the growth of the pathogen by growing over it, with Trichoderma spp. Exhibiting the strongest mycoparasitic and competitive effects. Three (3) interesting candidates (Trichoderma harzianum, Trichoderma citrinoviride and Clonostachys solani) were selected for further testing in greenhouse pathogenicity assays using A. lusitanica seedlings. T. citrinoviride provided the most effective protection, reducing necrosis progression as compared to other treatments and the control. This study provides the first experimental evidence that native endophytic fungi particularly T. citrinoviride can suppress P. plurivora infection in alder. These findings highlight the potential of native fungal mutualists as sustainable tools of managing oomycete driven decline in vulnerable riparian ecosystems.