Do colonization by dark septate endophytes and elevated temperature affect pathogenicity of oomycetes?

P hialocephala subalpina is one of the most frequent dark septate root endophytes in tree roots but its function in forest ecosystems is largely unknown. A full‐factorial infection experiment was performed, using six P . subalpina isolates, two pathogenic oomycetes ( P hytophthora plurivora [syn. P hytophthora citricola s.l.] and E longisporangium undulatum [syn. P ythium undulatum ]) and two temperature regimes (17.9 and 21.6 °C) to examine the ability of P . subalpina to protect Norway spruce seedlings against root pathogens. Seedling survival, disease intensity and seedling growth were affected by P . subalpina genotype, temperature and pathogen species. Some P . subalpina isolates effectively reduced mortality and disease intensity caused by the two pathogens. Elevated temperature adversely affected seedling growth but did not aggravate the effect of the pathogens. E longisporangium undulatum but not P . plurivora significantly reduced plant growth. Colonization density of P . subalpina measured by quantitative PCR was not affected by temperature or the presence of the pathogens. In conclusion, P . subalpina confers an indirect benefit to its host and might therefore be tolerated in natural ecosystems, despite negative effects on plant health and plant growth.