Strong in vitro antagonism by elm xylem endophytes is not accompanied by temporally stable in planta protection against a vascular pathogen under field conditions

Some endophytic fungi provide their host plants with protection against abiotic and biotic stressors, including pathogens. Endophyte-mediated mechanisms might be behind the environmental resistance shown in the field by some adult Ulmus minor trees to the Dutch elm disease (DED) pathogen, Ophiostoma novo-ulmi. We isolated and characterized seven endophyte fungi from the xylem of three adult U. minor trees that have survived the DED epidemics within areas in Spain ravaged by the disease. The antagonism of the isolated endophytes towards O. novo-ulmi was evaluated in vitro by means of dual culture assays. Six of the studied endophytes hindered the pathogen growth through antibiosis, competition for the substrate, or a combination of both mechanisms. Four of these endophytes were selected for in vivo tests where their protective effect was evaluated in field experiments during three successive years (2011-2013). The conditioning inoculation of two endophytes (Monographella nivalis and Alternaria tennuissima) reduced DED symptoms in 2011 and 2012, respectively. However, the same isolates did not show any prophylactic effect in 2013, which suggests that the repeatability of the treatments is low. A significant treatment x clone interaction was found, showing that the effectiveness of the treatments depended on the tree clone. The future use of endophytes in biocontrol strategies might be oriented towards taking into consideration the whole fungal microbiome in forest breeding programs rather than the external application of particular endophyte strains.