Effects of fungi isolated from Quercus robur roots on growth of oak seedlings

Effects of 62 isolates (of 41 taxa) of fungi on growth of 1-year-old oak (Quercus robur) seedlings were studied in an inoculation experiment. The fungi were isolated from roots of 80–96-year-old Q. robur that had been subjected to periodic flooding and had symptoms of oak decline. The fungal genera included Alternaria, Aspergillus, Calonectria, Chaetomium, Cladosporium, Clonostachys, Corynespora, Cylindrocarpon, Dicyma, Geotrichum, Ilyonectria, Isaria, Metarhizium, Oidiodendron, Ophiostoma, Pezicula, Phialocephala, Phialophora, Pyrenochaeta, Sporendocladia, Sporothrix, Thelonectria, Trichoderma and Trimmatostroma. Mycelial colonies of fungi growing in potato-dextrose broth were used for soil inoculation. Plant growth was assessed 2 years after inoculation, when the plants were 3 years old. Stem lengths, and dry weights of stems, roots and leaves were measured. Stem growth was inhibited by 31 isolates (50%) and root growth by 12 isolates (19%). Stem growth was stimulated by two isolates (3%) and root growth by 17 isolates (27%). The overall ratio of inhibitors to stimulants was 2.1. The proportion of taxa that inhibited stem growth was 16 times greater than that which promoted stem growth. The proportion of taxa that promoted root growth was only 1.5 times greater than that which inhibited root growth. The structure of the fungal communities in periodically flooded oak forests suggests that they are more likely to inhibit than to promote vigour in oaks.