Young trees are often associated with other fungi than older ones. Numerous fungi, like the fly agaric, associate with a range of tree species, only few fungus species are found in just one habitat. Ectomycorrhizas are not necessary for seed germination and the early stages of the tree seedlings. Nearly all seedlings that are a few month old live under natural conditions already in association with fungi, though. Controlled experiments showed that seedlings thrive far worse in sterile soil than their counterparts associated with mycorrhizal fungi.
Infected roots differ morphologically from uninfected ones.
The cortex cells develop no secondary walls, root hairs and calyptra are missing, and the endodermis is usually impregnated with tannins in order to prevent a further advance of the fungi. This complex hyphal branching pattern is considered to increase the fungal surface area in contact with the root. In contrast to saprophytic or parasitic fungi, most mycorrhizal fungi secrete no phenoloxidases required for the degradation of lignin and no cellulases necessary for breaking down cellulose. This is true as long as enough glucose is supplied. Cellulase is produced as soon as the supply stops, and the previously symbiotic fungus goes over to a parasitic life style.