The host range of Paxillus involulutus includes a wide range of species. These mycorrhizae can be identified in the field by their appearance. A positive correlation was found between the numbers of mycorrhizae and sporophores formed by the species. It is concluded that Paxillus involutus does not form sporophores when growing by a saprophytic mode of nutrition. In the presence of trees, the species fruits to varying extents: in poor closed stands hardly at all and in fertile stands profusely. After partial cutting, soil scarification, draining and application of nitrogen, its fruiting increases markedly. Consequently, growth of Paxillus involutus in raw humus is arrested primarily due to deficiency of nitrogen.
In pure culture the amount of submerged mycelium on agar is very limited, but the aerial mycelium profuse. In the latter, sclerotia are also formed. The pH and temperature requirements may vary between individual strains. The species is also able to utilize starch. Nitrogen is utilized in the form of both ammonium and nitrate, and organic nitrogen sources.
Paxillus involutus forms a balanced symbiosis, even when the host is relatively weak and the fungus relatively virulent. It survives rather well in Scots pine seedlings planted in various sites; moreover, the initial development of these seedlings is better than that of nonmycorrhizal seedlings.
This study emphasizes the need for thorough investigations concerning whether mycorrhizal fungi are capable of fruiting when subsisting by a saprophytic mode of nutrition. In pure culture experiments several strains should be used. Semi-aseptic synthesis is sometimes surprisingly rapid, its major handicap being the limited number of fungal symbionts that can be successfully inoculated. In both this and aseptic synthesis mycorrhizal associations can be formed whose existence in nature is questionable.