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The natural occurrence of Pisolithus tinctorius has been confirmed in 33 countries of the world and in 38 states in the United States. This ectomycorrhizal fungus is found associated with various tree species in nurseries, urban areas, orchards, forests, and strip-mined spoils. Experiments have proved that this fungal symbiont forms ectomycorrhizae with Abies procera, Betula pendula, Carya illnoensis, 11 species of Eucalyptus, 30 species of Pinus, Pseudotsuga menziesii var. meniziesii, 2 species of Quercus, and Tsuga heterophylla. Pisolithus has also been reported growing under natural conditions in association with three additional species of Betula, two species of Eucalyptus, nine species of Pinus, and eight species of Quercus, Populus tremuloides, Pseudotsuga grandidenta, and Salix humilis. This fungal symbiont has great potential in forestation efforts because of (1) the availability of practical techniques for artificially introducing in into nursery soils; (2) its ability to improve tree survival and growth in the nursery and the field; (3) its near worldwide distribution on a variety of sites; and (4) its broad host range encompassing many of the world's most important tree species.

Pinus taeda seedlings were grown for 10 weeks without ectomycorrhizae under low and high rates of complete soil fertility, as well as these rates minus N, P, K, or Ca. Seedling growth, inorganic chemical content of needles, and soluble-carbohydrate content of short roots were significantly affected by soil fertility, especially the high rates of N and P. Number and length of lateral and short roots were not affected by soil fertility. Sucrose and fructose contents of short roots were lowest at high levels of N and P; glucose was not detected. Seedlings from each of the 10 fertility combinations were inoculated with vegetative inoculum of Pisolithus tinctorius and incubated for 19 to 21 days at a moderate level of complete soil fertility. Significantly more ectomycorrhizae were formed on seedlings from the complete low fertility rate than on those from the high rates of N and P. Ectomycorrhizal development on seedlings from the other fertility combinations were intermediate from these extremes. Sucrose content of short roots determined before inoculation was significantly correlated with ectomycorrhizal development and accounted for 85% of the variation in susceptibility of short roots to infection by P. tinctorius. Fructose content of short roots was not correlated with ectomycorrhizal development. These results indicate that high levels of N and Pin soil decrease sucrose content of short roots of loblolly pine and decrease their susceptibility to ectomycorrhizal development by P. tinctorius.

In a special plant-growth room, isolates of Thelephora terrestris produced basidiocarps and formed typical ectomycorrhizae with seedlings of bristlecone, jack, sand, lodgepole, shortleaf, slash, sugar, Austrian, longleaf, cluster, ponderosa, red, pitch, eastern white, Scots, loblolly, and Virginia pines and Douglas fir. Atypical mycorrhizae (lacking mantle) were formed on seedlings of Norway spruce and jack, Japanese red, and Himalayan pines. The formation of atypical mycorrhizae was considered a result of differences in the symbiotic–parasitic nature of the fungal symbionts on different hosts. Pisolithus tinctorius formed typical mycorrhizae with seedlings of jack, sand, Japanese red, shortleaf, slash, Austrian, longleaf, cluster, red, pitch, eastern white, Scots, loblolly, and Virginia pines. Reisolation of specific fungal symbionts from mycorrhizae of several pine hosts was successful.Mycorrhizae formed by T. terrestris were macroscopically and microscopically different from those of P. tinctorius, but mycorrhizae formed by different isolates of T. terrestris were indistinguishable from each other, regardless of host. These results suggest that the fungal symbiont determines color and morphology of ectomycorrhizae.