Pinus sylvestris L. seedlings were grown in the laboratory in a forest humus which had been treated with different levels of CaO to give a soil pH gradient from 4 to 7. After 12 and 15 wk the resulting ectomycorrhizal infection was classified according to macroscopical morphology. The uniformity of the infection types distinguished was evaluated by isolating the fungi from the root tips and infecting seedlings with the isolates obtained. The isolation method was improved by using a medium containing benomyl and chlortetracyclin and by isolating from very small particles. For some mycorrhizal types the fungus was successfully isolated from 80% of the plated root particles.
The number of root tips that became mycorrhizal increased from 70% at pH 4 to nearly 100% around pH 5. Thereafter it decreased linearly to reach a minimum of slightly less than 40% at pH 7.5. Five different mycorrhizal types could be distinguished. They occurred with different abundance, distribution and pH optima. They were present throughout the entire pH range, except for a yellow type, identified as Piloderma croceum Erikss. & Hjortst., which was not found at values higher than 6–2. There was no great differences between infections on plants harvested after 12 and 15 wk. Seedling growth was constant to pH 5 then it increased up to about pH 7, at higher pH values growth declined.