Ectendomycorrhizae in Pinus resinosa seedlings. I. Characteristics of mycorrhizae produced by a black imperfect fungus

A black imperfect fungus isolated from 3-year-old nursery seedlings of Pinus resinosa was tested for its ability to form mycorrhizae on red pine in monoxenic culture. It was found to stimulate seedling growth. The fungal invasion passed through three successive stages of development during the 7 months after inoculation. At 3 months, intracellular hyphae occurred in all long- and short-root branches, with intercellular hyphae scattered only at isolated loci in the intercellular spaces. After 5 months the infection in the cortex changed abruptly from an intracellular infection to a completely intercellular Hartig net. Although the Hartig net was remarkably thin when first initiated, at its culmination it reached a massive thickness and sent penetrating hyphae into the cortical cells, resulting in the formation of distinctive ectendomycorrhizae. By the end of 7 months the secondary intracellular invasions were often found in the vicinity of the meristems in all root branches except the largest-diameter long-roots, and the fungus-root association had developed toward a predominant ectendomycorrhizal condition. The ectendomycorrhizal infection was distinctively different from the one produced by the so-called E-strains that have been commonly observed in coniferous nurseries.