The mycorrhizal association, known for its classic role in nutrient uptake, is shown to play an equally important role in the protection of seedlings against colonization by root-infecting fungi. This protection enhances the survival of seedlings. The survival of pine seedlings was tested in non-sterile and gnotobiotic systems when Pisolithus tinctorius and Rhizoctonia solani were added alone and in combination. When P. tinctorius alone was added to growth substrate of the pines, the survival of seedlings was greater than that of uninoculated controls. The increased survival of seedlings was probably due to a synergism between P. tinctorius and loblolly pine. If mycorrhizae were allowed to form on pine seedlings before R. solani was added to the growth substrate, survival was higher than treatments where R. solani alone was added or where R. solani and P. tinctorius were added simultaneously. The exact mechanism of the protection is unknown, but it is suggested that it might be any combination of resistance to penetration by root pathogens, mycorrhizae acting as sites of water and nutrient absorption during the period when the seedlings are susceptible to damping-off, production of antibiotics, and reduced growth stimulating chemicals in the rhizosphere. The effects of four fungicides, at two rates, on populations of mycorrhizal fungi were studied. In seedbeds receiving fungicides at low rates, the pine seedlings were larger and had fewer mycorrhizae than those in untreated seedbeds. In seedbeds receiving fungicides at high rates, the pine seedlings were smaller and had more mycorrhizae than the controls. The high rates of fungicides may have stunted the seedlings directly or may have selectively eliminated certain organisms, leaving others which were antagonistic to pines. The great numbers of mycorrhizae in seedbeds receiving high fungicide rates might have been due to the elimination of organisms that compete with mycorrhizal fungi. It should be possible to increase survival of seedlings in tree nurseries by mixing seeds with mycorrhizal fungi in the same way that legume seeds are inoculated with nodule forming bacteria.

Ph. D.