Actinomycetes inducing phytotoxic or fungistatic activity in a Douglas-fir Forest and in an adjacent area of repeated regeneration failure in Southwestern Oregon

Actinomycetes were isolated from the upper 1 - 3 cm of the soil layer in a well-developed forest and in an adjacent clearcut area where Douglas-fir [Pseudotsuga menziesii (MIRB.) Franco] regeneration had been impaired for two decades. The population density in the clearcut area was two times as high as that in the forested area. The percentage of actinomycetes that inhibited seed germination of the test plants was significantly higher in isolates obtained from the clearcut area than in those obtained from the forested area, and isolates from the clearcut showed five times the phytotoxic effect of those from the forest. Some actinomycete isolates, 4 % from the clearcut and 2.6 % from the forest, significantly reduced in vitro growth of two common ectomycorrhizal fungi of Douglas-fir, Laccaria laccata and Hebeloma ovstuliniforme. Two actinomycete isolates from the clearcut reduced fungal growth by 40 % and 73 %. Reduction of the nutrient in the growth medium did not affect the antifungal activity of the actinomycetes. The data support the idea that, along with other factors, phytotoxic and antifungal actinomycetes may suppress natural regeneration or establishment of planted seedlings - either directly or. indirectly - through inhibition of seed germination or of mycorrhizal fungi.