Production of Acid Is Responsible For Antibiosis by Some Ectomycorrhizal Fungi

The interaction of Pythium ultimum with a variety of ectomycorrhizal fungi was investigated in vitro. Out of 19 isolates of ectomycorrhizal fungi tested for inhibition of the growth of P. ultimum, 16 caused swelling and bursting of the tips of leading hyphae. These effects were eliminated by maintaining the culture medium at pH 6 with a biological buffer. This indicated that acidification of the medium was the cause of the inhibition. Hyphal growth of P. ultimum was inhibited below pH 3. In liquid culture, the ectomycorrhizal fungus Paxillus involutus reduced the pH of the medium to less than 3 before it reached stationary phase. It is suggested that the antibiotic effect exhibited by some ectomycorrhizal fungi is due to acid production. HPLC analyses of culture filtrates of Pax. involutus were shown not to contain significant amounts of oxalic acid, but other acids were present in large quantities.