FUNGAL DECOMPOSITION OF ATTACHED ANGIOSPERM TWIGS .4. EFFECT OF WATER POTENTIAL ON INTERACTIONS BETWEEN FUNGI ON AGAR AND IN WOOD

Interspecific interactions between fungi frequently isolated (isolation frequency > 5%) from dead, attached ash twigs were studied on agar and in twigs at different water potentials in the laboratory, and in twig lengths resuspended in the field. Under laboratory and field conditions the following observations were made. The main primary colonizers of dead attached ash twigs, Phomopsis platanoidis Died., Fusarium lateritium Nees. and Sp. 12, did not replace each other. Two of the common secondary colonizers, Libertella fraxinea Oganova and Acremonium sp. A, did not actively replace primary colonizers, except that L. fraxinea partially replaced P. platanoidis when associated with bacteria. The secondary colonizer Peniophora lycii (Pers.) v. Hohn & Litsch was, however, more combative and replaced all other species, provided that water potential was not lower than -4 MPa. There was some evidence that under field conditions P. lycii, arriving via the air spora could not replace P. platanoidis or Sp. 12; it apparently required a well established base in the resource. However, it could replace F. lateritium and consequently gain a suitable foothold in twigs naturally colonized in the field. Also, P. lycii demonstrated polar growth in twigs, progressively replacing other species. Active mechanisms by which fungi could resist replacement included the production of pseudosclerotial plates by P. platanoidis and the production of a volatile antifungal agent by Sp. 12. There was also some evidence that other fungi, in particular F. lateritium, produced diffusible antifungal substances, or altered growth conditions, which inhibited other fungi.