Latent Decay Fungi: The Hidden Foe?

Summary In the sapwood of branches and trunks of angiosperm trees, extensive (several, sometimes many metres) columns af decay, occupied by a single fungal individual frequently develop rapidly (often in less than one growing season), in the absence of major wounds. Such rapid and extensive colonization is unlikely to result from mycelial extension from a single focus. However, the patterns can be explained by the presence of latent fungus propagules (such as yeasts, hyphal fragments or spores) distributed widely but sparsely in the sapwood, but which do not develop when sapwood is functional in water conduction. The widespread presence of such latent fungi has been demonstrated in the laboratory where lengths of what had been healthy twigs, branches and trunks of ash, beech, oak and hazel were dried under controlled conditions. There is also considerable circumstantial evidence that development of these latent propagules into mycelia, which cause decay, occurs in the field when sapwood begins to dry, as a result of for example drought, root damage or loss of transpiring canopy. How these fungal propagules enter sapwood is uncertain, but possibilities include damaged roots, bud scars, insect activity, and passage through dead or living vascular cambium cells. Some of the fungi which are latently present in beech have been shown to affect growth of beech callus in tissue culture and in inoculated standing trees, and beech tissue affects growth and germination of some of these fungi.