ROBERT HARTIG developed the concept of tree decay almost a century ago. At that time, decay was well recognized as a serious economical problem. In tune with the theory of spontaneous generation, scientists believed that

Decay Caused Fungi.

Robert Hartig, in tune with the germ theory that emerged after 1845, said that

Fungi Caused Decay.

This simple reversal of two words set the stage for the decay concept and, in some ways, for the beginning of the science of FOREST PATHOLOGY.

The Classical Concept of Tree Decay has Three Major Parts:

1. WOUNDS started the processes (below left). (fig. 28)
2. DECAY FUNGI (Hymenomycetes) (below, upper right) (fig. 29) infected the heartwood through fresh wounds. (Robert Hartig showed that the sporophores on the wound surface (opposite page)(fig. 31) and the hyphae associated with the wood decay (below, upper right) were the same fungus.)
3. DECAYED WOOD resulted (below, lower right). (fig. 30)

	Figure 29 Hyphae of Phellinus tremulae
Figure 28 Wound on yellow birch	Figure 30 Decayed wood associated with Phellinus tremulae

Figure 31 Phellinus tremulae on Populus tremuloides