STEM AND BRANCH DISEASES

Ch14 10/5/99 2:19 PM Page 348
348 CHAPTER 14
Several species of fungi also cause galls. For example, Phomopsis spp. causes
globose galls on oaks, hickories, maples, and American elm that are similar to
crown gall. Macrophoma tumefaciens causes almost spherical galls on poplars in
the Rocky Mountains and Lake states.
DECAY
Figure 14.8
Bacterial gall on Douglas-fir.
(Source: From Scharpf 1993.)
Wood decay is the decomposition of wood by the enzymatic activities of microbes,
primarily fungi (Manion 1991). Bacteria can decompose wood, but are inefficient
compared to fungi. Insects, marine organisms, and UV light also can deteriorate
wood.
Knowledge of wood chemistry is extremely important in understanding the
decay process. Wood consists primarily of cellulose, hemicellulose, lignin, and
extractives. Cellulose, hemicellulose, and lignin are the major constituents of cell
walls, whereas extractives are not in the cell wall and typically confer decay resistance.
Extractives are primarily phenolic chemicals. Species, such as western redcedar,
redwood, and Douglas-fir, with colored heartwood typically have a high
extractive content and have higher decay resistance than species with white heartwood
such as hemlocks, spruces, and true firs.
Types of Decay
There are three basic types of decay: white, brown, and soft rot (Boyce 1961) as
shown in Box 14.1.
White rots are fibrous because some cellulose remains intact until late in the
decay process. It is whitish because of the loss of lignin, which is brown, and
because the remaining cellulose tends to be white. Cellulose is degraded by a complex
of enzymes known as cellulases, which typically break the cellulose into
shorter chains and eventually into glucose, the building block of cellulose. White