These diseases nearly always involve only the wood of the tree, and rarely the cambium or bark. Intact bark serves as an effective barrier to entry of the pathogens, and trees become susceptible only when bark is breached by another cause such as by canker, mechanical wound, root rot, sunburn etc. Because of the nature of the pathogens to avoid the cambium, and because the processes move slowly enough that the tree can produce new xylem to sustain itself, it is rare for trees to be injured physiologically by these diseases. Decline is associated with extensive wood decay, but not because the decay caused the decline. This will be discussed more fully below.
Wood decay is extremely common, but it is only occasionally the cause of tree failure.
An essential part of the diagnosis of wood decay then is determining when a tree with decay is at risk of failure. These cells are small, so that they are a small proportion of the wood volume but the are very numerous. As part of the process of conversion, parenchyma cells convert starch reserves into chemicals, some of which are anti-microbial. Tree defenses against the disease a. Discoloration reaction. When sapwood is injured the parenchyma cells are triggered into activity in which they produce chemicals similar to those produced on conversion to heartwood.