Here are two posts regarding CODIT that were sent to me by persons wishing anonymity.
1. Compartmentalization is a visual model but the cause-and effect
relations are very poorly, if at all, elucidated. Only wall 4 includes
a physiological cause-and-effect explanaation and that concept came
earlier than the CODIT-model. In my mind, CODIT visualizes the
situation but so many factor cause many deviations from CODIT in
rality, that not much hard facts remain once you dissect, as the
message, to which this is a quick reply, explored.

2.Compartmentalization is viewed from the tree side of things but not the
fungus side. In exposed xylem, microbes having gained access to
nutrition, may eventually try to prevent drying of the colonized substrate
by forming skin-like layers in the wood. These may form as a reaction to
living wood or other antagonists. The fungus thus directs itself, just as
the tree might be conceived of as having compartmentalized the decomposer.
This may occur with advanced decay. Anyhow, from the standpoint of our
ignorance, and here I lead the pack, one must view the whole
complexity--but what a wonderful system.

Also, for each organism one may find different offensive mechanisms that
lead to advantage for that particular species. The wound pathogens such
as annosus, purpureum, tomentosus, the mellea ecospecies and other
pathogenic decomposers might be viewed as being primary invaders.

Again, Rayner's idea of the fungus being limited by a cylinder of water in
newly formed wood is surely part of the whole picture because the
decomposition process is shut down. On the other hand, the fungi still
could invade and aerate the wood--the living wood must be doing something
else as well as we are given to understand.

Reply to post by Peter Torres, on February 24, 1999 at 21:49:46:

I am a PhD student quite new to the ideas of compartmentalization. I study the reaction zone in Eucalyptus and am trying to get a grip on characterizing it and understanding how it works. As such, I don't really have any words of wisdom but am thankful that these ideas are being discussed.

I agree that the idea of "successful" compartmentalization is whether the tree survives and can still produce seed etc. From the wood products perspective, we see that any decay or discolouration is undesirable - but the tree doesn't care!

I find that with decay entering from the heartwood in Eucalyptus, the reaction zone stays pretty much on the border of the sapwood, and I expect that as it becomes incorporated into heartwood it will decay, and the reaction zone will keep retreating. In this sense the reaction zone is being conquered but only because the cells are dying as they would anyway to become heartwood, not because the reaction zone wasn't effective. When a wound is made directly into the sapwood I think it is a very different mattter, and I'm not sure yet how stable the reaction zones are in eucalyptus. But there is some evidence that the barrier zones in eucalpytus is good for about 14 years. In sycamore (Acer pseudoplatanus to avoid the trans-atlantic confusion) Ray Pearce has found that reaction zones are more static and are ocassionally conquered to leave relic reaction zones. Francis Swarze has found that fungi like Inonotus hispidus can avoid reaction zone barriers by forming tunnels through the wood cell walls.

Also, the idea of general tree vigor and its effect on defence response is interesting. It would seem obvious that a tree already stressed by lack of resources (nutrients, water, whatever) wouldn't have the resources to give over to defence. But how about the fact that a tree with very vigorous growth (apparently very healthy) is actually using all these resources for growth, not defence. Is that clear? What I mean is that it depends on the life strategy of the plant - run (grow tall and produce lots of seed, like eucalyptus, die young from decay) or fight (grow slower and put more energy into defences, eg. the oldest living tree in the world is Huon Pine from Tasmania I believe, contend me if you will, which is extremely durable wood).

I agree that the CODIT model needs adaptation, but it is a thought provoking introduction. I am more inclined to the Rayner conception of zones of drying tissue etc. However, how can we understand the nature of reaction zones in this sense when in many cases they are in fact drier than surrounding sapwood(eg. pines and eucalyptus). I believe the fact that these trees have generally wetter woods explains it.

Also, a small addition to the topic of strong/weak parasite, when you create a wound (which lets pretend was sterile....) you might get alot of defence responses just from that, so if a weak pathogen/saprophyte invades you might still see discolouration and some typical defences.

So many ideas, so many possbilities. Unfortunately I should be writing all this in my thesis........

Look forward to more discussion
Karen Barry