Bacterial infected oak:
What it is and what to do about it
By Dr. Fred M. Lamb
Occasionally, I am mailed a sample of checked and honeycombed oak with a note that says something like, “I honeycombed this load of oak, but it’s bacterially infected. Right? Is that what caused the problem?” Unfortunately, for the operator, the sample of wood usually shows no sign of bacterial infection and the checking and honeycomb were caused by poor drying practices. Nevertheless, bacterial infected oak is a real problem that must be addressed by every oak drying operation. I believe that almost every operator drying oak today has at one time or another run into bacterial infected oak and its associated drying problems.
What is bacterial infected oak?
The old sawmilling and drying literature published in the trade journals of 40 to 50 years ago occasionally referred to what many old-time sawmillers called “sick oak.” The wood just did not look right, it smelled bad, and it split and cracked when drying. Most people back then knew it was not decay (which is caused by fungi), but they had no logical explanation for these unusual occurrences that seemed to affect some oak lumber. By the early 1960’s, we started to understand the role of bacteria and “wetwood” in trees. By the early 1970’s, researchers began to provide an explanation for the phenomenon of “sick oak.” Jim Ward did some of this early work at the U.S. Forest Products Laboratory in Madison, Wisconsin.
It seems that oak trees can be colonized (attacked) by anaerobic bacteria (that is, bacteria that live and grow without air). One such bacterium is Clostridium. It is speculated that the bacteria enter the tree through the root system, possibly by way of an injury. Once in the tree, they begin to colonize areas in the heartwood. Under normal conditions they progress slowly in the tree. However, if the tree is already under stress from other pathological agents, insects or environmental conditions, their growth may be faster and more widespread within the tree.
Normally, however, the bacterial infection is only in the lower portion of the butt log. The infection may be either in the center of the heartwood, near the heart-sap interface, or distributed generally across the heartwood cross-section. Wet sites and older trees seem to be more susceptible. However, it can be found wherever red oak is cut commercially. White oak seems to be less susceptible, but the occurrences in white oak seem to be growing. Nevertheless, at this time, red oak is the major problem.
What does it do?
It is assumed that the bacteria produce slime in the area of the tree they colonize. This slime isolates the area and the moisture content in that zone increases. The slime also seems to reduce the permeability of the wood. This situation is often called “wetwood” or “water pockets.” Furthermore, the enzymes secreted by the bacteria appear to weaken the bonds between the cells by attacking one of the chemicals—pectin. Thus, we have wood that is higher in moisture content, lower in permeability and somewhat weaker. That means in drying, there is more water to remove, but the water moves more slowly and is more difficult to remove and the wood is less able to withstand the drying stresses.
It must be remembered that these are anaerobic bacteria; they live without air. They do not spread from board to board nor can one or two boards infect an entire load. It is the tree that is infected. Sawing the logs and drying the lumber stops their growth and destroys them.
How to detect bacterial infected oak
There is no one easy way to determine bacterial infected oak, but it does have a number of characteristics that when taken together, provide a reasonable estimate. The following are some of the common indicators.
• Extremely high green moisture content, over 90 percent and generally over 100.
• A very strong disagreeable odor, like rancid butter, vinegar (acetic acid) or very old wet sweat socks.
• The presence of ring shake (sometimes called wind shake).
• Eyes sting and burn when going into the kiln (acetic acid vapors).
• Wood has a dark, off-color and translucent appearance.
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| Figure 2. Ring shake and honeycomb in a dry board of bacterial infected oak. |
A strong, disagreeable odor and a high moisture content are almost conclusive indicators of bacterial infection. So is the presence of ring shake. Since normal oak does not ring shake, this is a very good indicator. Color, growth ring pattern and growth rates are poor indicators and subject to much inaccuracy.
The big question: What to do about it?
The pattern by which the bacteria colonize the tree is such that all the lumber from an infected tree will not be bacterial infected. In most situations, probably less than 10 percent of the lumber from an infected tree would be bacterial infected. This means that in a kiln load, only a very small percent of the oak lumber would be bacterial infected. It could be as little as a few hundred board feet to as much as a couple percent. Only in unusual circumstances would the volume of bacterial infected lumber in a kiln charge be greater.
Thus, the big question is “do you slow an entire kiln charge for that small a volume of wood, or do you dry it normally and accept the heavy losses in surface checking and honeycomb that will occur in the bacterial infected material.” This is a decision that must be made on a load-by-load basis. The dry kiln operator needs some guidelines as to the company policy on this issue.
Some people have suggested that the bacterial infected lumber be sorted out and dried separately—a good technical alternative, but not very practical for most drying operations today. Still, companies need to decide on a policy as to how they will handle bacterial infected oak lumber. It should be noted that many people believe that this problem is only going to get worse in the future.
Drying bacterial infected oak requires slow drying under very mild conditions (very low temperatures, low air flows and higher humidities). Lumber 6/4 and thicker must be shed dried to well below 25 percent moisture content before going into the kiln. There is some concern, however, that 6/4 and especially 8/4 bacterial infected oak can really never be dried with any degree of satisfactory quality. Even 4/4 and 5/4 lumber can be dried with better quality if it is shed dried (or slowly air dried) first.
The dry bulb temperatures should be kept very low until the moisture content is well below 25 percent. Even slow drying at very mild temperatures is no guarantee that the bacterial infected oak will not still check and honeycomb. It is a difficult wood to dry and the chances of success are small.
Fred M. Lamb is Professor and Extension Specialist, Department of Wood Science and Forest Products, Brooks Forest Products Center, Virginia Tech, Blacksburg, Virginia. Dr. Lamb can be contacted at 540-231-7256; Fax: 540-231-8868
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