I continue to receive questions about defects caused by bacteria in the living the tree. We have discussed this many times, but perhaps I have not presented the total picture and have too focused on one issue. So, I worked on complete, detailed article. I suggest that you cut this out and share the contents with purchasing personnel, drying and grading technicians, manufacturing employees. Then, put the article in a place where you can quickly locate it in the future.

With this information, you and your employees can determine, when defects show up, if the defects are caused by the resource issues, which you can do little about other than try to receive a refund for defective material, or if the defects are caused by manufacturing issues of which you do have control.

Bacterial Infection. Living trees — especially trees growing in wetter sites, trees more than 75 years old, and trees that have cattle grazing underneath — are sometimes infected by bacteria that enter through the roots. These are anaerobic (no oxygen) bacteria, so they thrive in very wet, no-air conditions in the soil and tree. While living, the bacteria somehow increase the moisture content in the area or pocket (usually small pockets as wide as 6 inches, as deep as 2 inches and often several feet long) where they are active. 

The increase can be 50% MC, increasing the weight of infected lumber when first sawn and after drying as well. As the bacteria move an inch or two every year from the roots up the stem, it is rare to see bacterial damage on lumber cut from the butt log more than a foot or two up the log.

Once the tree is cut, the bacteria are not very active, so they cause no significant subsequent damage.

Moisture pockets. Bacteria apparently are able to encapsulate the infected area so the pockets do not dry like regular wood, but rather require many months or even longer to dry. In the finished flooring product, the wet regions are called wet pockets water pockets, bacterial wet wood, and other names. (Even when dried, the pockets are still called wet pockets, probably to reflect the source of an issue in dry wood.) The high moisture content is easily detected in manufacturing by the increased weight of the wood, as well as with an electronic moisture meter.

In older trees where the bacteria are quite active, the added moisture of a log over time can make the log so heavy that the log will no longer float in water. The logs are called sinkers. These logs sank to the bottom of a river, pond or lake; they are sometimes harvested today, sometimes a century after sinking, but usually the bacterial damage that caused the logs to sink also affects processing today. Western hemlock is one species that was noted for having many sinkers.

Usually with wood under 1-1/2-inch thick, the moisture dissipates slowly over six months or less. But in thick lumber, I have seen wet pockets in walnut, cedar, spruce, hemlock, and acacia (there are more), that were present two years after sawing and drying. I have also seen the pockets with enough moisture to support fungal growth within a supposed dry piece of wood.

Enzymes & shake. Bacteria create enzymes (also called a protein; probably Pectinase), that destroy part of the wood. Overall, the wood in the wet pocket becomes very weak and is prone to development of shake (cracks parallel to the growth rings). Shake is virtually a 100% indicator of a serious bacterial infection in the living tree. This shake in the trees, logs and green lumber is often called wind shake as, until the 1980s, shake was attributed to the wind blowing very hard.

Ends checks. As bacteria are only at one end of a log, and require years to work their way up the stem, it is very common to notice shake on only one end (the butt end of the butt log). Further, due to weakness of the infected wood, there will be a lot of normal appearing end checks. End coating may not control these end checks.
Note that end checks wider than ½ inch are due to stress in the living tree; this high stress level and the weak infected wood lead to shake and severe checking. Oftentimes, trimming two feet off the end of the lumber will eliminate the troublesome bacterial infections. Such action will have a big impact on production costs.

Surface checks/internal checks (honeycomb). The wet pockets eventually dry and shrink (or try to shrink) at the end of drying and even later after the final wood product is manufactured. So, the eventual shrinkage of this weakened wood occurs after manufacturing and creates many cracks even in dry wood. Normally, uninfected dry wood is too strong, and moisture related stress is too small, to create new checks or cracks in drying wood.
As the bacteria are in pockets and move slowly up the stem, the bacterial problems will not be seen in very much of the lumber — maybe 10% in an extreme case, usually less. A lot of checking can be attributed to incorrect drying environment, and not bacterial effects.

Foul odor. Further, the bacteria create some fatty acids that turn rancid, giving the wood a nasty odor, especially in high humidity conditions. The odor is more noticeable at high humidities and when the wood is used where air circulation is poor, as in a closet or inside a cabinet. Detection of the odor is a 100% indicator of bacteria. There is no way to coat the wood to stop the odor; replacement is the only option. Obviously, seeing shake and smelling the foul odor should suggest that such wood should be withdrawn from production.

Machining defects. Weakened wood can result in chipped grain and other defects in and around the wet pocket.

Discoloration of finish. Bacteria also create chemicals that might discolor wood and often discolor the finish, as well as cause finish failures. Often the outer rim of the wet pocket has a darker ring. Often, the discoloration develops over six months or longer after the wood product is finished.

Discoloration of wood. Sometimes the wet pocket has a grayish or other color that is different than the surrounding wood. Sometimes wet pockets will have some fungi that are present. They do not cause the defects, other than color changes. They merely find the wet pocket moisture helpful for the fungi growth.

Summary
Bacteria in the living tree can cause somewhat random occurrences of defects in manufacturing and in the final product. Three main defects to help in identification are the foul odor especially when humid; shake; and much higher than normal moisture in pockets. Increased end checking on one end of the lumber is common. Surface and end checking are often seen in and around wet pockets. Discolored wood and discoloration of finishes is common in and around the pockets. Machining defects can occur.

Q We have lost two larger sawmills as our main suppliers, so we are going to smaller mills. We seem to have more warp occurring in the kiln now. Side bend seems to be a yield loss. Comments and suggestions please.

A It is surprising how often I receive comments about warp during drying. It is a key point, mentioned in this column before, that all warp except cup cannot be controlled in the drying processes. The reason is that side bend, bow, twist, and kink are all caused by either the sawing procedure used by the sawmill’s sawyer (such as not sawing parallel to the bark or not rotating 180 degrees from the opening face to the next face) or by unusual grain in the log, such as crooked or sleepy log, large knot or branch, twisted grain). Also, although cupping is affected by drying speed, extremely low humidities near the end of drying, rewetting partly dried lumber, and low final MCs, the tendency for cupping is dramatically increased as lumber is sawn from near the pith (within 15 rings or so from the center); such lumber will,often be No. 2 Common or lower hardwood grade.

Side bend occurs when the annual ring pattern, viewed from the end of lumber, does not have the rings center, left to right. That is, the left edge is not a mirror image or the rings on the right edge. Incorrect log rotation at the sawmill is the cause of this.
In short, your lumber buyers need to learn the correct sawing procedures, and then work with their clients to assure that warp tendency is minimized. These procedures can also increase yield and value of the lumber as well. Sawmills, worried more about production volume and not quality might need special care, as they will mention many times how they have been sawing their way for 20 years.

Q Does the powderpost beetle get into wet wood?  I have read your articles about them and they are in dry wood, but I think I haven some that came in wet wood before we got into the kiln.  Thanks.

A There are several different powderpost beetles that affect wood, but normally the one we find in North American infects dry wood.  This one is the lyctus powderpost beetle that prefers 20% MC down to 7% MC and is found in hardwoods (leaf trees) only.  This MC range is where eggs will hatch, pupa and larva will grow and eventually (10 to 12 months) leave the wood as a mature insect.  It is common, after they leave, to find the fine powder dust, technically called frass, in their holes.  The dust can come out of the holes when the wood is hit with a hammer or other device, to loosen the frass.

There is one very common wet powderpost beetle that we find in logs, air drying stickers and lumber.  It prefers above 25% MC and both hardwoods and softwoods.  It is usually called an ambrosia beetle. However, it is a true powderpost beetle with frass in the exit holes. Its life cycle is several weeks, so they quickly multiply in warm weather air drying.  In a log missing some of its bark, ambrosia powderpost beetles can be found in warm weather.            

SPECIAL ALERT.  In any kiln that exceeds 140 F, we can be certain that all eggs, pupa, larva and insects in the wood are killed.  A few kilns for lumber may reach only 115 F or so, however, and so do not sterilize the wood.  Obviously, the kiln temperature does not affect preesixting damage. Frass from air drying will be present after kiln drying, potentially creating small piles that could be misinterpreted as coming from lyctid powderpost beetles — false conclusion.