Brown Rot, White Rot and other Wood Pestilence.

[Bob Smalser]

Bob,
Punky wood...is that the stuff that isn't really rotted yet, but it's on it's way? Dark, even black, solid but a little soft, hard to dry out. I wonder if my floors and keel are punky at this point? How does one define it?

Grow, mill and airdry enough lumber, plus restore several old boats and you eventually get to see it all, so I’ve saved some examples of pest damage to show you what’s what, what’s bad, what’s not, what you can do about it, and some why’s and wherefores. I’ll start with the worst.

Brown Rot

Often called “dry rot”, but there’s nothing dry about the results of this fungus attack that consumes both the cellulose that provides wood fibers their strength and the lignin that holds those fibers together. Brown Rot leaves some lignin residue that creates the distinctive cube pattern associated with this rot. Like all fungus, the spores of these species are everywhere in the environment, but they need warmth, oxygen, moisture and food to germinate and grow. Alter those conditions to inhibit it.

Temperature. Between 75 and 90 degrees F are ideal, and below or above that range germination tapers off on a linear scale until 40 or 105 degrees where it stops entirely. Where I am, 55 degrees and lower stops significant fungus activity in wood. High temperatures kill fungus entirely, which is one reason for kilning wood to the international standard of “56/30” or 56 degrees C for 30 minutes.

Oxygen. While oxygen is required, significant airflow deters spores from gaining a foothold, so ventilation is useful in prevention.

Moisture. Like all organisms, these need water, and 20% moisture content is their threshold. That’s another reason why construction lumber for domestic consumption is generally kilned to 19% – to prevent the stains and worse damage caused by fungus. Kilning to the 56/30 international standard for a “KD-HT” (heat treated) stamp also coincides nicely with the domestic “KD-19” (kiln dried 19%) stamp. “HT” wood here runs around 16% MC, which is adequately dry for immediate exterior or marine use without the danger of overkilning damage.

Food. While the species that cause Brown Rot eat any cellulose, most other fungus prefer the sugars found in sapwood, where most problems begin. Saturating or coating wood with preservatives like copper and lead poison their food to prevent growth, as do the natural extractives found in the heartwood of durable species. Extractive is a 5-dollar word describing the concentration of salts, colorful resins and acids that occur as the cells of sopping-wet, weak, sugar-laden sapwood transition to the relatively-dry, tough, heartwood that provides the tree’s stem its strength.

Because it consumes both cellulose and lignin and is self-generating, Brown Rot causes severe damage quickly if left unchecked. Here you see that planing the surface didn’t remove the rot pockets, which remain discolored and spongy. If I were to plane off a little more surface, I’d arrive at wood merely discolored. This is called “incipient rot” because the fungus species causing the discoloration are still there waiting for the right conditions to grow again.

Brown Rot is self-generating because removal of cellulose combined with lignin damage creates a sponginess that is hygroscopic as well as structurally unsound. Here you see the rotted area of this old cedar fence post is at 29% MC on the “A” Scale of my moisture meter….

…while the sound wood only a few inches above the rot is only at 16% MC on the “B” Scale, well below the 20% threshold for fungal growth, and a huge difference in moisture between sound and rotten. But not for long if that post is left intact and outdoors, because the wet area full of wood-destroying fungus will grow. A good moisture meter with pin electrodes can easily help you find hidden pockets of rot when surveying a boat. Just carry a small tube of 3M 5200 to fix the holes when you are done.

What to do when you find the soft wood that indicates rot? Remove it all the way back to good wood and replace or repair it. Brown Rot is almost always deeper than your first assessment:

Continued….

 

[Bob Smalser]

When excavation reaches the incipient stage of only discoloration of otherwise sound wood, CPES (an epoxy sealer) may be useful to isolate the remaining spores and reduce the wood’s potential to absorb moisture prior to replacing the structure lost to the rot.

White Rot

Sometimes called “slime” or “wet” rot, the fungus species responsible for White Rot are most often found in non-durable hardwoods where they cause spalting. Here you see large patches of White Rot in a sea of Blue Mold. White Rot also consumes both cellulose and lignin, consuming the lignin completely and bleaching out the wood’s color.

Like all fungi, tendrils or hyphae spread the fungus to areas favorable for its growth, these sufficiently large to be obvious.

When they attack softwoods like the Douglas Fir shown here, they don’t seem to penetrate as deeply or as quickly as does Brown Rot and can often be planed off to no ill affect to the use of the wood.

Incipient White Rot on a sapwood edge of roughcut Douglas Fir 2X stock.

Soft Rot

Soft Rot is caused by another group of fungus species and is generally found in wood with sustained high levels of moisture such as telephone poles and other buried posts immediately above ground level, and in water tanks. The entire surface of the wood becomes punky and can be lifted off in sheets. It’s not generally found in boats except in areas of high surface to thickness ratios such as plywood.

Continued….

 

[Bob Smalser]

Lichens

Interesting and sometimes colorful little algae-fungus combinations that are generally harmless and can be brushed off.

Blue Mold and Ambrosia Beetles

You’ll find Blue Mold (or Blue Stain) without Ambrosia Beetles but you’ll never find Ambrosia Beetles without the stain. Various species of molds and sapwood stains feed off the moist sugars in sapwood and discolor it in the process, often permanently.

Here you can see bluish discoloration in the sapwood edge of a Douglas Fir joist. As the joist was recently scrubbed with full-strength bleach, the stain is permanent. You can also see a few bluish pin holes in the sapwood from Ambrosia Beetles. These insects attack stacked logs and downed trees still having intact bark throughout the warm months of the year within as short a period as two weeks. The adult beetles bore through the bark into the wet sapwood, lay their eggs in the tunnels, and depart. Their secretions plus the sugar in the sapwood attracts Blue Mold development in the tunnels, which their larvae feed on after they hatch and also eventually exit the log to continue the cycle. In some woods the result is considered attractive, and there is a market for “Ambrosia Maple” as one example. Prevention measures include milling immediately in summer, debarking or ponding logs waiting to be milled, including continuous irrigation of the log stacks with water, which also inhibits other fungi due to oxygen depletion.

Carpenter Ants

Carpenter ants come in small and large sizes and also don’t eat wood, but use it for nesting. They require warmth and nearby food – remove one or both to drive them away.

The tiny species don’t have big munchers and seem to only attack wood already softened by fungus, here where a sapwood sticker in a board stack rotted and was attacked by the ants. The photo is of the mostly heartwood board beneath it and you can see where the ants attempted to bore, but only succeeded in a small area of sapwood. These aren’t a big problem here, and the damage can be planed off.

The large species are incredibly tough, can chew their way through almost anything, and here prefer to nest in the pith areas of live cedar trees, often forming centuries-old colonies of vast numbers. In the lumber yard, they usually don’t attack stickered stock through around 4X thickness, because there isn’t enough protection from the cold in relatively thin boards. They don’t usually attack stickered 4X and 6X beams like the one shown either, but if you are sufficiently absentminded to restack them touching each other after grading, you provide 8-12X of insulation and a perfect entry point for the ants. When they infest the wood or insulation in your house, what they are attracted to is the warmth. Sprinkle a persistent insecticide like Diazinon around the outside foundation of your house and beneath your lumber stacks when the temperature reaches 55 degrees in the spring.

Powder Post Beetles

A much more serious pest, the various species of Powder Post (or Furniture) Beetles eat the starch contained in wood instead of just nest in it, and penetrate throughout the heartwood as well as the sapwood. They are common in barn timbers, old furniture once stored in barns, and hardwood stacks overgrown with vegetation that inhibits air flow. One variety even penetrates hard varnish finishes to get to the wood beneath. There are various home remedies of fuming and borates but none are 100% reliable. To eliminate every stage of the beetle, infested wood must be kilned per the “KD-HT” schedule previously discussed.

Preventions are usually easier than cures, but knowing which pests are fatal and which are merely cosmetic helps you pick your battles.

Reference: A.J. Panshin and Carl de Zeeuw, Textbook of Wood Technology 3rd Ed, McGraw-Hill 1970.

 

[bjm]

Bob

I have to post a few corrections;

Brown rot fungi typically decay the cellulose component of wood leaving the lignin intact. The white rot fungi, however, attack all components of the wood and it is the removal of lignin that causes the bleaching that led to the term 'white' in white rot. Soft rot typically requires a wetter environment than brown/white rot and is a problem in wet environment, such as cooling towers (where it was originally studied in the 50's).

Brian

 

[Bob Smalser]

Thanks. I tweaked it accordingly.