mill and air dry 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, and end with the worst.
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
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 over-kilning 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.