16 BULLETIN 1163, U. S. DEPARTMENT OF AGRICULTURE. 
should be excluded from all lumber to be used for purposes requiring 
strength and durability. 
The incipient stage of conk-rot is often quite extensive. In one 
tree this stage of decay extended for 29.6 feet vertically in the heart- 
wood beyond the typical decay, while in several infections this figure 
ranged from 10 to 20 feet. The average extent upward of incipient 
decay beyond the typical decay was 3.3 feet and the average extent 
downward was 4 feet. The significance of this difference will be 
touched upon later. The average extent both up and down was 3.5 
feet, based on 145 measurements. 
Red-brown butt-rot is less variable in respect to the extent of 
incipient decay. The greatest extent found was 8.4 feet, while the 
average based on 44 measurements was 1.95 feet. This average is 
based on measurements of upward extent only, since most of the 
infections began in the stump or extended into it and no downward 
measurements were possible. 
Only meager data were available on brown trunk-rot. Based on 
13 measurements the upward extent of the incipient decay was found 
to be 3.5 feet, while the downward extent was 3.6 feet. The com- 
bined average was 3.6 feet. The extreme extent was 8.5 feet above 
typical decay. 
The incipient decay of yellow-brown top-rot had an average extent 
upward of 2.2 feet and downward of 3.8 feet, while the combined 
average was 3.1 feet, based on 58 measurements. In one tree incip- 
ient decay extended up beyond typical decay for a distance of 25.4 
feet. 
Trom the foregoing it can be seen that in all three of the rots in 
which it was possible to make a comparison between the upward 
and downward extent of incipient decay the downward extent 
exceeded the upward on the average, and the difference is most strik- 
ing in yellow-brown top-rot, where most of the infections occur in 
the upper part of the bole. This difference is probably explained by 
the well-known fact that older trees are much more subject to decay 
than younger ones, and therefore it follows that older heartwood is 
more susceptible than younger. As the fungus progresses downward 
in the heartwood of a tree it encounters wood gradually increasing 
in age and easier to decay, while as it moves upward younger wood 
which offers more resistance is continually invaded, decreasing the 
extent of both typical and incipient decay. This tendency for decay 
to work more rapidly downward than upward is in keeping with 
other observations (1, p. 21). 
The figures presented on the extent of incipient decay show that 
this is quite variable and indicate the need for careful inspection to 
eliminate this type of defect from timbers where durability and 
strength are a prerequisite. 
SUMMARY. 
The four principal decays in Douglas fir are conk-rot caused by 
the ring-scale fungus (Trametes pini), red-brown butt-rot caused by 
the velvet-top fungus (Poly poms schweinitzii) , brown trunk-rot 
caused by the quinine fungus (Fomes laricis), and yellow-brown top- 
rot caused by the rose-colored Fomes (Fomes roseus). Conk-rot and 
brown trunk-rot usually occur in the body of the trunk, red-brown 
butt-rot is commonly confined to the stump and first log, while yellow- 
