46 BULLETIN 871, U. S. DEPARTMENT OF AGRICULTURE. 



An examination of knots on the outside of a log or tree usually 

 does not give any reliable indication of the condition of the heart- 

 wood with respect to its degree of soundness. In this' respect dry- 

 rot differs markedly from the stringy brown-rot (Echinodontium 

 tinctorium) in white fir and the ring scale or red-rot (Trametes pini) 

 in Douglas fir. 



Fire and knots are responsible for over 90 per cent of the infections 

 and severe cull cases; other factors are of minor importance. 



About 3 per cent of the infections on the combined areas entered 

 through wounds the causes of which it was impossible to exactly 

 determine. Some of these may have been fire wounds, others 

 lightning. Of all such wounds 23 per cent subsequently became 

 infected. 



Lightning is of little importance as a means of entrance for dry-rot. 

 On the intermediate area only 2.5 per cent of the infections are 

 traced to this source, on the optimum area 1.5 per cent, and for the 

 combined areas 2 per cent. As a rule, lightning causes small super- 

 ficial scars offering little opportunity for inoculation, but at times 

 large areas of the cambium and bark are killed. This dead bark 

 then drops off, exposing the sapwood, which dries out. Cracks 

 opening up into the heartwood are formed, and such large areas 

 offer a good chance for the lodgment of fungous spores. This con- 

 dition is reflected in the percentage of risk of inoculation when it is 

 found that 19.2 per cent of the lightning-struck trees on the optimum 

 area, 31.8 per cent on the intermediate area, and 25 per cent for the 

 combined areas were infected by dry-rot through lightning wounds. 

 This figure is higher than that for all other factors except fire. The 

 chief reason, then, that lightning wounds are of so little importance 

 in relation to decay is not that the character of wounding on the 

 whole is such as to offer little opportunity for inoculation, but rather 

 that this type of wounding is rare. 



As an actual means of entrance of decay, frost cracks are even less 

 important than lightning wounds. These cracks, while often of 

 considerable length, even then present only a very narrow opening 

 exposed to the air, the chances of fungous spores lodging in such a 

 small opening being exceedingly small. Not quite 1 per cent of the 

 infections for the combined areas entered through frost cracks, 

 while the risk of infection is only 10.4 per cent lower than for all 

 others except broken and dead tops. But though a rare source of 

 infection, frost cracks sometimes carry an infection, entering through 

 some other type of wound, over a greater linear extent in the heart- 

 wood than might normally be expected, thus resulting in a large 

 proportion of cull. The pockets of dry-rot do not occur in the wood 

 immediately adjacent to the cleft or crack, but are usually found 

 some distance removed, leaving the wood around the crack sound. 



