Dec. 6,1915 
Honeycomb Heart-Rot of Oaks 
425 
COMPARISON OP ROTS OP STEREUM SUBPILEATUM AND POLYPORUS PILOTAE 
In the writer's investigation in the Ozarks no attempt was made in 
the field to separate the rot caused by P. pilotae from that caused by 
S. subpileatum , since both in their early stages produce small delignified 
areas in the diseased heartwood of living trees. It was therefore difficult 
to determine which fungus produced the rot unless the sporophores were 
present. Attention was called to this resemblance in a previous article 
by the writer. 1 However, the final stage of the rot produced by P. 
pilotae is quite distinct from that of 5 . subpileatum. The rot caused 
by P. pilotae usually moves upward in the infected wood, along certain 
well-defined zones consisting of several. annual rings of growth of the 
wood. These zones are usually separated by zones of apparently sound 
tissue—that is, the rot moves upward or longitudinally in the tree 
more rapidly than it does radially. The rot caused by S. subpileatum 
does not seem to form definite zones of infected wood separated by 
sound zones, at least in the white oak, but seems to move as rapidly 
radially as longitudinally in the attacked heartwood, thus forming a 
uniform cylinder of rotted wood in the heartwood of the trees. If 
this character should prove constant, one could use it in field work 
for differentiating this rot from the earlier stages of the rot of P. pilotae . 
However, in well-advanced stages of rot, the presence of typical lens¬ 
shaped to cylindrical pockets occupying practically all of the infected 
heartwood is fairly indicative that the rot in question is caused by S. 
subpileatum. 
ENTRANCE OP THE FUNGUS INTO THE HOST 
The fungus S. subpileatum , so far as the writer knows, enters the wood 
of the hosts only through wounds which expose the heartwood. The 
most common point of entrance is through wounds, usually fire scars, 
in the butt of the trees, although it also frequently enters through 
branch stubs. The writer found this rot several times in the tops of 
living white-oak and black-oak trees in the Ozark National Forest, 
Arkansas. In every case the fungus had undoubtedly entered through 
a branch stub. It produces the same type of rot (PI. XLI, fig. 4 and 
7) in the tops as it does in the butts, even to the peculiar honeycomb¬ 
like odor. 
No instances were found where this rot had entered a living tree 
through the dead sap wood of a wound, nor wher.e it had entered a dead 
tree or log through the sapwood. It is very probable, however, that 
the fungus does attack dead timber in this manner, since many examples 
were found where the fungus had grown from the heartwood into the 
dead sapwood of felled trees. 
1 Long, W. H. Three undescribed heart-rots of hardwood trees, especially of oak. In Jour. Agr. 
Research, v. i, no. 2, p. 109-128, pi. 7-8. 1913. 
