POLYPORUS DRYADEUS, A ROOT PARASITE 
ON THE OAK 
By W. H. Long, 
Forest Pathologist , Investigations in Forest Pathology, Bureau of Plant Industry 
Bulliard (1789, 1791) 1 figured and described under the name Boletus 
pseudo-igniarius a fungus which most European mycologists believe is 
the plant now called Polyporus dryadeus. Apparently the next record 
of this fungus is by Persoon (1799), where it is described as Boletus 
dryadeus . Again it is described by the same writer in his Synopsis 
Fungorum (1801), where Bulliard’s fungus is listed as a synonym. It 
is first named Polyporus dryadeus by Fries (1821), who describes the 
plant and cites as synonyms the names given by Bulliard and Persoon. 
Hussey in Illustrations of British Mycology (1849) gives a fairly good 
figure of the sporophore and a most excellent mycological description 
of the fungus, with its habitat. 
Since 1849, repeated references to this fungus are found in European 
mycological literature, but nothing was written concerning the rot 
produced by it in the oak until Robert Hartig in his epoch-making 
work on the true nature of the rots of woods (1878) described a heart 
rot of the oak which he attributed to Polyporus dryadeus . A careful 
study of Hartig’s figures and the description of the sporophore which 
he found associated with the white heart-rot so accurately described 
by him is sufficient to convince anyone who is familiar with the true 
P. dryadeus that Hartig’s fungus was not P. dryadeus. It is undoubt¬ 
edly identical with the heart-rotting fungus known in America as P. 
dryophUus and found by Hedgcock (1910 and 1912) to be associated with 
a whitish piped rot in oak. 
Polyporus dryophUus has one character, a hard, granular, sandstone¬ 
like core, that is unique and not possessed by any other polypore known 
to the writer. The sporophore of this plant, represented by numerous 
specimens collected by Hedgcock and the writer in western and south¬ 
western United States, shows this hard, granular core exactly as figured 
and described by Hartig in his article on P. dryadeus. This core extends 
back some distance into the tree in oaks; it is usually irregularly cylindrical 
while in the tree, but on its emergence from the tree it swells into a 
tuberous or spheroid mass and finally occupies the central and rear 
part of the sporophore. (PI. XXI, fig. 1.) If the sporophore is formed 
from a large branch hole, it is usually of the applanate type, with a 
small core, but when the sporophore forms directly on the body of the 
1 Bibliographic citations in parentheses in the text of this article refer to ” Literature cited,” p. 248. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
(*39) 
Vol. I, No. 3 
Dec. 10, 1013 
G—6 
