318 
wounds on mat-producing trees and on 
the inner sides of pieces of bark from dis- 
eased trees while these pieces lie on the 
moist forest floor. Perithecia were never 
found on such mats. 
It was well known before the present 
study was undertaken that the injection 
of either conidia or ascospores of Endo- 
conidiophora fagacearum into healthy 
oaks would result in diseased trees. How- 
ever, it was not realized that the irregu- 
larly shaped cells that constitute the 
pseudoparenchymatous interiors of myce- 
lial pads are capable of rapid germination 
and abundant sporulation comparable to 
that of conidia and ascospores. Like co- 
nidia and ascospores, the pad cells ger- 
minate well on plain water agar, but 
whether they will germinate and produce 
oak wilt symptoms when injected into 
healthy oak trees has not been determined. 
It has been shown that cultures obtained 
from pad cells are pathogenic. Pad cells 
do not seem to be adapted for insect trans- 
mission, but the possibility of their spread 
by squirrels or woodpeckers is not remote. 
While woodpecker damage to mycelial 
mats has not been observed in Illinois, 
damage caused by the feeding of squirrels 
is common. The spread of fungus diseases 
by these and other unusual agents has been 
reported by Gravatt & Marshall (1917), 
Heald & Studhalter (1914), and Talbot 
(1952). 
The decline of diseased oak trees and 
the development and decline of the result- 
ing mycelial mats seem to follow a pat- 
tern. ‘The brown streaks that are nor- 
mally found in diseased trees of the red 
oak group first become more pronounced, 
usually on one side of the tree; then they 
increase in width and length until a large 
area of wood is brown. During this proc- 
ess the bark loses its tight grip on the 
wood, which begins to emit the charac- 
teristic amyl acetate or ripe fruit odor of 
the fungus. Now having “room” to grow, 
the fungus appears in a macroscopic form, 
in most cases in a few days. The time re- 
quired for the conditioning process to 
occur before mats appear is dependent on 
the season of the year. In Illinois the 
first wilt symptoms of the year are seen 
on trees early in June. Trees wilting at 
this time decline rapidly and may produce 
mats late in August or in September. The 
decline of trees that wilt in the last part 
Intinors NaturRAL History SurvEY BULLETIN 
Vol. 26, Art. 3 
of July or in August is retarded by low 
winter temperatures, and these trees may 
not produce mats until the following 
spring or summer, unless the winter is 
unusually mild. Mycelial mats of Endo- 
conidiophora fagacearum have not been 
reported on trees of the white oak group 
in Illinois. The bark on diseased white 
oaks is thin and adheres to the wood, even 
after the trees have reached an advanced 
stage of decline. No ripe fruit odor was 
detected in the white oaks studied. 
In the period of this study, more mats 
were found during April, May, and June 
than in other months. These months were 
the ones during which large numbers of 
bark beetles (Scolytidae) were boring 
hundreds of tiny holes through the bark 
and wood of wilt-killed trees. Such per- 
forating of the bark may have been in- 
strumental in bringing about a subcorti- 
cal aeration that resulted in a condition 
favorable for rapid mat development. 
The inducement of mat formation by in- 
tentional wounding of trees that were 
nearing the mat-producing condition indi- 
cated that a supply of air from outside 
may hasten mat initiation. When the 
wood surface of mat-producing trees was 
covered with plastic, the fungus did not 
grow, at least to a visible form, on the 
wood but grew abundantly on the uncov- 
ered inner bark surface and on other areas 
of uncovered wood. 
Several factors seemed to be influential 
in determining the occurrence of mycelial 
mats and the longevity of inocula in na- 
ture. Weather conditions affected both 
the development and decline of the fun- 
gus. Mat production on individual trees 
seemed to be accelerated following periods 
of cool, rainy weather. Macroscopic 
growth of the fungus continued over a 
longer period of time during the winter 
months than during the summer, and ap- 
parently low winter temperatures, along 
with a minimum of insect activity, in- 
creased the longevity of the fungus mats. 
That the occurrence of new mats in July 
was rare was due probably to high tem-— 
peratures plus the fact that wilt-killed 
trees were, by that time, nearing a state 
of deterioration which favored the growth 
of various wood-rotting fungi other than — 
Endoconidiophora fagacearum. Observa- 
tions have shown that new mats are sel- 
dom found during August in Illinois. ; 
