278 
fungus in nature. It seems that such infor- 
mation would be applicable in finding the 
means of spread of the disease beyond root 
graft distances and in selecting and de- 
veloping effective control measures. 
ACKNOWLEDGMENTS 
The author gratefully acknowledges the 
advice and guidance given him during the 
course of this study by the late Dr. Leo R. 
Tehon. While the study was being made, 
Dr. Tehon was Botanist and Head of the 
Section of Applied Botany and Plant 
Pathology, Illinois Natural History Sur- 
vey, and Professor of Plant Pathology, 
University of Illinois. The author wishes 
to express sincere appreciation to Dr. J. 
C. Carter, Plant Pathologist and present 
Head of the Section of Applied Botany 
and Plant Pathology of the Illinois Nat- 
ural History Survey, for many helpful 
suggestions. 
Others to whom the author wishes to 
give special recognition for contributions 
to the progress of the research and the 
preparation of the data are Mr. Noel B. 
Wysong, Chief Forester of the Forest Pre- 
serve District of Cook County; Mr. H. 
W. Fox, Forester of the Sinnissippi 
Forest, Oregon, Illinois; Mr. R. E. 
Owens, Director of Parks, Peoria, IIli- 
nois; Mr. Ray R. Hamm, University of 
Illinois Photographic Laboratory; Dr. M. 
W. Sanderson, Mr. James S. Ayars, Mr. 
W. E. Clark, Mr. J. W. Curfman, and 
Mrs. Rovenia M. Fitz-Gerald, all at the 
time of the study members of the Illinois 
Natural History Survey staff. 
REVIEW OF LITERATURE 
A short time after the cause of oak wilt 
was established as a fungus (Anonymous 
1942), the conidial stage was named 
Chalara quercina by Henry (1944). Later 
Bretz (1951, 19524) succeeded in pro- 
ducing perithecia of the fungus in labora- 
tory cultures and named this stage Endo- 
conidiophora fagacearum. Symptoms of the 
disease have been adequately described by 
various workers (Henry & Moses 1943, 
Henry et al. 1944, Henry & Riker 1947, 
Riker 1948, Young 1949). 
In 1949 Kuntz & Riker (1950a) dem- 
onstrated local spread of the disease from 
ILtinois NATURAL History Survey BULLETIN 
Vol. 26, Art. 3 
diseased to healthy trees through natural 
root grafts. The means by which the 
pathogen is transmitted beyond root graft 
distances has not been determined. While 
transmission through root grafts is highly 
important in parts of the Midwest where 
the disease has become well established, it 
does not appear to be the primary method 
of spread in states such as Pennsylvania 
(Fergus 1953) and Ohio (Young ef al. 
1953). There many isolated single-tree in- 
fections occur, indicating long-distance 
transmission by a spore-carrying vector. 
For a decade following the identifica- 
tion of the causal fungus, the form of its 
fructification in nature was not known. In 
the summer and fall of 1951 Curl et al. 
(1952) discovered mycelial mats of En- 
doconidiophora fagacearum under the 
loose bark of diseased oak trees in Illinois. 
Endoconidia were present in large num- 
bers on these mats. A little later Stessel 
& Zuckerman (1953) discovered the ascig- 
erous stage on mats in nature. Reports of 
endoconidium- and __perithecium-bearing 
mats in other states (Barnett et al. 1952, 
Campbell & French 1953, Morris & Fer- 
gus 1952, Staley & True 1952) indicate 
that this type of growth is common on 
wilt-killed oak trees. The implication of 
the possible importance of mats in serving 
as reservoirs of inoculum from which the 
unknown vector or vectors might spread 
the disease is readily understood. Curl ef 
a’. (1953) described the thick, sclerotium- 
like pad that usually occupies the center of 
the mycelial mat, and they succeeded in 
producing a similar structure in labora- 
tory cultures. Zuckerman & Curl (1953) 
presented proof that the pad is a growth 
form of E. fagacearum and showed that 
single cells from laboratory-grown pads 
were capable of continuing growth. 
There are many physiological agents 
that affect the ability of fungus spores to 
germinate (Gottlieb 1950, Hawker 1950, 
Lilly & Barnett 1951, 
1947). Temperature and moisture have 
received more attention than other factors 
since they influence both the germinability 
of spores and the infection of the host. 
Many workers (Anderson et al. 1948, 
Heald & Gardner 1914, Heald & Stud- ; 
halter 1915, Ling 1945, McCrea 1931, 
Rosen & Weetman 1940) have shownill 
that spores of various fungi usually remain 
Wolf & Wolf — 
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