June, 1955 
Apparently the freezing of mats during 
the winter does not in itself have a marked 
deteriorating effect on the fungus in na- 
ture. Laboratory tests showed that a tem- 
perature of 0 degrees C. favored longevity 
of the fungus in both humid and dry at- 
mospheres; a continuous dry atmosphere 
was more favorable to the fungus than a 
continuous humid atmosphere. In _ the 
field, the fluctuating temperature and 
moisture conditions, combined with the 
effects of mat-invading microorganisms, 
may account in part for the fact that the 
germinability of spores collected during 
the winter months was lower than that of 
spores collected in spring and early sum- 
mer. Also, during the winter months, 
fewer new mats form than in spring and 
early summer, and it was found that 
spores from new or immature mats were 
more highly viable than those from older 
mats. Perithecia occurred most abun- 
dantly on aging mats at temperatures of 
about 16 degrees C. This was the aver- 
age temperature for the five study areas 
in May, 1953, the month during which 
the greatest number of perithecia occurred. 
The data obtained in this study indi- 
cate that the greatest inoculum potential 
of the oak wilt disease may be expected to 
occur in Illinois during April, May, and 
June. This indication is made clear by 
the following facts: (1) The greatest 
number of mats was found in May and 
June, (2) more viable conidia were pres- 
ent on mat samples taken in April than in 
other months, (3) the average size of 
mats that developed was greater in May 
and June than in other months, (4) the 
largest number of perithecia occurred on 
mats in May, (5) ascospores were most 
highly viable in April and May, (6) pad 
cells from immature mats were most 
highly viable and more mats of this class 
were found in March, April, and May 
than in other months. It is also interest- 
ing to note, from the standpoint of possi- 
ble transmission, that insect activity was 
greatest on mats during April, May, and 
June. 
Insects, particularly the Nitidulidae, in 
addition to being possible disease vectors 
and spermatizers of mycelial mats, are 
probably the most potent destroyers of 
naturally occurring oak wilt inocula. The 
feeding of squirrels on mycelial pads ap- 
pears to be of minor significance in reduc- 
Curt: Oak Witt INocuLa 
Ag. 
ing the amount of inoculum on trees, but 
the possibility of their spreading conidia, 
ascospores, or pad cells to healthy trees is 
evident. 
The perpetuation of the oak wilt dis- 
ease seems to be dependent not on the abil- 
ity of the fruiting mats of the fungus to 
survive long periods of adverse conditions 
but rather on the continued existence of 
the fungus in an oak wilt area where new 
inocula are produced from time to time. 
As some trees cease to produce mycelial 
mats, other trees, more recently wilted, 
continue the process. Thus, an almost con- 
stant supply of fresh inoculum is present. 
To control the spread of such a disease 
beyond root-graft distances, it would 
seem necessary either to prevent the mac- 
roscopic fruiting of the fungus, particu- 
larly during March, April, May, and 
June, or to prevent the feeding of insects 
and other possible vectors on the mycelial 
mats on both standing and felled trees. 
The possibility of the extensive spread of 
the oak wilt disease from fungus develop- 
ment on bark or wood chips on the forest 
floor seems negligible and might be elim- 
inated altogether by placing the pieces so 
that they dry rapidly. 
SUMMARY 
Need for information that wou!d be 
useful in explaining the spread of oak wilt 
prompted an intensive study of the avail- 
ability of oak wilt inocula. This study 
was made in five major wilt areas in IIli- 
nois and covered the 10-month period 
from October, 1952, through July, 1953. 
Results of preliminary laboratory tests 
showed that a 2 per cent water agar me- 
dium and an incubation temperature of 
28 degrees C. for 36 hours were best for 
determining the germinability of both 
conidia and ascospores of the oak wilt fun- 
gus, Endoconidiophora fagacearum. 
Good germination was obtained with 
cells from the interiors of pads of Endo- 
conidiophora fagacearum mycelial mats 
that had not started to decline. Mycelial 
pad cells were found to have a slightly 
lower optimum temperature for germina- 
tion than have conidia and ascospores. 
Germinated pad cells sporulated abun- 
dantly on water agar. 
Formation of Endoconidiophora faga- 
cearum perithecia on a wheat bran-agar 
