June, 1955 
viable longer under comparatively dry 
conditions than when kept moist, and the 
resistance of the spores to extremes of tem- 
perature is greater under dry conditions. 
Heat may affect reproduction by hasten- 
ing the fungus to maturity (Lilly & Bar- 
nett 1951). 
Weather conditions are known to influ- 
ence the incidence and relative prevalence 
of some plant diseases by influencing the 
availability of inoculum in the field. Ling 
(1945), working with stripe rust of wheat 
in China, found that the amount and dis- 
tribution of rainfall in late winter and 
spring are most important in determining 
rust epidemics. On the other hand, An- 
derson & Rankin (1914) found that win- 
ter conditions have little effect on the via- 
bility of pycnospores of Endothia para- 
sitica. 
Wilkins (1938) stated that the age of 
the spore is probably the most important 
single factor influencing germination of 
ascospores of Ustulina vulgaris. By mak- 
ing collections of ascospores and conidia of 
U. vulgaris at intervals during the autumn 
and winter, he showed that the spores soon 
lose the power to germinate. 
Henry (1944) found that the oak wilt 
fungus grew best within a range of 24 to 
28 degrees C. Young (1949) found the 
optimum range to be 22 to 26 degrees C. 
The optimum pH range for growth was 
found by Barnett & Lilly (1952) and 
Young (1949) to lie between pH 5 and 
pH 7, and to have limits at pH 3 and pH 
SE Young obtained maximum germination 
of conidia on agar of low dextrose content 
at 25 to 30 degrees C. Henry obtained 
best germination of conidia in 1.25 per 
cent malt solution at 24 degrees C. Little 
previous work has been reported dealing 
with germination requirements of asco- 
spores of the fungus. Bretz (1952d) 
found that the ascospores germinated rap- 
idly in 2 per cent dextrose solution at 25 
degrees C. Stessel & Zuckerman (1953), 
using ascospores taken in October from 
naturally occurring perithecia, obtained 
30 per cent germination in 2 per cent dex- 
trose solution at 25 degrees C. 
The availability of oak wilt isocula un- 
der natural conditions and the effects of 
environmental conditions on longevity of 
the fungus have received some attention. 
McLaughlin & True (1952) reported sur- 
Curt: Oak Witt INocuLA 
279 
vival of conidia of the oak wilt fungus for 
173 days on a glass surface when kept at 
10 degrees C. in controlled low relative 
humidities. The survival period was much 
shorter at temperatures above 25 degrees 
C. Jewell (1953) found that low tem- 
peratures and low relative humidity fa- 
vored the longevity of ascospores of the 
oak wilt fungus im vitro. Curl (1953) 
reported that the greatest concentration of 
viable, naturally occurring conidia and 
ascospores of the oak wilt fungus in IlIli- 
nois was found during March, April, and 
May. Young (1949) demonstrated in 
Iowa that the fungus lives over winter in 
trees that become infected late in the sum- 
mer. Young & Spilker (1952) in Ohio 
failed to obtain the fungus from lumber 
that had been cut from wilt-killed oaks 
and piled during the summer, but isolated 
the fungus from large twigs, slabs, and 
stump wedges for a period of 3 weeks. 
Bretz & Morison (1953) found that the 
survival of the oak wilt fungus in small- 
diameter infected twig samples is relatively 
short at temperatures of 20 to 25 degrees 
C. and above. Fergus (1953) reported 
the presence of mycelial mats of the fungus 
in nature in Pennsylvania from March 
through November, except in April and 
July. Morris & Fergus (1952) noted the 
appearance of mats in early March and 
found them to be viable a month later. 
They also observed that two mats collected 
in May continued to produce new peri- 
thecia and ascospores for approximately 2 
months when kept at 8 degrees C. in a 
moist chamber. Campbell & French 
(1953) found mycelial mats of the fungus 
in Minnesota in November after 244 
months of drought. 
The possible significance of mycelial 
mats of Endoconidiophora fagacearum in 
relation to transmission of the disease has 
been discussed by various workers. Curl 
et al. (1953) noted the presence of insects, 
which belonged to three families, beneath 
the bark of all mat-bearing trees exam- 
ined in Illinois. Some larvae and adu!ts 
were seen in direct contact with the fun- 
gus. True ef al. (1952) pointed out that 
mycelial mats might serve as natural res- 
ervoirs of oak wilt incculum. Craighead 
& Morris (1952) observed more than 20 
species of beetles, flies, and mites on mats 
of the oak wilt fungus. Morris & Fergus 
