Nov. 1, 1924 
Alternaria Leafspot of Cauliflower 
429 
Loops of a spore suspension of the 
desired concentration were placed upon 
clean cover slips, which were then 
inverted over glass rings cemented to 
slides with a mixture of beeswax and 
vaseline, the cover slips being sealed to 
the rings with vaseline. A small 
quantity of the same medium used in 
preparing the spore suspension was 
placed in the bottom of each cell. The 
hanging drops were placed in the 
electrically heated and controlled Alt¬ 
man incubators as soon as possible 
after they were prepared. The tem¬ 
perature of these incubators remained 
quite constant, varying for the most 
part less than a degree, except for the 
lower ones, which depended upon the 
amount of ice in the cooling chamber, 
where there was sometimes a variation 
of 2° to 2.5° C. A pan of water was 
kept in the bottom of each chamber to 
keep the air moist. From four to 
eight hanging drops were used at each 
temperature. These were removed 
from the incubators at frequent inter¬ 
vals and observed under the micro¬ 
scope. The mounts were examined 
near the incubators, and were kept out 
usually less than a minute. Only at 
the highest temperatures was there 
any appreciable fluctuation due to the 
opening of the doors, and in these 
cases the readjustment occurred very 
quickly. In the different trials the 
average time found necessary for the 
spores which germinated first to pro¬ 
duce germ tubes equal in length to the 
width of the spore was used as the 
time required for germination. The 
figures thus obtained, together with the 
average of the temperatures to which 
the spores were exposed in the different 
experiments, were used in plotting a 
curve which shows the variation in the 
time for germination to start due to 
the difference in temperature (fig. 1). 
Other criteria might have been used 
for determining the influence of tem¬ 
perature upon this vital phenomenon. 
However, the important thing in 
connection with the study of brown- 
rot is to know how soon infection can 
take place at the different temperatures 
and this obviously depends primarily 
upon the time necessary for the 
spores to germinate. In fact, it is 
usually of little practical importance 
to know whether or not all the spores 
germinate within a certain time at a 
given temperature since most fungi 
produce spores so profusely that if 
only a small percentage of them 
germinate a large amount of damage 
may result. About 99 pefr cent of the 
spores studied in these investigations 
germinated, except near the maximum 
temperature, where some of them 
were no doubt killed before germina¬ 
tion started. 
In the curve shown in Figure 1 the 
time in hours necessary for germination 
to take place is plotted on the abscissa, 
while the temperature in degress 
centigrade is plotted on the ordinate. 
The hanging drops were kept under 
observation for some time after ger¬ 
mination started, to make soire that 
the other spores germinated soon 
afterwards. No germination took 
place at the maximum temperature 
tried (46° C.). As no temperature 
between 40.5° and 46° was tried, the 
temperature at which germination 
would have just taken place was not 
determined. 
An examination of the curve shows 
that the optimum temperature for 
spore germination is somewhere be¬ 
tween 33° and 35° C. From the 
optimum the time necessary for germi¬ 
nation to begin gradually increased 
as the temperature became lower, but 
increased quite rapidly above this 
temperature until the maximum was 
reached. The minimum for germin¬ 
ation was not determined accurately 
for want of a sufficiently low tempera¬ 
ture. Germination took place in 48 
hours at an average temperature of 
1.5°. It is apparent from these data 
that so far as spore germination is a 
factor, infection by the brownrot 
fungus can take pk ce in transit. 
The effect of temperature upon the 
growth of the mycelium was studied 
in Petri dish cultures containing 20 cc. 
of a 2 per cent Irish potato agar. 
A small drop of a suspension of the 
spores in water was placed in the center 
of each plate with a 2 mm. platinum 
loop. The plates were placed in the 
incubators as soon as they were in¬ 
oculated and the rate of growth de- 
ermined by measuring twice daily 
the diameter of the mycelial disks 
formed. Five Petri dishes were used 
for each temperature. The experi¬ 
ment was repeated and the average 
diameters of the mycelial colonies as 
well as the average temperatures for 
the times that the cultures were 
grown were used in plotting the graphs 
in Figure 2. The base line of these 
EXPLANATORY LEGEND FOR PLATE 4 
Cauliflower head grown in the field, badly affected with brownrot resulting from artificial inoculation. 
This head has spread and the flower stalks have begun to elongate preparatory to blossoming. All of 
the blackened area was affected. About natural size 
99186—25t-2 
