May 16, 1921 
Temperature Relations of Cladosporium citri 
249 
Table III. — Number of days from inoculation to first indication of disease at different 
temperatures 1 
Days after inoculation at temperature (°C.) of— 
16 
18. 5 
19 
20 
21 
23 
24- s 
26.5 
27 - 5 
Sour-orange seedlings: 
f D May 16, 1917. 
7 
— 
17 
— 
— 
17 
— 
— 
— 
(2) June 5, 1917. 
14 
(3) Sept. 24, 1917. 
Detached leaves: 
6 
5 
— 
— 
5 
— 
— 
— 
— 
(4) In cornmeal agar. 
— 
5 
— 
— 
4 
— 
— 
— 
— 
(5) In water. 
6 
4 
_ 
3 
— 
10 
— 
IO 
1 A dash indicates that no scabs developed on plant placed at the respective temperatures. A blank 
space indicates that no plants were used in a given test at these respective temperatures. 
DISCUSSION 
Infection of sour-orange seedlings was limited to a range of about 8° C., 
16° to 23 0 , inclusive, in these experiments. This range could not have 
been over io° in extent under the same conditions even if more tempera¬ 
tures had been investigated, since plants inoculated at 14 0 and at 24.5 0 
failed to become diseased. This range is seen to include the temperature 
(21 0 ) at which the fungus was found to have the most rapid growth 
in water and in cornmeal agar medium. The temperatures just outside 
the range on the lower side (14 0 and below) are those at which the fungus 
grew somewhat more slowly than it did at the temperatures just outside 
the range on the upper side (24.5 0 and above). If, however, we consider 
the experiment with detached leaves in water (Table III) in which an 
exposure of 15 days was given at the different temperatures, the range 
for infection, though not extended downward, was extended upward to 
2 7-5°) a temperature at which the hyphal growth in water was approxi¬ 
mately the same as that at the lower limit of the range, 16 0 (Table II). 
This may indicate that the rate of extension of the hyphae must be above 
a certain minimum before infection of the liost can occur, a point that 
may have some significance. The rate of extension of the hyphae also 
appears to have a relation to the time-between inoculation and first 
appearance of scab as is shown, in a general way, in Table III. This 
time period, with a single exception, is shortest with temperature at or 
near 21 0 , the temperature for maximum growth if the tests on different 
dates be considered separately. 
It is, of course, not to be definitely concluded that infection can not 
occur with temperatures outside the range within which it was con¬ 
fined in these experiments, but it appears probable that scab is not to 
be expected with temperatures that remain outside this range. Of 
course, orchard temperatures may vary so as to subject the trees to 
temperatures within the range of infection for a sufficient time period to 
allow the penetration of the fungus. No attempt was made to determine 
