22 
BULLETIN 1474, U. 
S. DEPARTMENT OF AGRICULTURE 
Table 2. — Effect of definite maintained, temperatures for various periods on 
infection and, incubation of Phomopsis citri on grapefruit leaves at the end 
of a 3-week holding period 
Time exposed to 
Percentage and character of melanose infection 
inoculum 
10° C. 
15° C. 
20° C. 
25° C. 
30° C. 




0. 


15, very slight. 
91, very slight. 
100, very slight. 
100, medium. _ 
do 


0. 
12 hours 
14 hours 
16 hours 
pop 
67, slight 
100, very slight. 
100, medium. _ 
100, severe 
100, very slight. 
100, slight 
100, medium. . 
66, very slight. 
100, very slight. 
Do. 
33, very slight. 
20, very slight 
100, medium.. 
do_. 
100, medium. . 
do 
Do. 
100, medium. 
do 
100, severe 
100, severe 
Do. 
100, slight 
36 hours . 
do. . 
do 
do 
100, medium 
86, medium. _ 
From the data presented in Table 2 it is seen that infection took 
place at the four higher temperatures in 12 hours. At the lower 
temperature longer exposures were necessary to cause infection. As 
much infection occurred in 12 hours at 25° C. and in 14 hours at 
20° as in 30 hours at 10°. Thus it seems possible that natural infec- 
tion may take place in as short a time as 12 hours if conditions are 
favorable. Sour-orange leaves were as severely infected as the 
grapefruit. 
In order to determine the incubation period for melanose on plants 
held at definite maintained temperatures, five series of holding tests 
were conducted on potted grapefruit plants inoculated, placed imme- 
diately in the temperature compartments, and held there until the 
disease became visible. The results of these holding tests are shown 
in Figure 3. These show that maintained temperatures in the neigh- 
borhood of from 20° to 25° C. are optimum for melanose infection 
on leaves, not only with respect to promptness of development but 
also to severity of infection. At 25° the development was some- 
what slower, but approximately of equal intensity. At 30° it was 
slower than at 25° and of less severity; at 15° the rate of develop- 
ment was still slower, resulting in a slight degree of infection. At 
10° the development of the disease was very slow and resulted in 
a very slight infection. 
OTHEE INOCULATION TESTS ON POTTED CITEUS PLANTS 
A series of plants was inoculated in the usual manner, placed in 
an incubator, held at 25° C, and kept there for varying lengths of 
time, when the plants were removed without disinfecting to the 
relatively dry slat-shade house. Plants which were held at this 
temperature for 20, 54, 78, 102, 126, 154, and 174 hours, respec- 
tively, showed initial signs of infection in from 5 to 11 days from 
inoculations. The degree of infection ranged from very slight to 
medium. This indicates that, under natural conditions with the 
pathogen present, infection may be expected to occur if the suscepti- 
