20 BULLETIN 1474, U. S. DEPARTMENT OF AGRICULTURE 
TEMPERATURE RELATIONS 
Fawcett (8) has shown that the rate of mycelial growth of Pho- 
mopsis citri on artificial media is affected markedly by temperatures 
above and below the optimum for this organism. In general the 
maximum growth was obtained at temperatures around 27.5° C. 
In order to determine the effect of temperatures upon spore pro- 
duction, cultures of P. citri on sweet-clover stems were placed in 
temperature compartments held at fairly definite maintained tem- 
peratures for 75 days and examinations made at frequent intervals 
for spore production. 
At temperatures of 2° C. (35.6° F.), 4° C. (39.2° F.), 6° C. (42.8° 
F.), and 8° C. (46.4° F.), sporulation did not take place during the 
period under observation. Sporulation occurred at 10° C. (50° F.) 
in 33 days, at 12° C. (53.6° F.) in 30 davs, at 14° C. (57.2° F.) in 
24 days, at 16° C. (60.8° F.) in 18 days, at20° C. (68° F.) in 14 days, 
and 25° C. (77° F.) in 30 days; but at 28° C. (82.4° F.) and 33° C, 
respectively, sporulation had not taken place in 75 days. 
Thus it is evident that spore production was most rapid at about 
20° C. The length of time required for this increased gradually 
with a gradual reduction of temperature, but the necessary time 
increased rapidly as the temperature was raised slightly above its 
optimum, showing that spore development is greatly retarded if not 
absolutely inhibited at 8° above optimum and 12° below optimum. 
The 20° C. (68° F.) optimum for spore production is quite distinct 
from the 27.5° C. (81.5° F.) optimum for vegetative growth. 
SPORE GERMINATION 
Spore germination tests were also made at a number of tempera- 
tures. From pure cultures of P. citri growing on sweet-clover stems, 
fresh viable spores were placed on microscope slides, covered with 
plain agar, and put in Petri dishes. These slides were held in incu- 
bators and microscopic examinations made at frequent intervals. 
Spore germination was found to be most rapid at temperatures 
ranging from 20° C. (68° F.) to 27 p C. (80.6° F.). Good germina- 
tion had taken place within this temperature range in from 18 to 20 
hours. At a temperature of 33° C. (91.4° F.) and at 18° (60.8° F.) 
or below germination was greatly retarded. 
INFECTION AT MAINTAINED TEMPERATURES 
In order to determine the temperature range over which infection 
takes place, a series of inoculation experiments was conducted on the 
leaves of potted grapefruit seedlings. These plants were inoculated 
by saturating absorbent cotton with water rendered cloudy from 
heavy charges of viable spores of P. citri grown in pure culture. 
Swabs of this wetted cotton were placed on the parts to be infected 
and then wrapped in paraffin paper. The plants were then placed 
immediately in incubators for 48 hours, removed from the compart- 
ments, paper and swabs removed, washed for one minute in a 5 per 
cent solution cresol compound or in a 1 to 1,000 mercuric-chloride 
solution, rinsed in running water for two minutes, and placed under 
a slat shade to await developments. The progressive increases in 
