252 
Journal of Agricultural Research 
Vol. XXI, No. 4 
monthly means for the second period are well above the infection range 
as determined by the inoculation experiments. 
It is of interest to note also that the lowest average for February, 
March, and April was 17 0 C. in 1915, the year mentioned by Grossen- 
bacher (5) as one in which scab developed in great abundance on the 
growth starting in early spring. These data taken in connection with 
the maintained temperature experiments seem to explain the frequent 
absence of scab on tender growth of the second or third cycles even in 
the moist periods of summer. The spores may be present, the moisture 
and growth conditions may be favorable, but the temperature may be 
too high. 
The occurrence of severe late scab in some seasons is probably due to 
a decided drop in the temperature, bringing it well within the infection 
range for a sufficient length of time for infection. 
SUMMARY 
(1) Experiments to determine the influence of temperature on scab 
infection on young sour-orange (Citrus aurantium ) leaves and on the 
growth and spore formation of the causal organism (Cladosporium citri) 
are reported. 
(2) The inoculation temperatures resulting in infections of growing 
plants under conditions of rapid growth and abundant moisture were 
16°, 18.5°, 19 0 , 20 0 , 21 0 , and 23 0 C. No infections were obtained under 
the same conditions on plants inoculated at 12 0 , 13.5° 14 0 , 24.5 0 , 26.5°, 
27 - 5 °. 31 32 °, 32 - 5 °. 34 - 5 °, 36 °, 38.5°, 40°, 42.5°, and 44.5°. 
(3) Detached leaves floated in water with the scab fungus were infected 
at 16 0 , 18.5°, 21 0 , 24.5°, and 27.5 0 C. 
(4) The temperature at which the greatest extension of hyphae of 
the causal organism in cultures was observed was 21 0 C. The highest 
temperature at which extension was observed in water was 27.5°, and 
in commeal agar 32° 
( 5 ) Spores were observed in 48 hours or sooner in all the temperatures 
at which growth took place except 32 0 C. At certain temperatures the 
spores appeared to be ejected with considerable force from the ends of 
the hyphae. 
(6) The temperature at which the time was shortest between inocu¬ 
lation and first observance of signs of disease was usually 21 0 C. in the 
different tests. This time increased toward the upper and lower limits 
of the infection range. 
(7) This limited range of temperature at which infection of a sus¬ 
ceptible host took place under the presumably favorable conditions of 
the experiment appears to explain the great differences observed in 
the occurrence and severity of scab from year to year and from season 
to season in citrus orchards. It also explains the differences in results 
of previous inoculation experiments not hitherto understood. 
