SOME RELATIONS OF TEMPERATURE TO GROWTH 
AND INFECTION IN THE CITRUS SCAB FUNGUS 
CLADOSPORIUM CITRI 1 
By H. S. Fawcett 
Professor of Plant Pathology, Citrus Experiment Station, College^ of Agriculture , 
University of California 
INTRODUCTION 
In some previous work by the author (j) 2 it was found that the results 
of different inoculation tests on rapidly growing sour-orange trees (Citrus 
aurantiumb. ( ij ) with Cladosporium citri Massee were variable. It had 
previously been observed in the citrus orchards of Florida that abundant 
infections from scab did not inevitably follow the presence of abundant 
moisture on rapidly growing tissue, although these two conditions were 
usually present when abundant infection did occur. Seasons were 
encountered when scarcely any infection from scab occurred, even though 
conditions of moisture and growth appeared to be ideal for an outbreak. 
The author was led to suspect that temperature was also an important 
factor in infection. 
The experiments which form the basis of this paper were planned to 
determine what influence different temperatures might have, first, upon 
infection by the fungus when the two other conditions previously men¬ 
tioned, abundant moisture on the leaves and rapid growth were main¬ 
tained, and second, on the growth and spore formation of the causal 
fungus on culture media. These experiments with the scab fungus 
carried on during intervals in a more extended temperature investigation 
with other citrus fungi reported elsewhere (4) are seen to be somewhat 
incomplete, but they appear to throw considerable light on the possible 
relation of temperature to the occurrence of citrus scab and to offer a 
more complete explanation for the differences in the occurrence of scab 
from year to year or from one season to another. It is, of course, realized 
that in the orchard, under natural conditions, temperature is fluctuating 
and not constantly maintained as in these experiments. Nevertheless, 
the experiments indicate at least the comparatively narrow range of 
temperature within which infection of a very susceptible host is possible 
under the presumably ideal conditions. 
1 Paper No. 72, University of California, Graduate School of Tropical Agriculture and Citrus Experiment 
Station, Riverside, Calif. 
a Reference is made by number (italic) to ' * Literature cited,” p. 253. 
Journal of Agricultural Research, 
Washington, D. C. 
XV 
(243) 
Vol. XXI, No. 4 
May 16, 1921 
Key No. Calif.-30 
