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Journal of Agricultural Research voi. xxvm, No. 3 
While the season of 1916 was somewhat later than that of 1918 and more 
scab was noted, temperatures below those of 1918 were recorded during most of 
the development of the spring growth. By the time optimum temperatures for 
infection occurred growth was completed. However, the amount of growth was 
smaller and maturation of foliage was more rapid. 
The lateness of the season in 1920 was due to the low temperatures which 
occurred during the first week of March. All the young growth formed before 
this time was killed back. The mean monthly temperature for March was also 
below normal. Thus the forcing of growth from new buds was resorted to by 
the plants, which naturally delayed the development of the first spring growth. 
However, temperatures very much like those which prevailed during 1915 occurred 
for the rest of the season, which were very conducive to maximum infection. 
Thus, during the season of 1920, scab was almost as bad as in 1915. 
For practical purposes it appears that a mean monthly temperature below the 
normal for March in Alabama can be used as an indication of a scab epidemic, 
while temperatures above the normal for this month and with no freezing tem¬ 
peratures are indicative of a light scab season. The orchard grower then can 
forecast to some extent light or bad scab seasons from the mean monthly tempera¬ 
ture .and its departure from normal for March and can regulate his spraying 
program accordingly, depending on the prevailing conditions. 
It might be stated at this point that there is not much use in spraying after the 
weekly mean temperatures go above 75° F. nor after the first growth period is 
completed. 
WEATHER AND ITS EFFECT ON THE DISTRIBUTION AND 
PREVALENCE OF CITRUS SCAB 
In discussing the distribution of citrus scab it was stated that this disease 
was rather serious in the Gulf Coast States, Japan, and South China, while scab 
was not known in the Mediterranean countries, California, and the Philippine 
Islands, although scab had undoubtedly been introduced into California and 
possibly the Philippine Islands many times. It was further stated that there 
were certain factors which prevented the successful propagation of citrus scab 
in these localities. 
To determine just what some of these factors might be, the mean monthly 
temperature and precipitation and number of rainy days for Manila, P. I.; 
Los Angeles, Calif.; Mobile, Ala.; and Nagasaki, Japan; were obtained. 3 These 
data are tabulated in Table II and plotted in figure 2. For the sake of 
simplicity, in the discussion which follows, all references to the relation of these 
environmental factors to the development of the host plants will be eliminated. 
It should be understood, however, that young growth must be present as one 
of the prerequisites for successful infection. 
It can be readily seen that the principal reason why scab is not prevalent in 
the Philippines is the fact that the mean monthly temperatures for the whole 
year are above the maximum for infection. Even though scab was introduced 
on Citrus stock, the fungus could not propagate itself because of this fact. There 
is no doubt that citrus scab has been introduced into the Philippine Islands on 
nursery stock from Japan in the past, but owing to the prevailing high mean 
temperature militating against it, scab has never gained a foothold. 
If we follow the temperature curve for Mobile through we find that some 
scab could occur in March, but that by the end of May temperatures are already 
above the maximum. The period of optimum infection would be rather short. 
3 Climatic data were kindly supplied by J. Warren Smith, agricultural meteorologist, Weather 
Bureau, U. S. Department of Agriculture. 
