14 BULLETIN 1118, U. S. DEPARTMENT OF AGRICULTURE. 
occur on leaves, but should rainy weather occur during or soon after 
the bloom, the fruit is likely to be infected though the leaves escaped. 
If the weather reverses its order the leaves are likeh^ to be infected 
and the fruit remain free from scab. 
In localities where heavy dews or fogs occur normally during the 
critical period, or in orchards on low hammocks or flatwood soils, 
which are usually damp locations, scab is commonly quite severe 
even during springs of minimum rainfall; and the June bloom fruits, 
as well as the leaves expanding during the rainy season, are almost 
always very severely attacked. In the high, dry rolling hill country 
citrus scab is a negligible factor during ordinary years and only 
becomes serious during years which are unusually favorable for its 
development. Ordinarily in such localities the June bloom fruit 
is only moderately affected, if at all. 
In a very excellent article, Fawcett (6), among other data, gives 
the results of inoculations with the citrus-scab fungus held at definite 
maintained temperatures together with a statement of the failure 
of inoculations made in Florida during August under natural con- 
ditions and the success of those made in January. He concludes 
that other factors being favorable natural infection is largely de- 
pendent upon temperature ranging from 16° to 23° C. (61° to 73° F.) ; 
that infection is likely to occur when the mean temperatures are well 
within that range and unlikely to take place when the mean tem- 
peratures are outside of that range. He says : 
In the light of these temperature experiments, the greater severity of scab at low 
temperatures in Florida seems to be due largely to the fact that these temperatures 
fall mainly within the range for infection, while the later spring and summer tem- 
peratures are usually too high for infection to take place, even though other conditions 
are favorable. . . . This last statement may be translated into terms of mean 
temperatures, to the effect that severe infection will usually occur when the mean 
temperatures are well within the infection range and that infection will be unlikely 
to take place when the mean temperatures are outside this range. 
Such a conclusion is doubtless in a measure correct, but it is by 
no means entirely in keeping with the normal behavior of the disease 
on the several citrus hosts. On the same tree June bloom grapefruit 
is frequently more severely attacked than that resulting from the 
spring bloom. Citrus scab attacks nursery stock equally if not more 
severely in the hot summer months than is the case during the cooler 
seasons. This is to be expected, since even in warm months the 
night temperatures usually drop within the optimum range indicated 
by Fawcett. 
In recent years in Florida citrus scab was most prevalent during 
the seasons of 1914 and 1915 and least so during 1921. A study has 
been made of the precipitation and mean temperatures at Orlando 
during the months of January, February, March, and April of the 
years 191-A, 1915, and 1921. These are presented in Table 3. 
