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 likely 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 dm-ing 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 1914, 1915, and 1921. These are presented in Table 3. 



