Pomona College Journal of Economic Botany 



79 



but rather that its cause is more complicated than this, is shown by the appearance 

 of the trouble in this state, where it is limited almost entirely to a certain type of 

 soil. This is the coarse granitic, very porous soil, which is found particularly along 

 the base of the mountains in various places between Highlands and Pasadena. On 

 such soils the Florida die-back is of quite frequent occurence. There have been a 

 very few cases of this disease on heavier soils where the Florida experience seems 

 to hold true, all the instances of such soils of which we are aware having been as- 

 sociated with very heavy applications of fertilizers rich in organic nitrogen. In the 

 case of the type of soil mentioned, however, the disease occurs very severely in 

 many instances where only moderate amounts of fertilizers have been applied. The 

 most pronounced quality of this soil is the free passage which it affords to water. 

 It becomes very wet during irrigations, but unless water is applied very thoroughly 

 and carefully and quite frequently, the upper soil becomes absolutely dry between 

 irrigations. This soil also takes up water very poorly in a lateral direction, so that 

 the space between the trees and much of the whole body of the soil becomes ex- 

 tremely dry in many orchards. This disease appears to be brought about in some 

 manner through an irregular food supply, resulting from irregular water supply 

 which trees get in this soil, unless irrigation is particularly well carried out. 

 With trees that are not well fertilized, the effect is simply one of ordinary die- 

 back. True, exanthema seems to result where the soil is of the type described, and 

 a fair amount of fertilizing material had been appbed. It is apparently a sort 

 of indigestion in the tree, more food being taken up during irrigation than the 

 tree can take care of after it gets into the condition of dryness which follows. 

 On heavier soils the same thing occurs only where very excessive amounts of nitro- 

 gen have been applied." 



Improper Cultivation and Irrigation — Anything that tends to bring about mal- 

 nutrition or indigestion in the tree may be a favorable condition for the growth of 

 die-back. Proper methods of cultivation to aerate the soil and to conserve a regular 

 water supply is important in controlling die-back. Irregular irrigations, with too 

 much or not enough water at a time, may also bring about this disease. 



Destructiveness 



In Florida the die-back is more generally spread than any other citrus disease. 

 It is claimed to be in every locality and doing damage wherever it exists. All 

 writers make much of the amount of damage which is annually done. Swingle and 

 Webber estimate an annual loss of $100,000 up to the year 1896. Hume and later 

 writers make no estimate in dollars, but all state that the loss is very great. The 

 actual loss is not always as easily determined as in the case of more positive dis- 

 eases, but must be studied from a great many angles. Thus there must be con- 

 sidered the loss through the unproductiveness and slow development of the affected 

 trees as compared with normal healthy ones ; the excessive dropping of fruit ; the 

 coarseness and disfigurement of the fruit that does finally mature. Then there is 

 the case where groves have been planted on improper soils which have produced 

 a mild form of die-back, which is just sufficient to stunt the trees and cut down 

 the yield with no real manifestations or symptoms of the real trouble. Much capi- 

 tal has been spent to bring such groves into proper condition without the owner 



