Pomona College Journal of Economic Botany 



77 



deposited may be colorless or slightly brown. This condition is quite prevalent 

 in the Valencias on affected trees at Santa Paula, and cannot be told by any out- 

 ward appearance of the fruit. 



Dropping — The fruit first begins to fall before any staining may be noticed, 

 shortly after the first discoloration. It may continue to fall slightly until the first 

 signs of splitting, when most of the fruit drops to the ground. At the time of 

 maturity there is not enough left on the tree worth picking, and what is left is 

 usually thick-skinned and unfit for market. On the Valencia trees referred to 

 above, and which are seven years old, there are not more than two boxes of fruit 

 on twelve trees to date (March 20, 1911,) while neighboring trees are very heavily 

 loaded. Neither is there much upon the ground, for the trees have never borne a 

 good crop since they were planted. 



Figure 30. Die-back on Orange: A, Valencia, showing typical reddish-brown stains 



and cracks in peel. 



Causes and Conditions Favorable to Growth 



The exact causes of die-back are not all thoroughly understood, but enough is 

 known of the disease to tell almost with exactness the conditions which are favor- 

 able to its growth and which will nearly always produce its characteristic symptoms. 

 It is the general belief that the disease is caused by malnutrition, that is, an Ik 

 regular or improper food supply. 



