FUNGOUS AND PHYSIOLOGICAL DISEASES. 491 



eventually. The leaves produced on these shoots are usu- 

 ally quite yellow between the veins and have a peculiar 

 constricted appearance. 



Thus far, the cause of the disease remains unknown. 

 Trees may be attacked at any age, whether old or young, 

 but as a general thing it appears most commonly in old 

 bearing trees. Up to the time when they are attacked they 

 generally appear to be quite healthy. No external agent 

 has been found connected with it. It appears rather to be 

 the result of internal derangement of the functional pro- 

 cesses carried on within the tree. It may be that it is 

 caused by an enzyme, a poisonous substance present in 

 the soil. In fact, it appears at the present time that this 

 is about the only plausible explanation of its cause. A 

 number of other plant diseases, such as peach-yellows 

 and rosette, may belong to this same class of diseases, 

 though in none of these cases has this theory been act- 

 ually proven. 



Color is given to this theory of the cause of the blight 

 of citrus trees by the fact that it is usually more or less 

 localized in a grove and there may be a number of sepa- 

 rate centers of the disease in a single grove. The peculiar 

 nature of the new growth and the frenched appearance of 

 the foliage tend to strengthen this view. 



Treatment. Pruning does not stay the progress of 

 the disease. Diseased branches may be removed, leaving 

 only the healthy ones and yet the disease will reappear. 

 Observations have led to the belief that it is contagious, 

 at least, trees adjoining those diseased also become affected 

 in due time. In the premises the best suggestion that 

 can be made regarding the control of the disease is to 

 dig out and burn the diseased trees as fast as they show 

 symptoms of the disease. Dig them up and destroy them 



