PEAR GROWING IN CALIFORNIA. 



365 



CROWN GALL (Pseudomonas tumefaciens). 



While pear trees are not so susceptible to this disease as the stone 

 fruits, they are nevertheless frequently infected and it becomes neces- 

 sary to carefully inspect all nursery trees for this trouble. Crown 

 gall is a bacterial disease which corresponds very closely in its effect 

 upon plants to cancer in the human being. This disease is shown on 

 pear trees in Fig. 159. Like cancer, there is no cure after the disease 

 becomes well established in a tree. Experiments have been conducted 

 in which the galls have been cut from the trees and the wounds dis- 

 infected with Bordeaux paste or some other disinfectant. It seems 



FIG. 159. Pear nursery trees badly infected with crown gall. 



impossible to cut away every portion of the disease, consequently a 

 recurrence can always be expected after such treatment. Preventive 

 measures, therefore, must be carefully considered and all young trees 

 that show signs of crown gall should be discarded at the time of plant- 

 ing orchard. 



OAK ROOT FUNGUS (Armillaria mellea). 



This is a very serious disease which attacks most of the deciduous 

 as well as citrus fruit trees and olives. The pe'ar is apparently less 

 susceptible than many other kinds of trees, e. g., prunes, peaches and 



147 



