PLANT DISEASES 451 



by the statement that fungicides are appropriate only for 

 certain parasites ; that in each case there is a most effective 

 time for their application ; and that in some cases they are 

 only of supplementary use, not reaching all the sources of 

 infection. 



147. Surgery. This means the removal of infection and 

 guarding against further infection. Perhaps no treatment 

 of plants is done more thoughtlessly and needlessly than 

 surgery. Before any such operation, one must be sure of 

 three things : (1) whether the infection exists in the part 

 proposed to be removed; (2) if so, whether it will do any 

 good to remove it ; (3) and if so, whether it can be removed. 

 A few illustrations will make this plain. 



In the case of pear blight, the flowers are infected by in- 

 sects that obtain the bacteria from certain affected branches 

 in which they have passed the winter. It happens that 

 these branches show their character, for they are " blighted. " 

 It is obvious that such branches must be pruned out before 

 the opening of the flowers. 



Crown gall was once thought to be a case for surgery, but 

 now it is shown that the removal of a gall (tumor) is in- 

 effective because there are infecting strands (p. 444) which 

 cannot be removed. This illustrates a case in which the 

 infected area is known, but it cannot be removed. The 

 only surgery useful in crown gall is to destroy all affected 

 nursery stock. 



One of the most common applications of surgery is in 

 connection with the treatment of wounds on trees, to prevent 

 cankers and invasions of the water-conducting vessels. A 

 race of " tree surgeons " has been developed, some of whom 

 are reliable, and others are ignorant of their business. The 

 general method is to clean out the wound so that a fresh 

 surface is exposed for healing, and then to cover it so as to 

 prevent the entrance of wound-infecting fungi. 



148. Soil infection. When soil infection is involved in 



