PLANT DISEASES 



375 



weeks after flowering. The branches are at first slightly 

 swollen, then they turn dark brown, and soon the tissues 

 shrink and wilt conspicuously (Fig. 209). If it is not 

 checked at this time, the disease will extend into adjacent 

 tissues, and may readily be carried by insects to other 

 branches and to neighboring 

 trees. It is especially likely to 

 be spread by insects just after a 

 rain, because at such times a 

 gummy substance is excreted on 

 the surfaces of diseased branches, 

 in which substance are countless 

 living bacteria. The bacteria 

 live through the winter in the 

 diseased tissues, and in the fol- 

 lowing spring when the tree 

 resumes its growth they may 

 infect the living tissues of the 

 tree or may be carried by insects 

 to still other plants. The bac- 

 teria also live in cankers on vari- 

 ous parts of apple trees, and 

 these cankers are prolific sources 

 of new infections. 



The only means known for 

 controlling the disease is the 

 cutting out and burning of dis- 

 eased branches as soon as they 

 are discovered. An infected branch must be cut at some 

 distance below the place where the disease is evident, be- 

 cause some of the bacteria are usually present below the 

 darkened portions. Great care must also be taken to see 

 that the pruning knife or saw does not come into contact 

 with the diseased tissues, for in that case the cutting tool 

 itself will carry the disease from tree to tree. 



Fig. 209. — A branch of an 

 apple tree attacked by the fire 

 blight. Notice the character- 

 istic appearance of the younger 

 portions, which look as though 

 they had been scorched. 



