194 



Bulletin 236. 



5. How Trees Become Infected 



Aside from the nature of the organism causing the malady, perhaps 

 the most important question in taking up an investigation of a given plant 

 disease is that of how the parasite gains entrance into the host. The 

 answer to this question is the prime requisite for intelligent effort in 

 combating the disease. 



]\Iuch of my attention 

 while in the field during 

 the past season has been 

 directed to a solution of 

 this problem. Only 

 those ways of infection 

 which personal observa- 

 tion has discovered are 

 here recorded. Xo doubt 

 the bacteria gain en- 

 trance to the bark in still 

 other ways than those I 

 have observed. 



The bacteria fre- 

 quently get into the bark 

 of the limbs and body 

 by way of short spurs 

 and watersprouts. (Fig. 

 72.) Early in my in- 

 vestigations I came to 

 this conclusion. The 

 opinion was fully con- 

 firmed later in the sea- 

 son. Twig blight be- 

 came very prevalent 

 during July and August, 

 especially in the region 

 about Ithaca. It was then an easy matter to find blighted spurs and water- 

 sprouts with active cankers about their bases. (Fig. '/2)-) When these 

 watersprouts grew out from the trunks, as is often the case in young trees, 

 typical body cankers vere formed. (Fig. 74.) The infection of the 

 sprout itself is generally attributed to the work of insects, which after 

 visiting freshly cankered sprouts or blighted twigs introduce the bacteria 

 into the succulent tissues of the rapidly growing healthy shoots. The 



Fig. 64. — Large body canker on the side of an old tree. 



