How Trees Become Infected. 

 The next problem was to find out how the trees become in- 

 fected. Only those ways of infection which have come under my 

 personal observation will be mentioned, tho there is no doubt 

 that the bacteria gain entrance to the bark in still other ways. 



The bacteria frequently get into the bark of the limbs and 

 body by way of short spurs and watersprouts (Fig. 22). In 

 1905 twig blight became very prevalent during July and August, 

 especially in the region about Ithaca, and it was an easy matter 

 to find blighted spurs and watersprouts with active cankers about 

 their bases (Fig. 23). When these watersprouts grow out from 

 the trunks, as is often the case in young trees, typical body 

 cankers are formed. 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 

 blighted watersprout soon diries up and falls away, leaving often 

 a very indefinite scar in the cankered area so that the following 

 season it is usually impossible to tell with certainty the manner 

 of infection. Observation on a large number of trees this past 

 season, convinces me that the blighting of adventitious shoots 

 on trunk and limb is responsible for most of the cankers in such 

 locations. 



Another source of infection was found to be the pruning knife. 

 Along one side of an orchard of about 350 trees which was under 

 observation thruout the season, it was early noticed that the 

 pruned stubs, of 1904 especially, showed the collars of dead bark 

 often two or three inches in width (Fig. 13). Instead of forming 

 a callus and healing over the wound, as would normally occur, 

 the tissue had died and shriveled up but still clung to the stub. 

 In most cases the bacteria which had caused the death of the 

 bark had died out the first season. In a few instances, however, 

 the canker was observed to be active early in the spring, extend- 

 ing down the side of the adjoining limb. Two badly diseased 

 trees on this side of the orchard seem to have been the source of 

 infection. Owing to their diseased condition, they had been 

 severely pruned the previous season and very probably the knife 

 or saw had carried the bacteria to the healthy trees. Flies, 



