6 



states in the United States. Wherever the twig blight of apples 

 and pears occurs, I believe you will find the cankers on the 

 bodies of the trees. 



Cause of the Disease. 



Most orchardists regard the trouble as due to freezing or sun 

 scald. In seeking to discover the real cause of the trouble I 

 examined many cankers in different orchards at different times 

 of the year. One moist cloudy day early in May I discovered a 

 cankered apple tree from the diseased bark of which were exuding 

 drops of a sticky milky fluid (Fig. 6). Examined under a micro- 

 scope this fluid was found to be made up almost entirely of 

 bacteria, — minute rod-shaped plants (Fig. 17). The diseased 

 tissue within the bark was also found to be alive with these 

 minute plants. By their rapid growth and multiplication within 

 the cells of the bark they cause its death. They are not carried 

 along in the sap, but slowly work their way from cell to cell. 

 When the canker dries down they die and disappear, so that ex- 

 amination of the tissues of old cankers does not show them. That 

 they are the direct cause of the disease was proved in the fol- 

 lowing way : Bacteria from the cankered tissue were introduced 

 into the bark on the body of a healthy apple tree and also into 

 the bark- of a healthy pear tree, with the result that typical 

 cankers appeared in both cases (Fig. 18). Blossoms and growing 

 twigs of both pear and apple trees were also inoculated with 

 bacteria from this same canker. These nearly all developed good 

 eases of blight in about ten days (Figs. 19 and 20), while twigs 

 and blossoms punctured with a sterile needle' gave no infection. 

 This last experiment was twice repeated during the summer with 

 pure cultures of the bacteria from the apple tree canker. The 

 blight resulted in practically every case. Young fruits of both 

 the pear and the apple were also inoculated and gave well de- 

 veloped cases of the disease (Fig. 21). By a comparative study 

 in various culture media of the bacteria from cankers, twigs and 

 fruits of both pear and apple secured from different orchards 

 about Ithaca, the organism of the canker was shown to be iden- 

 tical with that of the well known " fire blight " of the pear and 

 " twig blight " of the apple, Bacillus amylovorus. 



