190 



Bulletin 236. 



4, The Cause of the Canker 



Fig. 62. — Typical New York apple tree can- 

 ker, caused by the fungus Sphacropsis malo- 

 ri(m. Comparatively young canker. Observe 

 the little pimples covering the diseased bark, 

 — the sporecases of the fungus- 



A microscopic examina- 

 tion of the viscid milky 

 drops that exude from 

 freshly cankered surfaces 

 (Fig. 51) on moist cloudy 

 days shows them to be com- 

 posed almost entirely of 

 minute rod-shaped bacteria. 

 The diseasetl tissue within 

 the bark is 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 

 examination of the tissue 

 of old cankers does not 

 show them. That they are 

 the direct cause of the dis- 

 ease was proved in the fol- 

 lowing way: Bacteria from 

 the cankered tissue were in- 

 troduced 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. (Figs. 65 and 66.) 

 Blossoms and growing 

 twigs of both pear and 

 apples were also inoculated 

 with bacteria from this 

 same canker. These nearly 

 all developed good cases of 

 blight (Figs. 67, 68 and 

 69) in about ten days, 



