17 
Origin of the cankers. Examine the specimens of cankers again 
-very carefully and OBSERVE :— 
12. That the cankers almost always surround the base of the 
spur, twig or watersprout, or that there is at least one or more within 
the area. Sometimes it is a fruit-spur, the bacteria had entered by way 
of the blossoms; more frequently it is a water-sprout, the tender suc- 
culent growth of which is very favorable for the rapid development 
of the disease. The bacteria kill the water-sprout down to the trunk 
or limb and spread out into the bark often for a considerable distance 
around the base of the sprout. (See illustration specimens and photograph 
8.) If the previous DRAWING of the canker does not show the dead spur 
or water-sprout, DRAW to show it. 
Cankers occasionally originate in other ways. Sometimes the pruning- 
kmife carries] the inoculum. Cankers may start from insect-wounds 
in the bark. (See Ontario Bul. 176:33-38.) 
ETIOLOGY 
The organism that causes this disease is Bacillus amylovorus (Burrill) 
Trevisan. 
Life-history. Laboratory studies in the life-history of this pathogene, 
aside from the facts brought out in the above study of symptoms, are 
necessarily limited to a short study of the morphology of the bacillus. 
Before proceeding with the following observations the student should 
have read carefully that portion of the text* dealing with the life-history 
of the organism. 
If pure cultures are available, examine them carefully and OBSERVE :— 
13. The character of the growth on the surface of the solid 
media. The effect on the bouillon. Why? How long has it taken to 
produce this effect? What does that indicate as to the rapidity of multi- 
plication? Make amount froma pure culture and before covering intro- 
duce a bee’s foot. OBSERVE:— 
14. The size and motility of the organism; cell-unions. Com- 
pare it in size with the claw of the bee’s foot. It may now be understood 
how a bee or fly might carry thousands of these on its feet. DRaw the 
bee’s claw and some bacteria beside it. Maintain relative proportions. 
Study photograph 9 or the stained mount under the demonstration 
microscope and MAKE OUT:— 
15. The cilia; their number, length and distribution over the 
thallus. They are peritrichic. It is this character that places the organism 
in the genus Bacillus. 
If fresh material is available crush a bit of the most recently affected 
tissue in water on a slide, cover and try to DEMONSTRATE :— 
16. The living bacteria. How do they compare in size, form and- 
abundance with those from the pure culture? Are they motile? 
Pathological Histology. Cross-sections of fresh growing twigs show- 
ing both healthy and diseased tissues are to be studied (supplemented with 
stained section provided). Examine the healthy portion and OBSERVE :— 
17. The reddish brown outer coat, made up of the epidermis 
and the several layers of cork-cells beneath. 
*This refers to any text which is available. 
