Blight Canker of the Apple Tree. 



197 



found them repeatedly hiding in the crevices of the dead hark that ac- 

 cumulates in the crotches, and one species seems to feed to some extent 

 on the living tissue in such places. I have also observed this same species 

 feeding on the exuding sap of cankered limbs and stubs. That it may 

 carry the bacteria to the crotches seems obvious. Besides this, many of 

 the crotches are of such a form that they readily retain moisture and thus 

 afford the best of conditions for bacterial growth. 



Fig. 72. — Cankers on limbs of apple tree which originalcd 

 through blighted spurs. 



At a general deduction, then, it may be stated that infection occurs 

 only through a wound of some sort. Moreover, the infection court must 

 be of such a nature that it will not dry out quickly. An abundance of 

 moisture is known to be necessary to the rapid growth and development of 

 the blight organism. This was repeatedly demonstrated in the large 

 number of pure cultures which I had under observation during the sum- 

 mer. The growth was most abundant and vigorous in liquid media. This 

 peculiarity accounts for the ease with which growing shoots are infected. 

 When the diseased tissue of an active canker was at once cut out and the 

 wound exposed to the drying heat of the sun without any other treatment, 

 the canker ceased to spread and the place healed rapidly. 



