Canker on Apple and Pear Trees. 



67 



the disorder usually spreads with far greater rapidity, and 

 trees are quickly destroyed. One form of affection of this 

 nature is undoubtedly due to bacterial agency, and to the 

 bacillus denned by Professor Burrill in the American 

 Naturalist as Micrococcus {Bacillus) amylovorus. This microbe 

 is most disastrous in its effects upon apple and pear trees in 

 the United States, and spreads with great rapidity. Pear 

 trees, perhaps, are more liable to this infection than apple 

 trees in the United States, but the liability of the two kinds of 

 trees varies in different States. It attacks chiefly the inner 

 bark and cambium of the body of the tree, as well as its 

 most important branches. Unlike the fungus Nectria ditissima, 

 producing the ordinary canker, which establishes itself only 

 in already existing wounds, scars, and cracks upon the bark 

 of trees made by pruning, hail, insects or other fungi, the 

 Bacillus amylovorus descends with the sap in the living bark, 

 through the twigs and branches, to the body of the tree. 

 Trees infected by this microbe are found to be perfectly 

 ihealthy at their roots and up to the part where the blight has 

 reached, showing that infection comes from the upper part of 

 the tree. It sometimes commences its attack in the blossom, 

 or on the tips of the shoots at the ends of the branches. In 

 the spring it is said to be always first noticed on the 

 blossoms, which turn black as if injured by frost. The 

 microbes stand cold well, and it has been found that the bark 

 of infected trees contains living colonies for a longer period 

 in the winter than in the summer. In spring-time, when the 

 trees are full of sap, the microbes invade new bark and 

 spread rapidly. At this period of the year, too, a viscous, 

 sweetish, brownish yellow substance exuded from parts of 

 the stem and branches attracts bees and other insects, 

 which convey the microbes to the blossoms, and thus 

 disseminate the infection extensively. 



In this country apple and pear trees are often seen with 

 the blossom blackened as if by frost, with withered or dying 

 tips of branches, with deep wounds in the bark, and with their 

 skin peeling and cracking in all directions. Frequently no 

 trace of Nectria ditissima can be found in these cases, and 

 the appearances correspond generally with those occasioned 



