302 The Canadian Horticulturist. 



BLIGHT ON THE APPLE AND PEAR. 



By John Craig, Horticulturist, Experimental Farm, Ottawa. 



URING the past five weeks a large number of specimens of 

 apple and pear twigs killed by " blight," variously known 

 under the names of " Apple Blight," " Pear Blight,' 

 "Twig Blight," and "Fire Blight," have been received at 

 this office. The leaves have a brown, scorched, but not 

 shrivelled, appearance; the bark, on the other hand, is 

 shrivelled and withered, in addition to being discolored. 

 The presence of this disease has been noticed in America 

 for more than loo years. It is easily recognized by the 

 manner of its growth and development upon apple trees, which is usually as 

 follows : — When the tree has made a growth of a few inches, the leaves on some' 

 of the young shoots may be observed to suddenly turn brown, in a single night, 

 as it were. If the tree is watched closely, this browning will be found to extend 

 downwards upon nearly all of the twigs which have been attacked. In some 

 cases the discoloration of the bark ceases when a larger branch is reached ; in 

 other instances the large branch becomes affected, communicating in turn the 

 disease to the stem, in which case the death of the tree may be looked upon as 

 a certainty. Often, however, the scorching and browning affects only the young 

 terminal twigs. Crab trees are frequently affected in this way. It is the least 

 injurious form of the disease. Pears are often attacked in the main branches 

 and on the stems, the disease finding entrance through spurs and tufts of leaves. 

 A tree attacked in this way usually succumbs. Until 1880 nothing was 

 definitely known regarding the nature of the malady. In that year Prof. Burrill-, 

 of Illinois, published the first authoritative account of the bacterial or " germ " 

 origin of this disease. Prof. Burrill's investigations were afterwards abundantly 

 corroborated by the careful work of Prof. Arthur at the New York Experiment 

 Station. The disease is known to be caused by a bacteria now recognized in 

 science by the name of Micrococcus amylovorus. Prof. Arthur proved the con- 

 tagious character of the disease, and also its bacterial nature, by demonstrating 

 that it could only be transmitted artificially by using the juices of branches 

 which contained the characteristic bacteria. This disease finds most congenial 

 conditions for rapid development in fast growing varieties, having an abundance 

 of succulent tissue. It usually obtains an entrance by way of the youngest 

 leaves, or through the blossoms. At this time, too, wood is in an immature 

 condition. The bacteria causing the disease may remain alive in dead branches 

 on the tree, and also in those which have been removed, and in this way com- 

 municate the disease to healthy trees. In addition to apple and pear trees, this 

 disease attacks other members of the rose family, notably Hawthorn and 

 Mountain ash. 



