II2 THE PEARS OF NEW YORK 



liquid oozes from lenticels newly attacked. On branches, the cankers 

 usually surround a smaller offshoot, sucker, or spur. The disease spreads 

 with great rapidity, by reason of which it is easily told from winter-killing. 

 Injury from cold is also more general, and the foliage browns rather than 



blackens. 



Pear-blight is an American disease, the history of which was briefly 

 given on page 51. Until recently it was confined to regions east of the 

 Rocky Mountains, but since about 1900 it has been a virulent epidemic 

 on the Pacific slope as well, and is now found from coast to coast wherever 

 pears are grown in North America. It seems not to be found in the pear 

 regions of other continents. It attacks the apple, quince, and other pomes 

 as well as the pear, and plant pathologists declare it to be the most destruc- 

 tive disease attacking the pome-fruits. Trees in the nursery suffer as well 

 as those in the orchard. Every variety of the pear bearing edible fruit 

 is attacked. Fortunately, some sorts are more immune than others. 

 Kieffer, Seckel, Winter Nelis, and Duchesse d'Angouleme are most resistant 

 of standard varieties, while Bartlett, Clapp Favorite, and Flemish Beauty 

 are little resistant. 



Pear-blight is caused by a bacterium, Bacillus amylovorous, the dis- 

 covery of which by Burrill in 1877 as a cause of this disease is one of the 

 landmarks in plant pathology. The organisms are dormant during the 

 winter, which they pass in the margins of blight-cankers where moisture 

 is sufficient to keep them alive. With the return of vegetative growth, 

 some sort of fermentation seems to set in and drops of a thick, opaque 

 liquid ooze out of the margins of blight-cankers. These contain countless 

 numbers of the blight bacteria which may swarm into the healthy tissues 

 adjoining, or be carried by any one of the great number of kinds of insects 

 which visit trees at flowering time to the pear-blossoms, to growing tips, 

 or to wounds in tender bark. The pruner with his tools may be an unwil- 

 ling agent in carrying the bacteria from tree to tree. The organisms now 

 multiply apace, killing tissues wherever they find entrance and causing the 

 several manifestations of the disease described under symptoms. Were 

 it not that the bacteria are killed by sunlight and even brief periods of 

 drying, the life of the plants attacked would be the only limits of the disease 

 unless checked by man. 



Theoretically, pear-blight can be controlled. Practically, pear-growers 

 fail to control it. Control consists in orchard sanitation whereby the 

 bacterium causing the disease is kept out of the orchard. This proves 



