112 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’Angouléme 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 
