Colorado Plant Diseases 
67 
PEAR 
Fire Blight (Bacillus amylovorus) (4), (5), (6)— This is without 
doubt the most destructive disease of the pear and is very prevalent 
in Colorado. Fire blight also affects the apple and quince and some- 
times the apricot and plum. Any part of the tree above ground may 
be affected. It becomes evident in the spring as soon as growth is 
started but seldom attracts attention until two or three weeks after 
the blossoming period. The twigs and fruit spurs shrivel, turn 
brown and then black, giving the appearance of having been burnt. 
Blight spreads down the twigs affecting the larger branches on 
which are produced dark sunken cankers. The entire tree is often 
killed. Fire blight is caused by bacteria which live in the growing 
soft tissues of affected plants. They are spread chiefly by insects 
and may enter the plant through insect bites or other wounds. 
The bacteria live over the winter in affected tissues of the larger 
limbs. Such places are called ‘‘hold over” cankers. In the spring 
when the sap rises the bacteria become active and drops of clear 
sticky fluid flow from the cracks in the canker. These drops of 
ooze are full of bacteria which are scattered by insects and other 
agencies to healthy twigs. 
Fire blight is .a very difficult disease to control. It can be held 
in check by cutting out in the fall diseased branches which might 
be a source of infection the following spring. Cankers which occur 
in crotches should be scraped down to healthy wood and disinfected 
with corrosive sublimate. All blighted twigs should be pruned out 
and destroyed as soon as they appear in the spring. Such twigs 
should be cut several inches below the blackened tissues. The 
pruning knife should be disinfected between each operation by 
dipping into a solution of corrosive sublimate (1-1000). 
Crown Gall— see Apple. 
Rust — see Apple. 
PINE 
White Pine Blister Rust (Cronartium ribicola) (66) — This is an 
extremely destructive disease of all five-needle pines. It is a native of 
Europe but was introduced into this country a little over ten years 
ago. Since that time it has practically destroyed all the white 
pines of the Eastern states. It has been spreading slowly westward 
and a few years ago it was discovered in Western Minnesota. There 
