CROWN GALL 349 
attacked. The Bacteria pass the winter in the infected regions, 
which are sources of further infection. When growth begins in 
the spring, a gummy substance carrying the Bacteria exudes 
from these dead portions, and insects visiting the exudations 
carry the disease to other trees. How would you combat Pear 
Blight? 
Crown Gall. — This disease is common on fruit trees, and 
occurs on Roses, Blackberries, Alfalfa, and a number of other 
plants. The presence of the Bac- 
teria causes an abnormal develop- 
ment of the infected tissues, resulting 
in the formation of cancer-like swell- 
ings. The disease may occur on 
any portion of the plant but is com- 
mon on the roots or on the stem 
near the surface of the ground. In 
general, nursery stock is more readily 
affected than older trees. Plants 
affected are much dwarfed or killed 
(Fig. 308). 
Beans, Tomatoes, Potatoes, Sugar 
Cane, Cotton, and most of our eco- 
nomic plants have some form of 
bacterial disease, but a further study 
of the disease-producing forms must 
be left to courses in Bacteriology 
and Pathology. 
In summarizing, the following fea- Frc. 303. — Crown Gall on 
tures should be noted. Bacteria are the Cherry tree. The cancer- 
the smallest of plants, and their like swellings are due to the 
plant body consists of a single cell i ee 
with protoplast poorly organized. Piste: 
The plant body may be globular 
or rod-shaped, either straight or curved. Some have cilia or 
flagella and are therefore motile. They are remarkably re- 
sistant, especially in the spore stage. With the exception of a 
few forms, they are saprophytes or parasites. The disease- 
producing forms are very destructive to both animals and plants. 
They reproduce by rapid cell division and are spread, partly by 
wind, partly by water currents, partly by their own locomotion, 
