PLUM DISEASES 375 
less curved (Fig. 109). When young they are smooth, and can 
be distinguished from the healthy fruit by their pale-yellow or 
reddish color. Later, the surface is gray and is wrinkled con- 
siderably. Finally, diseased fruits turn dark-brown or black, 
and rattle like bladders when brought in contact with any hard 
substance ; whence the name plum-bladders. In this condition 
they hang to the tree 
two or three days, then 
fall to the ground. 
The younger 
branches and_ leaves 
are affected, which in- 
creases the damage 
done to the tree. The 
growing limbs and leaf- 
lets become distorted 
and swollen. In June 
the diseased branches 
turn gray, the tissues 
soften, dry up and die. 
Shoots arising the next 
year just below these 
dead extremities are 
most frequently af- Fig. 109. — Plum-pockets. 
fected by the disease. 
As the new leaves unfold they turn red or yellow, when affected 
by the disease, and show an arching of the leaf-blade. In 
general a curl disease, similar to peach leaf-curl, is developed. 
Cause. 
The cause of these interesting malformations is the fungus 
Exoascus Pruni. Its growth in the plum-tissues causes their 
peculiar development, which finally result in the so-called pockets. 
For a long time the cause of the disease was not known, but was 
attributed to the work of some insect, to improper fertilization, 
