374 MANUAL OF FRUIT DISEASES 
Colorado, New Jersey, North Carolina, New York, Kentucky, 
Kansas and other states. The disease was observed as early 
as 1593, but it attracted little attention in America before 1888, 
at which time it occurred commonly in certain of the states 
already listed. The history of the disease indicates that it may 
never be expected to cause serious trouble over wide areas. 
It has never swept over any country, but rather affects individual 
trees here and there in isolated localities. In many cases, 
however, affected trees may not have a single healthy plum. 
Moreover, once a tree is diseased it seldom wholly recovers, but 
is attacked from year to year. The nature of the losses is as 
follows: (1) plums are distorted (Fig. 109) and may be rendered 
unmarketable; (2) affected fruits drop prematurely ; (3) leaves 
are curled; (4) twigs are swollen, distorted and finally killed. 
All kinds of plums are attacked, although the trouble is more 
abundant on the red and purple varieties. Wild species are 
said to be more commonly affected than cultivated ones. It is 
to be noted in connection with susceptibility that there is an 
apparent individual resistance which may be confusing in point- 
ing to resistant and susceptible varieties. Certain individuals of 
a given variety may be seriously affected, whereas others of the 
same variety, in close proximity to the affected trees, remain 
unaffected. 
Symptoms. 
In the case of affected fruits, there results, instead of normal 
plums, peculiarly enlarged, hollow deformities (Fig. 109). It 
is this condition which has given rise to the name plum-pockets. 
These consist merely of a thin shell, and in most cases there is 
no evidence of a seed. Wherever a stone is present at all, it is 
only rudimentary. The disease makes its appearance soon 
after the flowers have fallen. Toward the last of June the 
disease attains its full course of development and affected fruits 
drop from the tree. Affected plums are at first globoid in shape, 
but as they grow older they become oblong or oval, and more or 
