Plant Diseases 
339 
PLUM-LEAF BLISTER 
This disease is very easily detected in the Plum, as 
the leaves become blistered, the blisters being reddish in 
colour. Unlike many other plant diseases, the mycelium 
of this disease does not travel beyond the blister or out- 
ward appearance of it. Therefore it can be seen at once 
to what extent the trees are infected with the disease. 
Very tiny dark dots appear on the blisters about the end 
of June, and on examination under the microscope these 
little dots are seen to be the openings of little flask- 
shaped receptacles called spermagonia. Inside these little 
receptacles we find many curved bodies called spermatia. 
When matured, these spermatia exude from the mouths 
of the spermagonia in a sticky substance, which, when 
wetted with dew or rain, washes away from the spermatia, 
leaving them behind to germinate and produce new centres 
of the disease. These spermatia may be carried by in- 
sects to other healthy leaves or trees, there to start the 
disease, or the rain may wash them down on to lower 
leaves and thus infect the whole tree. 
Towards the end of the season, perithecia are formed 
on the blisters. As in many other plant diseases, this 
form of fruit is not matured in the leaves while fresh 
and on the trees, but gradually during the winter. When 
the spring returns, and the young leaves burst out, the 
spores inside the perithecia are matured and liberated, 
being transferred to the young leaves through the agency 
of wind, insects, or birds, then germinate, thus producing 
the disease again. 
If the trees are only slightly attacked by the disease, 
