375 
The Study of Plant Diseases. 
loss. Furthermore, the tendency of modern times is ever towards 
a more and more intensive cultivation of the soil, with the result 
that disease becomes increasingly prevalent. As an indication of 
the extreme possibilities of a fungus epidemic one may mention the 
well-known case of the destruction of the coffee plantation industry 
in Ceylon, on account of the attacks of the Rust Fungus Hemileia 
vastatrix. 
It was obvious that before any measures of control of plant 
diseases could be introduced, a knowledge of the life histories of the 
organisms responsible for the injuries was essential. Thanks to the 
labours of Brefeld, Hartig, Klebahn, Marshall Ward, Massee and 
many others, the main outlines of the life-history of most of the 
fungi which are the agents of disease are now well known. In many 
cases the fungus suspected of being the cause of injury has been 
isolated from diseased plants and has been grown in pure cultures 
from which healthy plants have been subsequently infected. 
Although in the case of obligate parasites like the Rust Fungi it is 
not possible to grow these organisms upon artificial media, yet the 
course of development of many of these fungi from spore stage to 
spore stage has been so carefully followed that the part they play 
in inducing diseased conditions is clearly known. 
Many kinds of treatment have been tried in regard to the 
control of such plant diseases as are here under consideration. 
Sometimes it happens that at one stage in the life-history of a 
destructive fungus the organism can be easily destroyed, whereas 
at other stages it is much more difficult to control. The Cherry 
Leaf Scorch Disease is a case in point. Cherry leaves attacked by 
this disease do not fall in the autumn because the presence of the 
fungus at the base of the leaf-stalk prevents the formation of tjie 
absciss layer. The only means of re-infection the following season 
is by spores formed in the perithecia which develop in the retained 
foliage of the previous year. Thus if leaves still hanging on the 
tree in winter time are collected and burnt there can be no danger 
of re-infection. But the matter is not always thus easy. With 
American Gooseberry Mildew, the practice which has been found 
most effective in control is the pruning of affected shoots from the 
bushes during the resting season and the destruction, by burning, 
of the perithecia attached to these shoots. If, however, the process 
of pruning is delayed beyond a certain time, the perithecia fall to 
the ground from the shoots, and there is great danger the following 
season of re-infection from the soil. 
