DISEASES DUE TO FUNGI 101 
die a very pretty effect is frequently seen on the under 
surface of the leaves where violet-purple areas are sur- 
rounded by the tawny-olive zones of the later infections. 
The fungus spreads rapidly over the leaf, which soon 
shrivels up and hangs as dead tissue covered with a 
fungus growth producing innumerable spores. In bad 
attacks the spores are so numerous and so easily dis- 
lodged from the leaves that by shaking the plants the 
air may be filled with them. It will readily be seen that 
infection may be both abundant and rapid. Generally 
the disease does not appear until July or August, but 
under specially favourable conditions, which are largely 
climatic in origin, it has been observed as early as April. 
Under normal conditions the older and more succulent 
leaves are first attacked, the disease passing from the 
lower parts of the plant to the top, but in severe attacks 
even the young leaves are attacked almost as soon as 
they are unfolded. The fungus occasionally attacks the 
flowers and young fruit, and while large fruit are un- 
injured those below the size of a pea have frequently 
been found to contain the filaments of C. fuluum. In 
severe cases the entire flower may be attacked and 
destroyed. Infection takes place by means of the 
stomata, through which the germ tube from the spore 
passes into the interior of the leaf. 
The effect of the disease upon the plant varies with 
the extent of the disease, which is again dependent upon 
the atmospheric conditions of the glasshouse. In slight 
attacks the old leaves only are attacked, and as these are 
being continually removed in the cultural process of 
defoliation, little injury is done to the plant. In severe 
cases the reduction in leaf area resulting from the action 
of the parasite is so great that the plants are weakened 
to a considerable degree, fruit production ceases, and the 
plants die prematurely. 
The extent and progress of the disease varies with 
the conditions of light, humidity, and temperature of the 
glasshouses. Light is unfavourable to the rapid develop- 
