DISEASES DUE TO FUNGI 95 
uncommon to see the soft tissues of the stem completely 
destroyed, leaving the vascular bundles exposed as dry 
fibres, and causing the death of the plant above the point 
of attack. On stem and petiole lesions, sporulation is 
abundant, a pinkish colour being produced, which turns 
black with age. 
The lesions on the fruits appear as pale green water- 
soaked, sunken areas, the surface of which, owing to 
abundant spore production, become pink in colour and 
finally black. The tissues under the lesion are destroyed 
and a cavity is produced, and this is exposed by the 
cracking of the surface above. When the leaves are 
attacked the health of the plant is impaired only by the 
serious reduction of ieaf area, but lesions on the stem are 
more serious and may cause the rapid death of the plant 
by destroying the tissues. 
The cause of the disease is a fungus, Colletotrichum 
oligochetum Cav., which has been shown by experiment 
to thrive upon such materials as new and rotten wood, 
straw, cotton wool, and paper, provided these are kept 
suitably moist (Fig. 24). Further tests have proved 
that following a severe outbreak of the disease the causal 
organism may tide over the winter by a saprophytic 
existence in various parts of the glasshouses. As the 
result of investigation (7), the following main conclusions 
have been arrived at: 
(1) The present methods of cleansing glasshouses 
during the winter months are not sufficient to exterminate 
centres of infection of Colletotrichum oligochetum, which 
may exist from a previous diseased crop. 
(2) Infection is more abundant immediately after the 
diseased crop has been removed than after the period of 
winter rest, but sufficient survives to carry the disease 
over from one season to another. 
(3) The fungus may live occasionally in the debris 
which collects in the overlap between two panes of glass, 
but except in old houses this does not form an important 
source of infection. 
