FUNGAL PARASITES — 173 
be specially favourable to the spread of 
infectious fungal disease. Diseases of this 
kind are apt very easily to get beyond control 
unless they can be checked in their early 
stages. Even with all our present precautions 
the annual loss from fungal disease is gigantic, 
amounting to many millions sterling in this 
country alone. 
Regarded from a biological point of view, 
the parasitic species are in many respects 
the most interesting of the group of fungi. 
In spite of their simple structure we find 
their physiological properties are very much 
specialised, and admirably adapted to their 
particular habits as parasites. In_ these 
respects, however, they show wide differences 
in behaviour. Some ruthlessly kill their host, 
reducing it to a mass of rottenness—for 
instance, the Phytophthora, which is the 
cause of potato disease. Others, while taxing 
their host for their own means of support, 
make no excessive demands, and may even 
stimulate a locally increased growth on the 
part of their hosts, at any rate during the 
earlier stages of their development. Some 
of the rust fungi furnish examples of this, 
causing local thickenings on the stems of 
roses, nettles and other plants. A _ very 
striking instance of the influence that a 
parasite may exert on its host is afforded by 
a species of smut (Ustilago violacea) which 
sometimes infests the Red Campion (Lychnis 
dioica) of the hedgerows. 
