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. 



