DISEASES CAUSED BY FUNGI 



55 



during the winter on tlie dead, fallen leaves, and the fungus thus 

 continues its annual cycle of development. 



In this instance the course of the fungus can be arrested by 

 clearing away and burning all fallen infected leaves before the 

 spores are liberated and dispersed by wind, animals, insects, etc., 

 in the spring. 



Going to the other extreme, where the parasite has evolved the 

 method of living along with its host-plant witliout causing injury, 

 but, on the other hand, actually enabling the host-plant to grow 

 more vigorously than plants of the same kind not infested with a 



" Smutted " wheat, caused by a fungus called Ustilago ti itici. 



parasite, and to pass uninterruptedly throughout its normal period 

 of growth, illustrates a much more highly developed phase of 

 parasitism than is manifested by the " damping-off " fungus 

 described above. Even in this advanced phase of parasitism, 

 marked differences of degree are observable. Ustilago avencB, the 

 fungus causing the soot-like " smut " in the ear of the cultivated 

 oat plant ; the spores of the fungus, present in the soil, infect the 

 seedling oat plant when it is only a few days old, in fact it is only 

 during the seedling stage that the plant can be infected. After 

 infection the mycelium of the fungus grows up along with the oat 

 plant, embedded in its tissues, and without any external evidence 

 of its presence, until the ear is formed, when the mycelium passes 



