668 



Violet Root-Rot. 



[FEB., 



its only mode of reproduction being vegetative by means of 

 mycelium. 



The way in which the fungus spreads in the soil and keeps its 

 hold can be easily seen. When a root or tuber has become 

 superficially coated with a felt of mycelium, sclerotia or con- 

 centrated masses of mycelium of two distinct kinds of structure, 

 and having different uses, are formed. Some sclerotia are of 

 considerable size, varying from that of a pea to a hazel nut ; 

 these become free from the root when fully formed, and remain 

 in the soil as centres of future infection. Other sclerotia, rarely 

 exceeding the size of an ordinary pin's head, are usually pro- 

 duced in considerable numbers under the felt of mycelium, and 

 in close contact with the root or tuber, to which they remain 

 firmly attached, and are removed along with it. If such infected 

 roots or tubers are eaten by some animal, the minute, compact 

 sclerotia are not injured by passing through it, and by such 

 means are often transported to new localities. In like manner 

 new districts are often infected by means of minute sclerotia 

 attached to potato tubers, carrots, &c. In some instances beans 

 and peas are attacked while yet in the pod, and minute sclerotia 

 are formed in the skin of the seed. 



The disease usually spreads from a point of infection equally 

 on every side, the mycelium gradually spreading through the 

 soil from diseased to healthy plants. 



The first sign of disease is the drooping and yellowing of the 

 foliage ; the presence of violet mycelium on the surface of a 

 carefully removed root proves the fact. 



The fungus can only develop in the presence of an acid, hence 

 can be held in check by the free use of lime. 



Good drainage and the prevention of sourness of the soil are 

 essential features in combating the disease, as is also the absence 

 of weeds, as these supply the main supply of food for the fungus 

 when a cereal crop is present. 



Care must be taken not to introduce the disease by means of 

 small sclerotia adhering to seeds or tubers. 



Seed obtained from dry, elevated districts should be selected. 



