68 



rest after it. While it is unknown how long they can 

 move about and live in the soil, they can certainly do so 

 for many days. 



In the field and garden the spores of this fungus are 

 liberated into the soil by the decay of the diseased tissues 

 of the infected roots. If diseased roots are left in the 

 ground for any length of time the decay takes place 

 rapidly and is generally accompanied by an offensive 

 odour. The spores pass into the soil and there remain 

 until conditions are favourable for their germination. If 

 liberated during the summer or early autumn they pro- 

 bably germinate at once, but if later it is likely that they 

 remain in the spore condition over the winter and 

 germinate in the warmer spring days. The smallest 

 quantity of water is sufficient to allow the minute specks 

 of protoplasm which are liberated, to swim about. If 

 young cabbages, turnips, etc., are grown in the infected 

 ground the organism soon gains entrance to the younger 

 roots and sets up the disturbances described above. When- 

 ever seeds of these cruciferous plants are sown in soil 

 known to contain the spores of this fungus the swellings 

 typical of the disease appear on the young roots in a few 

 weeks. 



We have seen that this organism differs from a fungus 

 like Pythium in several important respects. At no stage 

 in its life history does it possess filaments as do the vast 

 majority of fungi. It passes through the vegetative stage 

 of its life as a naked speck of protoplasm living and grow- 

 ing inside the protoplasm of the cells of a root. Unlike 

 Pythium it does not kill the cells of its host plant out- 

 right, but rather stimulates them to enlarge, divide and 

 draw food supplies to them which it then utilises. Like 

 Pythium, however, it rests in the form of spores in the 

 decaying roots and in the soil ; it also passes part of its 

 life as a naked free-swimming speck of protoplasm, but 

 in order to complete its life cycle it must enter the living 

 cells of the root of a cruciferous plant. 



In considering methods of preventing the attack of this 

 organism it is necessary, as in all such cases of plant 

 disease, to bear in mind the habits and life historv of the 

 parasite. As has already been shown, a single diseased 

 turnip or cabbage, if left to rot will liberate many 

 millions of spores into the soil. Some of these doubtless 

 die, but many remain as a source of infection for future 



