214 



DISEASES OF ECONOMIC PLANTS 



shown in the accompanying figure. It cannot be con- 

 founded with any other disease unless it be that caused by 

 eel worms (nematodes), and from this it may be easily dis- 

 tinguished by the greater magnitude of the club-root galls. 



These enlargments may 

 be noted in the seed bed, 

 though they are usually 

 overlooked until they be- 

 come larger in the field. 

 They so diminish the vigor 

 of the plant that it assumes 

 a sickly appearance, ceases 

 to develop, and fails to 

 head. The affected parts 

 usually succimib to offen- 

 sive secondary rots, the 

 causal organism is released, 

 and the soil is thus infected. 

 The proof of the parasitic 

 origin of this disease was 

 made by Woronin in 1873, 

 after three years of inten- 

 sive study. 



In general the precautions 

 against its spread are those 

 heading soil diseases. All 

 diseased refuse is infective, as is also manure to which it 

 has had access. Fields which are diseased remain so for 

 several years; therefore rotation to nonsusceptible crops 

 should be followed. Seedlings should be examined care- 

 fully to guard against the use of plants from an infected 



- Cabbage club root. Original. 



suggested under the general 



