VEGETABLE CROPS. 



669 



night. Plants are generally attacked when young or in the seed-bed, and 

 when so affected have a stunted and sickly appearance and seldom grow to 

 maturity. The malformations may be confused with root-knots caused by 

 eelworms (or gallworms), but these as a rule are not so large as the mal- 

 formations associated with Club Root. 



The parasite lives within the cells of the roots of certain plants, 

 especially these belonging bo the natural order Cruciferse. It is 

 mostly confined to the cortex, 

 and within the cells the parasite 

 passes through some of the stages 



of its development. At one 

 time the cell is filled with what 

 appears to be a number of shape- 

 less specks of jelly (protoplasm). 

 These later unite to form one 

 mass, the plasmodium, which 

 practically fills the cell, the 

 original contents of which have 

 been absorbed. Still later, the 

 plasmodium divides up to form 

 numerous spores covered with a 

 thin membrane. The parasit- 

 ised cells and also the neigh- 

 bouring ones are stimulated to 

 very active growth. They in- 

 crease in size and also in number 

 by rapid cell division, thus pro- 

 ducing the remarkable enlarge- 

 ments of the underground parts 

 of the plant. The parasite be- 

 comes distributed within the 

 tissues by this cell division, but 

 it is also able to penetrate the 

 cell walls. By the decay of the 

 attacked parts, the spores are 

 set free into the soil. Under 

 favourable conditions, the spore 

 germinates and gives rise to what 

 is known as a zoospore (or swarm 

 spore;, a speck of protoplasm 

 provided with a long appendage 

 or cilium. These swarm spores 

 move about in the soil and are 

 capable of entering a root apparently through a root hair. Once within the 

 plant the swarm spore goes through the stages outlined above. 



We thus see that the parasite is confined either to the soil or to the 

 underground parts of the plant, and therefore no sprayings of any kind 

 are of any use against this disease. The majority of Cruciferse are sus- 

 ceptible, but it appears that varieties of cabbage vary in their susceptibility. 

 Soils' of an acid nature are favourable to the development of the parasite. 

 Air-slaked lime is the most valuable substance to use to cheek it. Applica- 

 tions of 2 to 4 tons per acre are recommended, thoroughly incorporated in 

 the soil, after all refuse of diseased crops has been removed and burnt, not 



Club-root of Caobage. 

 (Flasmodiophora brassicx.) 



