'club-root' disease 76s 



true ' clubbing ' and insect-galls are frequently observed upon the 

 same plant. The latter are, however, usually found above, or 

 only just below, the surface of the ground and on the stems of 

 the plants, whereas ' clubbing' attacks the roots, generally some 

 distance below ground. Moreover when young the insect-galls 

 are hollow, and contain whitish grubs or larvae, which are readily 

 observed when the former are cut across with a knife. 



The insect-galls are practically harmless, and do not induce 

 the total decay of the roots, as in the case of true ' clubbing.' 

 ' Clubbed ' parts are solid when young, and very rarely if ever 

 contain larvae of insects. 



Wallflowers, candytuft and other cruciferous garden plants are 

 affected by the disease, but the latter does not attack carrots, 

 mangolds, or parsnips, although fanged, irregular-rooted speci- 

 mens of these plants, due to degenerate stock or imperfect 

 cultivation of the soil sometimes resemble plants suffering from 

 this disease. 



Cause. — ' Club-root ' is caused by an organism named Flas- 

 inodiophora Brasstcce Wor., which is usually classed with the 

 Myxomycetes or slime-fungi. 



On examining a section of a young diseased but undec^yed 

 root with the microscope, many large cells are noticed, filled with 

 a frothy, turbid, and somewhat brownish protoplasm distributed 

 irregularly among the smaller cells of the cortex and medullary- 

 ray parenchyma of the cabbage root {n, Fig. 259). Each such 

 piece of protoplasm represents the vegetative body or Plas- 

 modium of a single organism. 



The Plasmodium feeds and grows at the expense of the cell- 

 contents and food manufactured by the cabbage plant, and after 

 a time divides into a very large number of transparent, round, 

 thick walled spores (r, Fig. 259), which, when decay of the 

 diseased parts takes place, are set free into the ground in millions. 



When a spore germinates a small opening appears in its wall, 

 through which the protoplasm within makes its exit. 



