970 



MICROBIAL DISEASES OF PLANTS 



the tendency of the leaves to wilt or turn yellow. A microscopial 

 examination of the roots will reveal the distinctive protozoa which 

 cause the disease. 



The spores are liberated with the disintegration of the diseased roots 

 and become disseminated in the soil during cultivation. Under ap- 

 propriate conditions the spore is ruptured and a small flagellated, 

 amoeboid organism emerges. It is in this form that the parasites 



FIG. 199. Roots of Cabbage plant showing characteristic hypertrophy due to 

 Plasmodiophora brassica. (Woronin.') 



penetrate the roots of the young plants in which they complete their 

 development. The youngest forms seen within the vegetable cells 

 possess two nuclei each with a central mass of chroma tin or karyosome. 

 Several organisms frequently invade a single cell. As they grow, there 

 is a multiplication of nuclei and the associated organisms tend to fuse 

 together to form plasmodia. Subsequently there occurs a series of 

 changes. Some of these changes are readily distinguished; but others 

 are more difficult to follow. The nuclei first lose the greater part 



