6 THE FUNGI WHICH CAUSE PLANT DISEASE 



The Plasmodiophorales appear to include all of the true para- 

 sites of the Myxomycetes. 



Eet to Geneba of Plasmodiophorales 



Spores free, spherical 1. Plasmodiophora, p. 6. 



Spores united into groups 



Spores in groups of four 2. Tetramyza, p. 8. 



Spores in larger groups 



Spores forming a hollow sphere. ... 3. Sorosphtera, p. 8. 



Spores forming a spongy spore-ball 4. Spongospora, p. 8. 



Plasmodiophora Woronin 



This genus is parasitic in the living parenchyma of the roots 

 of plants, the plasmodia filling the cells and causing galls at 

 the point of attack. There are three species of the genus in 

 Europe and America. 



P. brassicse Wor.^"*' 2«>-203. 208 jjgg j^^g ^^gg^ known as a parasite 

 on the crucifers generally and recent work indicates that other 

 families, as the Umbelliferae and cucurbs, are also susceptible.^" 



The parasitised cells especially, and the adjacent cells as well, 

 are stimulated to enormous overgrowth; this hyi)ertrophy result- 

 ing in a characteristic root "clubbing." 



Study of diseased sections shows that the medullary rays and 

 cortex are abnormally thick (hypertrophy and hyperplasia) and 

 many of their cells are parasitized. Sclerenchyma cells are sup- 

 pressed by the parasite and the xylem is reduced and phloem in- 

 creased proportionately. The amount of stored starch is much 

 less than in normal tissues. 



Infection does not appear to pass from cell to cell but groups of 

 diseased cells are thought to arise from repeated division of a cell 

 after its infection. 



In the enlarged host cells the protoplasm appears abnormally 

 dense and fine grained. Eventually the whole lumen of the cell is 

 occupied by the crowded, amoeboid, individuals, each uninucleate 

 and unwalled, and still distinct from the other. These individuals 

 later fuse into a Plasmodium the nuclei of which enlarge and un- 

 dergo simultaneous mitotic division. Still later the mass divides 

 into uninuclear segments each of which matures to a spore 1.6 /* 

 in diameter, covered by a thin, smooth, colorless membrane. 



