102 FUNGOUS DISEASES OF PLANTS 



a pulsating vacuole are always seen. Fig. 22 shows a spore and 

 some swarmspore stages. Later, the protoplasmic mass moves 

 wholly by amoeboidal streaming. It is believed that the swarm- 

 spores may fuse into small amoeboidal plasmodia and that these may 

 also gain entrance to the host. Nevertheless, the true plasmodial 



stage is apparently that 

 which is developed im- 

 mediately preceding 

 spore formation. It has 

 been noted that in the 



FIG. 22. PLASMODIOPHORA BRASSIC&: SPORE, same cell the develoD- 

 GERMINATING SPORE, AND SWARMSPORES 



ment of the spores is 



simultaneous, and this may be true also of a whole cell group 

 (Krankheitsherde) occupying an area so large as to be visible to 

 the unaided eye. So it would seem probable that we may look 

 upon a plasmodium as extending through a considerable mass of 

 tissue. The mature spore possesses a refractive wall, or membrane, 

 the contents are granular, and include some differentiated bodies, 

 or globules, the nature of which has not been carefully determined. 

 Control. On account of the fact that this parasite gains en- 

 trance through the soil, numerous experiments have been made 

 in the treatment of soils with lime, sulfur, and other fungicidal 

 substances. In general it has been found that liming is the most 

 reliable method of prevention, lime being applied to ordinary soils 

 at the rate of about one hundred bushels per acre every few years. 

 It is further very important that all refuse from a previous crop 

 should be destroyed. It is especially advisable that such refuse 

 should not be thrown upon the compost heaps. Rotation of 

 crops, with destruction of weeds which may harbor the parasite, 

 should also receive attention. 



