THE FUNGI WHICH CAUSE PLANT DISEASE 69 



There are some twenty-five species most of which live as para- 

 sites on algae, worms, pollen grains, etc. 



O. brassicse (Wor.) Dang.* is parasitic on quite young cabbage 

 seedlings, sometimes infecting cells deeply seated in the host. 

 The same or a nearly related species also attacks tobacco and 

 several weeds. 



Sporangia solitary or several in each infected host cell, globular; 

 zoospores numerous, globose, uniciliate; resting spores globose, 

 with a wrinkled epispore which gives them more or less of a star- 

 like appearance. Fig. 40. 



Asterocystis de Wildeman (p. 68) 



There is a single species, A. radicis d. Wild.* which differs from 

 Olpidiimi in its stellate vegetative cell and the absence of the tube 

 for the escape of the zoospores, this being accomplished by the 

 breaking away of the tissues of the host. The fungus attacks the 

 roots of various plants, notably flax, Brassica and other crucifers, 

 Plantago, Veronica and numerous grasses, producing chlorosis. 

 It has not been reported from America. 



A Chjrtridiaceous fungus of unknown genus thought to stand 

 near the Olpidiaceae and Synchytriaceae has been described by 

 Home ' as the cause of an Irish potato disease. 



SjrnchytriaceaB (p. 67) 



The infecting zoospore invades the host cell and becomes 

 parasitic upon the still Uving protoplasm. Hypertrophy of this and 

 adjacent host cells is usually induced, resulting in the formation of 

 a small gall around the infected cell. This gall is often colored 

 and bears a superficial resemblance to a rust sorus. The parasite 

 enlarges until it occupies nearly the whole of the host cell. In 

 Synchytrium the one nucleus then enlarges and divides to produce 

 very numerous nuclei. '• '"• "• " The whole mass then divides 

 into segments regarded as sporangia, and each sporangiiun divides 

 into numerous uninucleate parts, each of which develops into a 

 zoospore. In some species development is arrested before the 

 division of the primary nucleus and the protoplast becomes 

 spherical, invests itself with a thick wall and becomes a resting 



