THE FUNGI WHICH CAUSE PLANT DISEASE 75 



Key to Families op Saprolegniales 

 Vegetative mycelium of thick tubular hy- 

 phae; aquatic; zoosporangia cylindrical 

 not much thicker than the mycelium 



Filaments uniform, not constricted 1. Saprolegniaces. 



Filaments constricted regularly 2. Leptomitacese. 



Vegetative mycelium of thin hyphae, mostly 

 parasitic or saprophytic on plant tis- 

 sues; zoosporangia much broader than 

 the mycelium, mostly globular 3. Pythiaces, p. 75. 



Dictyuchus Leitgeb. 



This genus of the Saprolegniacese contains the only parasite 

 genus in the first two families. 



Sporangia cylindric or clavate, swarm-spores becoming walled 

 within the sporangium and emerging singly through its lateral 

 walls. The genus is usually saprophytic but, D. monosporus 

 Leit. is said by Halsted to be a serious hyacinth enemy."' 



The other members are mainly on dead or diseased insects or 

 other animals that are in water or are on diseased algee or in water- 

 slime. 



Pythiaceae "' 



This family shows affinity with both the Peronosporales and 

 the Saprolegniales and is sometimes classed with the one, some- 

 times with the other. It consists of three genera and about twenty 

 species characterized by a mycelium of very delicate hyphse which 

 show no differentiation into sterile and fertile regions. The 

 species are either aquatic or terrestial; in the latter case they are 

 soil fungi that grow to maturity upon seedlings. When of aerial 

 habit the sporangia become conidial in character, that is, they are 

 detached from the hypha before the discharge of the zoospores. 



Zoosporangia elongate 1. Nematosporangium. 



Zoosporangia spherical or oval, not linear 

 Zoospores formed outside of the zoospo- 

 rangia 2. Pythium, p. 76. 



ZoSspores formed within the zoospo- 

 rangia 3. Pythiacystis, p. 77. 



