212 THE LOWER FUNGI— PHYCOMYCETES 



2. Branching dichotomoiis. 



a. Tips enlarged; sporangia apically papillate. 



(1) Tips disc-like, bordered by sterigmata bearing 



sporangia. 



5. Bremia, p. 219 



(2) Tii)s merely swollen. 



fi. Bremiella, p. 220 

 b. Tips acute; sporangia not papillate. 



7. Peronospora, p. 221 



1. Sclerospora Schroter (1889: 236). 



Mycelium intercellular, bearing small, usually knob-like, 

 unbranched haustoria; sporangiophores typically stout (Fig. 75), 

 composed of a main trunk and a compact group of rather short 

 apical branches which are once to several times divided ; branch- 

 ing dichotomous to indefinite; germination of sporangia by one 

 or more germ tubes or by swarmspores; oospores unlike those of 

 other genera of the family in that the exospore is confluent with 

 the wall of the oogonium; method of oospore germination 

 undetermined (Weston and Weber, 1928: 957; Melhus, Van 

 Haltern, and Bliss, 1928: 335). 



The genus includes about twelve species. They are more com- 

 monly found in the Orient than elsewhere. Several species occur 

 in the United States. The majority are parasitic on Gramineae. 



The type species, S. graminicola (Sacc.) Schroter, is common in 

 the United States on various species of foxtail grass {Setaria or 

 Chaetochloa) and has been reported (Melhus, Van Haltern, and 

 Bhss, 1928: 304) on Zea mays (maize), Euchlaena mexicana (teo- 

 sinte), Panicum miliaceum, and Saccharum officinarum. In other 

 parts of the world its host range is similar. The species is 

 characterized by abundant development of oospores (estimated at 

 three and one-half million in a medium sized leaf) and a poorly 

 developed, evanescent sporangial stage. The sporangia are 

 broadly ellipsoidal to globose and germinate by swarmspores. 

 The sporangiophore is stouter than in some other species, the 

 main branches being few and stout, and the terminal branches 

 conical. In later stages of the disease the terminal portion of the 

 host plant tends to take on a characteristic shredded aspect. This 

 is due to the disintegration of the tissue between the vascular 

 bundles and the consequent development of fibrous tangles 

 of dry bundles from which the oospores are gradually scattered. 

 The inflorescence is often transformed by the development of 

 elongate, leaf-like bracts which replace the usual floral parts. 



