140 



PHYCOMYCETEAE 



germinate by a stout germ tube or by the formation of a conidiophore 

 terminated by a single large conidium (as reported by Gregory, 1912, in 

 Rhysotheca viticola (B. & C.) G. W. Wilson), sometimes by the direct 

 formation of zoospores. (Fig. 46.) 



The six or more genera of Peronosporaceae are mainly distinguished 

 on the basis of their asexual characters. In the genus Basidiophora, with 

 two species parasitic on Composites (Family Asteraceae), the conidio- 

 phore is club-shaped with its slightly swollen 

 apex covered with numerous short sterigmata, 

 each bearing a nearly spherical conidium which 

 produces zoospores when it germinates. (Fig. 

 45 B.) 



In Sclerospora the more or less dichoto- 

 mously branched conidiophores are much 

 thickened. The conidia germinate typically by 

 zoospores in aS. graminicola (Sacc.) Schrot., but 

 in most of the other species of the genus by 

 germ tubes. The oogone wall may remain thin 

 and lie closely against, but not grown fast to, 

 the thick-walled oospore (in S. graminicola 

 (Sacc.) Schrot., according to McDonough, 

 1937) or it may become thickened and some- 

 what folded, separate from the oospore wall 

 which has a thin wrinkled outer layer and a 

 thick inner layer (in S. macrospora (Sacc.) 

 McDonough, according to McDonough, 1947). 

 The thirteen or more species are chiefly par- 

 asites of grasses (Family Poaceae). In the 

 East Indies they cause serious injury to sugar 

 cane (Saccharum officinariim L.) and to Indian 

 corn or maize {Zea mays L.). Sclerospora 

 graminicola is found frequently throughout 

 the north temperate regions on foxtail grasses 

 (Setaria). The conidiophores form a downy layer on the under side of 

 the infected leaves early in the morning but quickly dry down as the air 

 becomes warmer and drier. After the oospores develop the leaves die and 

 shred longitudinally into thread-like strips on which the oospores may be 

 seen readily by the aid of a hand lens. Weston (1920, 1921, 1923, 1924) 

 has given various species of this genus very careful study. (Fig. 47.) 



Plasmopara and Rhysotheca, usually united under the former name, 

 produce slender, much branched conidiophores whose branches arise 

 nearly at right angles. The tips of the branches are truncate. The two 

 genera differ, as mentioned above, by the mode of germination of the 



Fig. 47. Peronosporales, 

 Family Peronosporaceae. 

 Sclerospora graminicola 

 (Sacc.) Schrot., conidio- 

 phore. (After Weston: J. 

 Agr. Research, 27(10) :771- 

 784.) 



