218 THE LOWER FUNGI— PHYCOMYCETES 



Minor, the strenuous efforts to stop its ravages, and the incident 

 and accidental discovery of bordeaux mixture are now well 

 known to all students of plant pathology. The species also 

 occurs on the Virginia creeper {Psedera quinquefolia) and related 

 plants. Other well known species of the genus are: 



PI. geranii (Peck) Berl. & De Toni — on wild Geranium. 



PI. obducens Schroter — on Impatiens. 



PI. halstedii (Farlow) Berl. & De Toni — on Ambrosia, Bidens, Erigeron, 

 Eupatorium, Helianthus, Rudbeckia, Silphium, and other members of the 

 Carduaceae. This species has not been sufficiently' studied taxonomicaUy 

 and perhaps should be subdivided into several. It has been studied cytolog- 

 ically by Nishimura (1926). 



PI. viburni Peck — on- Viburnum. 



PL ribicola Schroter — on Ribes. 



PI. pygmaea (Unger) Schroter — ^n various genera of the Ranunculaceae. 



PI. nivea (Unger) Schroter — on various genera of the Umbelliferae. It 

 occurs on carrot and parsnip and has been reported in America from 

 (California. 



4. Peronoplasmopara Berlese, in Clinton (1905 a: 329). 

 syn. Pseudopero7iospora Rostowzew (1903: 422). 



In the earlier investigations of the Peronosporaceae two 

 species, Peronospora cuhe^isis B. & C. and Peronospora celtidis 

 Waite, were recognized as noteworthy in that their characters 

 show them to be intermediate between Peronospora and Plasmo- 

 para. The first of these is the well known and common mildew 

 of cucurbits. The other occurs on Celtis occidentalis. In the 

 monographic treatment of the family by Berlese (1902: 123) 

 both species are incorporated in the genus Plasmopara, a new 

 subgenus, Peronoplasmopara, being established there for their 

 reception. In these species, the sporangiophore branches in a 

 subdichotomous manner, the branches arise at acute angles, and 

 the terminal branchlets have subacute tips. The sporangia are 

 large and tinted as in Peronospora, but have a prominent papilla 

 and germinate chiefly by swarmspores as in Plasmopara. 



The year following the erection of the subgenus Peronoplas- 

 7nopara Berlese, the cucurbit organism Peronospora cuhensis 

 B. & C. was critically studied by Rostowzew. He pubUshed an 

 extensive discussion of it, expressed the belief that it represents 

 a distinct genus intermediate between Peronospora and Plasmo- 

 para, and applied to it the new name Pseudoperonospora (Rostow- 

 zew 1903: 422). Although he contrasted the species with those 

 of Peronospora and Plasmopara, he failed to give a generic 



