OEDER PERONOSPORALES 141 



conidia: by zoospores in Rhysotheca and by a single naked mass in Plas- 

 mopara. The most important species from the economic standpoint is 

 R. viticola (B. & C.) G. W. Wilson, which causes the downy mildew and 

 brown rot of the foliage and fruit, respectively, of grape (various species 

 of Vitis). 



Pseudopero?iospora (called Peronoplasmopara by some although the 

 former name has priority) has slender conidiophores branching at acute 

 angles and with pointed tips. The usually violet-tinged conidia germinate 

 by zoospores. Ps. cubensis (B. & C.) Rostow., first described from Cuba, 

 is probably native to Russia where its destructive effects have been known 

 for many years although the fungus was first recognized there in 1903 by 

 Rostowzew. It is a very serious enemy of the cucumber (Cucumis sativus 

 L.) and muskmelon (C. melo L.). Another species, Ps. ceUidis (Waite) 

 G. W. Wilson, attacks the hackberry {Celtis) while other species are found 

 on hemp {Cannabis), on hops (Humulus), and on nettle {Urtica). Hoerner 

 (1940) has shown that the species on hops {Ps. humuli Miyabe) is capable 

 of infecting Celtis, Cannabis, and Urtica and suggests that there is one 

 species infecting Urticaceae, perhaps with various physiological strains. 



Bremia and Peronospora have slender conidiophores, branching at 

 acute angles in a more or less dichotomous manner. Their conidia ger- 

 minate typically by germ tubes, although zoospore production also has 

 been reported for Bremia by Milbrath (1923) and for P. spinaciae (Grew.) 

 Laub. by Eriksson (1919). Schultz (1937) could not observe zoospore 

 production in the strains of B. lactucae Regel growing in Germany. In 

 Bremia the tips of the branches enlarge into disk-like structures bearing 

 sterigmata on their edges. B. lactucae is sometimes destructive to lettuce 

 grown under glass. In Peronospora the tips of the branches taper to a 

 point. Gaumann (1923) recognizes 268 species of this genus. They are of 

 economic importance in but few cases. P. spinaciae is sometimes destruc- 

 tive in plantings of spinach {Spinacia oleracea L.) and P. parasitica 

 (Pers.) de Bary on various crucifers (Brassicaceae). (Fig. 45 D-E.) 



Gaumann (1918a, b), Wartenweiler (1918) and others have shown 

 that the earlier recognized species of this family are separable by bio- 

 metric and cultural means into large numbers of closely related species 

 confined to very limited numbers of host species and differing constantly, 

 but only slightly, in the size and shape of the conidia and conidiophores. 

 When such studies have been extended to all parts of the world and to 

 all the forms occurring on different host species the number of species of 

 Peronosporaceae will doubtless be very greatly increased. 



Several other genera have been described which may be found to be 

 justified. The physiology of conidial germination must be studied care- 

 fully in these as well as in the older genera. Only after such studies can 

 we be certain that some of the generic distinctions now maintained, or 



