THE FUNGI WHICH CAUSE PLANT DISEASE 93 



P. obducens (Schr.) Schr. occurs on Impatiens, both wild and 

 cultivated, in North America, Europe and Asia. 



P. pygmea (Ung.) Schr. on various Ranunculacese, including 

 Aconitum in Europe and cultivated Hepaticas in America,^" is 

 of little economic importance. 



Peronoplasmopara (Berlese) Clinton (p. 83) 



Thiere are three species which have been variously designated 

 as Peronospora, Plasmopara, Pseudoplasmopara and Peronoplas- 

 mopara. The genus combines colored conidia and zoosporic germi- 

 nation with a type of conidiophores intermediate between those of 

 Peronospora and Plasmopara. 



Mycelium much branched, haustoria small, usually simple - 

 conidiophores pseudo-^monopodially branched, the ultimate branch- 

 lets acute, the primary arising at acute angles; conidia colored, 

 elliptic, conspicuously papillate both apically and basally; oospores 

 thin-walled, smooth or roughened; oogonium thin-walled. 



P. celtidis (Waite) Cl.'^ is unique in the family as the only 

 species infecting dicotyledonous trees. It occurs on hackberry in 

 the region about Chesapeake Bay, also in Japan. 



P. hiimuli Miy. & Taka ^^ causes a serious hop disease in Japan. 

 It has recently been found by Davis '^ on wild hops in Wisconsin. 



P. cubensis (B. & C), CL'^-'^- ">^ 



Hypophyllous, rarely amphigenous; discoloration of the host 

 yellowish, or water-soaked; conidiophores 1-2 rarely more from a 

 stoma, 180-400 x 5-9 fi, 3-4, rarely 2-5 times branched, the ulti- 

 mate branchlets recurved; apically acute, 5-20 /i long; conidia 

 gray, brownish or smoky, ovoid to ellipsoid, papillate, 20-40 x 

 14-25 fi; oospores spherical, yellowish, warty-papillate, 30-43 jn, 

 maturing in the decaying leaves. 



The myceUum abounds in the spongy parenchyma. The 

 conidiophores emerge through stomata, or rarely directly through 

 the cuticle, near the invasion line of the fungus. Fresh conidia 

 germinate in water in two to four hours forming flattish zoospores 

 with one anterior and one posterior ciUum. The zoospores later 

 become spherical, walled and develop a germ tube. These germ 

 tubes enter the host through the stomata or directly through the 

 cuticle from either above or below. Moist weather is favorable to 



