THE FUNGI WHICH CAUSE PLANT DISEASE 177 



in that it later becomes quite brown. It is found in closely felted 

 patches on stems and fruit. Perithecia begin to form in June. 



Amphigenous; mycelium persistent, at maturity forming dense 

 pannose patches of brownish hyphse; perithecia gregarious, more 

 or less immersed in the persistent mycelium, subglobose, 76-110 

 /i in diameter; cells large, at first well defined, then becoming 

 obscure, 10-25 fi wide; appendages usually few or even obsolete, 

 pale-brown, short, rarely longer, up to five times the diameter of 

 the perithecium, tortuous; ascus elliptic-oblong to subglobose, 

 70-92, rarely 100 x 50-62 fi; spores 20-25 x 12-15 fi. 



On wild and cultivated species of Ribes in America; recently 

 introduced into Europe where it is very destructive. 



S. lanestris Hark, occurs on various species of oaks in the United 

 States. 



Erysiphe Hedwig (p. 175) 



Perithecia globose, or slightly depressed, rarely concave; ap- 

 pendages floccose, simple or irregularly branched, sometimes 

 obsolete, usually more or less similar to the mycelium and inter- 

 woven with it; asci several, 2 to 8-spored. 



Salmon" recognizes eight species; Engler and Prantl,' twenty. 



E. polygoni D. C." 



Amphigenous; mycelium very variable, persistent, thin, effused 

 and arachnoid, rarely thick, or more often evanescent; perithecia 

 gregarious or scattered, usually rather small, 

 averaging 90 n, but ranging from 65 to 180 n; 

 cells usually distinct, 10-15 n wide; append- 

 ages very variable in number and length, 

 few or many, distinct or more or less inter- 

 woven with the mycelium, brown or coloriess; rm. i27.-^!^lygoni, 

 asci 2-8 or rarely as many as 22, variable in the asci. After Sal- 

 shape and size, usually small and ovate, with 

 or without a short stalk, 46-72 x 30-45 n; spores 3-8 rarely 2, 

 19-25 X 9-14 n. 



Conidiophores (=Oidium balsamii) medium; conidia ovate, 



hyaline. 



One of the commonest species, especially destructive to the pea 

 and turnip. It was studied by Salmon on one hundred ninety host 



