46 James G. Horsfall 



severely diseased leaves. These increase in size until the whole blade 

 is dead. 



Plants, if diseased when small, are markedly dwarfed. This is evident 

 if mildewed seedlings in the greenhouse are compared with unmildewed 

 ones kept healthy by dusting. 



Etiology 



Name, history, and classification of the pathogene. Powdery mildew of 

 clover is caused by Erysiphe polygoni DC. The perithecial stage has 

 been collected in the western part of the United States but has not yet 

 been found in the East. (Plant Disease Bulletin 6:53. 1922.) 



That the pathogene has long been known is indicated by the fact that 

 the causal organism is one of the few fungi named by Linnaeus, who 

 called it Mucor Erysiphe (Systema vegetabilium, ed. 13, p. 825. 1774). 

 The present name, Erysiphe polygoni, was applied by De Lamarck and 

 De Candolle (Flore Francaise 2:273. 1815). It is of interest historically 

 to note that Erysiphe polygoni is one of the organisms that Neger used 

 in 1902 in one of the early demonstrations of physiologic specialization 

 in the Erysiphaceae. 



The fungus is variable, but the presence of from four to eight spores in 

 the ascus separates it definitely from E. c ichor acearum with which it is 

 most easily confused. The latter has regularly only two spores in the 

 ascus. Because of the variability of the species even on the same plant, 

 an extensive synonomy has grown up (Salmon, 1900:174-178). Erysiphe 

 communis Grev. and E. martii Lev. are the most commonly used synonyms. 



Pathogenicity. That strains of E. polygoni highly specialized in their 

 pathogenicity occur was early recognized by Neger (1902), who showed, 

 for example, that conidia of E. polygoni from Trifolium incarnatum would 

 not infect T. repens or Vicia sepium, although they would infect the 

 species of Trifolium from which they came. Salmon (1903:290) stimu- 

 lated by this work, sowed conidia from red clover upon various closely 

 related species and concludes " that the Oidium on Trifolium pratense 

 is a ' biologic form ' specially adapted to this species, and incapable of 

 growing not only on species belonging to other genera of the Legum- 

 inosae, but also on numerous species, — if not on all other species — 

 within the genus Trifolium itself." He then predicts that "biologic 

 forms " exist on other species of Trifolium. Seventeen years elapsed 

 before this prediction was substantiated by experimental evidence. 

 Searles (1920:279) found indications of "biologic forms" on Trifolium 

 minus and T. hybridum. Mains (1923:311) confirms Salmon's form on 

 T. pratense. Hammarlund (1925) published a monumental piece of 

 work on the specialization of parasitism of Erysiphe polygoni and other 

 species. As a result of his experimentation extending over many years, 



