174 THE FUNGI WHICH CAUSE PLANT DISEASE 



still further by biologic specialization such that forms which are in- 

 distinguishable under the microscope show in inoculation tests dif- 

 ferent abilities regarding host infection. Thus Neger,™ Salmon,*"' ^ 

 Reed,*' and others have shown that spores borne on a particular 

 host are capable of infecting only that host or in other cases only 

 nearly related species of the same host genus. Forms which can 

 pass from one genus to another are less common. Forms morpho- 

 logically distinct are regarded as separate species. Differentiations 

 within such species, regarding the species of host plant which they 

 parasitize, give rise to "biologic species" or "biologic varieties." 



Reed *' writes of these biologic forms thus: 



"So far as investigated, Erysiphe cichoracearum, is the only 

 one with doubtful exceptions, . . . shown to be capable of in- 

 fecting plants belonging to more than one genus." 



"There are other cases where the mildew is limited closely to 

 plants of a single genus," and "Several cases are recorded where 

 the mildew from one species will not infect other species of the same 

 genus. Most of these claims, however, rest on insufficient data." 



Some morphological species show a very wide range of hosts; 

 one species, Phyllactinia corylea is known on forty-eight genera 

 in twenty-seven families, others are limited to single genera or 

 to single species of host plant. Two, three, and even five species 

 are recorded for some species of host. 



Geographically the Erysiphaceae are widely distributed, prac- 

 tically of world distribution, but they are more abundant in the 

 temperate zones than elsewhere. 



A pycnidium-bearing parasite, Cicinnobolus, p. 494, is quite 

 frequently found on the mycelium and conidiophores of the Ery- 

 siphaceae. 



Owing to the extreme variability of the perithecial characters 

 and the almost promiscuity of host selection this family presents 

 a most difficult problem to the taxonomist who must either segre- 

 gate or " Imnp " species. No middle ground seems open at present. 



Key to Subfamilies and Genera of Erysiphaces 

 Mycelium wholly external to the tissues of the 

 host plant, usually sending haustoria 

 into the epidermal cells only, perithecial 

 appendages various, more or less flaccid I. Erysiphese. 



