86 THE FUNGI WHICH CAUSE PLANT DISEASE 



subspherical with smooth, moderately thick walls, hyaline or 

 light yellow, 18-26 /*• It was described on lima beans in 1889. 

 The methods of infection were studied by Sturgis ^^ who showed 

 that spores are carried to the basal portion of the style and ovary 

 by visiting insects. Oospores were described and extensive arti- 

 ficial culture experiments made by Clinton ^' who first grew the 

 fungus successfully in pure culture on corn-meal-agar, and other 

 media, on which oospores were produced in abundance. 



The species is unique within the genus on account of the single 

 conidia which are borne at the apex of apparently simple conidio- 

 phores but subtended by several enlargements of the kind so 

 characteristic of the genus. 

 P. infestans (Mont.) de Bary.*^- »«. ne-m 

 Mycelium well developed, probably perennial; conidiophores 

 single or in groups of 2-4 from the stomata; scorpiose-cymosely 

 branched; conidia 27-30 x 15-20 n, ovoid, germinating by about 

 six to sixteen zoospores. 



On diseased solanaceous . hosts, particularly the potato and 

 tomato,®* this species is very destructive. It was first described 



in 1845 as a Botrytis and 

 has since been the subject 

 of many extensive papers. 



The conidiophores are 

 abundant on the lower sides 

 of infected leaves near the 

 invasion line. The myce- 

 lium migrates between the 

 fj« VY cells piercing them with 



haustoria. 

 The existence of oospores 

 Fia. 58.— Young and mature oospores of is a much Controverted 



p. phaseoh. After Clmton. . , ., 



pomt; the structures re- 

 ported by Smith 6^' as oospores probably belonged to some other 

 fungus. Recently Jones «» found peculiar thick-walled bodies, 

 somewhat resembling oospores, m undoubtedly pure cultures of 

 P. infestans. Whether they are oospores is not known. Clinton 

 has recently announced •» that he, in pure cultures, has obtained 

 "absolutely perfect oogonia, antheridia and even oospores." The 



