104 THE PHYCOMYCETES 



either through the stomata or through unbroken epidermis. Soon 

 lesions are formed, and after 3 to 5 days of favorable weather 

 P. infestans will again be sporulating. The short sporangial cycle 

 makes possible the rapid development of potato-foliage blight in 

 epiphytotic proportions. Presumably zoospores are transferred 

 through the soil and gain entrance through the lenticels, and 

 thus tuber decay is initiated. 



There are other very destructive species of Phytophthora. 

 Amon^ the better known are F. palmivora, studied by Seal (1928) 

 in connection with bud rot of coconut and other tropical plants. 



Nearly 50 years ago van Breda de Haan (1896) recorded a 

 disease of tobacco in Sumatra, the cause of which he identified 

 as P. nicotianae. The fungus has since spread throughout the 

 tropics, and about 15 years ago it appeared in the flue-cured to- 

 bacco district of North Carolina. Tucker (1931) regards it as 

 specifically identical with P. parasitica and uses nicotianae as a 

 varietal name. Phytophthora citrophthora, causing brown rot 

 and orummosis of lemons, was first described by Smith and Smith 

 (1906) as Pythiacystis citrophthora. Petersen's (1910) mono- 

 tA'pic Pythiomorpha gonapodioides, growing on twigs and fruits 

 immersed in water, has been indicated to be a Phytophthora. 



The Genus Phytophthora differs from Pythium in that on 

 germination the zoospores are fully formed within the 

 sporangium and escape through an opening at the papilla. In 

 Phytophthora palniivora, however, a vesicle may at times be 

 formed; at others the swarm spores escape directly. 



The taxonomy of both Phytophthora and Pythium is difficult, 

 since so much deoends on cultural characteristics and host rela- 

 tionships. 



LITERATURE CITED 



Breda de H.\.\x, J. van, "De bibitziekte in de Deli tabak veroorzaakt door 



Phytophthora nicotianae,'' Mededeel. s'Lands Plantentiiin, 15: 107 pp., 



1896. 

 Butler, E. J., "An account of the genus Pythium and some Chytridiaceae," 



Mein. Dept. Agr. India, Bot. Ser., 1: 1-160, 1907. 

 Clinton, G. P., "Oospores of the potato-blight fungus, Phytophthora in- 



jestmisr Kept. Conn. Agr. Expt. Sta., 1909-1910: 151-11^, 1911. 

 Edson, H. a., ''Rheosporangiinn aphanidermatiim, a new genus and species 



of fungus parasitic on sugar beets and radishes," /. Agr. Research, 4: 



279-291, 1915. 



