290 FUNGOUS DISEASES OF PLANTS 



spots and inhabits leaves only. This is also a very large genus, 

 consisting of about eight hundred species. 



Forms of both Phyllosticta and Phoma, occurring on the grape, 

 have been found to be stages of a Guignardia. Some species of 

 Phoma would seem to be imperfect stages of Diaporthe, and 

 others have been associated with still other ascigerous forms. 



Sphaeropsis includes species with relatively large, continuous, 

 colored conidia (phaeosporic). The conidia are generally elliptical. 

 The pycnidia are at first immersed and finally break through the 

 epidermis. They are black with papillate ostiolum. There are 

 nearly two hundred species of this genus, of which a few are 

 important parasites. 



Coniothyrium, which includes nearly as many species as 

 Sphaeropsis, differs from the latter chiefly in the smaller size of 

 the spores, which, moreover, are often less colored. 



Septoria. In this genus the pycnidium resembles closely that 

 of Phyllosticta or Sphaeropsis, but the spores are long and fili- 

 form, often slightly curved, usually pluriseptate. With respect to 

 spore characters, therefore, the genus corresponds more or less 

 to Cercosporella and Cylindrosporium of the imperfect fungi 

 here described. 



Leptothyrium is characterized by a more or less superficial, 

 shield-shaped, black pycnidium without definite ostiolum. The 

 spores are one-celled and hyaline. 



Entomosporium possesses relatively large, black pycnidia with- 

 out ostiola. The spores are four-celled in the form of a cross, the 

 horizontal cells smaller. Each cell is provided with a delicate awn- 

 like appendage. 



IV. POTATO SCAB 

 Oospora scabies Thaxter 



STURGIS, W. C. On the Susceptibility of Various Root Crops to Potato Scab, 

 etc. Conn. (N. H.) Agl. Exp. Sta. Kept. 20 : 263-266. 



THAXTKK, ROLAND. The Potato "Scab." Conn. Agl. Exp. Sta. (1890): 

 81-95. 



THAXTER, ROLAND. The Potato Scab. Conn. Agl. Exp. Sta. ( 1 89 1 ) : 1 5 3- 1 60. 



The scab of potatoes is a disease which is well known to grow- 

 ers, dealers, and consumers alike, for the conspicuous scab pits or 

 spots on the surface of tubers cannot fail to strike the attention. 



