256 



FUNGOUS DISEASES OF PLANTS 



darker in color, and pycnidia, appearing as black papillae, may be 

 produced over the entire surface. At this stage the effects of the 

 fungus are therefore unmistakable. Later the fruit shrivels in a 

 characteristic manner, but does not, as a rule, fall or shell. The 

 berries on bunches thus affected may hang on the vines through- 

 out the season. The pycnidia may also be easily observed with 

 the unaided eye upon the dried berries. 



Susceptibility of varieties. It seems to be the general experi- 

 ence that practically all of the more commonly cultivated varieties 



of grapes, particularly, how- 

 ever, the dark colored va- 

 rieties, including Concord, 

 Hartford, Roger's Hybrids, 

 etc., are susceptible. In some 

 districts certain light col- 

 ored varieties are more re- 

 sistant and the Scuppernong 

 is practically free from 

 attack. In this case, how- 

 ever, as in many others 

 already mentioned, there is 

 a great difference in the re- 

 sistance of varieties accord- 

 ing to their environmental 

 conditions. For all commer- 

 cial purposes grape growing 

 would be impossible in most 

 localities, on account of the 

 great losses entailed, if the 

 disease were not practically controllable by spraying operations. 



The fungus. The mycelium of this fungus is found in the 

 outer portions of affected berries, but mycelium is never abun- 

 dant. Under favorable weather conditions only about one week 

 may be required from the time of infection to the development of 

 the pycnidia, — ordinarily 8-12 days are necessary. The pycnidia 

 have long been known under the name Phoma uvicola B. & C. 

 Upon the leaves the pycnidial stage has passed under the name 

 Phyllosticta Labrusca. The pycnidium develops from a stromatic 



Fig. i 10. Grapes affected by Black Rot 

 (Photograph by F. C. Stewart) 



