Family Convolvulaceae 159 



very dry and more or less hardened, varying with 

 the nature of the fungi which act as secondary 

 invaders. The second form, or dry-ring rot, is 

 nothing more than the last or dried-out stage of 

 the soft ring in which the primary parasite has died. 



Dry Rot 



Caused by Diaporthe batatis (E. and H.) H. and F. 



Dry rot is a disease of stored sweet potatoes. 

 Although it has a wide distribution, the trouble is 

 not considered of great economic importance. The 

 disease usually follows a complication of others. 

 It begins at the end of the potato, producing a firm 

 dry rot which progresses slowly. The rotted potato 

 is brown, finally becoming hard and shriveled. The 

 surface later becomes black, rough, and uneven (fig. 

 26 d) and when examined under the microscope will 

 be found to be studded with numerous pycnidia. 

 From the mouths of the latter are seen to ooze out 

 whitish strings which are made up of millions of the 

 pycnidia or summer spores. The Diaporthe or as- 

 cospores are formed only when the infected roots are 

 allowed to hibernate. 



Foot Rot 



Caused by Plenodomus destruens Hart. 



Foot rot is a disease which, so far as is known, is 

 found only in Virginia, Ohio, Iowa, and Missouri. 



