THE BLACK ROT 173 



No appearance in the field has so far been observed, that 

 would distinguish hills diseased with black rot from 

 those attacked by some other of the numerous rots ; but 

 the dark sunken areas on the potato, and the black dis- 

 colorations of the sprouts, can scarcely be confused with 

 any other sweet potato disease." 



This black rot has been found seriously injurious in 

 New Jersey, Maryland, Delaware and Virginia, and it 

 probably occurs in many other portions of the United 

 States. It frequently destroys twenty-five per cent, of 

 the crop, the injury taking place mostly after the pota- 

 toes are stored, when the fungus can easily pass from 

 root to root. 



A microscopic examination of portions of diseased 

 roots shows the presence of threads of mycelium, which 

 push about between and through the cells, absorbing 

 the starchy matter with which the latter are normally 

 filled. These mycelial threads have thick walls, and are 

 olive-brown in color. Under suitable conditions of 

 warmth and moisture, they produce on the outside of 

 the skin of affected root or stem numerous spores (called 

 conidia) of two distinct forms ; and there are also 

 formed within the substance of the potato certain flask- 

 shaped bodies, within which are produced a third kind 

 of spores. The disease is likely to start from affected 

 potatoes planted in hotbeds to obtain sprouts. The 

 mycelium may pass directly from the root to the young 

 sprout, or the latter may be inoculated by means of 

 spores. Such sprouts, when planted, produce diseased 

 roots. 



Treatment. The following preventive measures 

 are recommended by Messrs. Halsted and Fairchild : 



1. The first and most important precaution to be 

 taken, in combating the disease, is to plant only per- 

 fectly healthy seed in the hotbed, even if it is necessary 

 to import such. This preventive measure is most essen- 



