348 FUNGOUS DISEASES OF PLANTS 



XLV. BLACK ROT OF SWEET POTATO 

 Spharonema fimbriatum (Ell. & Hals.) Sacc. 



HALSTED, B. D. Some Fungous Diseases of the Sweet Potato. The Black 

 Rot. N. J. Agl. Exp. Sta. Built. 76: 7-14. figs. 3-10. 1890. 



HALSTED, B. D., and FAIRCHILD, D. G. Sweet-Potato Black Rot. Journal of 

 Mycology 7 : i-n. pis. 1-3. 1891. 



The black rot of the sweet potato is one of the most destructive 

 diseases of this host, and it is known to occur from New Jersey 

 southward practically throughout the Atlantic coast region. The 

 distribution of the fungus, however, is not completely known. The 

 disease may appear in the seed bed, resulting from the use of 

 infested seed roots. The disease upon the seedlings is known as 

 black shank, due to the black spots or discolorations on the roots 

 and young stems. The commercial root may be infested either 

 as a result of planting diseased slips, or the infection may be due 

 to the presence of the fungus in the soil. Upon the full grown 

 root the disease appears in the form of dark patches or decayed 

 spots, which, upon more careful examination, and especially upon 

 removal of the skin, will appear green. These spots vary in size 

 from minute flecks to extensive areas involving practically the whole 

 root. When the roots are diseased there is no appearance of the 

 vegetative parts which suggests the presence of the parasite. 



The fungus. The mycelium consists of septate, much branched, 

 thick-walled, olivaceous hyphas, which are commonly intercellular. 

 The cells in the region invaded are robbed of starch, and the cell 

 walls are brown and often collapse. The fungus has many fruiting 

 stages which may be briefly referred to. Two kinds of conidia are 

 produced, one within the tissues consisting of simple, ovate, green- 

 ish cells, abscised from terminal or lateral branches. Upon the 

 surface of the diseased area, or in culture, there are also produced 

 minute, hyaline conidia. The latter are developed endogenously, 

 more or less as described for a similar phase in the case of the 

 root rot of tobacco, Thielavia basicola. The pycnidial form is 

 produced within the diseased areas, and it is also readily developed 

 in artificial cultures. It consists of a flask-shaped pycnidium, with 

 extremely long neck. The bulbous portion is from 96 to 224 /JL in 

 diameter and the neck from 395 to 608 p in length, and 24-34 /* 

 wide at the base, tapering to 12-14^-. The method of spore 



