Nov. II. i9i8 Sweet-Potato Storage-Rots 361 



of the roots when removed from the competition of other fungi. The 

 writers have seldom isolated the fungus, and in no case has it ever been 

 obtained from wounds or decayed tissue where it seemed the primary 

 cause of the rot. Cook and Taubenhaus were able to infect sweet pota- 

 toes artificially by inoculating them with pure cultures of T. koningi. 

 A rot differing somewhat from that caused by T. koningi in character 

 was produced by artificial inoculation with T. lignomm (Lode.) Harz. 

 The latter fungus, however, was not isolated from sweet potatoes, but 

 was obtained by the investigators mentioned above from Dr. Thaxter. 

 Both of these species of Trichoderma are widely distributed fungi, and 

 T. koningi is especially common in the soil. Therefore it is not surpris- 

 ing to find it associated with other fungi in the rotted tissue of sweet 

 potatoes. 



The symptoms caused by T. koningi are described by Taubenhaus 

 and Manns (57, p. 24) as follows : 



In the earliest stages the spots are circular and of a light brown color, w-ith a tendency 

 to wrinkle. The flesh is hard and watersoaked, brown in color, with a black zone in 

 the region between the healthy and diseased tissue. The spot enlarges in all direc- 

 tions and eventually destroys the entire root. 



DISCUSSION OF RESULTS 



In the foregoing pages 1 7 organisms have been found to bear some rela- 

 tion to storage-rots of sweet potatoes. Naturally, some of these are of 

 minor importance, but perhaps largely because the conditions suitable 

 to their needs are not usually maintained in the storage house. In fact, 

 there is not a single organism here discussed which did not exhibit a 

 preference for certain evironmental conditions. Such fungi as Rhizopus 

 nigricans and Sphaeronema fimbriatum find conditions suitable for them 

 to rot sweet potatoes at the usual storage temperature (50° F.) or at 

 higher temperatures. On the other hand, such organisms as Mucor race- 

 mosus, Boiryiis cinerea, and others find such tenmperatures unsuited to 

 their needs and bring about destruction of the host only at temperatures 

 considerably lower than that usually recommended for the storage of the 

 crop. 



Humidity was shown to play an important part. Rhizopus nigricans 

 requires a relatively high humidity until it has once started, after which 

 it will complete the destruction of the potato in an atmosphere almost 

 entirely free of moisture. Sphaeronema fimbriatum also grows better in 

 the presence of abundant moisture. Diplodia tuhericola, on the other 

 hand, will grow in a relatively dry atmosphere from the outset. R. 

 nigricans completes the destruction of the potato in a few days, while D. 

 tuhericola, Diaporthe bataiatis, and others are slow growers, requiring from 

 three to eight weeks. 



Although the writers have discussed at some length organisms which 

 thrive best at temperatures below that recommended for the storage of 

 the crop, sight must not be lost of the fact that the recommendations for 



