160 Diseases of Truck Crops 



Wherever present it causes serious losses to sweet 

 potatoes in the field. 



Symptoms. The disease first manifests itself as 

 sunken brown to black spots at the stem end of the 

 plant near the soil line. The spots gradually enlarge, 

 girdling the stem and working upwards. In a dis- 

 eased field, all the ends of the vines nearest the soil 

 line are rotted, so that the entire hill may be easily 

 pulled out. Although the feet of the vines in a dis- 

 eased hill rot off, the vines manage to remain partly 

 alive, owing to the nourishment obtained from the 

 secondary rootlets produced at the leaf nodes on the 

 vines which lie flat on the ground. Diseased hills 

 fail to produce any sweet potatoes, since the under- 

 ground roots are cut off from the main vines. In- 

 fection takes place in the field or in the seed bed. 

 Moisture appears to favor the disease. With the 

 death of the vines appears a pimply growth consist- 

 ing of numerous pycnidia. It is believed that foot rot, 

 like many other sweet potato diseases, is carried with 

 the seed. The fungus Plenodomus destruens so far as we 

 know possesses only the pycnidial or summer fruiting 

 stage. No ascospore stage has as yet been found. 



BLACK Rox 1 

 Caused by Sph&ronemafimbriatum (E. and H.) Sacc. 



Of all the diseases of the sweet potato, black rot is 

 the most dreaded by growers. It is found in all 



1 Material drawn from the author's work. Taubenhaus, J. J., 

 Delaware Agr. Expt. Sta. Bui. 109 : 3-56, 1915. 



