DISEASES OF IRISH AND SWEET POTATOES 237 



the soft-rot, which has an unpleasant odor. This fungus 

 takes advantage of the so-called sweating period, which 

 occurs a short time after the sweet potatoes are dug. It 

 then finds an easy entrance through the injuries on the sur- 

 face or through the broken end of the root. From a single 

 diseased or bruised spot it may spread with alarming 

 rapidity. This disease is readily prevented by proper dry- 

 ing of the potatoes for a few days after they are dug, be- 

 fore placing them in the lower temperature at which they 

 are to be stored. Infected potatoes should be destroyed. 



The black-rot of sweet potatoes. This disease is really 

 a soil rot. The fungus doubtless enters through the 

 young rootlets of the growing plant. It finally becomes 

 established in the potatoes themselves, producing circular 

 black patches. The disease may even extend its injuries 

 after the potatoes are stored. Black-rot is the most de- 

 structive fungous enemy of the sweet potato, but fortunately 

 it has not been found in all localities. In order to control 

 it, the potatoes should not be grown on any field where 

 sweet potatoes grew for one or two years before. More- 

 over, the seed or slip bed should be carefully watched, for 

 this disease may make its appearance there, producing upon 

 the shoots dark-colored spots known as "black shank." 

 Set out no slips from a badly diseased seed-bed. 



EXERCISE. Find Irish and sweet potatoes. Search them for any 

 unhealthy appearance. If any disease is found, does it seem to be one 

 of those described above? 



