Nov. II, igis Sweet-Potato Storage-Rots 345 



these potatoes, infected in the field, which carry the disease to the storage 

 house. Usually when the potatoes are put in the storage house they 

 show little or no evidence of blackrot. The potatoes with visible black- 

 rot spots are few, and such are generally thrown out. On the other 

 hand, those with infections too small to be seen pass along into the stor- 

 age house, where they continue their development. In the course of a 

 few weeks the spots attain a diameter of an inch or more, and the fun- 

 gus, under suitable conditions, has fruited abundantly. The spores are 

 scattered about the house on the bodies of insects, by the settling of the 

 potatoes in the bins, and probably by currents of air, and by other 

 means, such as picking the potatoes over and preparing them for the 

 market. In some houses a large percentage of the potatoes have black- 

 rot, though at the time they were put in storage they showed no evidence 

 of it. Plate 23, A, shows a potato infected with blackrot and Plate 23, 

 B, the same potato two months later. This potato was kept in an ice 

 box at a temperature of 10° to 13° C. During the two-month period the 

 spots had developed so as to envelop nearly the entire potato. 



That blackrot is transmitted through the soil was demonstrated by 

 bedding a large number of healthy potatoes in soil infested with black- 

 rot. The potatoes used in the experiment w-ere carefully examined for 

 soundness, and then disinfected in mercuric chlo rid (i : 1,000) for 10 

 minutes. Potatoes from the same lot, bedded in disease-free soil in the 

 same hotbed, were used as controls. The slips from blackrotted soil 

 were set alongside the control plants in a field where blackrot was not 

 known to occur. Only a few of the slips when set out showed blackrot 

 infection. About 25 bushels of potatoes were produced from these slips. 

 When the potatoes were dug, a few had visible blackrot spots on them 

 and were discarded. The apparently sound potatoes (25 bushels) w^ere 

 mixed with sound potatoes in a loo-bushel bin. When they were re- 

 moved in the spring, most of the 100 bushels in the one bin had to be 

 thrown out, and a fair percentage from bins on either side. The loss in 

 the adjoining bins was greatest on the side next to the bin containing 

 the blackrotted potatoes. The disease had not been communicated to 

 any extent to other bins in the same house. The potatoes from the 

 control plants stored in another part of the house all remained sound. 

 Further proof that it is disseminated in storage was shown by an experi- 

 ment in which blackrotted potatoes and healthy potatoes were mixed 

 in a bushel basket and stored. When the potatoes were removed in the 

 spring, all but one of the healthy potatoes were infected with blackrot. 



DESCRIPTION OF BLACKROT 



Blackrot is characterized by the formation of more or less circular, 

 somewhat sunken, black spots on the surface of the potato. Infection 

 takes place readily through wounds and through the dead rootlets. If 



