18 CIRCULAR NO. 114, BUREAU OF PLANT INDUSTRY. 



of the gas. Disinfected soil should be carefully protected to prevent 

 infestation before it is used. All instruments used in handling ster- 

 ilized soil, such as the wagon box, shovels, etc., should be disinfected. 

 It is believed by some fanners that better and stronger plants can 

 be secured by using manure as a source of heat in the hotbed. Others 

 insist that equally good, if not better, plants can be grown in a fire 

 bed. Which is right must remain unsettled, at least till reliable ex- 

 perimental evidence is produced. From the standpoint of disease, 

 there is far greater danger, however, from the use of manure, espe- 

 cially if it is produced on farms where sweet potatoes are grown. 

 It frequently happens that farmers are careless about the disposition 

 of decayed sweet potatoes and feed them to hogs or chickens or, 

 more often, throw them on the manure pile. From there they are 

 carried to the hotbed, and infection of the young plants becomes 

 easy. It is urged that all refuse sweet potatoes be cooked and not 

 thrown where they will be scattered in the manure and hauled on 

 the farm or used in the hotbed. 



CROP ROTATIONS. 



It is not known how long black-rot and stem-rot will live in the 

 soil without the sweet potato as a host. It is advisable, however, to 

 follow a system of crop rotation that will not bring this crop on 

 the same ground oftener than once in three or four years. It is 

 certain that the crop will not be free from disease so long as the 

 organisms are present in the fields. The only way to get them out 

 of the field is to starve them out by planting crops on which they can 

 not live. Experiments, however, have shown that the percentage of 

 diseased plants is much reduced .by planting healthy slips, although 

 the soil may be infested. This fact alone justifies all the precautions 

 necessary to grow healthy slips, even though the soil on which they 

 are planted is diseased. 

 [Cir. 114] 



