VEGETABLE AND FIELD CROPS 



293 



Above all, infected seed beds should 

 be avoided, and no infected potatoes 

 should be used for growing sets. 

 Crop rotation should be practiced, 

 and every precaution taken to get the 

 plants well established in the field. A 

 strong, well-started plant will often 

 resist the disease where a weak plant 

 would succumb. Waite^ says: " The 

 best remedy is to use slip seed. It is 

 advisable to grow the crop of vine 

 cuttings on new land which is not in- 

 fected or on land which has never 

 grown sweet potatoes, thus making 

 an absolutely clean start even though 

 the vine cuttings are taken from an 

 infected crop." 



Soil rot (Acrocystis Batatce (Ell. & 

 Halst.). — The loss from this rot is 

 sometimes almost total, and since this 

 is a soil disease, the raising of the crop 

 is prohibited for several years. 



The roots are attacked when quite 

 small, sometimes over the whole sur- 

 face. The part infected ceases to 

 grow, while adjoining parts enlarge. 

 This results in a condition such as 

 that shown in Fig. 128. The smallest 

 rootlets are the points of attack, thence 

 the disease proceeds to the potato and 

 causes the surface spots pictured. 



1 Waite, M. B., Encyc. of Agr., II, 622. 



f 



^ "1 



Fig. 128. — Soil rot of 

 sweet potato. After 

 Halsted. 



