248 



Diseases of Economic Plants 



with diseased roots. The fungus discharges myriads of 

 spores into the soil. This disease thus falls under the general 

 class of soil diseases. 



In addition to the suggestions given on page 26, especial 

 precaution should be exercised to 

 avoid sets already infected. Badly 

 diseased shoots are easily recognized, 

 hut the closest scrutiny is needed to 

 cull out those only slightly affected. 

 Al)Ove all, infested 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 

 jilants well established in the field. 

 A strong, well-started plant will often 

 r(>sist 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- 

 fested or on land which has never 

 gi-own sweet potatoes, thus making 

 an absolutely clean start even though 

 the vine cuttings are taken from an 

 infected crop." 



Root-rot '^^' (Plenodomus destruens 

 Hart.). — Small brown or black spots 

 appear on the stem near the soil line, 

 and eventually girdle the plant, which 

 soon wilts. In some regions of Ohio, 

 Virginia, and Iowa, losses of 50 per cent 

 occur, but the disease is not so widely 

 distributed as those mentioned above. 

 Texas root-rot {Ozonium). — A firm, brown rot is pro- 

 duced on th(> ]X)tatoes. Loss as high as 90 per cent is occa- 

 sional. Rotation involving cereals should be adopted. 





Fig. 135. — Black-rot of 

 sweet potato. Original. 



