MARKET DISEASES OF FRUITS AND VEGETABLES 31 



Soil Rot (Pox) 

 (Actinomyces ipomoea Person and W. J. Martin) 

 OCCURRENCE, SYMPTOMS, AND EFFECTS 



Soil rot, or pox, is found in all the important sweetpotato-growing 

 regions of the United States. Although the losses vary considerably 

 from season to season, in many localities soil rot is considered one of 

 the most important diseases. It seriously curtails the growth and 

 yield of the plants and blemishes the roots so that their value is greatly 

 reduced. 



Sweetpotatoes that show soil rot on the market generally are char- 

 acterized by dry, brown pits, or pox marks, of irregular size and shape 

 (pi. 14, B). The areas infected may vary from less than 14 inch to 1 

 inch in diameter. In the early stages the superficial, brown, circular 

 spots are smooth or slightly sunken, but as they enlarge the epidermis 

 over them cracks exposing a cavity with rough lining and irregular 

 margin. These exposed tissues are firm and dry when found on ma- 

 ture roots on the market. Ordinarily they are not followed by soft rot 

 or other secondary decays. Sometimes the root is almost girdled by 

 these blemishes, and frequently the growth of the tissues around the 

 deep pits results in the formation of misshaped potatoes. 



CAUSAL FACTORS 



Within the past few years Actinomyces ipomoea has been named as 

 the causal agent of soil rot. This organism makes very little growth 

 at 68° F. and grows best at 89° and only slightly at 107°. The high 

 temperatures in the Southern States apparently favor the develop- 

 ment of soil rot. 



The causal organism inhabits the soil and invades the potatoes 

 through the small secondary rootlets. Apparently most of the dis- 

 semination of this disease is through transportation of infested soil by 

 seed potatoes, farm implements, animals, drainage water, or wind- 

 blown dust. Experimental evidence indicates that the disease is not 

 transmitted by the use of diseased roots for seed purposes if they have 

 no infested soil on them. 



Soil rot is most serious during dry seasons. When sufficient mois- 

 ture is available the plants are able to keep growing and produce some 

 marketable potatoes even though they are diseased. 



CONTROL MEASURES 



If disease-free soil is not available for growing sweetpotatoes, it is 

 possible to control soil rot in infested soil by adding sufficient sulfur to 

 make it acid in reaction (pH 5.0) . 



(See 1, 22, 54, 102, 110, 136, 137.) 



Stem Rot (Wilt) 



(Fusarium oulbigenum batatas Wr. and F. oxysporum Schlecht. f. 2 Wr.) 



Stem rot, or wilt, is one of the important field diseases of sweet- 

 potatoes that rarely is found on the market. Like other types of 



