448 Journal of Agricultural Research voi. xiii. no. 9 



cent, available acid phosphate 7 to 8 per cent, potash 5 per cent. The 

 land was then divided into three plots. The middle remained as a con- 

 trol, and received no further treatment. The plot to the right received 

 an additional application of acid phosphate (guaranteed analysis, 14 per 

 cent of available phosphoric acid) at the rate of 2,000 pounds per acre. 

 The plot to the left received an additional application of hyd rated lime 

 (guaranteed analysis, 65 per cent of calcium oxid) at the rate of 2,000 

 pounds to the acre. The results obtained were very striking. The 

 control plot gave an average of almost 60 per cent affected roots. The 

 lime plot increased the amount of affected roots to about 85 per cent. 

 The percentage of diseased roots in the acid-phosphate plot was 32. 

 This seemed to indicate that an acid fertilizer has a tendency to keep 

 the pox in check, whereas lime has the opposite effect. 



The work of the writer would also seem to indicate that there exist 

 considerable differences in the resistance of varieties of sweet potatoes 

 to the disease. Of the limited number tested in 191 5 in Delaware the 

 following is a tentative classification of their resistance : (a) Total freedom 

 from disease (Dahomey, Red Brazil, Pearson) ; (b) from i to 20 per cent 

 infected (Big Stem Jersey, White Yam, Yellow Strassberger) ; (c) from 

 20 to 90 per cent infected (Goldskin, Big Leaf Upriver). 



Steaming the soil for six hours at 20 pounds' pressure will free it from 

 the pox organism. This also seems to be true when the infected soil is 

 treated with formaldehyde at the rate of i pint in 20 gallons of water, 

 applied at the rate of i gallon of the solution to each square foot of soil 

 space. However, since it is very doubtful if the disease is carried ^vith 

 the seed in the soil of the seed bed, soil sterilization would hardly seem 

 warranted unless it aimed also at controlling blackrot. Likewise it 

 seems hardly necessary to treat the seed for that alone. It is not defi- 

 nitely known how long Cysiospara batata would persist in the soil with- 

 out a suitable host. Observations of practical growers differ greatly in 

 this respect. Some assert that at least a lo-year rotation is necessary 

 to free the land from pox; others, and these seem to be in the majority, 

 maintain that a 3-year rotation is sufficient. Soil conditions, it seems, 

 play an important factor. Pox is more severe in the lightest of the 

 sandy soils, and less so in the heavy clay loams. 



SUMMARY 



(3) Although not definitely known, pox is probably prevalent wherever 

 sweet potatoes are grown. The name "soilrot" does not express the 

 symptoms of the disease and the terms "pox" or "pit " are recommended. 



(4) Pox disfigures the root and reduces yields. It destroys the feeding 

 rootlets and many of the roots which otherwise would make the crop. 



(5) Infections which result in blisters are apparently the exception, 

 and not the rule, and may be brought about by conditions unfavorable 

 for the parasite. 



