222 Diseases of Economic Plants 



to diminish the amount of seal), provided other susceptible 

 crops, as beets, turnips, cabbages, etc., are avoided. A 

 three-year rotation without a susceptible crop largely re- 

 duces it, though after five or even ten years the fungus 

 may not be entirely exterminated. During rotation any 

 practice that increases the acidity of the soil aids in reduc- 

 ing scab. Thus plowing under green crops is a favorable 

 practice. 



In general it may be said that scabby seed potatoes in any 

 soil will produce a scabby crop, l)ut the amount of scab is 

 determined somewhat by the conditions. Seed potatoes free 

 from scab, in a soil free from scab, will raise a crop free from 

 scab. Seed potatoes free from scab will not produce a healthy 

 crop if grown in infested soil; but less scab will result than 

 when diseased seed is used. 



There is much difference in varietal resistance to scab. 

 The more resistant varieties should be used on infested 

 soil. 



Two conditions arise reciuiring different cultural meth- 

 ods: — 



1. The management of clean soils. 



2. The management of soils already infested. 



If the land is free from infestation, great care should be 

 exercised to avoid introduction of the scab fungus by any of 

 the general means suggested under soil disease, p. 26. In 

 particular, infestation may occur by the use of scabljy seed, or 

 through the use of manure which has been infested by feed- 

 ing uncooked, scabby potatoes to stock. If seed must be 

 planted that cannot safely be regarded as free from scab, 

 and no seed from an unknown source can be so regarded, it 

 should be disinfected to free it of scab germs. See p. 240. 



No system of soil treatment has proved reliable in the 

 management of infested land. Long rotation, avoiding 

 susceptible root crops, should be practiced. The use of 

 fertiHzing mateiials which favor the fungus should be avoided 

 and resistant varieties should be planted. The turning 

 under of a green crop, e. g., rye, may gradually decrease the 



