VEGETABLE AND FIELD CROPS 269 



that increases the acidity of the soil aids in reducing 

 the 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, but the amount of scab is 

 determined somewhat by the soil condition. 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 an infected soil. 



There is much diiTerence in varietal resistance to scab. 

 The more resistant varieties should be used on soil at all 

 scab infested. Cambridge Russett, Carmen No. 3, Ameri- 

 can Giant, Sir Walter Raleigh, Irish Cobbler, Scab Proof, 

 and Aurora are recommended by the Maryland Station as 

 possessing some scab resistance.' 



Two conditions arise requiring different cultural 

 methods : — 



1. The management of clean soils. 



2. The management of soils already infected. 



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

 exercised to avoid introduction of the scab fungus by any of 

 the general means suggested under soil disease. In par- 

 ticular, infection may occur by the use of scabby seed, or 

 through the use of manure which has been infected 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 all scab 

 germs. 



To disinfect potato seed the most effective method is by 



1 Norton, J. B. S., Md. Agr. Exp. Sta. Bui. 108, p. 67, April, 1906. 



