268 Dlf^EASES OF ECONOMIC PLANTS 



name in Europe."^ It was first attributed to its proper 

 cause by Dr. Thaxter ^ in 1890. 



The scab is recognized by all potato growers as a char- 

 acteristic rough pitting of the tubers. If these are attacked 

 when quite young, the scabs are deep ; if the attack is later, 

 they are more shallow. In very severe cases the tuber is 

 furrowed or cracked. 



The scab at first appears as a minute reddish or brownish 

 surface spot, often when the potato is very young. It 

 extends outward, deepens in color, and is attended with 

 the development of an irregular corky incrustation. 



The disease is often prevalent to such extent as to pro- 

 hibit successful potato culture. The chief loss comes from 

 depreciation in sale value, though there is also loss in yield. 

 Estimates show that from 5 to 75 per cent of the potato 

 crop is unsalable on account of scab. The loss in one 

 county alone in California is placed at $300,000 yearly. 



Scab is most prevalent in alkaline soils and is favored by 

 moisture. Wood ashes, potash, soda, stable manure, lime 

 etc., increasing alkalinity, favor the growth of the fungus, 

 and cause increase in the amount of scab in soil already 

 infected. Materials which tend to decrease scab are com- 

 mercial fertilizers, potash salts (except carbonate), land 

 plaster, and ammonium sulphate. Rotation of crops tends 

 to diminish the amount of scab, provided other susceptible 

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

 three-year rotation without a susceptible crop largely 

 reduces it, though after five or even ten years the disease is 

 not entirely exterminated. During rotation any practice 



• Jones, L. R., U.S. Dept. Agr. Bur. Plant Indus. Bui. 87. 

 ' Thaxter, R., Ct. State Agr. Exp. Sta. Rpt. 14, pp. 81-95. 



