Vegetable and Field Crops 



221 



furrovvetl or cracked. Mites often follow scab, making the 

 pits much deeper. 



The scab at first appears as a minute reddish or brownish 

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

 tends outward, deepens in color, and is attended with the 

 development of an irregular, corky incrustation. 



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

 hibit successful potato culture. The chief loss comes from de- 

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



Fig. 121. — ^ Potato common-scab. After Melhus. 



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

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

 county alone is placed at $300,000 yearly. 



Common-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 infested. Materials which tend to decrease scab 

 are commercial fertilizers, potash salts (except carbonate), 

 land plaster, and ammonium sulfate. Rotation of crops tends 



