Diseases of the Potato 205 



three diseases are evidently widespread and may be 

 important factors in causing reduced yields and "running 

 out" of potatoes. Tubers from such vines are usually 

 small. For this reason the selection of small tubers from 

 the bin is likely to increase the percentage of such affected 

 plants. It is possible to avoid them by the selection out- 

 lined in the chapter on Control Measures. 



Constitutional degeneracy and other weakened conditions 



of plants 



Constitutional degeneracy is a name given to a condi- 

 tion of weakness of growth showing as small, often spin- 

 dling stems producing but a few small tubers to a hill. 

 Such a condition is inherent in the plant and is inherited 

 by the progeny. This trouble may be confused with 

 similar conditions associated with the non-parasitic 

 diseases already enumerated. Very small, spindling 

 plants producing no commercial tubers are often found 

 to be associated with an early rotting of the seed tuber 

 after being planted, and similar plants sometimes appear 

 when the only eye on the piece of tuber is near the cut 

 surface. Rotting of tubers may be due to such prac- 

 tices as keeping cut tubers undusted a week or more 

 before planting and as planting cut tubers in hot soil or 

 leaving them exposed to the sun after hand-planting. 

 Avoiding such practices may be of assistance in reducing 

 the number of weak plants in the field, but tubers from 

 degenerate hills should never be used for seed. The 

 elimination of these can be accomplished to some extent 

 by grading, but selection from the field is the surest way. 



