34 BULLETIN 195^ U. S. DEPARTMENT OF AGRICULTURE. 



' It is evident from the data presented that other causes than that of 

 inherent unproductiveness must have operated to lower the yield in 

 selections 1 to 5. This fact is made still clearer by the behavior of 

 selection No. 4, in which the first unit gave a reasonably good yield 

 of primes or merchantable tubers, while the remaining ones did not 

 produce any. It is believed that the seed tubers from the 1911 selec- 

 tions were either infected with some obscure disease or happened to 

 have been planted in previously infected soil. All the selections were 

 grown in the same or in contiguous rows, so there is httle likelihood 

 that the moisture or plant-food content of the soil was deficient in 

 the one case and not in the other. The behavior of the plants during 

 the growing season strongly corroborated the disease theory and 

 sustains a previous statement in this bulletin in connection with 

 varietal tuber-unit studies, namely, that the chief value of such 

 studies consisted in the ehmination of diseased and weakened plants. 

 It would seem probable from the behavior of No. 25 that a strong 

 vigorous, and productive strain had been isolated. The term ''prob- 

 able " is used advisedly, because, as previously stated, the best selec- 

 tions, including, of course. No. 25, were unfortunately cut to and 

 below the ground by a very severe freeze late in April, 1913. The 

 injury sustained was so severe that very few of the plants survived, 

 and those that did survive made a very unsatisfactory growth and 

 crop. These selections have therefore been lost, and a new start has 

 become necessary. 



Thus far, all selections that have been made for disease resistance 

 have proved undesirable, as they either did not retain this quahty or 

 else they were unproductive commercially or otherwise. The highest 

 degree of success can only be attained from either the tuber-unit or 

 hill-selection method by worldng with rather large numbers. It is 

 hardly conceivable that there exist many strains within a variety that 

 are especially productive or commercially desirable, or that are mark- 

 edly resistant to disease. Occasionally one may be fortunate enough 

 to isolate such a strain with a minimum amount of effort, but the 

 chances are strongly in the opposite direction. This statement is not 

 made for the purpose of discouragiag anyone from attempting to 

 improve his seed stock by up-to-date seed-selection methods. The 

 intention is rather to encourage the selectionist to make a larger num- 

 ber of selections and thereby increase his chances of securing a supe- 

 rior strain. 



The selection of a large number of high-yielding hills which are then 

 thrown together for mass planting the ensuing year is not likely to 

 result in any marked improvement except by the elimination of the 

 diseased or the unproductive plants. The only certain method of 

 secm-ing a superior strain is to plant each selection separately, as rec- 



