36 BULLETIN 64, U. S. DEPAETMENT OF AGEICULTUBE. 



It is not thought worth while to reproduce here the tables and sum- 

 maries of these variety tests. The varieties grown in Europe are 

 ahnost entirely different from those grown in the United States, and 

 repeated experiences have shown that few of them will thrive here if 

 introduced. In general, the indications are that varietal differences 

 in susceptibiUty to leaf-roll do exist, but that the tests need to be 

 carried on longer before any conclusions are drawn respectmg given 

 varieties. It seems certain that leaf-roll is not a result of ''running 

 out" of varieties through old age, for many quite recently originated 

 strams are affected. More striking still is its occurrence in seedlings, 

 which has been observed by several workers. 



There have been unusual opportunities to study the occurrence of 

 leaf-roll hi the Stuart collection of 10,000 seedliags, where perhaps 

 the most striking feature was that the leaf-roU was confined to certam 

 numbers. The five hills of a kind would be uniformly affected, while 

 those on either side were perfectly healthy. Clearly, the disease is 

 not due entirely to soil or climatic influences, and certainly there was 

 no indication of fungous infection. The marked contrast between 

 diseased and healthy rows is well shown in Plate VI, m which the 

 left-hand row is a hybrid (No. 2171) between Sophie and Keeper, the 

 healthy row on the right being from the same cross (No. 2165). 



Aix mteresting suggestion is put forward by Hedlund (1910), that 

 leaf-roU is a pathological, adaptative mutation, and, further, that since 

 acquired characters are not inherited the leaf-roU character must be 

 latent in normal potatoes. 



CONTROL OF LEAF-ROLL. 



No measure offers more hope of success in controlling leaf-roll than 

 the use of better seed stocks. Three means may be used to bring 

 this about : First and simplest, the importation of seed potatoes from 

 districts where the disease is unknown. This affords reUef but may 

 not greatly raise the standard of quaUty. Second, hiU selection, to 

 pick out from weak varieties strauas that will withstand the disease. 

 This has been done already by Von Lochow (1910), who took several 

 types from the variety Professor Wohltmann and bred them in pure 

 lines. The result was that certain of these pure strains showed sus- 

 ceptibility to leaf-roU, while others remained entirely or nearly free 

 from it. Third, new varieties may be bred from seed. This, while 

 requiring the most tune, may be the best means for meeting the 

 requirements where whole districts are attacked, as in the Colorado 

 outbreak. That such good varieties can be produced one can hardly 

 doubt after seeing the departmental collection of over 10,000 seed- 

 lings with its infinite variety of disease-resistant quahties. 



It is the prevailing opinion of European investigators that leaf-roll 

 is inherited — i. e., that the tubers from diseased hills will produce 

 diseased progeny. Cases are cited where the first crop after the 



