38 DISEASE RESISTANCE OB' POTATOES. 



(2) It seems related to general vegetative vigor, and is therefore in 

 a measure dependent upon cultural and developmental conditions and 

 tends to decrease with the age of the variety. 



(3) It can be restored b}^ originating new varieties from seed, 

 especiall}^ of hybrid origin. Not all seedlings show superior disease 

 resistance, 



(4) The use of other species of tuber-bearing Solanums for h3'brid- 

 izing offers some promise, but no practical results have 3^et been 

 secured. 



(5) Possibly the disease resistance in established varieties can be 

 improved by selection, but this has not been proved. 



(6) Early varieties may escape the disease by maturing before it 

 becomes epidemic, but when similarly exposed they are as a class less 

 resistant than late varieties. 



(7) The source of seed tubers is a matter of importance, northern- 

 grown seed giving plants of superior disease resistance in Europe. 

 Seed from a crop that was not too highly fertilized is probably prefer- 

 able. Possibl}^ tubers are better for seed purposes if dug before they 

 reach full maturity. 



(8) High fertilization, especialh^ with nitrogenous manures, lowers 

 the power of the plant to resist both blight and rot. 



(9) Varieties relatively rich in starch are more resistant to rot; those 

 richer in protein are more susceptible to it. 



(10) So far as skin characters are an index, the red varieties with 

 thick and rough skin seem more resistant as a class than the thin- 

 skinned white varieties. 



(11) So far as stem and foliage characters are concerned, the evidence 

 favors the stem that is hard, rough, and rather woody at the base, and 

 the leaf that is small, somewhat rough, and dark colored. 



The varieties rated highest as to disease resistance in England are 

 Evergood, Discovery, Royal Kidney, Northern Star, Sir John Lle- 

 wel3'^n, King Edward VII, Eldorado, and Factor. 



In German}' and Holland the following represent the best types: 

 Mohort, Irene, Geheimrat Thiel, Professor Wohltmann, Boncza, 

 Eigenheimer, and Paul Kriiger. 



In Belgium and France no improvement as to disease resistance has 

 been made over the best English and German t3^pes. 



In America, trials as to disease resistance have been conducted at 

 some of the experiment stations, notably in Vermont, where experi- 

 ments in breeding and selection for increased resistance are under way. 

 These results have been correlated with information recently secured 

 by a circular of inquiry addressed to a large number of potato special- 

 ists in the Northeastern States and in Canada. From these it appears 

 that a wide variation is shown in disease resistance among the varieties 

 now in cultivation in America, but that no one variet}' is preeminent. 



