26 DISEASE RESISTANCE OF POTATOES. 



thereafter very little variation occurs. kSome moditication is to be 

 expected, nevertheless, and among other things there ma}^ be varia- 

 tion in disease resistance. It seems worth while, therefore, to keep a 

 lookout for individual plants which show especial resistance to the 

 blight when this is epidemic about them. The tubers of such indi- 

 viduals deserve to be carefulh" saved apart and planted, in order that 

 it may be seen whether their resistance is a fixed and inheritable char- 

 acter or the result of some chance difterence in environment. Such 

 selection has alread}' been undertaken by three persons to our knowl- 

 edge: Appel in Germany,^' Stuart in Vermont,'' and Fraser in New 

 York;'' but no practical results have as yet been secured. 



ARE EARLY OR LATE VARIETIES THE MORE RESISTANT? 



The blight never becomes serious until the midseason of potato 

 growth is passed; thus in our Northern States its worst ravages come 

 in August and September. Therefore early varieties, as a rule, escape. 

 But this is simply because they mature before the blight is epidemic. 

 So far as evidence has been secured, both in America and Europe, 

 when the early varieties are planted late enough to expose them to 

 the disease alongside of the later ones, the early varieties as a class suf- 

 fer the worst. For example, the most complete destruction by rot 

 which the writer ever saw was with a late crop of Early Ohio potatoes 

 attacked by the disease in September. Woods,^' of Maine, has also 

 found early varieties especialh^ susceptible. 



RELATION OF SOURCE OF SEED AND CULTURAL METHODS TO DISEASE 



RESISTANCE. 



The opinions of highly intelligent potato growers in Great Britain 

 are especiall}^ worthy of note. One of the first concerns with them is 

 the source of their seed. Mention has already been made of the experi- 

 ments at Sutton's grounds, showing the superiority of northern-grown 

 seed. It was found to be the general verdict of practical men in 

 Europe that northern-grown seed is more highly disease resistant, as 

 well as more productive. If one is to aim for the best results in 

 health of crop, therefore, attention should be directed to quality and 

 source of seed as well as to variety. 



Practical men as well as scientists generally agree that methods of | 

 culture also determine to a considerable degree liabilitv to disease. ' 

 These act not only indirectly as the}' affect moisture content or other 

 ph3'^sical conditions of the soil, but more directly as they affect the \ 



"Appel, Otto. Die diesjiihrige Phytophthora-epidemie, Deutsche Landw. Presse, 

 XXIX: 685 (1902). 



«> Vermont Exp. Sta. Bui. 115, p. 139. 



<^ Reported in correspondence. 



<^ Woods, C. D., Maine Exp. Sta. Rpt., XIX: 181 (1903). 



