24 DISEASE RESISTANCE OF POTATOES. 



reproduction, the true seed produced in the berries or ''balls'' follow- 

 ing the blossoms, and the tubers produced below ground. Repro- 

 duction b}' seed is a sexual process, that l\y tubers is vegetative. 

 Both are exhaustive of vitality. The two are in a certain sense 

 physiologically opposed to each other and can not well be carried on 

 at the same time by the plant. Under the natural conditions of the 

 wild potato plant in Mexico, and doubtless elsewhere, seed production 

 precedes tuber formation. In Europe and northern America, with a 

 shorter season and intensive culture, the two processes overlap. As 

 a result there is, just after the potato plant comes into blossom, a period 

 when the natural tendencies within the plant toward seed production 

 above and tuber formation below are such as to subject it to unusual 

 phj'siological stress. This has been termed the "critical period" '^ in 

 the development of the potato plant. 



Usually the blossoms fall without the setting of the fruit, and the 

 plant then passes into a stage where its energies are devoted to tuber 

 formation alone. Once well started upon this vegetative period, its 

 growth is more or less indeterminate, i. e., there is no clearl}" defined 

 natural terminus to the life of the cultivated potato plant. Instead, 

 there is the gradual decline in vegetative vigor which may prepare the 

 way for earl3'-blight and other diseases characteristic of weakling- 

 plants. It is noteworth}' that the destructive attacks of the late-blight 

 fungus occur, as a rule, after the blossoming period has passed. So 

 far as the evidence goes it seems to suggest that high vegetative vigor 

 enables the plant to ward otf in some degree the fungus attack. 



There is, moreover, a natural decline in vigor, or "running out," 

 with the age of the variety. The length of life of a variety depends 

 upon numerous conditions, and is an indefinite matter in any case. It 

 is ordinarily placed by potato specialists at from twelve to twenty 

 3"ears. As a variety begins to ""run out" it apparently shows, among- 

 other things, a lessened degree of general disease resistance. Thus 

 Magnum Bonum had the highest reputation in this respect from its 

 origin in 1876 until about 1890 in Great Britain. On the Continent it 

 has remained longer in favor. Up-to-Date has held a like place during 

 the last decade in Great Britain, and Richter's Imperator has a similar 

 record in Germany. These statements are based upon the popular 

 verdict, not upon exact experiments; but the belief that disease resist- 

 ance decreases with the age of the variety is iirml}^ established in the 

 minds of specialists in potato culture in Great Britain, at least so far 

 as concerns resistance to Phytophthora. 



There is little definite evidence regarding the relation of vegetative 

 vigor to resistance to other diseases, but so far as it is formulated it 



« See Jones, L. R., Certain Potato Diseases and Their Remedies, Vermont Exp. Sta. 

 Bui. 72: 4 (1899); also The Diseases of the Potato in Relation to ItsDevelopment, 

 Trans. Mass. Hort. Soc. (1903), Part I: 144. 



