184 orton: pathology of solanum tuberosum 



ment influences disease. Late blight is limited by its require- 

 ments of abundant moisture and moderate temperature, hence it 

 is most common in the Northeastern States, occasionally extend- 

 ing southward in early summer or autumn, never in midsummer. 

 Hot or dry weather checks its spread. These well established 

 facts strongly support the hypothesis that Phytophthora infestans 

 is endemic in the native habitat of the potato. Our knowledge 

 of the general principles of immunity in plants further suggests 

 that there would be the place to seek strains of the host plant 

 possessing a high resistance to this parasite. 



In early blight, due to the fungus Alternaria solani, we find that 

 higher temperatures than those best for the potato appear to 

 promote infection. The range of greatest prevalence is well to 

 the south of that for late blight. 



Another instance of apparent geographical limitation of potato 

 diseases is afforded by the wilts due to Fusarium oxysporum and 

 Verticillium albo-atrum respectively. The Fusarium wilt is 

 southern in its general range, being most prevalent in California, 

 in the warmer irrigated valleys of the West and in the East Cen- 

 tral States. Verticillium, on the other hand, occurs in the most 

 northern districts from the Puget Sound to Maine. The two over- 

 lap in their distribution but have the general tendency stated. 



Pathological conditions not due to parasites are even more 

 conspicuously associated with attempts to grow the potato out- 

 side of its natural range. A heat and drought reaction common 

 in the United States is that known as tipburn, where the leaves 

 exposed to the hot sun and low relative humidity of midday 

 curl and burn at the margins, indicating an excessive transpira- 

 tion. This is seldom met with in Europe. 



Premature ripening follows when potatoes bearing half-grown 

 tubers are exposed to the midday heat of our Southern States. 

 There is ip. addition to the tipburn a yellowing and early death 

 of the foliage. Potatoes produced in these southern conditions 

 lose their constitutional vigor and germinate later, with small 

 weak sprouts and give a smaller yield than seed from northern 

 sources. This constitutional defect is not cured by restoration 

 to a northern environment. 



