CONTROL OF BLACKLEG DISEASE OF THE POTATO. 223 



the reason why infected tubers are not entirely destroyed by the 

 disease before planting time or at least before the young plants 

 which spring from such tubers can reach the surface of the soil 

 is that, while sufficient moisture is present to '-eep the organisms 

 alive, the temperature conditions are not right for their rapid 

 multiplication till the soil in which the tubers are planted be- 

 comes thoroughly warmed up. 



There is every reason to believe that in cutting seed the dis- 

 ease may be spread from diseased to healthy seed pieces by 

 contact of their freshly cut, moist surfaces and less frequently 

 from the hands of the operator or through the medium of 

 knives used. It is a common practice to cut seed tubers and 

 place them in barrels some weeks, or even months, before they 

 are needed for planting. Although plaster or lime is usually 

 sprinkled over such seed, which forms a more or less dry, pro- 

 tective coating over the cut surfaces, the conditions for the rapid 

 multiplication and spread of bacteria are often very favorable. 

 Seed cut in this way is very likely to "heat," especially if a few 

 days of warm weather are experienced before planting, unless 

 they are daily turned out of the barrels and exposed for a time 

 to the air. 



On one of the farms visited there was accidentally provided 

 an excellent opportunity to observe how the blackleg disease 

 may spread from diseased to healthy seed tubers after cutting 

 and while being planted. The variety in this case being Carmen 

 No. I, planted without disinfection. During the planting the 

 man in charge of the work was taken suddenly ill. After 2 or 3 

 days a substitute was obtained, but in the meantime several bar- 

 rels of cut seed were allowed to remain in the field, covered 

 with canvas. This provided right conditions of warmth and 

 moisture in the barrels for rapid multiplication of any bacteria 

 which might be present, and signs of "heating" and decay were 

 evident when the barrels were opened again. Before the fore- 

 man of the farm was aware of it the new man in charge of the 

 planting had planted 4 to 6 barrels of this seed. He at once 

 ordered it stopped and substituted freshly cut tubers from the 

 same bin from which the first lot was obtained. 



The field was examined by the writer on July 19. Where 

 the freshly cut tubers were used there was hardly a plant miss- 

 ing from the whole field and only about 15 or 20 plants per 



