THE PERSISTENCE OF THE POTATO LATE 

 BLIGHT FUNGUS IN THE SOIL.* 



F. C. STEWART. 



SUMMARY. 



This bulletin contains an account of some experiments the object 

 of which was to determine whether the fungus which causes the late 

 blight and rot of potatoes, Phytophthora infestans, lives over 

 winter in the soil. Boxes of soil containing blighted potato vines 

 and decaying tubers were left in the field exposed to the weather 

 until January 20, when they were transferred to a greenhouse and 

 planted with potatoes. Some of the resulting plants were placed 

 in a glass inoculation chamber and inoculated repeatedly by sprink- 

 ling or brushing the foliage with an infusion of the soil containing 

 the decaying potato tubers and stems. Meanwhile, the air in the 

 inoculation chamber was kept constantly at or near the point of satu- 

 ration. Notwithstanding the conditions were extremely favorable 

 for infection no Phytophthora appeared either on the leaves or 

 tubers. The conclusion reached is, that while the negative results 

 of these experiments do not prove that Phytophthora does not 

 persist in the soil they make such persistence appear highly improb- 

 able. 



THE PROBLEM. 



Does the potato blight fungus, Phytophthora infestans, persist in 

 the soil from one season to the next? Is it safe to plant potatoes 

 where potatoes blighted and rotted the previous year? These ques- 

 tions are frequently asked by potato growers. In giving an answer 

 it should be stated, first of all, that there are different kinds of potato 

 blight and rot. The Fusarium wilt and the tuber rot which accom- 

 panies it certainly do persist in the soil; while the dry weather blight, 

 called tip burn, does not persist. The fungus of early blight, 

 Alternaria solani, probably survives the winter in the soil, but 

 definite proof is lacking. Of course, it is the late blight and rot, 



* Reprint of Bulletin No. 367, October, 1913; for Popular Edition, see p. 770. 



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