INFLUENCE OF TEMPERATURE AND PRECIPITATION 

 ON THE BLACKLEG OF POTATO 



By J. RosENBAUM, Assistant Pathologist, Cotton, Truck, and Forage Crop Disease 

 Investigations, Bureau of Plant Industry, United States Department of Agriculture, 

 and G. B. Ramsey, Assistant Pathologist, Maine Agricultural Experiment Station 



OVERWINTERING IN THE SOIL 



It is still a question in the minds of some pathologists whether the 

 blackleg organism,^ Bacillus phytophthorus appel, can remain in the soil 

 over a winter and start the disease the following spring. In order to test 

 this point, about a bushel of potato tubers {Solanum tuberosum), badly dis- 

 eased with the blackleg, were collected and placed at the usual depth, about 



2 or 3 inches apart, on October ii, 1915, in Aroostook County, Maine. 

 In the spring of 1916, at the time when the general crop of potatbes was 

 being planted, some Irish Cobbler and Spaulding Rose tubers were care- 

 fully selected, disinfected in the usual manner, and planted in the rows 

 where the diseased potatoes had been planted the previous fall. When 

 these were being planted a large number of overwintered tubers were 

 found in the soil, either wholly or partially decayed. ' A careful watch was 

 kept of the plants throughout the growing season. In neither the Irish 

 Cobbler, an early-maturing variety, nor the Spaulding Rose, a late- 

 maturing variety, was a single case of blackleg developed. 



In the fall of 191 6 a similar experiment was begun. The same soil v/as 

 used, but instead of naturally infected tubers being used, approximately 



3 pecks of Irish Cobbler potatoes were inoculated by means of a hypo- 

 dermic syringe with a very virulent culture of the blackleg organism. 

 The inoculated tubers were then placed in a covered container and allowed 

 to remain there until each of them showed a marked rot at the point of 

 inoculation. The tubers were then planted whole, about i foot apart in 

 a trench 5 inches deep. On May 18,1917, this row was opened with a 

 hand hoe, care being taken not to disturb the decayed tubers any more 

 than necessary. Healthy Irish Cobbler potatoes, known to be free from 

 blackleg, were cut in the usual way, and one seed piece was placed in the 

 old seed bed with each of the decayed, overwintered tubers. A second 

 row was made by the side of the one mentioned and planted with the same 



1 The blackleg organism has been generally referred to as B./'Mo/'*^*^'^"^ Appel. According to the works 

 of Morse (JvIorse, W.J. Studies xtpon the blackleg disease of the potato, with special reference 

 To THE relationship OF THE CAUSAL ORGANISMS. In Joui. Agr. Research, v. 8, no. 3, p. 79-126. 1917) 

 all cultures of blackleg isolated by him in Maine agree with B. solanisaprus Harrison, but for certain reason, 

 he believes the name B. atrosepticus Van Hall should be adopted. According to the work of Smith (Smith, 

 Erwin F. bacillus ph\'TOphthorus appel. In Science, v. 31, no. 802, p. 748-749. 1910.), B. phytoph- 

 ihoTUs and B. solanisaprus are different. 



Journal of Agricultural Research Vcl. XIH, No. lo 



Washington, D. C. ^^^^ ^> '^18 



nq > (507) KeyNo.G-145 



