BLACKLEG POTATO TUBER-ROT UNDER IRRIGATION 



By M. Shapovalov and H. A. Edson, Pathologists, Office of Cotton, Truck, and Forage 

 Crop Disease Investigations, Bureau of Plant Industry, United States Department of 

 Agriculture . 



OCCURRENCE AND GENERAL APPEARANCE ' '■ 



A bacterial field decay of the potato tuber, the real nature of which 

 has not heretofore been adequately explained, prevails in certain irri- 

 gated sections of the West. In early harvest, when the diseased tubers 

 are apparently free from fungous invasion, the trouble has sometimes 

 been assumed to be "sunscald"; during the winter months it has fre- 

 quently been taken for a form of freezing injury. In other instances 

 it has been confused with the so-called "jelly-end rot" and attributed 

 either to Fusarium radicicola Wollenw. or to F. oxysporum Schlecht. It is 

 probable, also, that on superficial examination some such material has 

 been classed as "leak" {Pythium debar yanum Hesse), when conditions 

 favored an extremely rapid progress of the decay, whether in the field 

 or in transit. 



Specimens of this decay were received by the writers in 191 7 and 

 19 1 8 from Idaho, Nevada, and California. In material received in 

 August the decay was soft and mushy (PI. 12, A-C). The aflfected 

 tissues were in part brown to black, but mostly only slightly colored or 

 colorless, though with a darker margin on the border line between the 

 healthy and diseased portions. Disintegration, originating at one end 

 of the tuber, was advancing irregularly over the surface. In some 

 areas the decay was confined to the outer layer, just beneath the epi- 

 dermis, while in others the deeper tissues also were involved. As a 

 rule, the disease started at the stem end, but occasionally the eye end 

 became infected first (PI. 12, B). Decaying material usually possessed 

 a disagreeable odor. It is this soft type of the rot which some were 

 inclined to regard as sunscald injury. 



Specimens received later in the season, during the months of Novem- 

 ber and December, presented an entirely different appearance. The 

 affected portions were not mushy, but more or less tough or dry and 

 shrunken (PI. 12, D). The diseased area was dark brown in color, 

 except when a fresh decay developed under favorable conditions deeper 

 in the tissues. In the latter case it was practically of the same color as 

 the normal flesh of the tuber, but soft and mushy in consistency. When 

 such tubers were cut open and the cut surfaces exposed to the air, the 

 diseased portions turned brown or even black. If the progress of the 

 decay is completely anrested, the trouble may readil y be mistaken for 



Journal of Agricultural Research. Vol" ^^^^ ^°- ' 



Washington. D.C. Key No'.'g-»46 



(81) 



