PATHOLOGICAL ANATOMY OF POTATO BLACKLEG 



By Ernst F. Artschwager 



Scientific Assistant, Office of Cotton, Truck, and Forage Crop Disease Investigations, 



Bureau of Plant Industry, United States Department of Agriculture 



Blackleg, as has been shown by the researches of numerous investi- 

 gators, notably Appel (2), 1 Smith (7), and Morse (6), is a bacterial dis- 

 ease affecting the underground part of the potato stem and the tubers. 

 In its typical form the base of the potato stem shows a pronounced 

 blackening which may extend several inches above the ground. The 

 seed piece from which the diseased plants have been grown is found in a 

 state of decay or is already completely destroyed by rot. The external 

 symptoms are sufficiently striking to enable one to recognize diseased 

 plants even at a distance. Such plants are of a lighter color and usually 

 exhibit a xerophytic texture. They may be normal in size, but most 

 often they are dwarfed and stocky, so that the disease is easily mistaken 

 for leafroll. But while leafroll plants are firmly anchored in the ground, 

 blackleg stems are easily pulled and always show the characteristic 

 lesions on the underground part. The ease with which diseased plants 

 are removed is so striking that one unconciously looks for a contributing 

 mechanical cause, and that such a supposition is not altogether unfounded 

 is shown in a note by Heygi (4) who reports wire-worm injuries in almost 

 all the blackleg material that came under this observation. From the 

 results of his investigations, Hegyi is inclined to consider the presence of 

 the bacteria a secondary factor which has nothing to do with the original 

 cause. Yet while it is true that wire worms may cause a loss of many 

 potato hills, they are probably not responsible for the death of plants 

 suffering from blackleg. 



The xerophytic texture of the diseased plants is exhibited by stems 

 and leaves alike. The foliage is discolored, usually light and of a metallic 

 luster. The leaflets are folded, and the petiole and midrib are woody 

 and lacking the elasticity and softness which characterize the normal 

 organs. Not all the stems of a diseased plant are necessarily affected. 

 Healthy sprouts may appear side by side with diseased ones (PI. 57, A), 

 and the diseased sprouts may exhibit various degrees of injury. Plants 

 which have been attacked rather early often continue to live for a con- 

 siderable period. These plants remain naturally dwarfed, the stalks 

 are spindling, the internodes shortened, and the leaves small. Plants 

 attacked at a more mature age may attain full size, though they usually 

 succumb more quickly to the attack of the parasite than do many of the 



1 Reference is made by number (italic) to " Literature cited," p. 330. 



Journal of Agricultural Research, Vol. XX, No. 4 



Washington, D. C. N » v - *5. 1920 



vq Key No. G-210 



9508°— 20 7 



(3 2 5) 



