1923] n 



JENNISON — POTATO BLACKLEG * 



cies." A detailed description of the causal organism isolated by 



them was given. 



I. Diagnosis of the Disease 



As indicated in the preceding section, the blackleg disease of 

 potatoes is a common and destructive bacterial disease of many 

 varieties of Solatium tuberosum. The common name "blackleg" 

 has been widely used in the United States and Canada, since it 

 was first introduced by Jones in 1905. The term "blackleg" is 

 a rather free translation of "Schwarzbeinigkeit," under which 

 name the disease is known in Europe. The writer feels that 

 blackstem and rot would be more suitable for American use but 

 sees no point in suggesting that a change be made at this time, 

 because (1) the old name has become well established and (2) 

 because of its adoption by the American Phytopathological So- 

 ciety's Committee on Common Names for Plant Diseases. It 

 may be pointed out, however, that the writer has observed that 

 the term "blackleg" leads some farmers and others, especially in 

 the West, to think that it may be related to the blackleg disease 

 of cattle. More particularly is this the case when they learn 

 that both are caused by bacteria. 



Type— Potato blackleg is primarily a parenchyma necrosis, the 

 cortical and pith tissues of both stem and tuber being almost 

 exclusively involved. Occasionally, the vascular elements are 

 found to be invaded by the parasite and sometimes they are 

 somewhat browned. 



Signs of blackleg in the field.— Considerable importance at- 

 taches to general manifestations of the disease as it appears in 

 the field, since upon such manifestations depends the important 

 practice of roguing, advocated below. Not infrequently "skips" 

 or "missed hills" are due to this disease, for under favorable cir- 

 cumstances the young sprouts are destroyed by the parasite be- 

 fore they appear above ground; also the seed piece may be de- 

 stroyed before it sprouts. When the disease progresses rapidly 

 the plants wilt and become blackened to a considerable height. 

 Such plants are quickly prostrated and soon die (pi. 1, fig. 3). 



Secondary symptoms of the disease are likely to be the first to 

 appear. Undersized vines with more or less yellowed and rolled 

 leaves should be regarded with suspicion and more closely exam- 

 ined for the presence of black lesions upon the stem, both above 



