1923] 



JENNISON— POTATO BLACKLEG 11 



stalks a lateral development of a lesion promptly results in a 

 girdling of the stem, causing death. Under natural conditions 



stem is seldom 



Th 



tendency for the pathogen to migrate downward 



when stems are artificially inoculated at a point above ground. 

 This fact is strongly emphasized by Smith's ('20) figures 193 

 and 199. The strong tendency on the part of the parasite to mi- 

 grate upward along the main stem is shown in pi. 1, fig. 1. This 

 photograph illustrates a rather striking, though not uncommon, 

 case. Upon closer examination it will be found that the bacteria 

 are largely confined to the parenchymatous tissues of the stem 

 to the cortex and the pith. As it works upward in the pith this 

 colorless parenchyma also becomes blackened and necrotic (pi. 



1, fig. 4), easily observed by splitting the stem longitudinally. 

 In this manifestation we have one of the most striking as well 

 as reliable diagnostic features. The pith finally disintegrates, 

 leaving the stalk more or less hollow. Frequently worms will 

 be found feeding among the dead pith cells. 



Lateral spread in large mature stems is very slight. Even 

 though the stalk is severely diseased the plant may persist and 

 mature a crop of tubers (pi. 2, fig. 3). Under favorable circum- 

 stances the infective agent moves through the stolons to invade 

 the tubers. Sometimes the latter organs are infected at other 

 points, invasion probably taking place for the most part through 

 the lenticels, as has been experimentally demonstrated by Smith 

 ('20). Diseased tubers frequently rot in the ground, the entrance 

 of saprophytes bringing about the profound, slimy, putrid, soft- 

 rot frequently observed. Death of the roots is secondary and 

 follows a killing of the tops. 



Tuber symptoms. — As implied above, stem-end infection of 

 the tubers is most frequent under ordinary circumstances. As a 

 rule, decay begins at the point of attachment of the stolon (pi. 



2, figs. 3 and 4). Often, however, comparatively little decay 

 is visible. Experiments carried out by the writer showed that 

 the pathogen may be securely lodged in the tuber at the junc- 

 ture with the rhizome, even though there are no signs of rot. 

 He has also determined on many occasions that the blackleg 

 parasite is the cause of more or less discoloration in the tissues 

 associated with the vascular elements in the stem-end of the 

 tubers. Such signs of the disease may readily be mistaken for 

 symptoms of the Fusarium and Verticillium wilt diseases. In- 



