Pethybridge and Murphy — Bacterial Disease of the Potato. IB 



is not infrequently marked off sharply from that covering the healthy portion 

 by a sharp black line. The lenticels, too, on the diseased area are very 

 frequently much emphasized as distinct black dots. These points are 

 shown in Fig. 2, Plate I. Further, in badly diseased tubers there has 

 usually been an exudation of liquid, which causes the surrounding soil 

 to adhere to that part of the tuber which is diseased ; and hence, even from 

 a dry soil, such tubers do not come up clean. The diseased portion of the 

 tuber is not nearly so firm to the touch as the healthy portion, and when 

 squeezed a watery fluid is exuded. On cutting open an affected tuber the 

 diseased portion, which may show one or more cavities, quickly takes on a 

 pinkish tinge, owing to contact with the air. This tinge gradually deepens, 

 until at the end of a space of a few hours it is of a deep violet-brown colour, 

 almost black. The juice expressed from a healthy tuber is, as is well known, 

 slightly acid to litmus paper; but the juice from the diseased tissue is even 

 more distinctly acid at first. After standing in the air for some time, however, 

 and after blackening has occurred, the diseased tissue produces an alkaline 

 reaction with litmus. The diseased tissue in the interior of affected stalks we 

 have always found to be distinctly alkaline to litmus. Fig. 3, Plate I., shows 

 an affected tuber in longitvidinal section. 



Somewhat similar symptoms to those described, especially for the later 

 attacked plants, are to be found when pathogenic bacteria are not present. 

 Thus the wilting and withering of the foliage, together with the presence of 

 the three discoloured vascular bundles in a cross-section of the stem, are also 

 characteristic of the later stages of " leaf-roll." In this ease, however, pulling 

 the stalk immediately gives the clue ; for in Black Stalk- rot the portion of 

 the stalk below ground has decayed away, while this is not the case with 

 "leaf-roll." Again, the caterpillar of the Frosted Orange Moth {G-ortnya 

 ochracea) has been found on several occasions burrowing in the bases of 

 potato-stalks, and producing in the foliage symptoms strongly resembling 

 those of Black Stalk-rot. But the real cause is not difficult to discern in 

 such cases. 



Furthermore, it does not always happen that the decay of the stalk below 

 ground occasioned by the Black Stalk-rot leads to the immediate destruction 

 of the stalks and foliage. On the contrary, not infrequently there still 

 remains on the upper portion of the stalk below ground a tolerably good 

 development of healthy roots, suificient to keep the foliage suppUed, tem- 

 porarily at least, with enough water and dissolved salts to enable it to carry 

 on its proper functions. The consequence is that the manufactured food 

 passed downwards from the leaves, having its natural places of deposit — the 

 tubers — below ground cut off, accumulates in the buds formed in the axils of 



