pKTHYBKiDGE AND MuRPHY — Bacterial Disease of i/ie Potato. 11 



will be found to be absent on the portion of the stalk below ground, having 

 completely rotted away ; in others, these tissues will be found to be present, 

 but ill a state of decay, and consequently the stalk iu this region looks 

 black externally. The upper portion of the part of the stalk below ground 

 may still be provided with roots and with rhizomes with small tubers on them, 

 but these are absent from the lower portions. The parent "set," from which 

 the plant was derived, will be found to have decayed away, leaving nothing 

 but its skin. When a set has given rise to more than one stalk, it sometimes 

 happens that some of the stalks may be diseased, while, for a time at least, 

 one or more of the others remains healtliy. Ultimately, however, all the 

 stalks succumb. 



If the diseased stalk be split longitudinally with a knife, it will be found 

 that the soft tissues within the woody eyhnder have become destroyed and 

 blackened for a distance of some inches, usually extending up to and often a 

 couple of inclies beyond the level of the soil. Most frequently, at about this 

 level, there is a sharp line of demarkation between tlie lower diseased and the 

 upper apparently healthy tissues ; but in some cases this sharp line is absent, and 

 occasionally one finds that the pith has become disorganized and quite pulpy 

 for a considerable distance up the stem, without, however, losing its light 

 green colour and hence without becoming black, althougli, in other cases, the 

 blackening may extend for a considerable distance up the interior of the stalk. 



If diseased stalks be cut transversely, two tilings will be noticed : first, 

 the stem is tough to cut, and quite different in this respect from a healthy stalk, 

 which, at this stage, " cuts like a cucumber " ; secondly, on the cut surface, the 

 three principal vascular bundles, situated at the three angles of the stem, 

 display a strong brown discolouration, especially in the region of the wood. 



The distance to which this discolouration of the vascular bundles may 

 extend in the stalks varies according to the progress of the attack at a given 

 time. In severe cases it can be traced to the very topmost extremity of the 

 stem, and even into the petioles of the upper leaves. In other cases it can be 

 traced up to the penultimate internode ; and, in still others, where the attack 

 at the moment is but slight, it may have reached but a short distance above 

 soil-level. Examination of sections of the affected stalks with the microscope 

 shows in the majority of eases an absence of fungal hyphae. On the other 

 hand, actively motile bacteria are present in abundance. In the diseased 

 parenchymatous tissues, such as the pith, the cells are found to be isolated or 

 macerated, although they still retain tlieir starch grains intact. Between 

 them bacteria are present in great abundance. The brown discolouration of 

 the wood is found to be due partly to the formation of a yellowish brown 

 gummy material situated in the cavities of the vessels, but more particularly 



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