06 Bacterial Diseased of Plants 



Symjitoms of the disease. The first sign of disease is to be observed in 

 the foliage. The leaves turn yellow or much paler green than healthy 

 leaves and diseased plants can readily be recognised in the early stages 

 by this character alone. At the same time they retain a rather erect 

 position on the stem instead of spreading out horizontally and the edges 

 exhibit a rolling towards the mid-rib. The rolling should not be confused 

 with that of "Leaf-roll" since it is of cylindrical form rather than funnel 

 shaped and there is entire absence of the pink or purple pigmentation 

 characteristic of this disease. In the later stages of disease the leaves 

 wilt and the whole plant assumes a brown to black colour and a very 

 dejected appearance. The plants remain standing in the field in virtue 

 of a special hardening of the three main vascular bundles of the stem. 

 These quite early become browned for a considerable distance up the 

 stem and are so hard that a sharp knife is needed to cut them transversely. 

 The name "Blackleg" is derived from a characteristic blackening of the 

 tissues of the stem commencing in the underground portion and extend- 

 ing to a height of two to three inches above the soil level. In the lower 

 portion of the stem the pith is completely rotted away leaving a cavity 

 in which fungal mycelium, easily recognised as that of saprophytic forms, 

 is frequently present. The tissues in the upper portion of the stem appear 

 sound except for the browning of the vascular bundles above mentioned 

 and for a water-logged appearance of the pitli for a centimetre or so 

 above the upper limit of the cavity caused by the destruction of the pith 

 cells of the lower portion. If the attack has occurred early in the season 

 no tubers or only very small ones will have formed and anchorage in the 

 soil is so slight that the least pull is sufficient to lift the plant. If, how- 

 ever, the plant is attacked after the tubers have well formed these will 

 be infected by the organism through the vascular system of the under- 

 ground stems. Such infected tubers will in warm moist weather rot off 

 completely in the soil, but it more frequently happens that at the time 

 of " Ufting " the organism has not advanced from the vascular system 

 and outwardly the tuber appears quite sound. These infected tubers 

 give rise to the trouble in storage mentioned above and it is to them that 

 the chief losses incurred through this disease are attributable. The 

 organism may remain localised in the vascular ring of the tuber through- 

 out the winter and the planting of diseased "sets" is believed to be the 

 chief cause of dissemination of the disease. 



The parasife. Several organisms have been described as giving rise to 

 this disease but recent woik by Morse (:u) has shown that they are in all 

 probability all strains of one species B. afrosejHicus. Previously the name 



