180 



BACTERIAL DISEASES OF PLANTS 



the alkaline slime oozes to the surface but often the surface is 

 sound. On potato, narrow dark stripes corresponding to the 

 infected bundles often show through leaves and stems. On the 

 leaves of the potato, infection may be lateral, running out on one 

 side of the petiole and in particular veins of the leaflets as a 

 black stain: compare with Fire Blight of the Pear (No. XII). 

 The tubers are also subject to infection, principally in the Vascu- 



FiG. 123. — To])acco leaf wilted by Bacterium solanacearum. Results of a stem 

 inoculation made by the writer in 1906. 



lar region and usually by way of the stem, through the vessels 

 of the rhizome; cavities are formed and these rupture to the 

 surface. On cross-section of tubers in early stages there is, 

 especially at the stem end, a ring of brown stain and a gray bac- 

 terial ooze limited to the outer (vascular) part of the tuber 

 (Fig. 129). This stain often gives to portions of the surface of 

 the tuber a dusky hue even when the skin is unbroken and the 

 outer tissues are sound. Stems of attacked Helianthus, Tro- 



