BROWN ROT OF SOLANACEAE. 



177 



treme tips of the growing shoots, which are flabby or shriveled (plate 30). In less woody 

 plants the branches will be shriveled, or flaccid, as shown in plate 24. In the potato plant 

 the organisms pass down through the vascular system of the stems into the underground parts 

 and cause an internal brown rot of the tubers. This rot of the tuber begins in the vascu- 

 lar system at the stem end and 

 gradually extends to the oppo- 

 site end by way of the vessels, rot- 

 ting the tuber from within and 

 causing the appearance of dusky 

 patches on the smooth surface 

 of the tubers in advance of any 

 rupture of the superficial cork- 

 layer (pi. 23, figs. 9, 11). On 

 cross-section these patches are 

 seen to be due to the internal 

 brown stain centered in the vas- 

 cular ring and showing through 

 the more superficial layers of 

 white tissue. In later stages of 

 the disease the cork-layer is 

 ruptured and the rot continues 

 as a mixed infection due to the 

 entrance of other organisms di- 

 rectly from the soil. In tubers 

 less far advanced in the rot, 

 cross-sections often show the de- 

 cay pretty closely restricted to 

 the vascular ring (figs. 82, 83), 

 from which there is always a 

 gray-white or dirty white bac- 

 terial ooze (vol. I, plate 24, top). 

 In yet earlier stages of the dis- 

 ease the brown stain and bac- 

 terial decay are found only in 

 the stem end of the tuber (vas- 

 cular ring) or only in the vascular bundles of the underground stem 

 leading to the tuber, the surface of this rhizome and the whole of the 

 attached tuber being entirely sound (figs. 84, 85). On plants attacked 

 with great virulence and very early no tubers are formed. The tubers 

 on plants infected in the middle of the growing season are small and few 

 and are found at harvest time in all stages of decay (pis. 24, 25). Often 

 on such plants, as first noted by Dr. Halsted, tubers not larger than a 

 pea, together with their rhizome, will be found sound superficially but 

 brown-rotted in the vascular ring; in other cases, as already noted, the 



Fig. 84. : 



Fig. 85. f 



*Fio. 84. Cross-section of underground stem of a potato-plant leading to a tuber sound on the surface but bac- 

 terially rotted in the vascular system, i. e., a tuber such as that shown in fig. 83. Rhizome fixed in Carnoy's fluid, 

 embedded in paraffin, and sectioned on the microtome. Slide 348 a 1, stained with carbol-fuchsin. The bark and pith 

 are entirely free from bacterial occupation. The bacteria are confined principally to the vessels of the woody part of 

 the bundle, but there is a cavity in the outer phloem (at the top) and there are various small bacterial foci in the inner 

 phloem. The V-shaped marks in parenchyma-cells (use lens) denote crystal-sand. Material collected at Portsmouth, 

 Virginia, October 1905. Drawn with a Zeiss 16-mm. apochromatic objective, No. 4 comp. ocular, and the Abbe 

 camera. (For general appearance of the bacteria in these vessels see fig. 85.) 



fFio. 85. Bacterium solanacearum from one of the vessels in fig. 84 (cross-section rhizome, Portsmouth, Virginia, 

 potato). Slide 348 Ai. 



