BLACK ROT OP CRUCIFEROUS PLANTS. 



301 



SIGNS OF THE DISEASE. 



There is usually little difficulty in determining the existence of this disease. It is not 

 a soft rot, although it may be complicated by the appearance of soft rots. A striking 

 characteristic, especially in cabbages, is the yellowing of the foliage accompanied by a black 

 stain in the vascular system. This stain in the veins often causes patches of the leaf to 

 appear as a conspicuous black network on a yellowish or light brown background (fig. 98). 

 The reader may consult also the colored figure in Centralb. f. Bakt., 2 Abt., Bd. Ill, Taf. 

 VI. Such leaves are not wet or decayed but have a rather dry, somewhat leathery appear- 

 ance. When badly diseased, there is a gradual or successive shedding of such leaves, so that 

 the cabbage plant or cauliflower plant (fig. 99) may come to have a small terminal tuft of 

 leaves often more or less distorted, and separated from the root by a long stem bearing the 

 conspicuous scars of many cast-off leaves. The stems of such plants are browned internally 

 in the vascular ring (fig. 100 to 104). On cabbage-stems, etc., which have lost many leaves 

 there is frequently a slight pushing of shoots 

 from the axillary buds, but it is not known 

 whether such growths are stimulated by 

 the presence of the bacteria, or are due 

 solely to an unusual loss of leaves, the 

 latter being the most probable (fig. 105a;). 

 When the cabbage is attacked early in the 

 season and severely, it is either destroyed 

 outright in the course of a few weeks (seed- 

 ling stage), or is so injured that no head 

 forms. Dwarfing is a common sign of this 

 disease. Very often the plant is attacked 

 more on one side than on the other, the 

 result being unequal growth and a small 

 imperfect head. 



The thick petioles of infected leaves 

 may show no external evidence of the dis- 

 ease, but if examined in cross-section, the 

 vascular bundles or leaf-traces will be 

 found to be black or brown and occupied 

 by bacteria (fig. 102). On slender petioles 

 these blackened leaf-traces may show 

 through as dusky stripes; these pass into 

 the stem, which on cross-section is found 

 to have a blackened or browned woody 

 cylinder. This stain in the main axis may 

 involve the entire circumference of the xylem-cylinder or be confined to one side or even 

 to a few vessels on one side of the stem, the amount of stain depending on the place of 

 infection, on the level at which the section is made and on the stage of progress of the 

 disease, which is extremely slow in hard dry tissues and not very rapid even in soft watery 

 ones. Usually the bark and pith of such stems are free from bacterial occupation and normal 

 in appearance but not always (figs. 103, 105). After the first month or two the main roots 



*FiG. 98. — A cabbage-leaf with veins in middle part conspicuously blackened by Bacterium campestre. Marginal 

 portion free from signs of disease except toward base. The result of a pure culture inoculation made upon another 

 leaf of same plant by means of needle-punctures. The course of the organism was down the vascular system of the 

 inoculated leaf into the stem, up vessels of the stem and out into the vascular system of this leaf. The freedom of the 

 margin of the leaf from black venation is due therefore to the fact that the bacteria had not yet reached the final 

 ramifications of the vascular system, at least not in numbers sufficient to brown the veins. July, 1897. 



Fig. 98.' 



