BLACK ROT OF 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. III, 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 isfrequently aslight 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. 105%). 
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 oN 
vascular bundles or leaf-traces will be SONA 
found to be black or brown and occupied SN SOM 752 
by bacteria (fig. 102). On slender petioles Wa | tise iL 
these blackened leaf-traces may show NH | Ae, 
through as dusky stripes; these pass into aa | Hey 
the stem, which on cross-section is found Na 
to have a blackened or browned woody Fig. 98.* 
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 
*Fic. 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 thestem and out into the yascularsystemof 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. 
