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BLACK ROT OF CRUCIFEROUS PLANTS. 309 
on particular water-pores which were extruding fluid. Once past the incubation period, the 
downward progress of the disease is comparatively rapid (1 cm. or more per day), especially 
if the weather is warm and the soil is moist enough to induce a vigorous growth of the host- 
plant. The progress of the disease incool weather and in plants making aslow growth is less 
rapid (plate 2). On September 7, 1897, during weather very favorable to the progress of the 
disease, the writer examined a cabbage-plant at Racine, Wisconsin, which bore 170 separate 
water-pore infections all spreading rapidly. As yet there was no disease of the root, stem, 
or interior of the well-formed head, nor was there any black stain in the base of any of the 
petioles. These infections, therefore, probably took place not much earlier than the first 
of August. Such a plant might be expected to be badly rotted in stem and head in course 
of another six or eight weeks. 
Brenner states that when he inoculated cabbage-plants on a single leaf-tooth many 
other groups of water-pores subsequently contracted the disease, the organism being pre- 
sumably carried along the moist 
surface of the leaf-margin to 
the other groups of water-pores 
by capillary attraction. As 
noted in 1898, the writer saw one 
large cabbage-plant which bore 
more than 400 distinct marginal 
leaf-infections, while many other 
plants in the same field and in 
other fields showed from 50 to 
150 such separate infections. 
Sometimes nearly every serra- 
ture on a leaf would be infected. 
The writer has dissected hun- 
dreds of cabbage-leaves which 
were attacked at the time of 
observation only on their mar- 
gin, and other hundreds in 
which the organism was already 
well into the middle of the leaf 
or had alreadyentered thestem, 
as determined by cutting. An 
extensive marginal infection 
obtained by spraying, with the 
entire absence of infection by 
way of the ordinary stomata is shown in fig. 10. For an early stage of water pore infection 
see fig. 130a and Vol. I, fig. 87; for a late stage with disorganization, this volume, fig. 11. 
The leaf surface of many crucifers is covered with a waxy bloom repellant to 
water, and this preserves the leaf from injury when submerged for some hours, and also 
undoubtedly makes it difficult for the bacteria to enter through the ordinary stomata. 
At least, infections through such stomata have not been observed. Russell’s observations 
and experiments agree with those of the writer. 
In case of leaf-infections by way of the water-pores the general progress of the dis- 
ease is downward in the spiral vessels of the leaf (Vol. I, fig. 76, 77, 78). These are com- 
monly filled densely, many of them at least, by the rapid multiplication of the organism. 
From the blade of the leaf the bacteria pass into the leaf-traces of the petiole (fig. 109, 110) 
*Fic. 107.—Cross-section of a small portion of a blackened kohlrabi bundle, showing cavity due to presence of 
Bacterium campestre. Section fromenlarged edible part of plant. Material collected at Miami, Fla., March, 1904. 
x 500. Slide 293 b 18. 
