16 The Colorado Experiment Station. 
is peculiarly interesting, since a few years back a great deal of our 
cabbage seed was imported from Holland and Denmark. The com¬ 
paratively recent discovery of the infection in Europe is probably 
due to the better methods of diagnosing the disease than formerly, 
rather than to recent appearances of the trouble. Cabbage growers 
of Europe have observed it for many years, it seems, but they were 
accustomed to associate the cabbage worm with the rot, rather than 
the true bacterial cause. 
The disease can be recognized by the dwarfed, one-sided 
growth of the plants and in some cases, failure to produce heads. 
Sometimes the head will rot and fall off, but this is not a necessary 
accomplishment of the disease, for this symptom, together with the 
bad smelling head, may be due to the work of other bacteria which 
are living on the tissue already weakened by the true black rot 
organism. In the early stage of the disease, the leaves show a 
withered, dried appearance along the margin, followed, in time, by 
a yellowing. The small ribs that lead to the mid-rib are usually 
blackened first, and ultimately the larger ones and the mid-rib suc¬ 
cumb. On cutting across the stem of an infected leaf or sick 
stalk, one can see the blackened ends of the fibrous strands, known 
as the fibro vascular bundles, which lead from the stalk out into the 
leaf and supply it with water and soil foods. A microscopic ex¬ 
amination of these vascular bundles will show the tiny tubes of 
which they are composed to be alive with germs. As soon as these 
foods and water channels are destroyed, the blade of the leaf is no 
longer able to get the subsistance required and dies. Diseased 
leaves usually fall prematurely, leaving a long, naked stalk with a 
tuft of leaves at the tip. The old leaf scars will show the ends of 
blackened strands corresponding to the diseased fibro vascular 
bundles in the leaf. 
It has been found* that the most common method of infection 
in the field is through the very small water pores scattered over the 
blunt teeth on the margin of the leaf. It is by these water pores 
that a part of the soil moisture that is taken up by the root system 
escapes from the plant as water vapor, but if the surrounding at¬ 
mosphere is very moist, there will be no evaporation and the water 
will be seen to accumulate in tiny droplets just over the pores. 
However, if the soil is very dry, even though the air is moist, we 
do not have these water beads formed. They can be seen fre¬ 
quently in the early morning on the surface of the leaves and are 
frequently mistaken for dew. It must be borne in mind that the air 
is always filled with dust and numerous bacteria, and among these 
there are almost certain to be the germs of black rot, especially if 
*Russel—Bull. 65, Wis. Expt. Sta. Farmers’ Bull. 63, U. S. Dept. A g. 
