324 THE PARASITIC DISEASES OF PLANTS 
there are on the edges of the leaves minute openings called water 
pores. Through these pores the bacteria also can enter. Seem- 
ingly they can also enter through the roots, especially through the 
tips of the rootlets which are likely to be exposed and broken 
during transplanting. 
No effectual remedy against this disease has yet been found. 
Its method of distribution from field to field is not well known, and 
hitherto no means of checking it after it has made its appearance 
in a field has been discovered. Wet weather, which is best for the 
growth of the cabbages is, unfortunately, also best for the growth 
of this parasite. That it lives in the soil from year to year seems 
proved, and hence after it makes its appearance in a field it is 
likely to recur year after year. Since it is confined to members 
of the Crucifere, it is a natural suggestion that a change of crop 
to some kind of plant not in this family should be made when any 
particular plot of ground becomes infested with the disease. The 
destruction of all weeds of the mustard family in the vicinity of 
cabbage plots is also to be recommended. 
OTHER BACTERIAL DISEASES OF PLANTS 
The illustration given will serve to show the kind of evidence 
that is sought for in the study of plant diseases. In the list that 
follows, demonstrative evidence has been obtained in practically 
all cases, so that all of the diseases in this list may be accepted as 
caused by bacteria. The list given is a long one, and if it be 
compared with the list of animal diseases given in the last chapter 
it will be seen to surpass that list. These plant diseases have not 
the importance nor have they developed the interest that attains 
to some of the animal diseases, but nevertheless they are of great 
significance in farming operations, sometimes causing very large 
losses. As in the case of animal diseases the bacteria causing 
them are not confined to one plant host. The black rot, for ex- 
ample, attacks the cabbage, the cauliflower, the turnip, kale, 
Brussels sprouts, collards, rutabagas, radish, as well as some other 
