304 BACTERIA IN RELATION TO PLANT DISEASES. 
one to discriminate. The lapse of a little time between inspections will also help one to 
judge, since in its further progress the black rot is quite different from the other foliar 
diseases. In this connection see plate 18. 
ETIOLOGY. 
The cause of this disease is a yellow one-flagellate micro-organism, Bacterium campestre 
(Pammel) EFS. Conclusive proof of the infectious nature of this organism was obtained 
: : on turnips by Pammel in 1893 and pub- 
lished in 1895. He performed 20 direct 
inoculations successfully. Subsequently he 
isolated the organism. His pure cultures 
were derived from turnips and rutabagas, 
and 8 successful inoculations were made 
into rutabagas, as many plants being held 
for control. The signs of the disease ap- 
peared in the course of a few days, finally 
involving the whole plant; this same or- 
ganism was subsequently isolated from the 
diseased tissues, 7. e., from the blackened 
bundles and advancing margin of the rot. 
In 1896-97 the writer repeated and con- 
firmed Pammel’s experiments on turnips 
and rutabagas and extended the inocula- 
tions to cabbages, cauliflower, kale, rape, 
radish, and black mustard. Two strains of 
the organism were used for most of these 
infections, viz., pure culture isolations from 
diseased turnips obtained in Maryland and 
similar cultures from diseased cabbages 
from Wisconsin, but some cross-infections 
were also made with the organism obtained 
from charlock in Wisconsin. Microscopic 
examinations, bacteriological cultures and 
cross-inoculations showed the disease to be 
identical in all of these plants.* He also 
confirmed and considerably extended Pam- 
mel’s description of the organism. The 
writer at this time had obtained altogether 
more than 60 successful infections resulting 
in typical cases of the disease. From dis- 
eased plants at long distances from the 
point of inoculation he several times re- 
isolated the organism and obtained addi- 
tional infections with such cultures. Only 
one of his many control plants contracted 
the disease and this under circumstances 
which pointed clearly to neighboring inocu- 
lated plants as the source of the infection 
and to mollusks (Agriolimax agrestis) as the 
Fig. 102.7 carriers of the bacteria. 
*Punctures with a sterile needle never induced any disease in my experiments. Hecke also mentions this fact 
particularly. 
tFic. 102.—Cross-section of petiole of cabbage, showing every bundle blackened by Bact. campestre, the paren- 
chyma being free. Slide 8, plant No. 42. 
, a 
