New York Agricultural Experiment Station. 89 



disease usually reaches the stem of the plant. The progress of 

 the disease from this point is identical with that brought about 

 by infection through the root. 



In either form of infection the most reliable diagnostic char- 

 acter of the disease is the blackening of the fibro-vascular bun- 

 dles. These bundles may be readily inspected by cutting across 

 the leaf petiole or the stem. 



The failure to supply sufficient water checks growth and often 

 results in the death of the plant. The fibro-vascular bundles do 

 not branch freely in the stem and in cases where the disease gains 

 a foothold only on one side of the plant the growth on that side 

 is retarded so as to produce a marked curvature. The lower 

 leaves turn brown and drop ofif, but when the plant succeeds in 

 forming a head the upper leaves are held in place and often turn 

 black and decay thus destroying the commercial value of the 

 head. 



The extreme variation in the activity of this disease in different 

 years depends largely upon weather conditions. A combination 

 of abundant moisture with high temperature during August and 

 September is favorable for an epidemic. 



CAUSED BY BACTERIA. 



The blackening of the fibro-vascular bundles and the accom- 

 panying decrease in the water flow is due to the growth in the 

 tissue of a bacterium known as Pscudomonas campcstris (Pam.) 

 Smith. This is the fact in connection with the disease which 

 has been most carefully established. The tissue of healthy plants 

 is free from germ life, but large numbers of Pscudomonas campcs- 

 tris are constantly found in these blackened bundles. Germs 

 obtained in this way from diseased plants at such widely separated 

 points as Wisconsin, New York and Switzerland were carefully 

 studied and found to be of this species.^ When pure cultures of 

 P. campcstris were introduced into the stem of healthy plants 

 under circumstances which prevented the entrance of other forms 

 the characteristic phenomena of the disease were reproduced. 



3 Harding, H. A., Die schwarze Faulnis des Kohls und verwandter Pflanzen, 

 eine in Europa weit verbreitete bakterielle Pflanzenkrankheit. Centralbl.f. Baki., 

 II Abt., 6: 305. 1900. 



