340 PLANT DISEASES 



BLACK ROT OF CABBAGE 



(Pseudomonas campestris, E. F. Smith) 



A disease of cabbages and other Cruciferous plants, 

 known in the United States for the last dozen years, and 

 also recently detected in England, is diagnosed as follows 

 by Dr. Erwin F. Smith : — 



' This disease may appear on the plant at any stage of 

 growth, and is characterised by the following symptoms : 

 Dwarfing, or one-sided growth of the heads, or, if the 

 disease is very severe and has begun early in the season, 

 by the entire absence of any heads, and in extreme cases 

 by the death of the plant. Occasionally the heads rot and 

 fall off, but this is not a necessary consequence, the soft, 

 bad-smeUing rot being due to the entrance of other 

 organisms. If the stumps of affected plants are broken 

 or cut across, a brown or black ring will be observed 

 corresponding to the woody part of the stem, this being 

 the part of the stem specially subject to the disease. In 

 bad cases this blackening may be easily traced upwards into 

 the centre of the head, and is generally worst on one side. 

 In the leaves the symptoms usually begin at the margins, 

 and consist in yellowing of all affected parts except the 

 veins, which become decidedly brown or black; The leaves 

 appear to have " burnt edges." ' 



The disease is caused by a yellow bacterium which enters 

 the plant above ground, and usually at the margin of the 

 leaves, through minute openings called water-pores, situated 

 on the teeth of the margin of the leaf. 



Slugs and caterpillars also spread the disease by gnawing 

 alternately diseased and healthy plants, or by carrying the 



