BLACK ROT OF CRUCIFEROUS PLANTS. 



303 



venation of the fleshy edible part is very conspicuous (fig. 104) although externally this 

 portion of the plant may appear perfectly sound, the infection having taken place through 

 the leaf-traces. Cavities also appear in the flesh of the kohlrabi (fig. 107) . In those investi- 

 gated by Hecke in Austria and those seen by the writer in Florida, the fleshy part was not 

 dwarfed and was sound externally. In turnip roots a water-soaked appearance of the flesh 

 is not infrequent, but the brown stain also occurs. On cross-section a yellowish bacterial 

 slime frequently oozes from the blackened bundles of badly diseased stems. The writer 

 observed this ooze very frequently in charlock. The unmixed disease has no conspicuous 

 odor except possibly in turnips, but when the secondary white, rapidly disintegrating soft 

 or wet-rots set in the smell is usually very disagreeable. 



The only serious malady of cabbages likely to be confused with this is a Fusarium 

 disease first described by the writer (Bull 17, Wilt disease of cotton, watermelon and cowpea, 

 1899, footnote, p. 41) and more recently by Mr. Harter (Science). This occurs from New 



Fig. lOl.t 



Fig. 100.* 



York to South Carolina and is an almost equally serious disease, but is easily distinguished 

 by the presence of the fungus in the vascular system. In' the absence of a microscope, 

 bad cases of the two diseases can usually be distinguished by cross-cutting the stems with 

 a clean sharp knife and leaving them under a clean bowl or pan for 24 hours in a moist 

 warm place. In the one case there is then often a slight yellowish ooze from the blackened 

 bundles; in the other case there will be often a ring of white fungous threads extruded 

 from the brown woody cylinder. 



Slowly extending and relatively unimportant marginal leaf-injuries due to fungi and 

 to other bacteria have been seen by the writer occasionally on the cabbage, and in some 

 fields these might be mistaken perhaps for the marginal infections of the black rot, particu- 

 larly by persons not very familiar with the latter. A little experience will generally enable 



*FiG. ioo. Cabbage head showing in the stem a conspicuous ring of black bundles due to Bacterium campestre. 

 From a field in Holland. After van Hall. 



fFlG. 101. Cauliflower-stem parasitized by Bacterium campestre. Organism confined to vascular bundles, which 

 are stained black. Beeville, Tex., Dec. 1902. Natural size. 



