402 13ACTERIU.M CAMPESTKE IN SOUTH AFRICA. 



it. In discussing the subject with local gardeners, 1 have found 

 it extremely difficult to convince these men that their losses 

 were due to a specific disease which might be prevented, and 

 not owing to an inherent tendenc}- of cabbages to " go wrong on 

 account of the climate." 



Up to the end of 1914, the " black rot " had only been 

 observed in cabbage and cauliflower plants, but at the beginning 

 of this year some experimental plots of kohl-rabi at Groenkloof 

 were found to be badly infected. This was not to be wondered 

 at, as the plots were not far distant from some cabbage plants 

 which had been severely attacked. 



In March of this year a crop of swedes in the neighbour- 

 hood of Johannesburg was attacked ; the tops were fairly healthy, 

 but the roots failed to swell out, and began to rot. The source 

 of infection in this case is not evident ; the discolouration was 

 only present in the lower part of the root, and no blackened 

 veins were observed in the leaves ; but Smith and other investi- 

 gators state that infection through the root system is not known. 

 Inoculation experiments, which will be described later, showed 

 this trouble, and that in kohl-rabi were identical with the " black 

 rot " of cabbage and cauliflower. 



Signs of the Disease. 



Some of the signs of the disease have been indicated in the 

 previous paragraphs, and there is usually little difficulty in deter- 

 mining its presence ; a more detailed description o f the appear- 

 ance of affected plants will, however, not be out of j^lace here. 



In cabbage and cauliflower the first sign of infection is tlie 

 blackening of some of the smaller veins, usually in the neigh- 

 bourhood of a water pore or of an insect injury. The tissues 

 in the afl^ected area become yellow, wilted, and rather leathery 

 in texture (Plate 8). The dark brown or black stain soreads 

 along the vascular system, and eventually invades the midrib of 

 the leaf and the main stem of the plant. The stain in the thick 

 petioles and main stem is frequently only visible on cutting them 

 through (Plate 10. a). 



Severely affected leaves fall to the ground, leaving con- 

 spicuous leaf scars, and badly diseased plants present the appear- 

 ance of a small terminal head separated from the roots by a long 

 stem bearing the conspicuous scars of many cast-ofif leaves. 

 Plants attacked early in the season or in the seedling stage are 

 either killed outright or become so deformed and dwarfed that 

 no head forms. 



Cauliflower plants are affected in a similar way : the leaves 

 show blackening of the veins and yellowing of the surrounding 

 tissues ; when badly diseased they fall, leaving conspicuous leaf 

 scars, and frequently at the time when the head should be form- 

 ing the plant consists of a scarred stem 2 to 3 feet long, sur- 

 mounted by a loose terminal tuft of leaves. Cavities are formed 

 in the stem, and the whole of the pith mav be destroyed. 



