Relation of xylem plugging to black rot lesion development in cabbage

Abstract

Black rot lesion development in cabbage leaves was correlated with accumulation of plugging material in the xylem vessels as Xanthomonas campestris multiplied and spread. Plug formation preceded a partial disorganization of the xylem, and spillage of bacteria from the vessels. The movement of eosin and 32PO43− was blocked within the vessels, and plugs caused water shortage in affected cells. Vein blackening, the first symptom of the disease, occurred after plugs had formed, and was associated with an accumulation of melanins among collapsing cells of the vascular bundle. The population of viable bacteria within the veins declined rapidly as the veins blackened.Advance of the bacteria in the xylem vessels accounted for the relationship between venation and lesion spread. A weakly virulent isolate spread extensively in the xylem vessels if inoculated with a virulent isolate, but not when inoculated alone. Acid mucopolysaccharide plugging material was not formed in vessels infected with the weakly virulent isolate.Water stress in cells of interveinal tissues of inoculated leaves resulted in chlorosis, loss in turgor, and loss of electrolytes. If net water loss from infected leaves was prevented by high atmospheric relative humidity, affected cells remained green and turgid and maintained their electrolyte balance.