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Factors Influencing Black Rot Lesion Development in Resistant and Susceptible Cabbage. T. Staub, Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; P. H. Williams, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 62:722-728. Accepted for publication 3 February 1972. DOI: 10.1094/Phyto-62-722.

The effects of light, temperature, and inoculum level on lesion development on resistant and susceptible cabbage were determined. Resistance in veins was distinguished from that in hydathodes by inoculating injured vein endings and noninjured hydathodes. The hydathode was implicated as the site at which resistance normally operates. In resistant plants, the inoculation of injured vein endings resulted in black rot lesions, but the cessation of lesion enlargement 9 to 12 days after inoculation indicated an induced inhibition of bacterial multiplication in the major veins. Multiplication of Xanthomonas campestris in the veins of resistant and susceptible plants paralleled lesion progress. In resistant plants, inhibition of both lesion progress and bacterial growth were more pronounced at 20 than at 28 C. The generation times of X. campestris were the same in xylem fluid from resistant and susceptible genotypes. Both genotypes had similar percentage infection at varying inoculum doses when either veins or hydathodes were inoculated, suggesting that there was no preformed resistance in the cabbage. The concomitant appearance of lesions in both resistant and susceptible plants after vein inoculation together with the subsequent divergence in the rate of lesion growth also suggested that a period of time was required after inoculation for resistance to become operative. The decline of resistance under low light intensity (600 ft-c) and the delay of induced resistance at temperatures above the optimum for cabbage growth (22 to 24 C) indicated the importance of the physiologic condition of the host in resistance.