Black rot of mustard is caused by

Black rot caused by the bacterium Xanthomonas campestris is a significant disease of cabbage and other crucifer crops worldwide including broccoli, cauliflower, radish, turnip, mustard, etc. Symptoms of black rot generally begin with yellowing at the leaf margins which expand into characteristic V-shaped lesion. The bacterial infection becomes systemic, meaning that the bacterium can enter the veins of plant which can lead to serious losses.