Black rot of mustard is caused by?

Correct option is Xanthomonas campestris. Black rot is caused by the bacterium Xanthomonas campestris and is considered the most serious disease of crucifer crops worldwide. This disease is also known as blight, black stem, black vein, stem rot, and stump rot. Hence, Black rot of mustard is caused by Xanthomonas campestris. So, the correct answer is 'Xanthomonas campestris'.