A plant disease which is not caused by a fungus is:

Explanation

Red rot disease is caused by the fungus Glomerella tucumanensis. It produces a structure called acervuli. This results in profuse sporulation. Late blight of potatoes is caused by the fungus Phytophthora infestans. It infects and destroys the leaves, stems, fruits, and tubers of potato and tomato plants. The black rot of crucifers is caused by the bacterium Xanthomonas campestris. It results in dark brown discolouration of the leaves of fruits and vegetables. Wheat leaf rust is a disease caused by the fungus Puccinia triticina. It affects wheat, barley, and rye stems, leaves, and grains. Thus, the correct answer is 'Black rot of crucifers.'Although option B may seem correct, the question specifically asks about plant diseases not caused by fungi. Late blight of potato, mentioned in option B, is caused by a fungus-like protist. While it is not a true fungus, some textbooks classify it as a type of primitive fungus.