In plants, several diseases can be produced by different types of pathogens like viruses, fungus, mycoplasma, bacteria, viroids, etc. Citrus canker is a bacterial disease which is caused by bacteria Xanthomonas citri in members of the citrus family (lemon family). The mosaic disease of tobacco is caused by Tobacco Mosaic Virus (TMV). Late blight of potato disease is caused by a fungus named Phytophthora infestans. Potato spindle tuber disease is caused by viroids (infectious low molecular weight RNA).