Citrus canker is a foliar disease of citrus caused by the bacterium Xanthomonas citri subsp. citri. It is a leaf, fruit, and stem blemishing disease that affects most citrus. Citrus Canker causes premature leaf and fruit drop. It is a highly contagious disease that is easily spread by: Wind borne rain. People carrying the infection on their hands, clothing or equipment. Moving infected or exposed plants or plant parts.