Multiple choice Identify the mismatched pair of disease and its causative agent. Black root rot of cotton : Thielaviopsis basicola Black rot of crucifers : Xanthomonas campestris Gray mold of hemp : Agrobacterium tumefaciens Downy mildew of mustard : Peronospora parasitica Bottom rot of lettuce : Rhizoctonia solani Reveal answer Fill a bubble to check yourself C Correct answer Explanation Correct answer. The most important disease of hemp is gray mold, caused by the fungus Botrytis cinerea. Crown gall of hemp is caused by Agrobacterium tumefaciens.