大麦散黑粉菌（学名：Ustilago nuda）是属于黑粉菌目黑粉菌科黑粉菌属的一种真菌，寄生在大麦属植物上，可引起大麦散黑穗病。该种分布于全世界各地。

Loose smut of barley is caused by Ustilago nuda. It is a disease that can destroy a large proportion of a barley crop. Loose smut replaces grain heads with smut, or masses of spores which infect the open flowers of healthy plants and grow into the seed, without showing any symptoms. Seeds appear healthy and only when they reach maturity the following season is it clear that they were infected. Systemic fungicides are the major control method for loose smut.