Dispensers should trigger an event similar to BlockFertilizeEvent, but with the block which performed the action (the dispenser) instead of a player.

Dispensers can use bone meal on grass blocks as well as nylium and generate plants such as grass, flowers, fungus, roots etc. It's the same behaviour as if a player would do it. We can easily check all the affected blocks when players use bone meal by listening for a BlockFertilizeEvent, though there's no relevant event triggered when a dispenser fertilizes a block.