We don't have synonyms for decurrent yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are current, recurrent and deterrent.
(adj) :
pertaining to plant parts that extend downward, most often applied to leaf blades that partly wrap or have wings around the stem or petiole and extend down along the stem; pertaining to lamellae (the gills of a mushroom) that are broadly attached and extend down the stipe of the mushroom
"in this group of mushrooms, the attachment of the gills to the stipe is decurrent."
(adj) :
pertaining to plant parts that extend downward, most often applied to leaf blades that partly wrap or have wings around the stem or petiole and extend down along the stem; pertaining to lamellae (the gills of a mushroom) that are broadly attached and extend down the stipe of the mushroom
"in this group of mushrooms, the attachment of the gills to the stipe is decurrent."