We do not currently know of any antonyms for rowanberry.
The noun rowanberry is defined as:
The small spherical fruits of the rowan or European rowan (Sorbus aucuparia), a species of rowan (Sorbus subg. Sorbus), native to most of Europe except for the far south, and northern Asia; the fruits are bitter and inedible if uncooked, but may be used in the making of jellies and jams.