The Collaborative International Dictionary of English v.0.48:

mayhaw \mayhaw\ n.
   The hawthorn tree (Crataegus aestivalis) of the Southern U.
   S. bearing a juicy acid scarlet fruit often used in jellies
   or preserves.

   Syn: summer haw, Crataegus aestivalis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Crataegus aestivalis
    n 1: hawthorn of southern United States bearing a juicy, acidic,
         scarlet fruit that is often used in jellies or preserves
         [syn: mayhaw, summer haw, Crataegus aestivalis]