Jordan Matsuo algebras over fields of characteristic 3

The Matsuo algebra associated with a connected Fischer space is shown to be a Jordan algebra over a field of characteristic 3 if and only if the Fischer space is isomorphic to either the affine space of order 3 or the Fischer space associated with the symmetric group. The proof uses a characterization of the affine spaces of order 3 and equivalence of Jordan and linearized Jordan identities over a field of characteristic 3 in case the algebra is spanned by idempotents.