Abstract: .alpha.-D-Galacturonide derivatives, salts thereof, and plant disease control agents containing the derivative or the salt as an active component. The derivatives are represented by the formula ##STR1## wherein R.sup.1 is alkyl having 1 to 8 carbon atoms, alkenyl having 2 to 8 carbon atoms, cycloalkyl having 3 to 8 carbon atoms, aryl or a 5-membered or 6-membered unsaturated heterocyclic ring residue, R.sup.2 is a hydrogen atom or methyl, A is an oxygen atom or sulfur atom, and the hydroxyl attached to the carbon atom at the 4-position of the galacturonide skeleton and the hydrogen atom attached to the carbon atom at the 5-position may be eliminated to form a double bond between the carbon atoms at the 4- and 5-positions.The compounds represented by formula (I) are prepared by the processes represented by reaction schemes 1 to 3 as described below. ##STR2## wherein R.sup.3 is a protective group for hydroxyl group.