Disclosed are a decoy peptide which is a partial peptide of ASYMMETRIC LEAVES1 (AS1), the decoy peptide being capable of interacting with βC1 encoded by a betasatellite of a virus belonging to the family Geminiviridae, a nucleic acid encoding the decoy peptide, an expression vector comprising the nucleic acid, a geminivirus disease control agent comprising the decoy peptide or the nucleic acid, a method for controlling a geminivirus disease using the geminivirus disease control agent, a transgenic plant into which the nucleic acid is introduced, and a method for reducing a disease symptom caused by a virus that comprises βC1 and belongs to the family Geminiviridae, using the nucleic acid. Also disclosed is a method for evaluating the degree of reduction in a disease symptom caused by a virus that comprises βC1 and belongs to the family Geminiviridae.