An isolated nucleic acid molecule comprising one of a nucleotide sequence set forth in SEQ ID NO: 1 and a nucleotide sequence set forth in SEQ ID NO: 2, the nucleotide sequence complementary to a Rpg3Dun gene sequence. A method for screening a plurality of plant cultivars to identify individual cultivars containing Rpg3Dun gene sequences, comprising the steps of: (a) collecting a biomass sample from individual cultivars; (b) preparing a nucleic acid preparation from each biomass sample; (c) contacting each nucleic acid preparation with at least one of the nucleic acid molecules set forth in SEQ ID NO: 1 and SEQ ID NO: 2, (d) performing a PCR amplification reaction therein; and (e) assessing each amplified sample to detect hybridization with one of the nucleic acid molecules set forth in SEQ ID NO: 1 and SEQ ID NO: 2. A hybridization reaction indicates the cultivar contains therein Rpg3Dun gene sequences.
