Crown gall disease can severely limit the growth and productivity of many tree crops and ornamental plants. We have developed a novel crown gall disease resistance strategy that is based upon the silencing of two highly conserved Agrobacterium tumefaciens oncogenes which are required for crown gall development. The tryptophan monooxygenase (iaaM) and isopentenyl transferase (ipt) genes are horizontally transferred from A. tumefaciens to the plant cell, mediating the de novo auxin and cytokinin production that initiates tumorigenesis. By expressing inducers of post-transcriptional gene silencing homologous to iaaM and ipt in plant cells, the corresponding oncogene mRNA transcripts are degraded in planta, providing functional resistance to crown gall disease. This strategy is highly efficacious in English (Persian) walnut (Juglans regia L.), as several transformed lines expressing self-complementary iaaM and ipt transgenes display a highly specific accumulation of small-interfering RNAs and a broad-spectrum suppression of tumorigenesis. Potential applications and limitations of this technology in the future development of crown gall-resistant fruit and nut crop are also discussed.