This document discusses using the CRISPR-Cas9 system to engineer plant genomes for disease resistance. It describes how CRISPR-Cas9 uses RNA-guided nucleases to introduce targeted double-strand breaks, which can then be repaired through non-homologous end joining or homology-directed repair. This allows for knocking out or editing genes. The document outlines different components of the CRISPR-Cas9 system and various methods for delivering it to plant cells and tissues to modify plant genomes.