Most cultivars of Vitis vinifera are highly susceptible to several diseases. We sought to develop disease resistant forms of important cultivars by genetic engineering with an endochitinase gene (ThEn-42) from Trichoderma harzianum strain P-1, a magainin gene (mag-2) from Xenopus laevis, and a synthetic magainin gene (MSI-99). Endochitinase-expressing 'Chardonnay' vines were evaluated for resistance in the greenhouse and have been grown outdoors in New York since 2000. Significant resistance to Botrytis cinerea was observed in endochitinase-expressing 'Chardonnay' fruit clusters inoculated at the post-bloom stage. Among magainin-expressing 'Chardonnay' lines, significant reductions in crown gall size were observed. When greenhouse-grown potted vines were needle-inoculated with Agrobacterium vitis, gall size reduction was greater with strain TM4 than with strain CG450. When each strain was grown in vitro in the presence of a range of concentrations of Mag-2 and MSI-99, the same relationship was observed; strain TM4 was more sensitive to antimicrobial peptide inhibition than was strain CG450. Taken together, these results help to illustrate that the reductions observed in gall size are likely due to antimicrobial peptide production. A survey of peptide effects on grapevine pathogens A. vitis, B. cinerea, Uncinula necator and Xylella fastidiosa was undertaken with Mag-2 and MSI-99 plus several other natural and synthetic peptides (PGL, ESF12, ESF39, MsrA3, and Cecropin B). Results will be used to help design a new group of gene cassettes for the development of transgenic disease resistant vines.