Incidence of Pierce's disease (PD), caused by the bacterium Xylella fastidiosa, continues to increase in many vineyards in California due to the establishment and spread of the vector Homalodisca coagulata (Say) (glassy winged sharpshooter, GWSS). Commercially available materials, including kaolin, harpin, and imidacloprid, were evaluated in field experiments for their effect PD development. PD incidence was 6% in plants treated with kaolin and 14% in plots treated with conventional insecticides. Incidence of PD in plants treated with harpin were 13%, 7%, and 6% when 160, 320, and 460 g harpin a.i ha(-1) were used, respectively. PD incidence in untreated control plants was 19%. Plants treated with harpin grew more vigorously than plants treated with kaolin, imidacloprid, and control plants not treated with chemicals or exposed to GWSS. Higher GWSS mortality rates were observed on plants treated with kaolin. Results from field studies as validated by a greenhouse study show that kaolin, harpin, and imidacloprid would be useful in reducing the development of PD in grape. (c) 2006 Elsevier Ltd. All rights reserved.