Evaluating pistachio rootstock tolerance to Phytophthora root and crown rot

Pistachio (<i>Pistacia vera</i>) is one of the most widely cultivated nut crops in California with approximately 115,000 hectares of bearing trees. In recent years, Phytophthora root and crown rot has emerged as a severe and widespread disease of pistachio, particularly in orchards established on the industry-standard UCBI rootstock. To promote sustainable management strategies against Phytophthora root and crown rot, this study investigated the relative tolerance of three commercial rootstocks [UCBI (<i>P. atlantica</i> × <i>P. integerrima</i>), PGI (<i>P. integerrima</i> x <i>P. integerrima</i>) and Platinum (<i>P. integerrima</i> × <i>P. atlantica</i>)] to <i>Phytophthora niederhauserii</i>, <i>P. mediterranea</i>, and <i>P.</i> taxon walnut. Stem and soil inoculation experiments were conducted and lesion lengths on rootstocks stems, root loss, and crown rot development were assessed to determine differences in rootstock susceptibility. Platinum consistently exhibited significantly smaller lesion lengths, a lower percentage of root loss, and little to no crown rot development relative to UCBI and PGI rootstocks. In contrast, UCBI and PGI were confirmed to be highly susceptible to Phytophthora root and crown rot across all experiments. These findings demonstrate clear differences in rootstock tolerance and support the use of Platinum rootstocks in areas prone to Phytophthora root and crown rot infections caused by <i>P. niederhauserii</i>, <i>P. mediterranea</i>, and <i>P.</i> taxon walnut.