Evaluation of Rhyme Fungicide for Controlling Cotton Root Rot of Pistachio in Arizona, 2025

Cotton root rot (commonly known as Texas root rot, Ozonium root rot, Phymatotrichopsis root rot, or Phymatotrichum root rot), caused by Phymatotrichopsis omnivora, is a major soil-borne disease of pistachio in Arizona. The fungus produces necrotic lesions on the bark of the root and crown and destroys root systems, resulting in rapid tree decline and mortality. This report evaluates Rhyme fungicide for managing cotton root rot. The field trial was conducted in 2025 on 3-year-old Pistacia vera ‘Golden Hills’ trees on Platinum rootstocks. This study was initiated as an IR-4 priority. IR-4 facilitates regulatory approval of sustainable pest management technology for specialty crops and specialty uses to promote public well-being. The results showed that four monthly applications of Rhyme reduced mortality from 90% to 10% in inoculated trees. This study provided efficacy and crop safety data to support the manufacturer’s registration of Rhyme fungicide for pistachios. Rhyme will be available to pistachio growers to manage cotton root rot and should reduce tree and yield losses.