Pathogenicity of Phytophthora palmivora to Edgeworthia chrysantha: Importance of a conducive environment for disease development

In 2023, a homeowner in South Carolina had a group of four young paperbush shrubs, Edgeworthia chrysantha, that were unthrifty and declining and submitted samples to the Clemson University Plant and Pest Diagnostic Clinic for diagnosis. Symptoms included wilting and chlorosis of the foliage and distinct decaying of the roots and root-crowns. A species of Phytophthora was consistently isolated from plant roots and recovered from the soil and debris associated with the root system. Isolates were identified as P. palmivora based on morphological characters, DNA sequences, and phylogenetic characteristics. Koch’s postulates were completed in a greenhouse experiment using two independent trials to confirm pathogenicity of P. palmivora to E. chrysantha, but only after identifying conducive environmental conditions, which were subjecting roots to prolonged periods of flooding. E. chrysantha is native to the Himalaya Mountains and southwest China where plants are adapted to moist environments, so excessive flooding was needed for successful disease development. In the southeastern USA, paperbush is a unique but relatively uncommon landscape plant that prefers moist but well-drained soil. This is the first report of P. palmivora causing root and crown rot on E. chrysantha.