Morphological and Molecular Characterization of Phytophthora cinnamomi Causing Root Rot on Avocado

Abstract

Root rot symptoms observed in avocado (Persea americana Mill.) orchards were investigated to determine the causal agent and characterize its morphological and molecular attributes. Field surveys revealed progressive decline of infected trees, including leaf chlorosis, canopy thinning, reduced vigor, and fine root necrosis under poorly drained soil conditions. Oomycete isolates obtained from diseased root tissues produced fast-growing, coenocytic mycelium on selective media, consistent with Phytophthora species. Microscopic examination revealed non-papillate to semi-papillate sporangia, internal proliferation, and abundant chlamydospore formation characteristic of Phytophthora cinnamomi. Pathogenicity assays conducted on healthy avocado seedlings under controlled conditions successfully reproduced root rot symptoms, confirming virulence and fulfilling Koch’s postulates. Molecular identification using ITS rDNA and cytochrome oxidase subunit I (cox1) gene sequencing confirmed the isolates as P. cinnamomi, showing high similarity with global reference sequences. Phylogenetic analysis placed all isolates within a well-supported P. cinnamomi clade, indicating low genetic variability among populations from different orchards. Disease development was strongly associated with high soil moisture, poor drainage, and warm temperatures, which favored zoospore production and root infection. The study highlights the significant threat posed by P. cinnamomi to avocado production systems and emphasizes its role as a major soilborne pathogen responsible for tree decline. These findings underscore the need for integrated disease management strategies, including improved drainage, use of resistant rootstocks, soil sanitation, and careful irrigation management to reduce disease incidence and ensure sustainable avocado cultivation.