Morphological and Molecular Characterization of Phytophthora ramorum Causing Sudden Oak Death in Forest Nurseries

Abstract

Sudden oak death symptoms observed in forest nursery systems were investigated to determine the causal agent and characterize its morphological and molecular features. Field surveys in nursery-grown and adjacent forest vegetation revealed leaf blight, shoot dieback, stem cankers, and progressive decline in susceptible woody hosts under cool and moist environmental conditions. Oomycete isolates obtained from infected tissues produced fast-growing, coenocytic mycelium on selective media, consistent with Phytophthora species. Microscopic examination revealed ovoid to semi-papillate sporangia with internal proliferation and occasional chlamydospore formation, characteristic of Phytophthora ramorum. Pathogenicity tests conducted on healthy nursery seedlings under controlled humidity and temperature conditions successfully reproduced foliar blight and stem necrosis 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. ramorum, showing high similarity with global reference isolates. Phylogenetic analysis placed all isolates within a well-supported P. ramorum clade, indicating low genetic diversity and a relatively conserved population structure. Disease development was strongly associated with prolonged leaf wetness, high humidity, and moderate temperatures, which favor sporangial production and zoospore dispersal. The study highlights the significant threat posed by P. ramorum in forest nursery production systems and its potential role in forest ecosystem decline. These findings emphasize the need for strict phytosanitary measures, early molecular diagnostics, and integrated disease management strategies, including sanitation, water management, and monitoring of plant material movement to prevent pathogen spread.