Morphological and Molecular Characterization of Phoma clematidina Causing Black Stem on Clematis Ornamentals

Abstract

Black stem symptoms observed on Clematis ornamental plants were investigated to identify the causal agent and characterize its morphological and molecular features. Field surveys in ornamental nurseries revealed stem lesions, dark necrotic streaks, wilting, and progressive dieback of shoots, leading to reduced plant vigor and marketability under humid cultivation conditions. Fungal isolates obtained from infected stem tissues produced slow-growing, dark olivaceous colonies with abundant pycnidia on potato dextrose agar. Microscopic examination revealed hyaline, oval to cylindrical conidia consistent with Phoma clematidina. Pathogenicity tests conducted on healthy Clematis plants under controlled greenhouse conditions successfully reproduced black stem symptoms, confirming virulence and fulfilling Koch’s postulates. Molecular identification using ITS rDNA and β-tubulin gene sequencing confirmed the isolates as P. clematidina, showing high similarity with reference sequences from global databases. Phylogenetic analysis placed all isolates within a well-supported Phoma clematidina clade, indicating limited intraspecific variation among populations from different nurseries. Observed minor sequence polymorphisms suggest potential adaptation to local environmental conditions and nursery production practices. Disease development was strongly associated with high humidity, poor air circulation, and overhead irrigation, which favored pathogen sporulation and spread. The study highlights the increasing importance of P. clematidina as a pathogen in ornamental Clematis production systems and emphasizes the need for early detection and integrated disease management. Recommended strategies include sanitation, pruning of infected tissues, improved ventilation, and judicious fungicide application to reduce disease incidence and improve plant quality in ornamental nurseries.