Morphological and Molecular Characterization of Septoria apiicola Causing Late Blight on Celery in Temperate Regions

Abstract

Late blight symptoms characterized by small, irregular brown to black necrotic lesions, chlorosis, foliar blighting, and premature defoliation were observed in celery (Apium graveolens) crops cultivated under temperate climatic conditions, resulting in significant yield and quality losses. Diseased leaf samples collected from affected fields were used for pathogen isolation and characterization through morphological, pathogenicity, and molecular approaches. Fungal isolates consistently produced slow-growing colonies with greyish mycelia and abundant pycnidia embedded in host tissue, typical of Septoria apiicola. Microscopic examination revealed hyaline, filiform, multiseptate conidia consistent with diagnostic features of the pathogen. Considerable variation in colony morphology, pycnidial density, and lesion severity was observed among isolates obtained from different celery-growing regions. Pathogenicity assays conducted on healthy celery plants reproduced characteristic late blight symptoms, including leaf spot development and progressive blighting, whereas control plants remained symptom-free. Re-isolation of the fungus from inoculated tissues fulfilled Koch’s postulates, confirming its pathogenic role. Molecular characterization was performed using amplification and sequencing of the internal transcribed spacer (ITS) region along with additional conserved loci relevant for fungal phylogeny. Sequence analysis revealed high similarity with authenticated S. apiicola isolates available in global databases. Phylogenetic analysis grouped field isolates within the S. apiicola clade with strong bootstrap support, confirming species identity and indicating limited genetic variability among regional populations. Disease development was strongly favored by cool temperatures, prolonged leaf wetness, and high humidity conditions typical of temperate production environments. Severe infections resulted in significant reduction in photosynthetic area, plant vigor, and marketable yield.