Morphological and Molecular Characterization of Alternaria porri Causing Purple Blotch Disease on Onion and Leek

Abstract

Purple blotch disease causing severe foliar damage was frequently observed on onion and leek crops cultivated under humid and moderate temperature conditions, resulting in substantial reductions in bulb quality and marketable yield. Symptomatic leaves exhibiting elongated purple lesions with concentric rings and chlorotic margins were collected from affected fields for pathogen isolation and identification. Fungal isolates consistently produced dark olivaceous colonies with abundant sporulation on culture media. Microscopic examination revealed septate conidiophores and characteristic muriform conidia with elongated beaks, corresponding to morphological descriptions of Alternaria porri. Considerable variability in colony growth, pigmentation, and sporulation intensity was observed among isolates obtained from different host plants and geographic locations. Pathogenicity tests conducted on healthy onion and leek plants reproduced typical purple blotch symptoms, including necrotic lesions and progressive leaf blighting, confirming the pathogenic nature of the isolates. Re-isolation of the pathogen from infected tissues fulfilled Koch’s postulates. Molecular characterization was performed using amplification and sequencing of the internal transcribed spacer (ITS) region and additional conserved gene loci associated with species differentiation. Sequence analysis demonstrated high similarity between the obtained isolates and authenticated A. porri sequences available in public databases. Phylogenetic analysis clustered the isolates within the A. porri lineage with strong bootstrap support, confirming species identity and genetic relatedness among isolates. Environmental conditions characterized by prolonged leaf wetness and high relative humidity significantly favored disease development and pathogen dissemination. The combined use of morphological and molecular techniques provided reliable identification of the pathogen and improved discrimination from closely related Alternaria species infecting Allium crops.