Morphological and Molecular Identification of Alternaria dauci Causing Leaf Blight on Carrot in Irrigated Production

Abstract

Leaf blight symptoms characterized by dark brown to black necrotic lesions, chlorosis, foliar curling, and progressive defoliation were observed in carrot (Daucus carota) fields under irrigated production systems, leading to significant reductions in photosynthetic activity and root yield. Diseased leaf samples collected from affected fields were used for pathogen isolation and characterization through morphological and molecular approaches. Fungal isolates consistently produced dark olivaceous colonies with septate, branched hyphae and abundant conidia typical of Alternaria dauci. Microscopic examination revealed elongated, muriform, and beaked conidia consistent with diagnostic characteristics of the pathogen. Considerable variation in colony morphology, sporulation intensity, and lesion development was observed among isolates obtained from different carrot-growing regions. Pathogenicity assays conducted on healthy carrot plants reproduced characteristic leaf blight symptoms, including necrotic spotting and extensive leaf blighting, whereas control plants remained symptom-free. Re-isolation of the fungus from inoculated tissues fulfilled Koch’s postulates, confirming its pathogenic role. Molecular identification was performed using amplification and sequencing of the internal transcribed spacer (ITS) region and additional gene loci including glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and translation elongation factor 1-α (TEF1-α). Sequence analysis revealed high similarity with authenticated A. dauci isolates available in global databases. Phylogenetic analysis grouped field isolates within the A. dauci clade with strong bootstrap support, confirming species identity and revealing limited genetic variability among regional populations. Disease development was strongly favored by warm temperatures, prolonged leaf wetness, and frequent irrigation, which enhanced spore germination and infection efficiency. Severe infections resulted in significant reduction in leaf area, root development, and overall yield quality.