First Report of Alternaria arborescens Causing Leaf Spot on Basil in Protected Cultivation Systems

Abstract

Leaf spot symptoms observed on basil (Ocimum basilicum L.) cultivated under protected cultivation systems were investigated to identify the causal agent and confirm its pathogenicity. Initial field observations revealed small, dark brown to black necrotic lesions on leaves, which progressively enlarged and coalesced under high humidity conditions commonly associated with greenhouse environments. Fungal isolates obtained from infected tissues consistently produced dark, septate mycelium with short, branched conidiophores and muriform, obclavate conidia typical of Alternaria species. Pathogenicity assays conducted on healthy basil plants under controlled greenhouse conditions reproduced characteristic leaf spot symptoms within 7–10 days post inoculation, thereby fulfilling Koch’s postulates. Molecular identification based on ITS rDNA, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and Alternaria major allergen (Alt a 1) gene sequencing confirmed the isolates as Alternaria arborescens. Phylogenetic analysis placed all isolates within the A. arborescens species complex with strong bootstrap support, clearly separating them from closely related Alternaria taxa. This study represents the first confirmed report of A. arborescens causing leaf spot on basil in protected cultivation systems in the region, expanding the known host range of this pathogen. Disease incidence was strongly associated with high relative humidity, dense canopy structure, and limited air circulation within protected environments. The findings highlight the epidemiological risk posed by Alternaria species in intensive basil production systems and underscore the need for early detection and integrated disease management strategies. Implementation of cultural practices, environmental regulation, and targeted fungicidal interventions is recommended to reduce disease severity and prevent further spread in greenhouse herb production systems.