Etiology and Pathogenicity of Botrytis squamosa Causing Leaf Blight on Onion in Irrigated Production Systems

Abstract

Leaf blight symptoms characterized by elongated necrotic lesions, chlorosis, leaf tip dieback, and premature senescence were frequently observed in onion fields cultivated under irrigated production systems. Diseased leaf samples collected from affected fields were used to determine the etiology and pathogenic characteristics of the associated fungal pathogen. Fungal isolates consistently produced grayish colonies with abundant aerial mycelia and dark sporulating structures typical of Botrytis squamosa. Microscopic examination revealed branched conidiophores and single-celled conidia corresponding to morphological descriptions of the pathogen. Pathogenicity assays conducted on healthy onion plants under controlled environmental conditions reproduced characteristic leaf blight symptoms, including necrotic spotting and progressive tissue collapse, whereas control plants remained symptom-free. Re-isolation of the fungus from inoculated tissues fulfilled Koch’s postulates and confirmed the pathogenic role of the isolates. Disease severity increased significantly under conditions of prolonged leaf wetness, moderate temperature, and high relative humidity commonly associated with irrigated onion cultivation. The pathogen exhibited rapid colonization of senescing leaf tissues and produced abundant conidia that facilitated secondary spread within the crop canopy. Infected plants showed substantial reduction in photosynthetic area and bulb development, leading to decreased marketable yield and quality. Variability in aggressiveness among isolates suggested differences in pathogenic potential within the regional pathogen population. Continuous irrigation and dense crop canopy conditions created favorable microclimatic environments for disease establishment and epidemic development. Accurate identification and pathogenicity assessment of B. squamosa are essential for improving disease diagnosis and implementation of effective integrated management practices.