Virulence and Molecular Diversity of Alternaria brassicicola Isolates Causing Dark Leaf Spot on Canola

Abstract

Dark leaf spot caused by Alternaria brassicicola is one of the most destructive foliar diseases affecting canola production worldwide, leading to significant reductions in seed quality and yield. The present study investigated the virulence and molecular diversity of A. brassicicola isolates collected from major canola-growing regions to understand the variability among pathogen populations and their potential impact on disease management strategies. Diseased leaf samples exhibiting typical dark leaf spot symptoms were collected, and fungal isolates were purified and identified based on morphological and molecular characteristics. Pathogenic variability among isolates was evaluated through artificial inoculation on susceptible canola cultivars under controlled environmental conditions. Molecular diversity was assessed using polymerase chain reaction (PCR)-based molecular markers and sequence analysis to determine genetic relationships among isolates. Considerable variation in colony morphology, sporulation, lesion development, and aggressiveness was observed among the isolates, indicating the presence of diverse virulence patterns within the pathogen population. Molecular analysis further revealed substantial genetic polymorphism, with several isolates forming distinct clusters based on geographic origin and pathogenic behavior. Highly virulent isolates produced extensive necrotic lesions and rapid disease progression, whereas less virulent isolates induced comparatively mild symptoms. The findings demonstrate a significant association between molecular diversity and pathogenic variability in A. brassicicola populations infecting canola. The study highlights the evolutionary adaptability of the pathogen and emphasizes the necessity for continuous monitoring of pathogen diversity for the development of durable disease-resistant canola cultivars and effective integrated disease management strategies.