Pathogenicity and Cultural Variability of Diaporthe spp. Causing Stem Canker and Seed Decay on Sunflower

Abstract

Stem canker and seed decay symptoms associated with Diaporthe species were frequently observed in sunflower-growing areas, causing significant reductions in plant vigor, seed quality, and overall crop productivity. Diseased stem and seed samples collected from infected fields were used for isolation and characterization of the causal pathogens. Fungal isolates exhibited considerable variation in colony morphology, pigmentation, mycelial texture, growth rate, and sporulation patterns when cultured on different nutrient media. Microscopic examination revealed characteristic alpha and beta conidia typical of the genus Diaporthe. Pathogenicity assays performed on healthy sunflower plants and seeds demonstrated substantial differences in virulence among isolates, with highly aggressive isolates inducing severe stem lesions, tissue necrosis, wilting, and extensive seed decay. Re-isolation of the pathogen from artificially inoculated tissues confirmed the pathogenic nature of the isolates and fulfilled Koch’s postulates. Cultural variability studies indicated that temperature and growth medium significantly influenced colony development and sporulation behavior of the pathogen. Isolates showing rapid mycelial growth also produced greater disease severity, suggesting a positive association between cultural characteristics and pathogenic potential. Environmental conditions characterized by moderate temperature and high humidity favored infection and disease progression under both laboratory and greenhouse conditions. The observed diversity among Diaporthe isolates indicates the presence of genetically and biologically variable populations capable of adapting to diverse agroecological environments. Accurate characterization of pathogenic and cultural variability is essential for disease diagnosis, screening of resistant sunflower germplasm, and formulation of effective disease management strategies.