Intraspecific Variation and Host Range of Diaporthe helianthi on Sunflower Genotypes with Differing Susceptibility

Abstract

Intraspecific variation and host range of Diaporthe helianthi, the causal agent of sunflower (Helianthus annuus L.) stem canker, were investigated across sunflower genotypes with differing levels of susceptibility. Field surveys and isolate collections were conducted from symptomatic plants exhibiting stem lesions, cortical necrosis, and premature plant senescence under field conditions. Fungal isolates were characterized based on colony morphology, pigmentation, pycnidial development, and conidial features typical of Diaporthe species. Pathogenicity assays performed on sunflower genotypes with known differential resistance responses confirmed variable aggressiveness among isolates, indicating significant intraspecific variation in virulence. Highly aggressive isolates caused extensive stem cankers and rapid tissue necrosis, whereas less virulent isolates induced restricted lesion development, suggesting quantitative variation in pathogenic fitness. Molecular characterization using ITS rDNA and translation elongation factor 1-alpha (TEF1-α) gene sequencing confirmed all isolates as D. helianthi, with phylogenetic analysis revealing distinct sub-clusters corresponding to differences in virulence profiles rather than geographic origin. Host range assessment demonstrated primary specialization on sunflower, with limited pathogenicity observed on closely related Helianthus species, indicating a relatively narrow host spectrum. Genotype-specific reactions highlighted significant variation in host susceptibility, with resistant genotypes showing restricted pathogen colonization and reduced disease severity. Environmental conditions, particularly high humidity and moderate temperatures, enhanced infection efficiency and disease progression. The study underscores the dynamic nature of D. helianthi populations and their adaptive variability within sunflower agroecosystems.