Pathogenicity and Virulence of Phomopsis viticola Causing Phomopsis Cane and Leaf Spot on Grapevine

Abstract

Phomopsis cane and leaf spot is an economically important grapevine disease responsible for cane lesions, leaf necrosis, shoot blight, and reduction in vine productivity under humid viticultural conditions. Diseased grapevine tissues exhibiting characteristic symptoms were collected from commercial vineyards to investigate the pathogenicity and virulence variability of Phomopsis viticola isolates. Fungal cultures obtained from infected canes and leaves produced whitish to gray colonies with abundant pycnidial structures and alpha conidia typical of the pathogen. Pathogenicity assays were conducted on healthy grapevine shoots, leaves, and detached canes under controlled environmental conditions to evaluate symptom development and aggressiveness of different isolates. Inoculated tissues developed dark elongated lesions, necrotic spotting, cane cracking, and shoot blight symptoms similar to those observed under field conditions, whereas control plants remained symptom-free. Re-isolation of the fungus from infected tissues fulfilled Koch’s postulates and confirmed the pathogenic nature of the isolates. Considerable variability in virulence was detected among isolates, with highly aggressive isolates producing rapid lesion expansion and severe tissue necrosis compared with moderately virulent strains. Environmental conditions characterized by prolonged moisture and moderate temperature significantly enhanced infection efficiency and disease severity. Virulence differences among isolates suggest the existence of pathogenic diversity within P. viticola populations infecting grapevine. The pathogen’s ability to colonize young shoots and overwinter in infected canes contributes to recurring disease outbreaks in vineyards. Understanding pathogenic variability and aggressiveness of P. viticola isolates is essential for accurate disease forecasting, screening of resistant grapevine cultivars, and development of effective integrated disease management strategies.