Screening and histopathological characterization of sunflower germplasm for resistance to Macrophomina phaseolina

Twenty-two sunflower germplasms were screened for resistance to Macrophomina phaseolina to select parental genetic resources useful for the development of charcoal rot-resistant sunflower cultivars. Potting soil inoculated with pathogen (10 mL pot^-1, 2 × 10⁵ sclerotia mL^-1) sown with sunflower seeds was examined for disease severity index (%), disease incidence (%), mortality (%), and growth inhibition index (%) 90 d after inoculation. None of the germplasm was disease-free; four were found to be resistant, five moderately resistant, six moderately susceptible, five susceptible, and two highly susceptible. All inoculated plants exhibited disease symptoms both externally and internally. Mild to severe symptoms included brown lesions on aboveground plant, pith disintegration in stem, and shredded appearance of tap root. Histopathological features exposed different colonization mechanism of the pathogen in the resistant and susceptible cultivars. Physical blockage, tissue disintegration, blackening and rupturing of cortical, pith and vascular regions by fungal mycelia, and sclerotia and pycnidia causing large spaces in the center of stem rendered it a hollow structure in all susceptible germplasm. However, stem and root tissues of the resistant germplasm indicated local infection restricted to few cells. This suggested expression of true resistance genes in resistant germplasm. Therefore, the sunflower lines resistant to the M. phaseolina infection are potential genetic resources for the development of quality sunflower cultivars resistant to charcoal rot disease.