Antifungal Effect of Saussurea lappa Roots Against Phytopathogenic Fungi and Resulting Morphological and Ultrastructural Changes

Abstract

Phytopathogenic fungi infect plants during growth, development, and harvest and their products during postharvest storage and transport. Fungicides are applied indiscriminately to combat these pathogens, posing a serious threat to humans, animals, and the environment. Some filamentous fungi also produce mycotoxins, which pose further problems. In this study we evaluated the antifungal activity of Saussurea lappa root extracts against several potent phytopathogenic fungi (Fusarium solani, F. moniliforme, F. oxysporum, Helminthosporium sativum, and Macrophomina phaseoloni). Various solvent extracts (methanol, ethanol, dichloromethane, ethyl acetate, and water) of the S. lappa root were used. We found that methanol, ethanol, and dichloromethane extracts had potent antifungal activity against all tested isolates. M. phaseolina showed the highest inhibition (89%) with a 5% methanol extract of S. lappa, and severe damage to the morphology and ultrastructure of its cells was also observed. SEM micrographs revealed that cells treated with S. lappa extracts had distorted, peeled, twisted, and shortened mycelium and peeled, completely distorted, and underdeveloped conidia. Moreover, the dichloromethane S. lappa extract (5%) arrested conidial development completely. Similarly, TEM studies showed that treated cells lacked clear demarcations between the cell membrane and cell wall, and had heavy vacuolation, a detached cytoplasm, ruptured and distorted mycelium and conidia, and a clear deposition of electron dense granules on the outer surface. GC-MS and FTIR studies indicated the presence of important bioactive compounds and functional groups in the extracts. Our results indicate that S. lappa roots are a rich source of bioactive compounds that can affect the integrity, stability, and structure of the fungal cell membrane, ultimately leading to cell death. Thus, root extracts could serve as eco-friendly, safe alternatives to the fungicides that are applied at both the harvest and postharvest stages.