The Antifungal Activity and Mechanism of Dehydroabietic Acid Against Alternaria alternata Causing Poplar Leaf Spot

Dehydroabietic acid (DHA) is a secondary metabolite isolated from rosin, which has certain antifungal activity, but its inhibitory effects against <i>Alternaria alternata</i> are unclear. In the present study, we found that DHA inhibited the mycelial growth of <i>A. alternata</i>, <i>Botrytis cinerea</i>, <i>Valsa mali</i>, <i>Pestalotiopsis neglecta</i>, and <i>Fusarium oxysporum</i> in a concentration-dependent manner, with the best inhibitory effect against <i>A. alternata</i>. Moreover, DHA can also inhibit the spore germination of <i>A. alternata</i>. Then, in vivo inoculation experiments showed that the leaf lesions of <i>Populus alba</i> gradually decreased with the increase in DHA concentration. The disease of <i>P. alba</i> leaves inoculated with <i>A. alternata</i> was not obvious after treatment with 800 mg/L DHA. The scanning electron microscopy showed that the mycelial morphology was abnormal, with crinkles and depressions. Meanwhile, the relative conductivity, soluble protein content, malondialdehyde content and hydrogen peroxide content of <i>A. alternata</i> were significantly increased after DHA treatment, which affected the integrity of the cell membrane and increased the permeability of <i>A. alternata</i>, resulting in a large leakage of intracellular substances, exacerbating the degree of lipid peroxidation of the cell membrane of <i>A. alternata</i> and causing oxidative damage to cells. The enzyme activity assay showed that treatment with 56.015 mg/L (EC₅₀) DHA significantly reduced the activities of antioxidant enzymes (superoxide dismutase, catalase, peroxidase) and cell-wall-degrading enzymes (endoglucanase, polygalacturonase, pectin lyase) in <i>A. alternata</i> (<i>p</i> < 0.05), resulting in a decrease in the activity of pathogenic fungi, as well as a reduction in the ability of the <i>A. alternata</i> to degrade the cell wall of the host plant, which led to a decrease in the ability of the <i>A. alternata</i> to infest the host plant. Moreover, the decrease in the relative expression of defense-related enzyme genes (<i>AaSOD, AaPOD, AaCAT</i>) and pathogenicity-related enzyme genes (<i>AaPL, AaPG</i>) was consistent with the enzyme activity results. Thus, the present study revealed the fungicidal activity and mechanism of DHA against <i>A. alternata</i> and the potential of DHA to be developed as a plant-derived antifungal agent was established.