Selenium Improved Phenylacetic Acid Content in Oilseed Rape and Thus Enhanced the Prevention of <i>Sclerotinia sclerotiorum</i> by Dimethachlon

<i>Sclerotinia sclerotiorum</i> is a broad-spectrum necrotrophic phytopathogen that can infect many plant species worldwide. The application of fungicides is a common measure for controlling <i>Sclerotinia sclerotiorum</i>. Due to the risk of developing resistance to fungicides, it is imperative to find ways to be environmentally friendly and even effective. Using bioactive compounds in plants to reduce the amounts of fungicides has become a clean and sustainable strategy of controlling <i>Sclerotinia sclerotiorum</i>. Our study found that selenium in soil mediated the phenylacetic acid-related metabolic pathway in oilseed rape and reduced the incidence rate of <i>Sclerotinia sclerotiorum</i>. The growth-inhibition rates of <i>Sclerotinia sclerotiorum</i> were observed at 25.82%, 19.67%, and 52.61% for treatments of 0.8 mg·L⁻¹ dimethachlon, 0.1 mg·mL⁻¹ phenylacetic acid, and dimethachlon (0.8 mg·L⁻¹) + phenylacetic acid (0.1 mg·mL⁻¹), respectively. Phenylacetic acid reduced the application amount of dimethachlon and enhanced the inhibition effect for <i>Sclerotinia sclerotiorum</i>. Results also suggested that phenylacetic acid severely damaged the morphological structure, changed the electrical conductivity, and reduced the capacity of acid production and oxalic acid secretion of <i>Sclerotinia sclerotiorum</i> mycelium. Further studies revealed that phenylacetic acid increased the gene-expression level of <i>Ssodc1</i>, <i>Ssodc2</i>, <i>CWDE2</i> and <i>CWDE10</i> in mycelium while decreasing the expression level of <i>SsGgt1</i>, and phenylacetic acid + dimethachlon reduced the relative expression level of <i>SsBil</i>. These findings verified that phenylacetic acid could partially replace the amount of dimethachlon, as well as enhance the prevention of <i>Sclerotinia sclerotiorum</i> by dimethachlon, which provides evidence for developing an environment-friendly method for <i>Sclerotinia sclerotiorum</i> control.