Biodiversity of Rhizosphere Fungi from Suaeda glauca in the Yellow River Delta and Their Agricultural Antifungal and Herbicidal Potentials

<i>Suaeda glauca</i> is a typical halophyte distributed in coastal and inland zones of the Yellow River Delta. Its rhizosphere soil is a potential source for exploring various fungi and their metabolites. In this study, the rhizosphere fungal community of <i>S. glauca</i> was evaluated with high-throughput sequencing, suggesting that it was tightly associated with seasonal variation and soil physicochemical factors. The fungal diversity at the genus level when sampling in May was better than that in July and October. The physicochemical factors TK and TP exerted relatively positive effects on the fungal diversity, while SOM, pH and TDS exhibited negative ones. Using the dilution plating method, 55 cultivable fungal strains were further isolated from the rhizosphere soil of <i>S. glauca</i>, in which <i>Aspergillus</i> and <i>Penicillium</i> were the dominant ones. A total of 47 and 20 strains showed antifungal and herbicidal activity, respectively. Finally, bioassay-guided isolation from the representative strain <i>A. tabacinus</i> GD-25 obtained three polyketides (<b>1</b>-<b>3</b>) and one diphenyl ether (<b>4</b>). <b>1</b> (sydonic acid) and <b>4</b> (diorcinol) greatly inhibited mycelial vitality of <i>Bortrytis cinerea</i>, with IC₅₀ values of 75.4 and 67.4 mg/L, respectively. In addition, 50 μg/mL of <b>4</b> could almost inhibit seedling growth of <i>Echinochloa crusgalli</i>.