Pythium myriotylum is a destructive soil-borne phytopathogen, causing yield losses in ginger and many other crops. Environmental and regulatory concerns drive the need to find biological alternatives to conventional pesticides used to manage P. myriotylum. Trans-cinnamic acid (TCA) alone, and fermentation broth from strains of symbiotic bacteria of eight species of entomopathogenic nematodes alone, and in combination with TCA, were tested for their effect on zoospore germination and mycelial growth of P. myriotylum. TCA significantly inhibited mycelial growth. Fermentation broths from seven of the eight strains of symbiotic bacteria directly inhibited mycelial growth, especially those isolated from Steinernema feltiae (strain SN) and S. riobrave (strain 7–12). Moreover, adding TCA significantly increased the inhibitory effect on mycelial growth of the fermentation broths of seven of the strains tested. All bacteria fermentation broths showed inhibitory effects on zoospore germination. However, TCA alone was not inhibitory to zoospore germination but was inhibitory to mycelial growth. Antimicrobial effects on mycelial growth and zoospore germination were proportional to the concentration of symbiotic bacteria isolated from S. feltiae (strain SN). These results show that TCA and symbiotic bacteria of entomopathogenic nematodes may have potential to provide biorational control of P. myriotylum. © 2020, Koninklijke Nederlandse Planteziektenkundige Vereniging.