Oomycetes are microorganisms found in diverse ecosystems, including tropical rainforests, oceans, deserts, and even extreme environments, for example in Antarctica (Belonoznikova et al., 2022). They have adapted to survive under a wide range of conditions and play important roles in these ecosystems. Most oomycetes are devastating pathogens of plants and animals. Interestingly, the nonpathogenic oomycete Pythium oligandrum is an effective mycoparasite that colonizes the root ecosystems of diverse plants to induce defense responses against pathogens. As a sterol auxotroph, P. oligandrum relies on external sources of sterols from other host pathogens or plants, similar to other oomycetes. It also produces antimicrobial compounds, competes for nutrients and space with pathogens, and promotes plant growth via producing the auxin precursor tryptamine (Bělonožníková et al., 2022). Since the initial discovery of P. oligandrum in the root system of peas in 1930, it has served as a biological control agent (BCA) for plant disease control and has been widely and successfully used in agricultural production and ecosystem management. The application of P. oligandrum is an environmentally friendly crop disease control strategy, is naturally present in the rhizosphere of many plants, and has shown the potential to change the microbial diversity in soil through its interactions with other microorganisms (Bělonožníková et al., 2022). Meanwhile, it is able to spread into the plant root tissues without inducing any symptoms and also beneficial for the plant to defend various stresses (Le Floch et al., 2003; Bělonožníková et al., 2022). Polyversum® is currently the only biopesticide product registered for use in disease control based on P. oligandrum oospores. It has been registered as a biopesticide in the US and China and as a fungicide in plant protection products by EU authorities. According to different crop types, Polyversum® can be used in various manners, such as foliar application, root dipping/drenching, seed treatment, and watering. Polyversum® has been successfully applied in preventive and emergency applications in the control of fungal diseases in a wide spectrum of crops, such as lawns, fruit trees, and bushes and ornamental plants.