Pythium oligandrum is an Oomycete. It is a parasite of many fungi and other oomycetes including Botrytis

Botrytis

Botrytis may refer to:*Botrytis, the anamorphs of fungi of the genus Botryotinia**Botrytis cinerea, a mold important in wine making*Botrytis, the cauliflower cultivar group of Brassica oleracea...

, Fusarium

Fusarium

Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health...

and Phytophthora

Phytophthora

Phytophthora is a genus of plant-damaging Oomycetes , whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The genus was first described by Heinrich Anton de Bary in 1875...

. It has been licensed as a biocontrol

BioControl

BioControl is a peer-reviewed scientific journal published by Springer Science+Business Media covering all aspects of basic and applied research in biological control of invertebrate, vertebrate, and weed pests, and plant diseases. The journal was established in 1956 as Entomophaga and published by...

 agent in the form of an oospore soil treatment, which reduces pathogen load and concomitant plant disease.
P. oligandrum can grow within the roots of certain plants, including tomato and sugar beet. Production of auxin

Auxin

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

-like substances stimulate plant growth. Defense responses can be induced in the plant, which primes the plant from further infection by pathogenic fungi, oomycetes or bacteria.