Oomycota Are Not Fungi, And That Mistake Is Costing You Data

By Dr. Franziska Böwer, Scientist at Soilytix | Updated June 2026

Oomycota are not fungi. They look like fungi, grow like fungi, and even share a German nickname (Eierpilze, literally "egg fungi"), but they belong to a completely different domain of life. And if your soil sequencing workflow relies on 16S and ITS alone, you are systematically missing them.

TLDR

• Oomycota are protists, not fungi. They belong to the stramenopile lineage, closer to brown algae than to Aspergillus
• ITS sequencing targets true fungi and does not reliably capture oomycetes like Phytophthora or Plasmodiophora
• 16S targets bacteria only. Oomycetes are invisible there too
• Clubroot (Plasmodiophora brassicae) is present in 35 European countries and causes yield losses of up to 60% in oilseed rape. Most standard sequencing workflows cannot detect it
• 18S metabarcoding is the only standard method that covers the full eukaryotic picture, including oomycetes, in a single assay

Why does everyone think oomycetes are fungi?

The confusion is understandable and deeply embedded in the language. In German, Oomycota are called Eierpilze (egg fungi), a name that stuck from classical morphology when researchers observed hyphal growth and assumed fungal ancestry. In English, field guides and plant pathology textbooks still routinely group them under "fungal pathogens" for practical convenience. Phytophthora infestans, responsible for the Irish potato famine, was studied as a fungus for over a century before molecular phylogenetics placed oomycetes firmly in the stramenopiles, a lineage that also includes diatoms and brown algae.

The cell wall tells the real story. True fungi build their walls from chitin. Oomycetes use cellulose, the same structural polymer as plants. This single biochemical difference explains why fungicides targeting chitin synthesis don't work against Phytophthora or Pythium. It also explains why ITS primers, designed around conserved regions in true fungal ribosomal DNA, fail to consistently amplify oomycete sequences. The naming legacy is a scientific liability hiding in plain sight.

What does 16S and ITS actually miss?

In practice: quite a lot, and often exactly what someone is looking for.

We encountered this directly at Soilytix. A client came to us having run standard 16S and ITS sequencing on their soil samples. At the end of the analysis they asked a straightforward question: did we find Phytophthora or Plasmodiophora? The honest answer was that we hadn't looked. Neither organism shows up reliably in a 16S bacterial profile or an ITS fungal profile. Plasmodiophora brassicae, the clubroot pathogen that devastates brassica crops across northern Europe, is not a fungus. It is a phytomyxean protist. Phytophthora is an oomycete. Both require 18S sequencing to detect with any confidence.

The economic stakes are not small. Clubroot is present in 35 European countries and is considered a pandemic disease, causing average yield losses of around 15% worldwide. In heavily infested fields, losses range from 30% up to total crop failure. Field surveys in oilseed rape across Europe document losses between 5 and 60% depending on inoculum load and cultivar susceptibility (Wallenhammar, 1998; McGrann et al., 2016, via doi:10.3389/fagro.2020.590908). In southern Sweden alone, P. brassicae DNA was detected in 60% of oilseed rape fields surveyed, with nearly half of those fields carrying spore loads sufficient to cause more than 10% yield loss in susceptible cultivars (Wallenhammar et al., 2016, doi:10.3390/plants5020021).

These are not exotic edge cases. They are common field conditions across northern and central Europe, and they are invisible to the two most widely used soil sequencing methods.

What actually works, and what the trade-offs are

Two 18S metabarcoding approaches are worth knowing about, and they serve different purposes.

The first is a short-amplicon pan-eukaryote approach targeting the 18S V9 region. Because the amplicon is small it performs well on degraded environmental DNA, which is the reality for most soil samples. It captures the full eukaryotic community in a single assay: fungi, oomycetes, protists, micro-algae, and more. Resolution lands at genus or family level for most groups, though for oomycetes specifically it can be patchy depending on reference database coverage for the lineage in question. For screening and community-level questions (is Phytophthora present, what is the general protist pressure in this field) it is more than sufficient.

The second is a dedicated protist metabarcode targeting the 18S V5-V9 region, producing an amplicon of around 900 base pairs. The longer read gives substantially better taxonomic resolution for protist groups, including oomycetes, at the cost of being more sensitive to DNA quality. If the diagnostic question is specific (which Pythium species, is this Plasmodiophora brassicae or a closely related species) the longer amplicon is the right tool.

In practice the choice depends on the question. Community screening and baseline profiling: short V9. Targeted pathogen identification or high-resolution protist work: V5-V9. Neither replaces the other, and neither is ITS.

Does this mean ITS is useless?

No. ITS remains the gold standard for true fungal community profiling. It has deeper reference databases for fungi and higher taxonomic resolution within the fungal kingdom than 18S V9. The point is not to retire ITS but to be clear about what it covers and what it doesn't. For any study where oomycete or protist pathogens are a diagnostic question (clubroot in brassicas, late blight pressure, water mould activity in irrigated fields) 18S needs to be in the assay stack alongside it.

The broader point is this: the taxonomic misclassification of oomycetes is not just an academic footnote. It is baked into sequencing conventions, diagnostic assumptions, and product recommendations across the industry. Protist diversity, including oomycetes, cercozoans, flagellates, and others, is being systematically missed by the two most widely used soil sequencing methods. That gap has consequences for how trials are interpreted, how pathogens are identified, and how crop protection decisions get made. 18S metabarcoding closes it.

Working on a soil health study or trial where oomycete or protist detection matters? We are happy to talk through which approach fits your question. [Get in touch with the Soilytix team.]

References

• Wallenhammar AC et al. (2016). Quantification of Plasmodiophora brassicae using a DNA-based soil test facilitates sustainable oilseed rape production. Plants 5(2):21. doi:10.3390/plants5020021
• Dunker S et al. (2020). A global survey on diseases and pests in oilseed rape. Frontiers in Agronomy. doi:10.3389/fagro.2020.590908
• Czubatka-Bieńkowska A, Kaczmarek J, Marzec-Schmidt K, Nieróbca A, Czajka A, Jędryczka M (2020). Country-wide qPCR based assessment of Plasmodiophora brassicae spread in agricultural soils and recommendations for the cultivation of Brassicaceae crops in Poland. Pathogens 9(12):1070. doi:10.3390/pathogens9121070
• Beakes GW, Glockling SL, Sekimoto S (2012). The evolutionary phylogeny of the oomycete "fungi". Protoplasma 249(1):3-19. doi:10.1007/s00709-011-0269-2
• Kageyama K (2014). Molecular taxonomy and its application to ecological studies of Pythium species. Journal of General Plant Pathology 80:314-326. doi:10.1007/s10327-014-0526-2

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