The seed looked clean. The test said otherwise: Oregon study exposes a hidden potato disease pathway

A nine-year survey found the powdery scab pathogen and potato mop-top virus in seed lots that showed no visible signs of either—strengthening the case for more targeted molecular testing.

By Lukie Pieterse, Editor/Publisher of Potato News Today

A newly published study from Oregon State University has delivered an uncomfortable reminder to the potato industry: seed tubers that appear healthy can still carry pathogens capable of following them into commercial production fields.

Researchers examining seed potato lots entering Oregon detected the powdery scab pathogen, Spongospora subterranea f. sp. subterranea, at high frequency. Potato mop-top virus, or PMTV, was detected less often but appeared sporadically in seed originating from several production regions.

Most significantly, neither pathogen produced visible symptoms in the seed tubers examined during the nine-year survey.

The open-access study, published on August 19, 2026, in the American Journal of Potato Research, concludes that visual inspection alone may not be sufficient to detect some economically important threats moving through the seed system.

Its authors argue that molecular diagnostics could provide growers and seed certification authorities with an additional layer of information—particularly for soilborne pathogens and viruses capable of remaining concealed in apparently healthy material.

Nine years of testing seed entering Oregon

The research was led by Daniella M. Echeverria and Kenneth Frost of Oregon State University’s Department of Botany and Plant Pathology, working with Victoria Skillman and Cassandra Funke at OSU’s Hermiston Agricultural Research and Extension Center and Hannah Rivedal of the USDA Agricultural Research Service.

Between 2016 and 2025, commercial growers in Oregon’s southern Columbia Basin submitted samples of seed lots purchased for planting. Each submission generally contained between 100 and 300 whole seed tubers.

The samples were collected opportunistically from growers participating in import seed lot trials. They were therefore not drawn through a statistically random survey intended to represent every seed lot entering Oregon.

That limitation matters. The percentages reported in the study should not automatically be applied to the entire North American seed potato supply.

Even so, the long sampling period, geographic range and repeated detection of pathogens provide evidence of a pathway that the industry cannot afford to dismiss.

The seed lots came from 11 US states and two Canadian provinces, although the researchers kept individual origins anonymous by assigning random state numbers.

Powdery scab pathogen found at high frequency

The team collected soil adhering to the surface of seed tubers—commonly called tare soil—and tested it using polymerase chain reaction, or PCR, methods.

Across the study, Spongospora subterranea was detected on its own in 77.1% of seed lots. It was found together with PMTV in another 5.3%.

Detection of the powdery scab pathogen increased during the later years of the programme. From 2018 through 2020, it was found in 60% to 75% of tested lots. From 2021 onward, the figure remained above 82% and reached as high as 95% in one year.

The pathogen was detected in tare soil associated with every potato variety represented sufficiently for varietal comparison. Detection ranged from 60% of Ivory Russet lots to 96% of Shepody lots.

This does not mean that every positive lot would inevitably produce severe powdery scab. Disease expression depends on several interacting factors, including inoculum levels, soil moisture, temperature, cultivar susceptibility and field history.

Detection does mean that pathogen material was present. For a field not previously known to be infested, that distinction is important.

Spongospora produces long-lived resting spores that can remain viable in soil for many years. The researchers warn that repeated introductions on seed or in tare soil could eventually establish the organism in previously unaffected production fields.

PMTV presents a less frequent but serious risk

PMTV was detected less frequently than Spongospora, but its distribution was highly variable among years, seed lots and origins.

The virus was detected on its own in 4.6% of lots and together with Spongospora in another 5.3%. The proportion of lots testing positive for PMTV fluctuated from zero to 12.5% in individual survey years.

Among lots in which PMTV was found, researchers estimated that the proportion of infected tubers ranged from 1% to 100%. Thirteen of the 20 PMTV-positive lots had an estimated prevalence of 5% or less, while four exceeded 10%.

The extreme estimates require careful interpretation because the confidence intervals were sometimes wide. In two 2019 lots estimated at 100% prevalence, for example, the reported 95% confidence interval extended from 14.3% to 100%.

Nevertheless, even a relatively low incidence may matter when infected seed introduces the virus into a field where its vector is already present.

Spongospora subterranea is the only known vector of PMTV. Once the pathogen has acquired the virus, its persistent resting spores can provide PMTV with a long-term refuge in the soil.

The resulting tuber disorder, commonly known as spraing, can cause internal necrotic markings that make potatoes unacceptable for fresh markets and processing. The severity of symptoms varies considerably by variety and growing conditions, and infected tubers do not always display visible damage.

That ability to remain asymptomatic is part of the challenge.

Clean appearance did not mean pathogen-free

During the entire survey, the researchers observed no obvious powdery scab or PMTV symptoms on or inside the tested seed tubers.

Yet molecular analysis detected both organisms.

Removing visibly affected tubers during grading therefore cannot be assumed to eliminate the threat. An apparently clean lot may include asymptomatically infected tubers, while small quantities of contaminated soil can remain attached to the seed.

This finding does not invalidate seed certification. Certification systems have been highly valuable in reducing the movement of numerous diseases, particularly those that can be identified through field inspection, post-harvest examination or established laboratory testing.

The study instead draws attention to a specific gap. Certification programmes have historically depended heavily on visual inspections, while powdery scab and PMTV may remain invisible. According to the researchers, seed certification programmes generally do not test routine commercial lots for these two organisms.

The study also detected potato virus Y in 55 of 369 lots tested for it, equivalent to 14.9%. Tobacco rattle virus was detected in two lots, one in 2020 and another in 2025. The authors included these results as further evidence that seed potatoes can transport several pathogens with very different epidemiologies.

The case for targeted molecular testing

The researchers stop short of prescribing a universal testing requirement. Instead, they suggest that molecular examination of seed tubers and tare soil could support more informed decisions by growers, seed producers and certification authorities.

Targeted testing may be particularly valuable when seed is destined for land with no known history of powdery scab or PMTV, where protecting clean soil carries substantial long-term value.

Testing could also help growers distinguish between immediate disease risk and the less visible risk of introducing persistent inoculum. A positive molecular result does not predict the severity of a future outbreak, but it provides information that visual inspection cannot.

That information could influence seed sourcing, field selection, sanitation planning and variety choice.

The research also exposes important gaps in scientific understanding. Researchers do not yet fully understand how Spongospora acquires PMTV, what proportion of its resting spores carry the virus or how environmental and biological conditions affect transmission.

Nor did this study establish that geographic origin or potato variety caused the observed differences in PMTV detection. Seed origin and cultivar were partly confounded, while submissions were not collected through systematic sampling of every source region.

Those qualifications should prevent the results from being used to stigmatise particular seed-producing areas or cultivars.

A warning that extends beyond Oregon

Although the investigation centred on seed entering Oregon, its central message applies much more widely.

Modern potato production depends upon the movement of vegetatively propagated material across farms, regions and national borders. That system allows superior varieties and high-quality seed to reach growers, but it also creates a biological transport network.

The organisms moving through that network are not always visible.

The study’s most important finding may therefore be the simplest: the absence of symptoms is not proof of the absence of risk.

Molecular diagnostics will add expense and cannot replace field inspection, certification or experienced agronomic judgement. But as diagnostic technology becomes more accessible, the industry faces a practical question: where would targeted testing cost less than allowing a persistent soilborne pathogen or virus to enter a clean field?

For powdery scab and PMTV, that calculation deserves considerably more attention.