UK researchers have confirmed resistance to succinate dehydrogenase inhibitor fungicides in Plenodomus biglobosus, one of the two closely related pathogens that cause phoma stem canker in oilseed rape.
The peer-reviewed study, published in Pest Management Science on 22 September 2026, tested 20 UK isolates collected in 2025 and 17 from Poland collected in 2024. Nineteen of the 20 UK isolates-95%-were classified as resistant to boscalid and fluxapyroxad, compared with one of the 17 Polish isolates. Some UK isolates showed an approximately 500-fold shift in laboratory sensitivity to boscalid.
That is a serious warning, but it needs careful interpretation. The study tested a limited collection of isolates under laboratory conditions. It does not show that 95% of UK fields are affected, that every phoma population behaves in the same way, or that every SDHI treatment will fail in the field. Rothamsted Research says further work is needed to establish the effect on practical disease control.
The useful response is therefore neither panic nor business as usual. It is a more disciplined approach to diagnosis, treatment decisions and records.
1. Separate the confirmed facts from the open questions
The research confirms SDHI resistance in P. biglobosus for the first time anywhere in the world. It links resistance with mutations in the pathogen's succinate dehydrogenase genes and found little consistent evidence that those mutations reduce growth or sporulation in the laboratory. That suggests resistant strains may persist.
What remains uncertain is the scale and effect in commercial UK crops. The study sample was small, the isolates were not a statistically representative survey of every oilseed rape region, and laboratory sensitivity does not translate directly into a field-performance percentage.
Record the finding as a change in risk, not as a universal diagnosis.
2. Confirm that the symptom really is phoma
Treatment decisions depend on recognising the disease correctly. AHDB describes P. lingam leaf spots as typically tawny with dark specks, while P. biglobosus usually produces darker spots with few or no specks. Downy mildew can be distinguished by white fungal growth on the underside of the leaf.
Walk each field rather than relying on a regional headline. Photograph representative lesions, note the percentage of plants affected and record crop growth, variety, drilling date and recent weather. If performance is unexpectedly poor or symptoms are unusual, involve a BASIS-qualified agronomist and consider diagnostic testing before assuming resistance.
3. Use thresholds, not anxiety, to time decisions
AHDB's current phoma guidance remains based on crop symptoms, varietal resistance and plant size. It advises prioritising susceptible varieties and backward crops, with treatment thresholds linked to the percentage of plants showing leaf spots.
The new study does not itself replace those thresholds or create a new label recommendation. Continue to use current authorised labels, official guidance and qualified advice. Avoid an automatic “insurance” application simply because resistance has been reported: unnecessary exposure adds cost and selection pressure without proving a benefit.
Timing matters. A correctly chosen treatment applied near the appropriate threshold is more defensible than a late response to established infection or a routine spray with no recorded trigger.
4. Recheck the whole disease-risk picture
Phoma risk is shaped by more than chemistry. Infected crop residues are a major source of spores, and warm, wet late-summer or autumn conditions encourage infection. Small plants are more vulnerable because the pathogen can reach the stem more quickly. Susceptible varieties and short rotations can add pressure.
For each field, review:
- the previous oilseed rape crop and nearby stubbles;
- the interval since oilseed rape was last grown;
- variety resistance ratings;
- crop size and establishment;
- local rainfall and disease forecasts; and
- whether new lesions are continuing to appear.
This turns a national research result into a field-specific decision.
5. Protect every effective mode of action
Do not respond by simply replacing one repeated programme with another repeated programme. AHDB advises using a range of modes of action to reduce resistance risk, alongside non-chemical measures such as varietal resistance, wider rotations and management of infected debris.
Product choice, dose, mixture, sequence and maximum application numbers must follow the current label and professional advice. Never assume that two brand names provide different modes of action; check the active substances and FRAC groups.
The study found strong cross-resistance between boscalid and fluxapyroxad in the tested isolates. It also found smaller sensitivity shifts to fluopyram, but that is not a field recommendation and should not be treated as a simple substitute. The responsible conclusion is to review the entire programme with an agronomist, not to improvise from one laboratory result.
6. Make spray and outcome records useful
A product name and date are not enough to investigate a suspected loss of performance. Record the field, variety, growth stage, disease level, product, active substances, rate, water volume, nozzle, weather, application time and operator. Add photographs before treatment and at a planned follow-up.
Also record machinery condition and calibration. Poor coverage, unsuitable conditions, delayed timing or an application fault can resemble resistance. Keeping those details lets the grower and agronomist separate a biological change from an operational one.
If records are completed in the cab, keep the device or paperwork accessible without obstructing controls, visibility or safety equipment. The priority is reliable information captured without distracting the operator.
7. Escalate suspected reduced performance properly
If disease progresses unexpectedly after a correctly timed and applied treatment, do not immediately repeat the same mode of action. Preserve the evidence: photographs, field maps, spray records, product batch information and untreated comparisons where available.
Contact the agronomist and product supplier, and follow their reporting route. Laboratory confirmation may require suitable samples collected at the right time, so ask before removing or storing material. A structured report is more useful to surveillance than an anecdote shared after the crop has moved on.
What growers should do now
This discovery changes the evidence base, but it does not justify blanket claims of field failure. The immediate job is to monitor crops well, diagnose symptoms accurately, use established thresholds, protect modes of action and keep records capable of explaining the result.
The research also exposes an information gap. Current public guidance that predates the paper may still state that UK phoma resistance has not been recorded. Growers and advisers should therefore watch Rothamsted, AHDB, FRAG-UK, product labels and agronomy updates for revised advice as field evidence develops.
The strongest disease programme will not be the one with the most applications. It will be the one that combines crop resistance, rotation, field monitoring, well-timed authorised treatments and evidence good enough to spot when performance genuinely changes.
This article provides general information, not crop-protection advice for a specific field. Always follow the current product label and consult a suitably qualified agronomist.
Sources
King KM and West JS, Pest Management Science: “Discovery and characterisation of succinate dehydrogenase inhibitor resistance in Plenodomus biglobosus” - published 22 September 2026
Rothamsted Research: “Fungicide resistance discovered in major oilseed rape pathogen” - published 25 September 2026
Farmers Weekly: “SDHI fungicide-resistant phoma discovered in OSR” - published 28 September 2026
FarmingUK: “Major oilseed rape disease pathogen shows widespread SDHI resistance” - published 30 September 2026
