The summer of 2018 offered a wakeup call for the UK potato industry, as levels of mosaic virus uncovered by seed crop inspections reached their highest for a number of years.
This had a knock-on effect in 2019, with some ware crops having in excess of 60% of plants visibly infected.
Growth cracking – which can also be a symptom of infection by some mosaic viruses – was also reported in some varieties, leading to loss of marketable yield.
In England, a significant 25% of seed stocks entered into the certification scheme in 2019 were downgraded because of virus or virus in adjacent crops.
While the 2019 figures in Scotland were not as startling with 4.7% of seed crops downgraded due to virus infection, it was the highest level there since 2000 with the predominant virus strain being PVYNTN.
This isn’t just a UK problem, either. In the same year, a massive 38% of Dutch certified seed crops entered at S grade were downgraded as a result of virus infection.
The cause is complex, but a perfect storm of changes in aphid vector pressure, insecticide resistance and loss of insecticides are all playing a part.
Critically, there is also genetic variability in the PVY virus that makes it more challenging to detect in visual inspections prior to burndown allowing infection to go undetected.
So, what does this mean for ware growers? For Adrian Fox, senior plant virologist at agri-food science firm FERA, keeping faith and sourcing Certified seed is the first and most obvious piece of advice.
Secondly, where there is a concern about the virus status of seed, growers should ask for it to be tested and to see the ensuing laboratory report.
“That is particularly important if the intention is to grow that seed stock on a further year or as home saved seed,” warns Dr Fox.
For seed destined for 2020 plantings, virus testing will now have to be carried out on samples taken from boxes in store.
Dr Fox says every effort should be made to take the 100 tubers required from as many boxes as possible to get a truly representative sample.
In future seasons, ware growers should request a post-burndown, pre-harvest test, which is the most reliable and accurate way of determining virus levels in seed crops.
Testing at this time picks up both secondary virus infection, which is present in the input stock, and any primary infection, which is spread by aphids through the crop during the growing season.
To make a meaningful interpretation of results, Dr Fox adds that the sample has to be as representative of the field as possible.
“About 100 tubers should be taken in a “W” pattern across the field and one tuber taken from each plant root. Don’t take the small tubers or the really big tubers. Go for the average size tuber from each plant,” says Dr Fox.
While this advice can help ware growers source healthier seed, Scottish Agronomy’s Eric Anderson believes the legacy of the last two seasons should prompt more systematic action.
Although these were years of exceptional aphid pressure, he says the virus problem isn’t going away and changes to the Seed Potato Classification Scheme (SPCS) are required.
He believes the current system is not fit for purpose, as crop inspections are visual and only reliable for detecting secondary infection, not primary infection that tends to be more challenging to identify within the growing season.
Some varieties express virus symptoms less readily too and can be missed.
Mr Anderson also raises the potential issue of there being two tests at various laboratories available to growers, with NIAB, FERA and SASA all offering both a growing on and rapid molecular test.
He questions whether all are comparable, although work carried out by Dr Fox a number of years ago shows that both methods should provide similar results.
The UK also uses a sampling rate of 100 tubers, which is divided into four tuber sub-samples or bulks.
An algorithm then estimates the level of virus. This assumes that if a number of bulked samples are tested, and only a few of these are positive, then in each positive bulks there will only be a low number of positive tubers.
As the number of positive bulks increases, the algorithm accounts for this by assuming that a greater number of individual tubers in each bulk are also positive.
Mr Anderson says the tuber sampling rate needs to be increased to provide better resolution of results at low infection frequencies and give actionable data to confidently flush potyvirus out of the seed multiplication system.
“It would be costly to make the wrong decision on either a false positive or false negative result for a seed stock,” he notes.
“For ring rot testing, samples of 400 tubers are used and for virus testing in the Netherlands, they use a minimum of 200. We need to be using at least 200 and if we want better seed health, we should potentially test more.”
In addition, Mr Anderson would like to see an agreed sampling and testing protocol between the two certification authorities FERA and SASA, plus other testing providers such as NIAB.
As with recent potato cyst nematode (PCN) testing improvements he advocates a ring test based on all participating laboratories using the same agreed methodology.
This would be reinforced by a further proficiency test where everyone tests the same material for tuber virus testing using seeded tuber virus samples.
He believes there is also a case for fewer generations of certified seed crops, which would decrease overall virus loading in stocks.
“A compulsory post-burndown, pre-harvest testing of seed might also be sensible. There will be increased cost and value returned to the industry, but doing nothing is untenable,” concludes Mr Anderson.
No silver bullets
However, SASA senior virologist Christophe Lacomme cautions that compulsory testing is no silver bullet, as both virus tests have their limitations.
With any test, results are only as good as the sample taken and not all sampling is carried out correctly he fears.
Furthermore, primary infection is far less likely to be evenly distributed through a crop, so neither the molecular nor growing on test will give a 100% accurate prediction of final virus levels in the following crop
“This may be part of the reason why, in some areas such as the Netherlands, even if you have [compulsory] post-harvest testing you will still have some levels of infection, either secondary or primary, undetected.
“That is something we have to live with and the limitation of every test.
“There is a danger in thinking introducing post-harvest testing will solve all the problems. It is more a situation of appropriate crop management, of low virus and aphid pressure and, most importantly, sourcing high quality seed to grow with the lowest virus incidence possible,” explains Dr Lacomme.
Integrated pest management
Achieving low virus and vector pressure within UK potato-growing areas will require an industry wide integrated pest management (IPM) approach to virus control.
This includes adequate segregation of ware and carrot crops and seed fields, ware growers sourcing the cleanest input seed and adequate control of volunteers to reduce the virus reservoir.
Varieties could also play a useful role, with some more tolerant to viruses than others, but this is an area that requires more targeted research to make an impact in the field.
Currently, National List (NL) trials aren’t using the dominant strains of mosaic virus to test varieties as they come through the system, so published information is not relevant to what is happening at farm level.
“Like with PCN, we need further knowledge on the tolerance and resistance status of potato varieties to the most prevalent virus species and more trials using the right inoculum, like with late blight.
“To minimize the impact of virus and aphid-borne infections, planting varieties resistant to PVY should be an important option in areas of known high virus and aphid pressure,” adds Dr Lacomme.
Nick Badger, potato portfolio manager at Certis – who recently launched its Tubercare initiative to promote seed tuber health through the production chain – agrees these IPM measures will be increasingly important as chemical options for vector control dwindle.
News that foliar insecticide Biscaya (thiacloprid) will be withdrawn in 2020 will largely restrict potato growers to just two insecticides unaffected by resistance in important aphid species by 2021.
These are Insyst (acetamiprid) and Movento (spirotetramat), although the latter has the label restriction: 'Applications in potato varieties that produce flowers can only be made from the end of flowering (BBCH 69)’.
Other options include pyrethroids – although the insecticide group offers limited efficacy against a range of PVY vectors – and mineral oils, which have been shown by recent AHDB work to improve control of potyviruses when used as part of a wider programme.
“It means growers will have to be much more targeted in how they use these available products to optimise vector control,” adds Mr Badger.
Virus pressure – key points