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Consider seed testing to allay potato virus fears

Concerns are growing about high levels of virus transmission in potato crops this season. Can tuber testing play a role in managing the problem? Advice on how to get the best and most relevant results.

Many 2020 seed potato crops have emerged into high aphid vector pressure and combined with significant virus carryover from 2019, risk of transmission is high.

This is leading some experts to recommended additional post-burndown, pre-harvest tuber testing this autumn to identify and manage problem stocks.

Rothamsted Research suction trap data shows both the Peach-Potato aphid Myzus persicae and the Willow-Carrot aphid Cavariella aegopodii have been presen in significant numbers across large parts of the country, encouraged by the warm and dry spring.

Both these species, along with cereal aphids, are important vectors of non-persistent potato viruses, such as PVY, and seed potato growers will have been using robust insecticide programmes to limit their ability to spread virus in growing crops.

However, the high pressure – combined with widespread resistance to pyrethroids and further loss of aphicides – programmes are not guaranteed to be watertight.

Compounding matters is the news that the COVID-19 pandemic has seen Animal and Plant Health Agency (APHA) inspections of most basic seed crops across England and Wales cut from two to one. Scotland’s protocol remains unchanged.

This has led to worries that virus infection may go unnoticed in English and Welsh seed stocks and impact on further seed multiplication or ware yield and quality in 2021.

Buy certified

At SASA, senior virologist Dr Christophe Lacomme says the best solution to ensure healthy seed is to purchase high-quality certified stocks that have been subject to robust inspection.

At the growing crop inspection, the tolerance within certified seed for PVY and PVA is 0% in pre-basic crops, and for basic seed; 0.02% in S-grade, 0.1% in SE-grade and 0.4% in E-grade, so growers should have confidence that certified seed has low levels of virus.

“Above anything else, we would highly recommend using certified seed that has been inspected and graded according to the strict tolerances of the seed potato classification scheme,” adds Dr Lacomme.

However, after the high levels of virus symptoms seen in UK potato crops during 2019 and potentially high in-season transmission in 2020, growers may want to consider post-harvest testing before planting in 2021.

Nick Badger of Certis, who’s Tubercare initiative seeks to promote seed health across Britain, says the extraordinary aphid pressure and fewer inspections across England and Wales will increase the demand for tuber testing.

He says testing tubers is a good way of seeking that extra assurance in a difficult year and provides valuable information that will help with decisions on the intended use of seed next spring.

“The industry has suffered significant yield and quality loss as a result of virus infection in recent seasons, particularly from certain strains of PVY.

“Requesting a test by your seed supplier or arranging the test yourself is a good way of identifying problem seed stocks early. This allows growers to react accordingly to reduce its impact next year by taking infected material out of production,” explains Mr Badger.

Testing methods

There are three laboratories that provide a tuber testing service for potato growers (see panel), all covering a wide range of diseases including soil-borne pests, fungal pathogens and persistent and non-persistent viruses.

For non-persistent viruses such as PVY, two test types are available. These include a growing on test, where plants are grown from tubers to bio-amplify any virus infection. The ELISA method is then used to detect the presence of specific viruses in the plant material.

In all, the whole process takes up to six weeks from delivery of the sample to receiving results.

The second method is a direct tuber test, which uses a real-time polymerase chain reaction (PCR) to detect specifically each virus species in tuber tissue. This process is much faster and take just five days to turn results around.

FERA’s principal virologist Adrian Fox says demand for real-time PCR tests is greater early (September) and late (March) in the testing season when customers require a fast turnaround.

The more cost effective but less timely growing on test then fills the bulk of the lab’s work over the winter.

Dr Fox says those requesting tests are a mix of seed producers looking to take a belt and braces approach over and above the SPCS, or ware growers saving their own seed.

He adds that results from the two testing methods are comparable but does caution that because only 100 tubers – sub-divided into five bulks – are tested, it only gives a crude indication of the level of virus in a particular stock.

“The result is only as representative as the sample that you sent to the lab, so using the correct sampling method will help give the most accurate indication of virus levels in a given stock,” says Dr Fox.

Best practice sampling

The first key component of good sampling is timing. The best time to sample is post-burndown, pre-harvest, as the whole crop is still accessible, and the tuber sample can be collected over a suitable range of plants representative of the whole crop (or stock), says Dr Lacomme.

There is debate about whether samplers should follow a traditional “W” pattern across the field, or a grid pattern along the drills to obtain a representative sample. However, this is unlikely to be critical.

Dr Lacomme adds it is important that when digging up single plants at points along the W or grid, an average sized tuber is taken.

This is because, in the case of aphid-borne infection of a plant during the growing season, virus might not be evenly distributed in larger tubers, potentially skewing results.

Similarly, virus may not have yet translocated into more recently formed (smaller) tubers and could underestimate virus levels.

Where a post-burndown, pre-harvest sample cannot be taken, the next best means is to take tubers from store post-harvest and NIAB’s head of pathology and entomology Jane Thomas says every effort should be made to make it as representative as possible.

This may mean shifting boxes around with a forklift and not just picking tubers off the top layer.

“Try and take a representative sample from each box and then bring all those samples together,” she adds.

All laboratories require a minimum of 100 clean tubers but request 120 tubers to allow some wriggle room for technicians. These should be sent in nylon nets or paper sacks, accompanied by submission forms available on the Fera, NIAB and SASA websites.

Which virus?

Virus testing submission forms should stipulate which method is used and the virus tested for, with labs typically offering PVY and potato leaf roll virus (PLRV) as standard and each additional virus or strain commanding an additional fee (see table).

Dr Fox says the virus or viruses tested for will depend on the grower’s geographical location, environmental conditions and variety.

For most growers, PVY is the main concern, particularly the PVYNTN strain of PVY which current knowledge suggests is the predominant strain in UK potato fields and can cause severe tuber cracking and necrosis in some varieties.

“In some varieties other viruses are a bigger consideration [because they are more susceptible], such as PVA in Estima, Desiree and Hermes. For newer varieties, we’ve got limited information on susceptibility.

“If growers contact the lab, we can talk through the testing requirements to ensure they are getting the most relevant information,” he adds.

Interpreting results

Results are typically provided electronically via email and labs can provide information and assistance in interpreting results.

Dr Fox explains that ultimately, what growers do with stocks based on testing results is entirely a commercial decision, but as a guide he suggest two cut-off points.

Stocks with up to 4% virus are typically OK for further production, but anything over 10% should not be planted whatsoever. 

“Between 4% and 10%, you need to consider what virus you have, or whether you have a combination of viruses, and the potential impact on the specific variety you are growing,” he adds.

Dr Lacomme stresses that different rules apply for different classification schemes. Only pre-basic and basic seed produced north of the border, with strict tolerances to their grade.

“In addition, ware crops with unacceptable levels of virus in proximity to seed crops may be burned down in Scotland,” he adds.

Tuber test providers in Great Britain

 

Test offered

Turnaround time

Potyviruses identified*

Price**

Tubers required

Fera, York

Growing on (ELISA)

Up to 40 days

All PVY strains as standard. Customer can specify PLRV, PVYO, PVYN, PVA, PVX, PVM, PMTV

£250 for single virus, add £65 for each additional virus/strain

 

 

 

 

 

120 tubers

Rapid direct tuber test

5 days

PVY as standard. Customer can specify PLRV, PVA, PVV, PVX

£230 for single virus, add £75 for each additional virus/strain

NIAB, Cambridge

Growing on (ELISA)

4-6 weeks

PLRV and PVY as standard. Customer can specify PVX, PVS, PVA, PVV and breakdown of PVY sub strains

£153 for standard PLRV and PVY test, add £29.50 for each additional virus/strain

 

 

 

 

 

120 tubers

Rapid direct tuber test

5 days

PVY

£334

 

 

PVY and PLRV

£385

SASA, Edinburgh

Growing on (ELISA)

4-6 weeks

PVY (all strains), PVA, PLRV, PVV, PVM, PVX, PVS, PMTV

£158 for single virus, £16 for each additional virus

 

 

 

120 tubers

Rapid direct tuber test

5 days

PVY (all strains), PVA, PVV, PLRV, PVX, PMTV, TRV

£245 for single virus, £30 for each additional virus

*Note that all labs offer testing advice and packages tailored to specific needs.

**All prices ex-VAT. Bulk orders may reduce cost.