Potato Review

www.potatoreview.com POTATO REVIEW JULY/AUGUST 2020 15 VIRUS TESTING e more cost e ective but less timely growing on test then lls the bulk of the lab’s work over the winter. Adrian 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 four bulks – are tested, it only gives a crude indication of the level of virus in a particular stock. “ e 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,” said Adrian. Best practice sampling e rst key component of good sampling is timing. e 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 Christophe. ere is debate about whether samplers should follow a traditional “W” pattern across the eld, or a grid pattern along the drills to obtain a representative sample. However, this is unlikely to be critical. Christophe adds it is important that when digging up single plants at points along theWor grid, an average sized tuber is taken. is 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, virusmay not have yet translocated intomore 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 fromstore post-harvest and NIAB’s head of pathology and entomology Jane omas says every e ort should bemade tomake it as representative as possible. ismaymean shifting boxes aroundwith a forklift and not just picking tubers o the top layer. “Try and take a representative sample from each box and then bring all those samples together,” she adds. All laboratories require aminimumof 100 clean tubers but request 120 tubers to allow some wriggle room for technicians. ese should be sent in nylon nets or paper sacks, accompanied by submission forms available on the Fera, NIAB and SASAwebsites. Which virus? Virus testing submission forms should stipulate which method is used and the virus tested for, with labs typically o ering PVY and potato leaf roll virus (PLRV) as standard and each additional virus or strain commanding an additional fee (see table). Adrian says the virus or viruses tested for will depend on the grower’s geographical location, environmental conditions and variety. For most growers, PVY is themain concern, particularly the PVYNTN strain of PVYwhich current knowledge suggests is the predominant strain inUK potato elds and can cause severe tuber cracking and necrosis in some varieties. “In some varieties other viruses are a bigger consideration [because they aremore 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. Adrian 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-o 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 speci c variety you are growing,” he adds. Christophe stresses that di erent rules apply for di erent classi cation 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. Test o ered Turn- around time Potyviruses identi ed* 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 tes 5 day PV £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 o er testing advice and packages tailored to speci c needs. **All prices ex-VAT. Bulk orders may reduce cost. Tuber test providers in Great Britain “Ware crops with unacceptable levels of virus in proximity to seed crops may be burned down.”