Potato Review

www.potatoreview.com POTATO REVIEW JANUARY/FEBRUARY 2021 9 VIRUS TESTING Are there any recommendations for sampling potato tuberswhile in storage? Sample fromasmany boxes as possible. emore widespread the sampling themore representative this will be. Virus YNTN symptoms on tubers in a stock that was planned to be retained for planting, are theseworth testing or just move on? Depends on the levels. Not all infected tubers will develop symptoms, so this is a good indication of an underlying problem, and the levels of infection are likely to be greater in the stock than symptoms would suggest. Additionally, this will forma source of inoculum for the next eld generation. So if at high levels it may be better to cut your losses, though a test is always worthwhile to ensure that is the root cause of the problem. Typically, virus infectedplants are distributed randomly or areweakly aggregatedwithin the eldpopulation. Plants positive for virus occur signi cantly closer together than expected if infection was randomamong the plants tested. What scienti c papers have been published on validating appropriate systematic sampling patterns for tuber sampling fromthe eld? I amnot aware of any papers covering this speci c subject for tuber sampling for virus, but the general principles have been investigated for other issues such as PCNand bacterial infections. Visual observation for thepresenceof virus symptomsduring thegrowing seasonwill giveagood impressionof thehealthof the cropas far as viruses are concerned.While thismayhavebeen true in the1970’s, but with the emergenceandspreadof PVYn strains that havemildsymptoms and the useof cultivars that havepoorhonesty in expressionofmosaic symptoms, visual observation isperhapsbecominga less reliable indicatorof plant health. Inyour opinion is it di cult or impossible to determine the true virus incidence increase by visuallyobserving foliage ina seedcrop bya single inspectionbyAPHA inspectors? Any visual observation, as with any diagnostic test, is a snapshot in time. e relativemerits of visual observation in some varieties and with some strain/isolate types is a complex discussion. ere is evidence that PVYN is expressed to a lesser extent than some other strains, but there will also be a dependence on timing of infection and ongoing aphid activity in identifying virus levels. e best practice approachwould be to combine visual inspection and diagnostic testing as a ‘belt and braces’ approach to limiting virus incidence in the eld. Does itmatterwhendirect tuber tests aredone? No.Earlyinthestorageseasontheremaybea slightlyhigherlevelofvirusdetectedattheheelend ofthetuber,sothiscouldaffectcomparativetesting, butthedirecttestshouldgiveeffectivedetection throughoutthepost-harvesttestingseason. Whydo laboratories typicallyonlyprovide anestimateof the virus ina seed lotwithout reporting theassociatedstandarddeviation within theprocedureof bulkedsubsamples to enable correct interpretationof the result? is has traditionally beendue to the desire to give an ‘easy to understand’ answer, but I appreciate giving the ‘average’ result could be a littlemisleading depending uponwhat you arewanting to dowith the results. I think growers and consultant these days aremare familiarwith the data, so I would like us tomove towards reporting both average and upper/lower 95%Con dence limits. InEngland, a key risk is the interval betweeno cial inspection andburndown. Crops that exceed 1/3rd virus severe virus tolerance for SE grade = 0.1%or E grade = 0.4%or 1/3 of othermosaics virus tolerance of 0.5% for SE grade or 0.8% for Elite grade, so 0.16% for SEor 0.26% for E, will be subject to compulsory burndown 28 days after nal single date inspection. For those stocks subject to a compulsory burndown, what are the likely chances of meeting the threshold for Elite this season withhigh vector pressure experienced as well aswillowcarrot aphid, grain aphid and bird cherry oat aphidpyrethroid resistance or at least sensitivity shifts. On the basis of testing 100 tubers, the current interpretation wouldbe that 3.20%mean calculated positive PVY fromthe tuber test wouldhave anupper 95%con dence limit of 9.09%. us, less than 3%mean calculated result froma tuber test be required tomaintainE grade classi cation? is is correct. See also the discussion from the webinar on interpreting results. Doesmolecular testing allow to poolmore than tubers inone sample andhowmany would you recommendpool together? Intheoryyes.AtFerawehavelookedatpoolingup to10tubers,butitispossibletogobeyondthis-the Netherlandscertificationschemetests200tubers as4poolsof50tubers.However,thishastobe balancedwiththepurposeofthetestingandwhat theresultsareusedfor.Largebulksaregoodfor apresence/absenceandcertifyingalowlevelof virus,butdonotgivetherequiredresolutionforlow tomoderateviruslevelsthatgrowersworktofor commercialdecisionmakingonhomesavedseed. WhichELISAkit would you recommend for PVY andPLRV testing? I cannot recommend any speci c kits. All commercial suppliers should have carried out internal validation on antisera prior too release, and then it would be up to each laboratory to validate the tests being used to endure they are t for the purpose that are being used for. How long canwe store leaf samples at -20 Cbefore performing ELISA? We keep samples in the -20 freezer for several months without any detection issues, however I would not recommend this as a routine testing strategy. Are all diagnostics tools able to detect the strains andwhat is the recommended tool for the detectionof the strain? No, di erent PCRprimers andELISAantisera will have di erent speci cities. ese shouldbe checked against themanufacturer’s claims and also validated for use in the lab. For instance, molecular primers can check for silico aswell as other known isilatels to ensure range of detection. Is themechanical transmission important like othermeans of virus transmission? It can be, but this will be dependent upon the pathosystemyou are in. InNorthern Europe theremay be somemechanical transmission during the season, but themain transmission route is via aphids. What informs themethod of testing you will use for detectionof possible potato viruses? Based on grower’s request or your owndiscretion? is is at grower’s request, thoughwe can advise growers as to themost appropriate tests tomeet their needs.