Potato Virus Y - Testing Facts and FAQs

Wednesday, 19 August 2020

Testing your potato seeds for Potato Virus Y (PVY) is an important step in developing healthy, disease-free crops.  In this article Adrian Fox, of Fera, discusses what is involved in testing and answers questions (FAQs). 

Testing - general facts

Testing - what it can do?

  • Identify strain (definitive, not subjective but this is also dependent upon definition of strain)
  • Identify stock with excessive virus irrespective of symptom
  • Predict what’s likely to be there at end of season

What testing cannot do

  • Detect something which is not being tested for (complete form correctly)
  • Detect something that is not in the tubers sent for testing (take a representative sample and remember that the sample is a tiny proportion of the total tubers)
  • Predict impact
  • Predict what will be in the daughter stocks

 Visual crop inspection

  • Cost effective
  • Tight tolerance 0.02% is one plant in 5,000
  • Wholly depends on symptom expression
  • Not likely to pick up primary infection (some varieties with primary infection show symptoms, e.g. Maris Peer)
  • Some varieties are tolerant and don’t show signs of virus

 Sampling must be representative

  • Ideally sample after burn down before harvest.
  • Use a W pattern across field
  • Take one tuber per plant, of representative size
  • Best to avoid sample from boxes (if necessary, take from at least 10% boxes)

Testing - FAQ 

Q: Is there an effect of field size on sampling?

A: If sample is representative it doesn’t matter whether a large or small area. The statistics are calculated on the basis of sampling from an “unlimited” population

Q: If infection is coming into field from one side what should the pattern be?

A: If for detection, sample on edges preferably focussing on symptomatic plants. If seeking the percentage infection in the field, then use a W or grid.

Q: At harvest should 1-2 tubers be tested from every box?

A: No direct comparison has been made of this approach compared with the W sampling. But if the sample is ‘representative’ of the stock then this approach should work, but would need validation.

Test Methods 

Growing out test

Stages:

  1. Cut section of tuber rose end
  2. Treat with Ggibberellic acid 
  3. Grow
  4. Sample leaves
  5. ELISA (a serological test with antibodies)
  6. Samples from 4 tubers are combined.

Direct test

Stages:

  1. Cut core from stolon attachment point of tuber
  2. Break up tuber material with automated ball bearing system
  3. Extract virus with bead extraction method using iron filings coated with silicon
  4. PCR
  5. Samples from 4 tubers are combined

Differences between the 2 methods are due to:

  • Virus strain differences
  • Primary vs. secondary infection
  • Virus distribution in tuber
  • Virus titre in tuber and in plantlet

Timing

Early in season real time PCR may indicate higher levels of virus than grow out/ELISA but later on in storage the 2 methods give more equal results.

Test Methods - FAQ 

Q: How do the 2 methods compare?

A: The 2 tests may not appear to align because the samples are from different tubers or from different parts of the tuber. So Fera sampled the same tubers by the 2 methods for a true comparison. When the plantlets are tested by PCR this gives the same results as the ELISA test.

Q: Why does Fera not provide confidence limits as routine with test results?

A: As part of a service review Fera is planning to offer full interpretative data on the website and will move to reporting confidence limits as well as calculated virus.

Interpretation of test results 

England and Wales:

  • >10% - don’t plant
  • Less than 4% - OK
  • Inbetween 4% and 10% action depends on variety and its risk

Scotland:

Farm saved seed should be less than 4% infected.

Interpretation of test results - FAQ

Q: Was 2019 a blip?

A: Yes, but on the background of rising risk due to:

  • Loss of actives
  • More vectors with insecticide resistance
  • Virus in input seed and virus in volunteers

Q: Should there be a single test per tuber? There could be a market demand for such a service.

A: It is possible, but it would only make a difference at higher infection levels, when the seed shouldn’t be grown anyway.

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