There are different approaches to virus assessment. Samples of tubers can be tested directly using a molecular test based on the polymerase chain reaction (real-time PCR methods often termed qPCR).
Alternatively, plants are grown from the tuber sample and sap from the leaves of each test plant is tested using an antibody- based technique (ELISA). The latter has a longer turn-around time as breaking dormancy, growing plants and testing usually takes 4-6 weeks. The direct tuber testing of a single crop sample usually requires ~ 2-5 days.
Do the direct tuber testing and growing-on tests give the same result or is one more/less sensitive?
The two tests use different tissues from different parts of the tuber and so there will be variability related to virus distribution in the dormant tuber, especially in tubers from plants infected late during the growing season (late primary infections). Overall, they should give comparable results.
Technically, the ELISA test is less sensitive in terms of analytical sensitivity (compared to the molecular technique) but the growing on of the tissue results in an amplification of the virus in the growing plant, resulting in an increase of the virus level, compensating for the inherent differences in sensitivity between the two tests.
Does it matter when direct tuber tests are done?
Early work suggested that the type of molecular techniques used initially (conventional PCR) dropped off in efficiency up to 20 weeks, but this is not something seen by the labs now. Based on current information it is thought that growing-on and direct tuber testing qPCR are relatively comparable when they have been done side by side throughout the off-season.
Where can I get tubers virus tested?
- NIAB - NIAB Potato Disease Testing
- Fera - https://www.fera.co.uk/crop-health/plant-clinic
- SASA -https://www.sasa.gov.uk/diagnostics/virus-testing
How do we interpret test results?
Commercial testing labs will be able to provide more information about the test results. The information below is provided courtesy of Fera and refers to a 100 tuber sample where, 25 sub-samples (of four tubers each [referred to as bulks]) are tested for virus.
Software is used to give a mean calculated result (% virus). It assumes that if a number of bulks are tested, and only a few of these are positive, then in each of those positive bulks there will only be a low number of positive tubers. As the number of positive bulks increases it is assumed that a greater number of individual tubers in each bulk are also positive.
If 0 bulks out of 25 are positive, then there is 95% confidence that the stock has less than 4% virus. If 24 out of 25 bulks are positive, the value will be estimated as 55% virus content (with lower and upper confidence intervals of 32% and 82%, respectively. This is shown in the graph below:
Fig. 1. Calculated % virus based on the number of bulks (out of 25) testing positive for virus.
How do % virus values translate into effects on yield and quality?
This will vary with virus strain, potato variety and whether infection is primary or secondary. There isn’t much information publicly available for GB varieties. A review from 2007 quoted that estimated yield losses of 10-15% would be anticipated if the incidence of PVY-infected seed tubers was 30% (in Spain). Unpublished work from Switzerland suggests a mean figure of just over 0.2 t/ha per % virus infection, but the varieties studied my not reflect those grown in GB or the PVY strains that predominate here. A similar figure (~ 0.18t/ha in each variety) was reported from North America for secondary infections of PVY in Russet Burbank, Russet Norkotah and Shepody.
Test methods in discussion
In the podcast below, Dr Adrian Fox, senior plant viroligist at Fera discusses the pros and cons of each test method. Scroll to 11 mins 45 seconds for the section on testing.