Tasmania's potato industry has fewer virus disease issues than its mainland counterparts. This is believed to be due to the State’s isolation (maintained by strict potato import requirements), and also due to rigorous adherence to the Tasmanian Certified Seed Potato Scheme.
The Tasmanian Certified Seed Potato Scheme was introduced in the 1920s to ensure varietal purity, and to control the tuber-borne viruses that once devastated crop yields. The
scheme works on a flow through basis, in which minitubers are produced from virus-free tissue culture stock. This stock is then multiplied through a maximum of five field generations before it is used to produce a commercial crop. Visual inspections and testing monitor virus levels, and the absence or near absence of virus (usually < 1% infection) ensures that the material is suitable for seed for the next generation. Without the regular crop inspections and testing, virus levels would soon reach high levels resulting in inferior seed and subsequent lower-yielding disease-affected crops – impacting on the entire industry.
The TASAG ELISA testing service, located at DPIPWE New Town Laboratories, performs virus certification testing of seed potato crops as part of supporting the Tasmanian Certified Seed Potato Scheme. Seed certification officers from the Tasmanian Institute of Agriculture (TIA) submit samples from each crop during Summer and early Autumn. The suite of viruses tested has varied since 2002 depending on the major issues perceived during each season, and have included combinations of:
- Potato virus S (PVS)
- Potato virus X (PVX)
- Potato virus Y (PVY)
- Potato leaf roll virus (PLRV)
- Potato virus A (PVA)
- Tomato spotted wilt virus (TSWV)
The testing is conducted using ELISA (Enzyme-Linked ImmunoSorbent Assay) which takes a minimum of two consecutive days to complete (longer for large numbers of samples). This assay incorporates specific antibodies to trap virus to the wells of a microtitre plate. Positives are indicated by reagent wells turning yellow in the final step, whilst negative wells remain clear. ELISA is well suited to large numbers of samples.
Potato growers outside the scheme including some mainland growers also use the service to gauge virus infection levels in crops.
Typically, sampling of a seed crop has consisted of 300 leaves collected randomly, via a transect pattern, and submitted/tested as either 15 lots of twenty leaves, or 30 lots of ten leaves (subject to variation). A percentage infection for that crop can then be calculated on the number of positives detected.
Testing since 2002 indicates that PVS is the most prevalent and enduring (but also considered less of a risk to yield). It is spread by mechanical transmission and, to a lesser extent, by aphids. PVX occurs each year in a very small number of crops and is spread only by mechanical transmission/contact. PLRV and PVY (considered more important) are largely absent from Tasmanian seed crops and both are spread by aphids.
PVY necrotic strains are particularly serious causing problems on the Mainland but absent here. TSWV was once sporadic but hasn’t been a problem for a number of years, and is spread by thrips. However PVA was detected in 2013 in a few crops (spread by aphids).
The Tasmanian Certified Seed Potato Scheme is now overseen by TIA. Together with key industry stakeholders, TIA research scientists make decisions on sampling regimes and the viruses to be tested each season – based on which viruses are considered to be the most important at the time.