Potato Leaf Roll Virus (PLRV) and Mosaic Viruses (PVX and PVY)
Why is it a problem? Several potato viruses have been reported in Bhutan. Serious PLRV infections can greatly reduce yields, but can normally be easily avoided through good management practices, especially seed potato renewal and hygiene practices. More importantly, virus infections can contribute to degeneration in seed quality. Management of viruses during the production of seed potato is therefore critical. Although as yet unquantified in Bhutan the effect of viruses on potato quality and therefore yield and price is expected to be significant.
Where and when is it a problem? Viruses are expected to be worse at lower altitudes where vectors are more abundant, and in situations where effective seed potato renewal are not practiced (such as private food gardens). PLRV, the most damaging virus in Bhutan, appears to be less prevalent at high-elevations, probably due to absence of vectors.
There are a range of laboratory tests that can be used to confirm the identification of viruses. The easiest is ELISA tests.
Symptoms of PLRV are of two types: primary and secondary. Primary symptoms occur due to infection in the current season as a result of transmission by aphids while secondary symptoms appear in plants emerging from infected tubers.
Primary infection can be recognised by:
- Upright, rolled and pale green young leaves, with chlorosis (loss of colour) of top leaves.
- Young leaves may have pink to reddish margins in some cultivars.
- Primary symptoms may not be visible in late season infection which hampers diagnosis.
Secondary infection can be recognised by:
- PLRV may cause stunting of shoots in some cultivars.
- Severe upward rolling of leaflets of older lower leaves.
- Rolled leaves turn chlorotic (lose their colour), become leathery and stiff, giving a rustling sound when disturbed and break easily when crushed between the fingers.
- Some cultivars show reddening or purpling of leaves.
- Tubers upon storage develop internal (net) necrosis.
Confusion with other pests: PLRV can be confused with moisture stress and nutrient deficiencies, but leaf roll type is different. PLRV symptoms, especially when rolled-leaves are purplish, may be mistaken for purple top wilt or aster yellows which is caused by a phytoplasma. Node swelling and production of aerial tubers are other diagnostic symptoms of aster yellow. A specialist may be required to confirm that PLRV is the cause of observed symptoms.
Lifecycle: Potato plants can be infected in two ways: primary infection which occurs during the growing season, and secondary infection which occurs from infected tubers. Primary infection is exclusively through transmission by infected aphids and begins in the youngest leaves. Symptoms from secondary infection can be more severe.
Transmission: Transmission is either through infected seed potato or aphid transmission. Transmission studies have not been conducted in Bhutan, but it is expected that the peach-potato aphid is the most important vector for transmission between plants within and between potato fields. PLRV cannot be transmitted between plants through mechanical rubbing.
When can damage be expected? High virus levels within a field can occur in areas where aphids reach high densities (especially at low altitudes), where infected seed potato are used, and where seed potatoes are not replaced regularly enough, resulting in a build-up in virus loads.
Hosts: In Bhutan, potato viruses have only been recorded on potato, although elsewhere they are known to also utilise other hosts in the Solanaceae family such as tomato and chilli.
Virus management is most critical when producing certified seed potatoes, but they are currently not being produced in Bhutan. Existing seed rules and standards will need to be followed for seed potatoes to be certified. Virus monitoring, testing and management are an important aspect of certification.
Virus problems can be avoided when growing table potatoes, and during informal production of seed potatoes, through sourcing virus-free potatoes, and removing any plants that show symptoms. Vector (aphid) management may be required at low altitudes.
Regular monitoring for symptoms is necessary for seed-potato production. It is also recommended in potato production areas at lower altitudes where viruses can also be a production issue.
Effect of variety
Virus resistance varies between varieties although the resistance status of the main varieties in Bhutan is yet to be confirmed. Seed replacement does not need to be as frequent when growing virus resistant varieties.
Grow, or source, seed potatoes in areas free of viruses and/or where aphid vectors are uncommon. This includes areas above 2,800 m asl. Certified seed potato is currently not available in Bhutan.
Dig out, remove and destroy (burn or deep burial) any infected plant as well as volunteer plants.
If infected plants are becoming common in an area it may be because seed potatoes have not been renewed often enough.
Viruses cannot be managed with chemicals.
Use of insecticides to manage virus vectors is not necessary in mid to high-altitude Bhutan. See aphid management for guidance at low altitude.
Image acknowledgements: NPPC.