Potato leafroll virus (PLRV) causes a disease of potatoes worldwide and occurs in Western Australia.
High levels of infection within a crop reduce returns as the virus greatly reduces tuber yield, size and marketability.
In Western Australia, as part of the seed potato certification scheme, leaves from Generation 2 seed potato crops are sampled late in the growing season. The leaves are tested in the laboratory for PLRV and four other viruses using enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) .
From 2005 to 2015 the level of PLRV detected in the Generation 2 seed potato crops ranged from 0 to 0.5% of total potato plants tested. In 2015 no PLRV was found in the G2 seed potato crop survey, but growers need to remain vigilant to contain this disease to low levels.
Infection
Initial infection of potato crops by PLRV occurs when plants become infected by virus-carrying aphids during the growing season (primary infection). Infection also occurs when seed stocks containing infected tubers are planted, and infected potato plants grow from them (secondary infection). Aphids then spread the infection further.
All tubers produced from an infected plant will carry the virus, so when they sprout the plant is already infected and can act as a virus source to neighbouring plants.
Symptoms
Symptoms of primary infection are visible in the young leaves with upward rolling of the leaf margins. This occurs mainly in the part of the leaf near the base. The affected leaves are slightly pale and may show purpling or reddening. The leaves are often crunchy when touched.
Symptoms of secondary infection are visible on all leaves, with upward rolling of lower leaves, while young leaves are often upright and pale. Lower leaves are stiff and crunchy when touched and often have purpled or reddened undersides (Figure 1). Infected plants are usually stunted (Figure 2).
[1]
[2]Visible symptoms vary with the age at which the plant becomes infected, the variety, environmental conditions and crop vigour.
Recently in some varieties there have been instances where visual foliage symptoms are subtle or not expressed in infected plants. This is a serious issue as it can lead to virus infection being missed in visual inspections of seed potato or ware crops. This can then lead to crops having unacceptably high levels of PLRV.
Although such symptoms may be subtle or invisible, these apparently healthy plants can act as sources of the virus for spread by aphids to nearby plants. The seed tubers they produce will carry the virus providing infection sources for subsequent crops. The cause of the poor symptom expression is not known.
Depending on the variety, PLRV-infected plants produce few and smaller tubers, and ‘russet’ varieties sometimes show internal browning that affects marketable yield.
Sources of PLRV
Infected ware and processing potato crops and self-sown potato plants often act as a source of infection for young potato crops. In 2012, self-sown potatoes were collected from four sites and PLRV was detected at three of these at levels of 2–19%.
Other plant hosts of PLRV include tomato, capsicum and solanaceous weeds such as nightshade, wild gooseberry and thornapple.
Only a few aphid species transmit PLRV. Among these, the green peach aphid (Figure 3) is highly efficient in spreading PLRV and commonly occurs in potato growing areas. There is more information on Green peach aphid: potato pest in Indonesia and Western Australia [3].
[4]Spread by aphids
PLRV is persistently transmitted by aphids which means that once infected an aphid can transmit PLRV for the rest of its life and spread the virus over long distances. An aphid needs to feed on an infected plant for several hours to acquire the virus which then spreads through its body and lodges in the salivary glands. The virus-carrying aphid then flies off, and when it lands and feeds on another plant it transmits the virus to the new plant.
Management strategies
- Plant only certified or virus-tested tubers. See Potato seed certification. [5]
- Remove all self-sown potatoes and solanaceous weeds.
- Avoid planting new and seed potato crops near old, ware or processing crops.
- Rogue (remove) infected plants showing virus symptoms from seed potato crops.
- Apply neonicotinyl insecticides early in the life of the crop to control early aphid arrivals, use as directed on label. Do not wait until aphids are visible as this will be too late.
Correct diagnosis is essential for effect disease control. A commercial diagnostics service is available through DDLS - Seed testing and certification services [6].
Acknowledgment
Funding for the Generation 2 seed potato crop virus survey is supplied by the Agricultural Produce Commission - Potato Producers Committee.