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Symptoms of PLRV are of two types: primary (current season or primary infection) and secondary (infection from infected tubers). Primary symptoms develop in plants infected with the virus during the current season and are usually are restricted to paleness (chlorosis) or anthocianescence (purpling) of the tops of the plants and slight leafrolling of the leaflets. Secondary symptoms develop in plants arising from infected tubers. Typical secondary symptoms are stunting of the plants and severe upward rolling of the basal leaves which progresses during season to the upper leaves. Rolled leaves are leathery and break easily when crushed between the fingers due to the excessive accumulation of carbohydrates. In some cultivars, severe chlorosis may develop in apical leaves. Some varieties may develop internal necrosis (net necrosis) in the tubers as a result of primary, secondary or tertiary infection with PLRV (Douglas and Pavek, 1972).
Conspicuous variations in symptoms can be observed in some genotypes. The most common symptoms are those induced in Solanum tuberosum ssp. tuberosum (i.e. the chlorosis and/or leafroll symptoms described above). However, varieties derived from S. tuberosum ssp. andigena and other wild species do not show leafrolling but do develop severe chlorosis and stunting; this symptom is called 'enanismo amarillo' (yellow dwarfing) in the Andes (Rodriguez and Jones, 1978). A combination of the two types of symptoms may develop in hybrid potatoes between the two subspecies and between S. tuberosum ssp. tuberosum and some wild species.
Control of PLRV is based primarily on the use of virus-free or certified seed potatoes (Slack and Singh, 1998). Tolerance levels to PLRV in certified seed planted in several countries differs. In most stringent programmes, tolerances are not usually higher than 0.5% although in others up to 3-5% is considered to be the threshold level.
Early warning systems for increases in aphid vector populations exist only in a few developed countries. These systems are especially helpful in detecting late-season aphid migration in seed production programmes.
Two approaches have been used to develop resistance to PLRV: resistance to the virus (resistance to infection and multiplication) and resistance to the aphid vectors (Harrewijn, 1986; Barker et al., 1994; Franco Lara and Barker, 1999). In addition, transgenic resistance to the virus is being studied, alone or in combination with natural resistance (Derrick and Barker, 1997). Natural resistance sources have been described at the International Potato Centre in Lima, Peru (Jayasinghe and Salazar, 1998). However, due to the multicomponent nature of the resistance to PLRV, cultivars with high overall resistance to PLRV are not easy to develop.
Roguing of infected plants and the elimination of weeds and volunteer potatoes are common practices used to prevent or minimise the spread of leafroll. Control of aphids in the field by the application of insecticides also helps to reduce or prevent the spread of PLRV.
PLRV is considered to be the most serious and damaging virus of potatoes. It causes severe yield losses (up to 90%) and in some cultivars also reduces the marketable quality due to internal net necrosis (Jeffries, 1998).