Potato virus Y (PVY) is a plant virus of the family Potyviridae, and one of the most important plant viruses affecting potato production. PVY infection of potato plants results in a variety of symptoms depending on the viral strain. The mildest of these symptoms is production loss, but the most detrimental is 'potato tuber necrotic ringspot disease' (PTNRD). Necrotic ringspots render potatoes unmarketable and can therefore result in a significant loss of income. PVY is transmissible by aphid vectors but may also remain dormant in seed potatoes. This means that using the same line of potato for production of seed potatoes for several consecutive generations will lead to a progressive increase in viral load and subsequent loss of crop. An increase in potato plant infection with viruses over the past few years has led to considerable losses to the South African potato industry. The increased rate of infection may be attributed to several factors. These include a marked decrease in the effectiveness and administration of chemicals used in vector control, the use of infected seed potatoes in cultivation, incorrect irrigation and farming methods as well as a lack of a sensitive, rapid and reliable method of detection. An increase in the average temperature of winters as a consequence of global warming has also led to an increase in aphid numbers, which in turn has led to an increase in viral distribution.
Hosts, strains, and symptoms
PVY belongs to the genus Potyvirus, the largest genus of plant viruses and possibly the most destructive one in potato crops. The genus includes more than 200 species that bring about significant losses in the agricultural arena. PVY infects many economically important plant species. These include potato (Solanum tuberosum), tobacco (Nicotiana tabacum), tomato (Solanum lycopersicum) and pepper (Capsicum spp.). The level of damage to crop is determined by the strain of PVY infecting the plants, the viral load, the time at which infection occurs as well as the tolerance the host possesses toward the virus. Resistance to PVY infection by hosts is low in many cases. Infection of a potato field with PVY may ultimately result in 10–100% loss in yield. It has been shown that the PVY has different isolates according to the symptoms they induce in various potato plant species. Extensive biological, serological and molecular variability of PVY isolates makes the classification of isolates as particular strains particularly difficult. Occurrence of a variety of symptoms and the emergence of the necrotic PVYNTN has led to a search for more reliable classification tools than simple serological identification. Traditionally three chief strains of PVY are recognized: PVYC, PVYN and PVYO. PVYC, originally known as Potato Virus C, was the first to be recognized and was identified in the 1930s. PVYC induces hypersensitive responses in a wide range of potato cultivars. These reactions include the formation of mild mosaic patterns or stipple streak. Unlike the other strains of PVY, some PVYC strains are non-aphid transmissible. Previous studies by Visser et al. did not identify any of the local isolates as being PVYC but it has been reported to occur in potatoes in South Africa. A second strain of PVY is PVYN. This strain was described in tobacco plants growing close to potato plants. PVYN results in leaf necrosis and mild or even no damage to the tubers. The ordinary strain of PVY is denoted as PVYO. Infection of a potato plant with the PVYO strain results in mild tuber damage and does not cause leaf necrosis. Both PVYN and PVYO are aphid transmissible and occur in South Africa. In Europe these two strains have been shown to have recombined to form PVYNTN. The PVYNTN has been accredited with the ability to induce potato tuber necrotic ringspot disease (PTNRD). Tubers damaged by PTNRD become unmarketable and infection by PVYNTN thus results in a larger economic impact than infection by the other strains.
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