Potato virus Y

Potato virus Y (PVY) is a plant pathogenic 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. PVY belongs to the genus Potyvirus, of which it is the type member. Potyvirus is 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 to in South Africa. A second strain of PVY is PVYN. Some notes on suspected variant of Solanum virus 2 (Potato virus Y). 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. PVY may be transmitted to potato plants through grafting, plant sap inoculation and through aphid transmission. The most common manner of PVY infection of plant material in the field is through the aphid and although aphids on their own can directly damage potato plants it is their role as viral vectors which has the greatest economic impact. In cold climates aphids spend the winter either as wingless aphids giving birth to live young (viviparae) or as eggs. Hosts such as weeds and other crops serve as breeding grounds for these aphids and form a temporary area of colonization before the aphids migrate to the potato fields. In moderate climates, such as in South Africa, aphids are thought to reproduce asexually on weeds, other crops, indigenous plants and garden plants. This means that there are a number of aphids present year-round. The importance in effective and stringent monitoring of aphid populations is stressed in a review by Radcliffe and Ragsdale (2002) as PVY virions are introduced to potato fields almost solely by winged aphids from a virus source outside these fields. Wingless aphids have not yet been linked to the spread of PVY in potato fields. The green peach aphid (Myzus persicae) has been found to be most effective in its role as viral vector, but others such as Aphis fabae, Aphis gossypii, Aphis nasturtii, Macrosiphum euphorbiae, Myzus (Nectarosiphon) certus, Myzus (Phorodon) humuli and Rhopalosiphum insertum are also strongly associated with viral transmission. The Agricultural Research Council-Vegetable and Ornamental Plant Institute (ARC-VOPI) 6 of South Africa identified twenty five species of aphid able to function as PVY vectors. The efficiencies of some of these aphids to function as PVY vectors were also established (Ragsdale et al., 2001) and were found to vary between the different species. In South Africa, Aphis fabae, Aphis gossypii and Aphis nasturtii are the most common and efficient PVY vectors found in the field. Apart from being classed according to efficiency as vectors, aphids can also be divided into two subgroups, namely colonizing and non-colonizing species. Colonizing aphids are aphids which reproduce and establish themselves on potato plants, specifically, while non-colonizing aphids do not reproduce nor establish colonies on potato plants. Colonizing aphids are better adapted to life on potato plants and are thus generally considered as better PVY vectors than non-colonizing aphids. Noncolonizing aphids do not primarily feed on potato plants but do occasionally feed on them while searching for a more suitable host. Their lower efficiency as PVY vector is cancelled out by the sheer numbers in which they occur. Because of this, all aphids present in and around potato fields must be considered as possible vectors and their numbers carefully monitored. Transmission of PVY by aphids occurs in a non-persistent, non-circulative manner which suggests a less intimate interaction between virion and vector than is the case of circulative virions. The fact that the virions are transmitted in a non-persistent fashion means that viral replication does not occur within the aphid vector and that, unless the aphid feeds on infected plants, it loses its ability to infect plants after two to three feedings. The virions attach to the aphid stylet in a matter of seconds and may remain infectious for four to seventeen hours. The distance over which the virions can be transmitted is limited due to the short period for which they remain infectious. Although the short life span outside plants inhibits long-distance viral transmission, it does not reduce the transmission efficiency bestowed by the quick rate of viral acquisition and inoculation within a field.