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. 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. Potato virus Y transmission 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