Depending on virus strain and potato variety, symptoms vary from mild mosaic to severe foliar necrosis to plant death. Primary symptoms from the initial infection may differ from a secondary infection of the progeny plants. PVYN can appear symptomless or as a mild mosaic. Primary symptoms of PVYO and PVYC may be severe mosaic or leaflet necrosis. Youngest leaves may be mottled or distorted and petioles lower on the plant become brittle and leaves fall off or hang by a thread. Bare stems with tufts of leaves on top may give a palm tree effect. Symptoms of secondary infection may be small crinkled, mottled and twisted leaves with mature plants being dwarf, weak and sprawling.
Primary infection of PVYNTN shows mild mosaic, chlorotic foliage, crinkled leaves and necrotic rings on the tuber surface. These might be isolated or coalesce to cover the whole tuber. Necrosis increases during storage and rings may become sunken, facilitating rotting. Secondary symptoms of PVYNTN show a distinct mosaic on leaves and severe necrosis on stems and veins leading to leaf drop.
The virus can be spread by mechanical contact but is more extensively spread by winged aphids, in particular the peach potato aphid (Myzus persicae). Non-persistent in the aphid, the virus can be acquired and spread within seconds. The virus is lost from the aphid within a few hours. Consequently the virus is only transmitted a short distance, up to 100 metres. The main source of PVY inoculum is infected seed tubers.
Apart from growing resistant varieties, the most effective way to control PVY is to use clean certified seed grown in low risk areas. Eliminate infection sources by early roguing of diseased plants. Minimising cultivation damage to plants will prevent contact infection. Systemic or contact insecticides are of limited use as the virus is transmitted, and potentially spread, long before the aphid succumbs to the toxin. Weekly applications of mineral oil can reduce virus spread by slowing virus uptake by aphids and modifying their behaviour (Link). Virus testing and field kits are commercially available.