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PVY & PLRV

Pototo Grower - May 2010 Juan Alvarex

"Currently available insecticides cannot prevent transmission of PVY since the aphid vectors require only a few seconds of probing for the acquisition and transmission of the virus. However, they can reduce the spread of PVY by preventing aphid colonization on the plants.”

Viruses transmitted in this way are located on the surface layer of plant leaves.

non-persistently transmitted viruses are acquired readily from plants, usually in minutes or seconds. Since the virus is located on the surface layer of infected plant leaves, the mouthparts of the aphid get contaminated with virus particles in the brief process of probing (sampling of the sap in the epidermal cells in order to find a suitable host). The probing behaviour is ideally suited to rapid acquisition of non-persistent viruses. This is why cereal aphids can spread PVY.

Once the virus is acquired by the aphid mouthparts, aphids transmit these viruses in a few seconds while feeding on other plants, since there is no latent period. The virus remains on the aphid for a short time (<2 hours). Longer retention times have been reported for non-feeding aphids, which could explain long distance transport of PVY by aphids. The age of the plants is known to have an effect on virus transmission, with acquisition from and inoculation to young plants usually presenting higher transmission rates. Also, it has been demonstrated in Idaho that some PVY necrotic strains are better transmitted than other strains. Two potato-colonizing aphids, the green peach aphid and the potato aphid, are the most efficient vectors of PVY. Although the bird cherry oat aphid, a cereal aphid, is not an efficient PVY vector, it is still considered a very important vector because of its high numbers, which usually start increasing by the first week of July and reach maximum levels near the end of the month, coinciding with the maturing of cereal crops. Winged bird cherry oat aphid forms are obviously more important in transporting the virus into and within fields when they migrate long distances to find suitable plant hosts.

The molecular details of the aphid virus interactions are becoming better understood and recent studies have demonstrated that most processes are determined by specific attachments between proteins embedded in the mouth parts of the aphids and the genomic components of the viruses (viral coat proteins and non-structural virus-encoded proteins). All aphid-borne potato viruses (except the PLRV) are non-persistently transmitted.

PLRV infects the phloem tissue of potato plants and is the most important example of persistently transmitted potato virus.

Aphids have to feed on the phloem to acquire the virus, not just probe the leaf surface. This means that the aphid must consider the plant as a host and stay on the plant for as little as 20 minutes, but it usually takes several hours. Acquisition depends on the aphid species—only aphids that colonize potatoes are able to acquire and transmit this virus. Green peach and potato aphids transmit PLRV in the Pacific Northwest.

The feeding time required to inoculate the virus to an uninfected host is about the same as the acquisition time. However, a “latent” period is needed before the virus can be transmitted to healthy plants and cause infections. During the latent period, the ingested virus moves from the gut of the aphid into the haemolymph (blood of the aphid) and then into the salivary glands. When the aphid feeds on a healthy plant, virus particles move with the salivary fluid into the plant tissue. The latent period in green peach aphids has been reported as two days. The latent period is shorter in younger nymphs and when virus particles are acquired from young leaves. Winged aphids coming into a field that are already carrying the virus can probably transmit PLRV within 20 minutes.

Although the percentage of transmission progressively declines, PLRV could be retained and transmitted through the life of the aphid. It has not been demonstrated that the virus does not pass from the mother to the young

Currently available insecticides cannot prevent transmission of PVY since the aphid vectors require only a few seconds of probing for the acquisition and transmission of the virus. However, they can reduce the spread of PVY by preventing aphid colonization on the plants. Seed and in-furrow, at-planting treatments with neonecotinoids insecticides have drastically reduced the spread of PLRV.

Even the most intensive aphid control regime may not prevent spread of potato virus unless measures are also taken to keep virus-source plants at a minimum. A level of 0.05 percent seed infection translates to 900 infected plants in a 120 acre field. When the virus is abundant, the aphid population may appear insignificant, yet it can still cause severe losses. Therefore, buying clean seed is critical to reducing the spread of PVY and PLRV. Research in Idaho shows that hairy nightshade, an abundantly occurring weed in Idaho and the Pacific Northwest may become infected with PVY and PLRV. Studies reveal that aphids prefer hairy nightshade as a host over potato. Hairy nightshade plays in important role in the epidemiology of PLRV and PVY, with transmission of these viruses from hairy nightshade to potato at equal or higher rates than from potato to potato. Thus hairy nightshade should be considered in a comprehensive PVY management plan.

Resistant varieties may be the most practical method of controlling losses from PVY and PLRV. PVY spread could be limited by using cultivars that decrease the chance for aphid vectors to penetrate to leave surface cells during probing.