Potato virus X

PVX belongs to the plant viruses of the Potexvirus genus, family Flexiviridae. It was first discovered in 1931 in the United Kingdom. Once very common in Poland, it is now virtually absent due to the effective efforts of Polish seed production and breeding, which introduced resistant varieties. It sporadically appears with planting material imported from abroad. From 2006 to 2012, it was quite prevalent in Northern Ireland.

The primary hosts of the X virus are plants from the Solanaceae family (such as tobacco, pepper, tomato), but also turnip (Brassica rapa), some legumes (currently in the Fabaceae family), and weeds. The main source of PVX infection is seed potatoes. The X virus is mainly spread mechanically through leaf contact and abrasions during cultivation practices. It can be easily introduced into a plantation by humans, animals, or tools that have had contact with infected plants. Aphids do not transmit this virus. Some insects, such as the Colorado potato beetle, leafhoppers, and the green peach aphid, may be incidental vectors. There are reports that PVX can also be transmitted by the spores of Synchytrium endobioticum (potato wart).

With mild or latent infection, yield losses are slight, up to a few percent. Reductions of up to 30% are often reported. Older studies indicate that yields can be even lower, up to 75%. The harmfulness of PVX significantly increases with co-infection with other viruses. Since 2015, planting material in Poland is officially evaluated for this virus. Current legal regulations do not specify the maximum infection of PVX for different qualification grades, only the sum for six assessed viruses (PVY, PVM, PVS, PVA, PVX, PLRV). The highest permissible percentage of infected tubers, including PVX, cannot exceed 10% for the lowest class – EU class B. In the Czech Republic, due to its low harmfulness, the recorded infection is multiplied by a factor of 0.33.

Symptoms of Infection:

The intensity of PVX infection symptoms depends on the susceptibility of varieties and weather conditions. Symptoms are more visible in cloudy weather and lower temperatures (16-20°C). Infection can also be asymptomatic. The most common symptoms during the growing season include:

Mild to severe mosaics on leaves, irregular spots of light and dark green (photo 1X). Other colors, such as brown or yellow, are not associated with viral infection. Mosaics can be more pronounced when PVX occurs in conjunction with PVA or PVY. In the presence of the latter, PVX concentration multiplies. Mosaics often have a light green hue.
Deformations and wrinkling of the leaf blade (photo 2X), waving of its edges – more common when this virus is present with PVA or in susceptible varieties.
Varieties with hypersensitivity (HR) may show more severe necrosis and leaf deformation at the infection site, but the virus does not move further into the plant.
Point or streak necroses on leaves or in tubers.
Protection Methods:

The basic method is the frequent exchange of seed potatoes for qualified virus-free material. Additionally, planting resistant varieties reduces the risk of infection. Since 2015, no research has been conducted in Poland to determine the resistance of potato varieties to PVX. However, this resistance is often specified for many varieties registered in Poland. For many foreign, unregistered varieties, this resistance is frequently provided in the characteristics on the websites of breeders.

Applying as much spatial isolation (distance from other potato plantations) for this virus is of little importance. It is essential to frequently disinfect tools, clothing, and equipment after leaving the plantation due to the possibility of virus transfer. This applies mainly to plantations of unknown health status.

It is also crucial to remove sources of infection, i.e., plants infected with viruses, along with tubers, from the field – this is called negative selection. However, one should not be too hesitant, as leaving a suspicious plant in the field in the hope of achieving higher yields may result in the harmful increase of virus infection in neighboring plants and, consequently, in seed plantations – degradation or even disqualification.

A very effective method to limit the spread of viruses from above-ground parts to tubers is the early destruction of haulms. This is a commonly used practice in seed plantations. For PVX, due to its mode of transmission, the only recommended method is the chemical method, popularly known as desiccation. It involves applying appropriate sprays to plants, causing them to dry up. Applying the agent in inappropriate weather conditions can contribute to defects in tubers. This is particularly important in the cultivation of potatoes for food processing (chips and fries). Research from abroad shows that during a prolonged drought, spraying with Reglone 200 SL or its equivalent at the full single dose may cause internal discoloration of the pulp on some varieties. Contact of tubers with this agent can also cause noticeable damage. Such situations can occur when tubers are very shallow in the soil and, after the treatment, intense rain washed the preparation from the stems to the tubers. For the best results, split doses are recommended. The first dose is intended to destroy the upper leaf mass so that the agent can precisely cover the exposed stems during the second treatment. The second half of the recommended dose of the preparation should be applied 3-6 days after the first treatment. The speed of absorption of agents by leaves and stems varies and depends, among other things, on the soil on which potatoes grow. They are most rapidly absorbed by plants growing on heavier soils (clay, loamy sand). In the case of Reglone 200 SL, rainfall after 15 minutes from application generally does not affect its effectiveness, while after applying Basta 150 SL, according to the label, 4 hours of rain-free weather are required.

Compiled by: Dr. Hab. Sławomir Wróbel