Potato M virus, similar to the leaf roll virus, spreads much less in Poland than it did 30 years ago. Currently, it occurs quite sporadically. Most strains affecting potatoes in Poland are weak, causing mild disease symptoms in plants under field conditions. This virus belongs to the Carlavirus genus of the Flexiviridae family.

The primary source of M virus infection is seed potatoes. From infected plants, it is mainly transmitted by aphids, but mechanical damage to plants can also lead to transmission with plant sap. Important species in Poland include the nasturtium-feeding potato aphid (Aphis nasturtii), alder buckthorn-feeding potato aphid (Aphis frangulae), peach aphid (Myzus persicae), and many non-potato aphids, such as the bean aphid (Aphis fabae), which is abundant in May. Although potatoes are not a host plant for them, these aphids, while searching for a suitable plant, can also transmit the virus by probing. A few seconds of probing are sufficient for aphids associated with potatoes to effectively transmit the virus, with an optimum time of just 2 minutes.

The harm caused by the virus includes a reduction and dwarfing of yields and significant weakening of the plant, making it susceptible to other pests, such as the Colorado potato beetle. However, yield reductions are not as significant as in the case of PLRV or PVY, reaching a maximum of 30% sporadically. Legal regulations define the maximum virus infection of seed potatoes for different qualification classes. In the Czech Republic, due to its low harmfulness, the observed infection is multiplied by a factor of 0.33. In Poland, there is no such reduction, and the actual share of tubers infected with the virus is taken into account. The highest allowable share of tubers infected with viruses, including PVM, cannot exceed 10% for class B.

Symptoms of infection

PVM is considered a mild virus in seed production due to its weak disease symptoms in plants. Most registered varieties in Poland show mild to moderate symptoms after infection with a strong strain of PVM. The intensity of symptoms often depends on the susceptibility of varieties; generally, the more susceptible the variety, the more pronounced the disease symptoms. Although symptoms often disappear after the acute phase, a phenomenon known as apparent recovery is observed. The most common symptoms during vegetation on plants include:

• Mosaic on leaves, irregular patches of light and dark green. Other colors, such as brown or yellow, are not associated with viral infection.
• Clearing of veins, best seen under light on upper leaves.
• Curling of upper leaves, which are soft and do not have a spoon-shaped appearance like in the case of PLRV. The curling is similar to the symptoms of rhizoctonia infection in potatoes.
• Wrinkling of the leaf blade, shrinking of leaves, and, in severe infections, even reddening or necrosis of the plant.

Protection methods

One method is to plant resistant varieties, although resistance in potatoes is rare. Currently, field resistance studies to PVM are no longer conducted, and only laboratory research is conducted to evaluate varieties’ reactions to infection with strong PVM strains. Therefore, many varieties do not have specified resistance to this virus. High resistance can be identified in laboratory studies, often associated with the presence of the Rm gene that confers complete resistance.

There are no effective direct methods for controlling viruses. Their spread can only be limited. In commercial production, cultivating varieties with low resistance in central and southern regions of the country will require more frequent exchange of seed potatoes. In seed production, while cultivating resistant varieties is possible in central Poland, susceptible varieties can only be grown in the north (Fig. 1). The least favorable conditions are in the southwest part of the country (zone 4). In zone 3 (central and southern parts of the country) and zone 4, producing high-quality seeds of susceptible virus varieties is virtually impossible, although there are occasional non-specific years.

Fig. 1 – Division of Poland into favorable zones (zone 1 and 2) and unfavorable zones (zone 3 and 4) for seed potato cultivation (source: S. Wróbel, A. Wąsik, Am. J. Potato Res. 91 (3)/2013)

The most effective measure is to use maximum spatial isolation (distance from other potato plantations). Additionally, removing potential sources of infection, i.e., virus-infected plants, along with tubers, is essential, known as negative selection. However, one should not be too hesitant, as leaving a suspicious plant in the field in hopes of achieving higher yields can lead to the virus spreading to neighboring plants, and ultimately, in the case of seed plantations, degradation or even disqualification.

Controlling aphids on the plantation aims to limit the number of potential vectors (virus carriers). This method is most effective in limiting the spread of leaf roll virus (PLRV). However, it is much less effective for viruses transmitted by piercing-sucking insects, so-called non-persistent viruses – PVY, PVM, PVS, PVA. If diseased plants (virus-infected) are growing on the plantation or in its vicinity, chemical protection may be ineffective. The number of aphid control treatments depends on the earliness of the variety and the intensity of aphid occurrence. Usually, 2 to 5 treatments are applied. By treating potatoes with the Prestige Forte 370 FS agent, the first treatment can be delayed due to its insecticidal action for the first 6-8 weeks after planting. Additionally, this agent protects the plantation against the Colorado potato beetle and soil pests.

Fig. 2M – Impact of various insecticides and mineral oil on reducing aphid numbers on the potato plantation and final infection of PVM seed potatoes (source: S. Wróbel, Am. J. Potato Res. 91(6)/2014)

For susceptible varieties of PVM, similar to PVY, weekly sprays with mineral oil (e.g., Olemix 84 EC, Sunspray 850 EC) should be applied. Treatments should begin when approximately 80-90% of the plants have emerged. The recommended oil dose is 7 l/ha. This is currently the only, but very effective, chemical method to limit the spread of PVM on seed potato plantations. In commercial production, it is much cheaper to exchange seed material for healthy, qualified material every 2-4 years.

An effective method to limit virus movement from aboveground parts to tubers is the early destruction of stems. This is a commonly performed procedure in seed plantations. In commercial production, its effectiveness in limiting M virus infection in tubers is significantly lower due to the considerably later timing of execution.

In practice, three methods are applied:

Mechanical method

The mechanical method involves physically crushing the aboveground parts of the potato. Special 2-, 4-, or 6-row devices mounted on the tractor’s three-point linkage, powered by the power take-off (PTO), are used for this purpose (Fig. 1mech). The device has a system of rotating blades (beaters) adapted to the shape of the ridges. It is advisable to leave stem residues approximately 10 cm long. Setting the cutting elements too low can damage the top layer of ridges, exposing and damaging the tubers. It also promotes secondary plant regrowth, especially in conditions of high moisture (rainfall). This is crucial for seed plantations, as new shoots are vulnerable to viral infections. Leaving stems too long significantly extends drying time and also promotes regrowth. Importantly, this method should not be used on plantations heavily affected by late blight due to the high risk of spreading spores with plant residues.

Chemical method

The chemical method, commonly known as desiccation, involves applying specific agents as a spray on plants, causing them to dry up. Applying the agent in inappropriate weather conditions can contribute to defects in the tubers. This is particularly important in potato cultivation for food processing (chips and fries). Foreign studies indicate that during prolonged drought, spraying with Reglone 200 SL or its equivalent at the full single dose may cause internal discoloration of the flesh on some varieties. Contact with the agent can cause significant tuber damage. Such situations may occur when tubers are very shallow in the ridges, and after treatment, intense rain washes the agent down the stems to the tubers in the soil. For optimal results, it is recommended to use split doses. The first dose aims to destroy the upper leaf mass so that, in the second treatment, the agent can precisely cover the exposed stems. The second half of the recommended agent dose should be applied 3-6 days after the first treatment. The absorption rate of agents by leaves and stems varies depending 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 15 minutes after application does not significantly affect its effectiveness, while after using Basta 150 SL, a rain-free period of 4 hours is required according to the label.

Mechanical-chemical method

The most advantageous method due to the rapid drying of stems and effective limitation of tuber infection with viruses. It involves combining both methods described above. After breaking the stems and leaving slightly longer stem sections (15-25 cm), a reduced-dose desiccant spray should be applied. The drying of stems after using this method is the fastest (Fig. 4).

Fig. 4. Rate of stem drying depending on the applied method (source: S. Wróbel, Biul. IHAR 237/238/2005)

Compiled by: Dr. hab. Sławomir Wróbel