Sergey Banadysev, Doctor of Agricultural Sciences, Head of the breeding program of Doka-Gene Technologies LLC
The harmfulness of potato viral diseases is steadily increasing. In recent years, the damage from them is expressed not only in a decrease in yield, but also in a deterioration in the marketability of tubers. Necrotic strains of YBK, AFLK, mop-top, rattle viruses make potato non-standard, while the symptoms of new strains on the leaves are becoming less pronounced. The Y-virus becomes especially problematic (photo 1-4). There are no resistance genes for its necrotic strains, the creation of varieties with high resistance by classical selection methods is impossible. Consideration of the biology and characteristics of the causative agents of viral diseases is not the subject of this article (there is more than enough information on this topic), we will focus on the analysis of the possibilities for obtaining high-quality seed material for viral diseases.
First of all, you need to clarify the very concept of high quality. The requirements of the standards provide for severe restrictions on the external manifestation of viral diseases, and in a number of countries, including the Russian Federation, the internal, latent infection of tubers is additionally limited. At the same time, there are many nuances of regulating tolerances for viral diseases and some of them are significant (Table 1). We will understand the details.
At first glance, the Dutch standards for latent infection seem to be very stringent: only a 1,4% tolerance in the elite. But the NAK’s internal analysis procedure manual explains this and makes it clear. Mandatory is the control of latent infection of the superelite category and above. The analysis is carried out by PCR and only on the Y-virus. 50 tubers are combined in one PCR sample. A batch is considered standard if no more than one sample out of four, or no more than two samples out of eight, or three samples out of 12 shows a positive reaction, depending on the total number of tubers in the sample being analyzed. A tolerance of 0,6% is obtained from the assumption that in a sample of 50 tubers there is only one infected. But this is in the ideal case, theoretically, all 50 tubers in a combined sample of 50 tubers can be infected. And then the figure of the actual hidden infection of the party should be 25%, not 0,6%.
The realists in assessing latent infection are the Scots and the Americans. Scotland is home to one of the five Highly Approved High Grade (HG) EU Potato Seed Protected Areas. That is, this is a zone of production of the highest quality material. However, certification rules there do not require mandatory verification of latent infection. Yes, it is recommended to do it, but the level of latent infection does not affect the certification results. In the United States, tuber infections are assessed using the classic soil control method. In the fall, samples are taken from all batches subject to certification and shipped to Hawaii or Florida. There is no winter, tubers are immediately planted in the soil and evaluated by the visual manifestation of viral diseases during the growing season. And rightly so, not every hidden infection is reflected in the leaves and crops, sometimes it remains in a latent form for many years, without causing any harm. By February, results are obtained with a specific actual manifestation of the existing viral diseases on the offspring plants. Buyers of seeds will have the same picture, but later. This information determines the final result of annual certification, and the main spring implementation is carried out taking into account this information.
The UNECE, Germany, France standards S-1 also provide for and use the assessment of viruses in direct progeny. After harvesting, it is necessary to take samples of 100 tubers, grow in a greenhouse and evaluate the manifestation of viral diseases on the leaves. In essence, this is also soil control, only in closed ground, a more expensive option. Laboratory testing is carried out only in controversial cases. But there is a contradiction in the same S-1 standard, since the possibility of using not only the visual, but also the laboratory method is mentioned in the section on the evaluation procedure. Specialists understand that by ELISA or PCR several times more viral plants are detected than visually.
As for the norms of hidden infection introduced by GOST 33996-2016 in 2018 in the Russian Federation, they provide for laboratory testing, without a visual assessment of the grown offspring, and are unique in terms of explanation. According to the new standard, only an assessment of the latent contamination of the original seed material is mandatory: SSE and higher. But the note opens up immense opportunities for impromptu and improvisation: “Note. For lots of superelite, elite and reproduction seed potatoes coming into circulation in countries that have adopted the standard, laboratory testing is carried out at the request of the producer or supplier of seed potatoes. The maximum allowable norms for limiting viral and / or bacterial infection according to the results of laboratory testing of tuber samples can be established in agreements (contracts) for the supply of seed potatoes by agreement of the parties. For the parties of the ES category, the maximum level of UVK restriction according to the results of laboratory testing shall not exceed 10%. ” These three proposals have transformed the standard from a strict and unambiguous document into legal nonsense, a source for clarifying the relations of producers and customers, a complaints tool. How can one allow (essentially replace the standard) and rely on the parties ’agreement on quality issues, because the interests of the seller and the buyer can not coincide here. How can the same standards be established for different categories, because a decrease in quality as it reproduces inevitably. And another provision of standard 33996-2016 - the assessment of latent infection with viral diseases - is allowed to be performed by both ELISA and PCR. Despite the fact that the sensitivity of these methods varies by an order of magnitude. But the standard is approved and put into effect, compliance with its norms and requirements is mandatory for all seed enterprises.
Potato seed production is currently in all countries with developed potato growing on a so-called virus-free basis. The culture of cells and tissues in combination with thermo-chemotherapy and microclonal propagation allows one to obtain initial seed material with zero virus content in potato cells. But two points must be understood: zero means not total absence, but the possibility of being present in an amount below the sensitivity threshold of the available molecular diagnostic methods. And getting rid of viruses doesn't mean giving potatoes resistance to viruses. Without reliable protection against re-infection with viral diseases, improved potato seed becomes a variety of F-1 hybrids, in which only the first generation gives excellent results, and with subsequent reproduction, a sharp decrease in performance occurs. The acquisition of seed potatoes for one year of cultivation in regions with a stressful climate and a high infectious background is widely practiced, including in the south of the Russian Federation. But with such a scheme for providing seed material, the seeds should be in sufficient quantity and at a reasonable, affordable price. The whole point of the field stage of potato seed production is to obtain the maximum volumes of affordable seeds with strict containment of reinfestation. Almost all mandatory and special organizational, methodological and technological measures and operations are aimed at this. It is impossible to cure a plant and tubers infected with the virus, and the fight against viral diseases is expressed in preventing plant aphids from becoming infected mechanically. Consider some of the most important aspects of viral disease management, for which work needs to be done at the highest level.
1. Accounting and response to the dynamics and species composition of virus aphids. The transfer of viruses by aphids from diseased plants to healthy plants, along with the initial infection of planting material and mechanical transfer, is the main cause of reinfection of seed potatoes. Most viruses retain virulence and remain on the aphid's mouth apparatus for several hours. During this time, aphids can fly tens of kilometers from the garden, landfills, low-quality sowing and end up in seed nurseries. It is necessary to observe this process on seed fields virtually continuously - from planting to complete drying of the tops. It should be noted that in all countries of developed potato growing aphids are monitored with significant participation of budget funding and the involvement of leading research institutes. The results of monitoring and response to them by seed enterprises are necessarily taken into account during certification. Once upon a time, the state plant protection service worked here too. Currently, monitoring in the Russian Federation for potatoes is for some reason conducted only by the Colorado potato beetle, which is not a problem in commercial potato growing. Therefore, any professional seed-producing enterprise should independently carry out the control of aphids, using the standard method. But it is quite transparent and unified, you only need to know the important details. The aphid monitoring system is based on Merike's yellow traps (photo 5). They need to be installed just above the height of the plants, on a site cleared of all plants in the immediate vicinity of the potato field. (Plant protection specialists can obtain fairly accurate information about the number of aphids by directly viewing 100 leaves of the upper, middle and lower tier along the diagonal of the field).
Trapped winged aphids should be collected weekly and species composition determined. The ability to carry viruses in different species of aphids differs significantly. The most harmful - green peach aphid (photo 6,7) has an efficiency equal to one, all others have much lower coefficients (Table 2,3). The harmfulness factors of aphid species were proposed by English scientists (Felton B. et al., 2013) and are used everywhere. They differ for the Y-virus and VLK, that is, for different mechanisms of virus transmission. Particularly dangerous species colonizing potatoes, i.e. those that inhabit potatoes form wingless forms that also carry viruses. These include peach, potato, large potato, buckthorn, buckthorn aphids. The remaining species considered do not colonize potatoes, but carry viruses in transit, in the process of test injections in search of habitual forage crops. Determination of the species composition of winged aphids requires high qualification and the presence of a microscope. Fundamental identifiers of aphids were published 40-50 years ago and are now a bibliographic rarity. The latest full-color atlases of Germany (2000) and the Netherlands (2008) can be purchased in online stores of publishers. There are specialized sites on aphids, for example, aphid.aphidnet.org, but on them purely potato species are presented fragmentary. When analyzing the contents of traps, one should not lose sight of the possibility of getting into them of cicadas and psyllids (photo 8,9), which carry the column and the Zebra chips virus that has already penetrated Europe.
The number of aphids caught is converted to the vector pressure index (IVD). Peach aphids are taken into account in real units, all other species are multiplied by the harmfulness coefficient. In fact, they are converted into one unit of measurement, the format of the harmfulness of peach aphids. So, if 20 pieces of potato aphid are caught, then to calculate the index, their number will be 20x0,2 = 4 units. The total amount in terms of peach aphid is the vector pressure index. It is more logical to consider the IVD for non-persistent aphid species as the most problematic. An index of less than two units is completely safe, green. From 2 to 10 - yellow color, state of readiness, taking protective measures at the discretion of the enterprise. An IED above 10 requires the use of protective equipment against aphids. This is verified by the certificate authority. Failure to take protective measures is the basis for lowering the class of seeds or issuing orders to destroy the tops. The aphid harmfulness threshold (10 units of peach aphid) was established in the course of special studies of the degree of infection of seed material at different vector pressures. Infection with viruses increases significantly if the number of aphids exceeds the threshold, with smaller amounts the difference between infection levels is not significant. Accordingly, with a green and yellow level of an IVD, protective measures do not give an effect, do not affect the level of infection and there is no point in conducting them.
In some European countries, for all monitoring points, the total (accumulated) IRI from the beginning of the growing season is calculated (Fig. 1). The limit values of the total IVD are established, after which the certification service issues an order for mowing, desiccation of tops on seed plots. What is the ultimate value? In the Netherlands, there are currently 80 units of peach aphids (Haverkort A., 2018). The national official certification rules do not indicate these indicators. But it is emphasized that when issuing a prescription for mowing tops, not only the accumulated vector pressure index is taken into account, but also the resistance of the varieties to viral infection (there is an official gradation of cultivated varieties by this criterion), the number of plants with virus symptoms detected during the field inspection, and protective measures against aphids. Farmers are given two (Netherlands) to three (Scotland) days to fulfill the order. Failure to comply with the requirement leads to a refusal of certification or a decrease in the class of seeds.
Qualified continuous monitoring of aphids that transmit viral diseases provides an operational understanding of the level of infectious background and allows you to carry out protective measures not routinely, but as necessary, thereby saving significant funds on plant protection. In addition, an aphid warning service is provided to seed farmers in the EU. In the system of budgetary research institutes, several suction traps function (photo 10), which detect carriers in the air at high altitude. One suction trap gives an accurate forecast within a radius of 100 km. You can only dream of such a service, but the information provided allows you to correctly monitor at the level of an individual enterprise, transforming the information received into operational measures to control viral diseases, taking into account the situation in a particular year and in specific fields. The list of these measures includes various methods of isolation and protection of seed plots.
2. Spatial isolation. The distribution of seed potato production in regions with a minimal infectious background of viral disease vectors is practiced in all countries of developed potato growing. So, in the Netherlands, seed production is concentrated on the northern coastal polders (Fig. 2).
Long-term observations have confirmed that in this area the infectious background is one and a half times lower, and the critical seasonal level of temporary respiratory infections occurs on average on July 26, two weeks later compared to the south of the country. (Haverkort A., 2018). This can be clearly seen in Fig. 1: in the north, the IED is green, in the south, red. It can be concluded that in the south of the Netherlands, seed production of potatoes is extremely risky, and there is nothing to say about more southern countries. But do not forget about relativity. If you do not rest against the border of the Netherlands, and drive south all of Belgium, then in the north of France, in the coastal region of the English Channel, there is a zone of seed production of potatoes of this country. France ranks second in the world in terms of seed potato exports. This implies a high quality product, while the French seed production area is located 700 km south of the Dutch polders. Seed growers, due to the high level of technology and organization, produce worthy goods, although for obvious reasons it is not possible to see the results of aphid monitoring in the public domain.
The Scottish potato seed region is concentrated in the northeast, on the coast (Fig. 3). If you look at the English monitoring of aphids in recent years, then in this zone the infectious background is two times lower compared to central England. But it turns out that in Scotland itself there are areas with even fewer aphids, in the southeast, the border with Northern Ireland. Why are seed potatoes grown there?
Obviously, due to the lesser suitability of the land for mechanical composition and rockiness. If we focus only on aphids, then all the seed production of EU potatoes would be logical to place in Scandinavia. Certain steps in this direction have been taken. Literally 200 km to the Arctic Circle, on the 64th parallel, there is a Finnish, one of the five European HG, zone of seed production of potatoes (photo 14). Up to 1000 thousand tons of seed potatoes are grown there annually on an area of 30 hectares. Climatic conditions are minimally sufficient: the growing season is from May to October, a very long daylight hours at the beginning of the growing season, it is cool and enough rainfall is up to 300 mm (Fig. 4).
The aphid infectious background has been thoroughly studied by independent researchers (Kirchner S. et al., 2013) using yellow and suction traps. In a week, from 20 to 3000 aphids fall into the trap (Fig. 5). But virus carriers among them are only 10-20%, the rest are non-specific tree-shrub species. There is practically no peach aphid in the north; potato is rarely found.
The continuation of the article will be published in the journal Potato System No. 3, 2020.