Continue to work on virus strategy

In recent years, a lot of new knowledge has been gathered about the approach to Y-virus in potatoes. What is the latest state of affairs? And how should we actually view the return of the leaf roll virus? We asked WUR researchers Martin Verbeek and Klaas van Rozen for the latest insights.

Martin Verbeek and Klaas van Rozen have both been involved in a large-scale four-year project in the field of viruses and aphids in recent years: the PPS Virus & Vector Control. Verbeek on the fundamental side, from Wageningen, and Van Rozen as a practical researcher in Lelystad. For four years, the project provided a stream of information, on the basis of which growers and advisors could adjust their course. And because not all questions have been answered yet, a follow-up project is in the making.

In recent years, much attention has been paid to the potato Y virus (PVY). Last season, the leaf roll virus suddenly reared its head en masse. Were you surprised?
Verbeek: "No, that was definitely coming. After the ban on neonics in 2019, you immediately saw the yellowing virus return to sugar beets the following year. Just like the potato leaf roll virus, this is a persistently transmitted virus. This means that once an aphid is infected, it continues to release the virus. For the record: Y-virus is a non-persistent virus, which is picked up by the aphid during test drilling and can be released again immediately. With a persistent virus, it takes a while before the virus is absorbed by the aphid and can be spread. But because seed potato growers also have fewer insecticides at their disposal, aphids were given more and longer opportunities to pick up the virus. The fact that it took a number of years for the virus to show itself on a larger scale is also easy to explain. The leaf roll virus was really gone. Growers had not seen it for years. Yet it was only a matter of time before the virus would start to build up again."

What have been the most important lessons from the many studies into the Y-virus in recent years?
From Rozen: "The take home message was: the problem is in the potatoes. The potato itself is the most important source of the virus; we have not found PVY in other plants until August. The virus usually spreads within the plot or from other potato plots and storage plants. In addition, an important insight is that viral disease is a problem that you should tackle with integrated management. This means that stacking preventive measures is the most important strategy. Fortunately, we also see and hear this from the sector. More growers have started covering their basic material with mesh, straw is used and maintaining distance between different batches is also better remembered.

Verbeek: "And don't forget the use of resistant varieties. That is the most certain route. As an individual grower, you have no direct influence on that, but you do see that breeders are working hard on this. Although they also have to take into account a whole range of other desired properties."

Covering with straw or sowing grain on the ridges is being taken up quite widely. Also spraying white ridges is being investigated. Is that the right way?
Verbeek: "Confusion seems like a good approach; we have seen good results from it. To be clear: it is primarily about contrast. I also sometimes get questions from tulip growers who say: 'It doesn't work for me'. But their straw has been outside all winter and has almost turned the same colour as the soil. The visual effect counts. Incidentally, we do not know exactly how it all works. We hope to gain a better understanding of the mechanisms with additional research."

What research is currently underway in the field of tackling virus Y?
Verbeek: "We have continued research into the use of silicon, commissioned by BO-Akkerbouw and the potato sector. Within the PPS Virus & Vector, we saw favourable effects of this. The use of the fertiliser led to smaller numbers of aphids, but that did not immediately translate into noticeably less virus infestation. We know that silicon makes the cell walls of the plant stronger, but the precise mechanism behind this is also not yet clear. And we do not yet know how silicon can best be absorbed by the plant. So there are still quite a few fundamental questions that we want to investigate further."

Van Rozen: "This year we are conducting a field trial on a practical scale, in which we apply silicon in the planting furrow. We are also going to create an additional object with the application of silicon via fertigation. We are using two varieties and, in addition to the effect on Y-virus, we are also taking into account the effect on leaf roll virus. You can see that it is a topic that is alive. We get a lot of questions about it."

Early selection is also important for controlling the Y-virus. The hope is that cameras will soon be able to see the first symptoms in the leaf earlier than the human eye. Is that hope justified?
Verbeek: "Early detection is certainly possible, although it is still laborious to do that in the field. For that, you need to be able to lower a hood over the crop, so that you can measure under controlled light conditions. With the help of a flash of light, you can then see where the chlorophyll has been affected. Damage by viruses, but also bacterial diseases or phytophthora, cause a certain stress reaction in the plant. You can make that visible."

You have also looked at models that can better predict the first aphid flights, using a temperature sum. How are things going with that?
Verbeek: "We did indeed find a relationship with the number of cold days. But a real tool has not been found. We use traps ourselves, for the accurate monitoring of insect flights. In them we see how the numbers of various insects develop. We start with that in mid-March. We always see the first peak of the aphids in May; normally in mid-May and after a mild start of the year a few weeks earlier."

There is a trend to leave green manures and field margins over the winter. Does this pose a risk to the survival of aphids and possibly also leafroll virus?
Van Rozen: "I think that's not too bad. As winter progresses, many green manures disappear and are incorporated. Delicate insects such as aphids do not survive this and without regrowth I do not expect a population in those plots. We have not found any PVY in field margins, but there may be small numbers of aphids that fly a little earlier. In addition, greenhouses and sheds can also be a more important source of early aphid flights."

Verbeek: "An aphid lives for a few months at most. It is unlikely that the leaf roll virus in an aphid survives the winter. I expect a longer growing season to have a greater effect, as a result of which the females produce more offspring. That contributes to a higher vector pressure and, depending on the number of virus sources, virus pressure."

The effect of mineral virus oil varies greatly in practice. It has also been shown that it hardly protects clean plants. How important is the role of oil?
Verbeek: "That role remains important. We are increasingly understanding how mineral virus oil works and what its limitations are. We know that it mainly helps prevent the aphid from picking up the virus. Apparently, the oil layer ensures that the virus attaches less quickly to the stylet (the aphid's sucking snout). And what you don't pick up, you can't transmit. But it is also true that a layer of oil does not sufficiently protect clean plants. The stylet is not 'washed clean'. Once the virus is on the inside of the stylet, it attaches itself there with the help of a sticky protein. Apparently, it can therefore get through the oil layer without any problems."

Van Rozen: "The beneficial effect of mineral oil is beyond dispute, but it is important to realise how it works. It can still significantly slow down further spread within your own plot. But the other side is: you cannot keep the virus out with it."

There are plans for a new PPS around virus and potatoes. How is that going and what should be in it as far as you are concerned?
Verbeek: "We are indeed holding discussions. The intention is to also develop more knowledge about the leaf roll virus and in particular how we can control that virus in a sustainable way. We would also like to know more about the behaviour of aphids; what do they respond to? Which scents and which colours influence the behaviour? With the help of new techniques, we can map that even more accurately." 

Van Rozen: "We still have to discuss the exact content. Fertilisation is also a point of attention. For example, we would like to measure the colour of the leaf and the ingredients. These influence the landing behaviour of the aphids, while the nutrients in the plant determine the development speed of the aphids. Furthermore, when leaf roll virus is transmitted, controlling aphids is more important than with PVY. The use of natural enemies and the use of green agents are therefore also on the agenda. In addition, there will again be attention for techniques for early detection, both of Y-virus and leaf roll; when and how do you see the leaf roll symptoms? That will hopefully give growers tools to intervene earlier."

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