RE SEARCH Variety choice test helps grower to tailored PCN-resistant varieties Het telen van resistente rassen is voor aardappeltelers een belangrijk middel om aardappelmoeheid de baas te blijven. Maar soms valt het bestrijdende effect tegen. Een rassenkeuzetoets kan dan helpen. “Tussen op papier gelijkwaardige AM-rassen zitten verschillen, net als tussen aaltjespopulaties. Wij toetsen de rassen daarom met aaltjes van het perceel zelf”, legt Egbert Schepel van HLB uit. F or potato growers growing resistant varieties is an important way of controlling potato cyst nematodes. However, the effect of the PCN control is sometimes disappointing. In such a situation, a variety choice test can help. ‘There are differences between PCN-resistant varieties which, on paper, are shown as being the same, as is the case with the nematode populations. This is why we test the varieties with nematodes from the grower’s own plot’, Egbert Schepel of the Dutch research and consultancy company HLB explains. Nematode specialist Egbert Schepel from HLB consultancy laboratory in Wijster has talked to several potato growers in recent years to discuss potato cyst nematode control. For many growers using resistant varieties forms an important part of the strategy. ‘Whether it’s a seed potato grower who works towards a zero situation, a consumption potato grower who wants to keep on growing a popular, susceptible variety, or a starch potato grower who can accept a certain level of infection with the current supply of resistant varieties everything is possible’, says Schepel. ‘But’, he says, ‘in some cases, a variety doesn’t exactly perform as you had hoped and the infections are higher than expected, in which you case you have to look further. A resistance figure is not conclusive, you see. We have to deal with genetic variation of both the nematodes and the varieties. To establish the relative susceptibility of a variety for the European varieties lists, fixed protocols are used. This means that standard populations of nematodes are used, which correctly reflects the average situation, but may disappoint in individual cases. Moreover, the controlling effect may decrease after a few crop rotations because virulent nematodes manage to multiply changing the genetic composition of the population. Growers are increasingly having to deal with this. The trick is to find the right match: the right variety and the right infection. And that’s what we’re doing with out variety choice test.’ ‘It’s often thought that starch varieties always score better than consumption varieties where potato cyst nematode is concerned. That is not so. There’s quite a lot of variation’, according to Egbert Schepel. Globedera pallida (white potato cyst nematode) In most cases, the problems occur with a Globodera pallida infection. The genetic diversity of this group is large and variety resistance is not absolute, as is the resistance to G.rostochiensis. Yet, G.rostochiensis may give an atypical picture as well, is Schepel’s experience. In order to be able to carry out the variety choice test, cysts with live content are collected in problem spots. In the HLB laboratory, the larvae from the cysts are released onto different varieties (see box), after which the multiplication speed on those varieties is determined. In order to give an idea of the differences found, Schepel describes a situation in which the cultivation of the susceptible standard variety Désirée would lead to a density of 100,000 live larvae per 200 ml of soil. A very serious infection. From the highly-resistant varieties which were examined in the same test, the prognosis of the worst-scoring variety was 50,000 larvae. ‘This still means a large problem area. The best-scoring variety - which on paper has the same relative susceptibility - appeared to be able to reduce the infection to fewer than 1,000 live larvae. This is quite a big difference. For a starch or consumption potato grower, this is a situation which, in combination with row-spray treatment, is reasonably promising.’ The variety choice test is not new though. In the 1980s, when the Peat District (province of Drenthe) in particular encountered high PCN infections, HLB was already testing widely for soil samples originating from infected plots. Following that period, the highly-resistant varieties were developed, and the nematode problems quickly decreased. ‘You cultivated a resis28 Potato World 2017 • number 1 Pagina 27
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