Variety choice test helps grower to tailored PCN-resistant varieties RE SEARCH tant variety and that was it. Those days seem to be over. We’ll have to start working in a targeted way’, says Schepel. A permanent place in the recommendations. In the Peat District, the test will have a permanent place in the recommendations in the coming years. The Stichting TBM (Foundation for Crop Protection Measures), Avebe (Potato Sales Cooperative) and LTO Noord (Federation of Agricultural and Horticultural Organisations) endorse its usefulness. The TBM Foundation had the variety choice test validated last winter. In order to do this, certain ‘problem populations’ have been doubly researched in combination with a few varieties. i.e. by HLB with their standard variety choice test and by the Averis breeding station in an official test. The difference between both tests, among other things, is the size of the jars, the duration and subsequently the cost of the test. The relative differences that came up from the tests appear to correspond well with each other. ‘For us this meant further confirmation that the test is giving accurate forecasts’, Schepel says. The recommendations for the starch potato area have been drawn up in such a way that, if there are fewer than 500 live larvae, it remains possible to grow highly-resistant varieties. Sampling is repeated every two years. If more than 2,000 live larvae are found, the advice is to have a variety choice test carried out, to ensure that the infection doesn’t get out of control. Schepel advises the consumption potato growers to follow similar procedures. ‘A variety choice test gives you important information. Although the variety is a blessing for the potato sector, growing Innovator twice doesn’t necessarily mean that your infection is gone. You still have to watch it carefully. The sooner you know what a variety is doing, the sooner you can consult with your buyers about growing a specific variety.’ Happy with variation Schepel doesn’t like naming varieties because varieties produce different results under different conditions. He can mention varieties from every trading company that sometimes do better or, in fact, worse than expected. He would also very much like to clear up a misunderstanding. ‘Many people think that starch varieties always score better than consumption varieties where UP TO 500 LARVAE PER CYST To make clear what we’re talking about, Schepel shows a jar with cysts: the survival structure of the nematode. The remains of a female filled with eggs. Each cyst can contain 50 to 500 larvae. ‘With five litres of soil on your machine, you can drag a lot of problems around’, Schepel emphasises. THE VARIETY CHOICE TEST The HLB variety choice test illustrates how a specific nematode population reacts on various varieties. To do the test, after the potato harvest, soil samples are collected from problem areas. On receipt at HLB, staff rinse the samples and collect the cysts with live contents. They are temporarily stored. The tests are started in February. The test is carried out in jars filled with 55 ml of sterile soil. A mini tuber is placed in each jar, which is in the white-dot-stage. With a syringe needle, 300 larvae are injected. After eight to ten weeks in the climatic test chamber, the samples are assessed and the number of cysts counted, after which a score can be determined. This reflects the relative susceptibility of a variety for the situation in the relevant plot. How many varieties can be tested depends on the number of available live larvae. One cyst contains 50 to 500 larvae. The test is repeated eight times for each variety. The control is the susceptible Désirée variety. Usually, apart from Désirée, seven varieties are tested for which the test costs 300 euros. potato cyst nematode is concerned. That is not so. There is also a lot of variation in that.’ Schepel has to laugh at the fact that the differences in relative susceptibility are expressed to three decimal places on the official list. ‘This provides some sort of pseudo-accuracy, which you can’t actually do much with. As long as populations stick to the rules, you don’t have to worry. But if there’s a problem, that list isn’t going to offer the solution. You’ll then have to start experimenting. And don’t forget to continue sampling to keep an eye on the situation.’ The seed potato growers that he advises are already the furthest ahead with this, which is logical because they have only one objective: a zero situation. Unfortunately, it’s the seed potato growers that are confronted with the limitations of the variety choice test first, because it needs sufficient live larvae to be able to test an extensive series of varieties. According to Schepel, the PCN approach continues to be a quest. ‘You’ll have to look at it on a case-by-case basis. If this variety is performing well on your land we’ll continue with it. We’ll then look again to see whether this corresponds with the nematode report. This is the way to put your finger on the right spot. An increasing number of trading companies are coming up with new resistances, which means that there’s more to choose from. However, there will never be an ultimate variety. There’s too much variation in soil life for that. In fact, I’m quite happy with all that variation in the variety range. It means that there’ll always be possibilities for solving individual problems.’ Egbert Jonkheer Potato World 2017 • number 1 29 Pagina 28
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