Preventive Fusarium test for onions

Research Institute HLB - De Groene Vlieg has been offering a new service for several months: a soil analysis to detect Fusarium. "With this test, onion growers can gain insight into the disease risk of their planned onion fields for the new growing season," explains Account Manager Sanne Graafstra, who is closely involved in the development and rollout of the test. Bayer CropScience spoke with her.

The Fusarium test is performed on plots that growers have registered with us. Two sub-samples are first made from a single soil sample. One is analyzed immediately for the presence of Fusarium, while the other is first cultured for three days at 25 degrees, creating an optimal climate for the fungus. Should the fungus be present in insufficient quantities in the direct sample, it can still be detected via the cultured sample. This allows even very low levels of contamination to be detected.

The PCR technique is used for the testing. This is a globally known technique that allows fragments of genetic material – DNA – to be detected in very low concentrations. "We have been using the PCR technique for many years to identify Globodera pallida nematodes and to demonstrate virus in potato tubers. And now also to detect the fungus Fusarium oxysporum f. sp. cepae."

How reliable is the test?
The PCR technique is highly reliable. This specific Fusarium species is detected from all fungi present in the soil sample. If there is any risk to reliability at all, it lies at most with the soil sampling. According to the protocol drawn up by Wageningen UR, one sample must be taken per maximum of 4 hectares. If this is done properly and 'honestly', it yields a reliable analysis regarding the presence of Fusarium. To ensure that this protocol is adhered to as closely as possible – and to more or less rule out unreliable analysis and reporting – the samples are currently being taken by employees of HLB - De Groene Vlieg.

The test provides – as you describe it yourselves – 'insight into the disease risk of the planned onion plot'. How concrete do you make that insight for the grower?
Based on our analysis, each sample is classified into an infection class. Currently, there are five: undetectable infection, lightly infected, moderately infected, heavily infected, and very heavily infected. The grower is shown in a simple and compact report which class their sample has been assigned to.

Do you also provide advice on how the grower can best proceed based on the indicated infection class?
No, we are currently only indicating the infection class. We have been designated by the research group Uireka to carry out this test on a larger scale. The advisory model still needs to be developed. Consequently, the interpretation of the outcome currently lies with the grower themselves – whether or not in consultation with their advisor. Based on the results, they can, for example, switch plots or adjust their choice of varieties. It may also prompt them to approach cultivation differently, for instance by starting to grow onions on ridges.

In any case, any form of advice regarding fusarium in onions remains very complex. There are so many factors involved in predicting the disease. Consider weather conditions, crop rotation, and field history. And let's not forget the role that cover crops and weeds play as host plants for the fungus. How should you weigh and assess these?

I view the Fusarium test as an additional tool for growers to assess the Fusarium risk. Nothing more and nothing less. This does not alter the fact that we also want to gain more knowledge about the disease, including from growers who have had the test performed. Therefore, this coming autumn we intend to send them a questionnaire in order to translate the risk class into advice. How were the onions grown? Were special measures taken against Fusarium? What do you see reflected in the batch from a tested plot? Questions like these can help to further refine the test and potentially provide more direction for the decisions growers can make based on the test.

When is it worthwhile to have a fusarium test performed?
"I would say: it makes sense for all plots where onions will be grown. Fusarium is present in all soils and can therefore—certainly under unfavorable conditions—strike hard anywhere. Last year, we saw several examples of batches that appeared perfectly clean when they went into storage, only to have to be scraped out a few weeks later with a lot of tare. Something like that costs the grower a lot of money. What I am trying to say is: fusarium is and remains a very elusive fungus, and with a test prior to cultivation, you at least have more knowledge at hand to weigh the risks."

How long does it take for growers to receive the results?
Usually, that is within two or three weeks. This depends partly on the number of samples we receive. If there are many in a certain period, we can fill a sample plate faster. The sooner the plate is full, the sooner we can analyze and report.

How great is the interest in the test?
"Since early November – when we started offering the test – we have carried out over two hundred tests (up to the second week of February). Many samples originate from older or more intensive onion growing areas such as Zeeland and Flevoland. But we are also receiving requests for testing from the newer growing areas on the Eastern and Southern sandy soils. As stated earlier: fusarium is present in every soil – even when there is no history whatsoever with onion cultivation."

Are only onion growers having land tested, or are there other interested parties as well?
The vast majority of the soil samples – I estimate around 95% – come from onion growers. But we also test for other interested parties. For example, last winter we carried out a number of analyses for onion trial fields of pesticide manufacturers and plots where they want to grow mother bulbs for the production of onion seed.

What expectations do you have of the test for the coming years?
Fusarium remains a hot topic in onion cultivation – especially in years when it rears its head on a massive scale. We also notice this in the attention it receives within Uireka; research into fusarium is very high on the agenda. Hopefully, in the coming years, using data from growers who have taken the test, we can contribute to better control of the disease and – in the longer term – also to an advisory model for growers.

Finally: what does the fusarium test cost?
The complete assessment – ​​sampling, analysis, and reporting – costs €200 for plots up to 4 hectares. If the onion plot is larger, it costs €155 more per 4 hectares. For a plot of 10 hectares, for example, this amounts to €200 + €155 + €155 = €510 per hectare.

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