All in the battle to keep Fusarium at bay

"We're seeing more fusarium in the onions than we expected based on the growing season. And white rot is now also occurring in places where we've never seen it before," say Koos Arens and Rick den Dekker of Van Iperen. They are clearly concerned about the increase and, even more so, the elusiveness of both soil-borne fungi. "We must do everything we can to keep fusarium under control, with healthy soil, undisturbed growth, and a strong and resilient crop as the foundation," they say.

"The soil structure was simply good on most plots. And it wasn't particularly wet during the growing season. That's why we're surprised that there's so much fusarium in the onions again. Perhaps the dry growing season combined with high temperatures caused stress in the crops. But even so, we didn't expect this level of damage." Koos Arens briefly outlines how fusarium – and also white rot – has once again caused many unpleasant surprises. Colleague Rick den Dekker confirms this picture, but adds that there are significant differences. For example, plots on ridges and with fertigation show significantly less fusarium, he observes. "The onions there simply grow better and more efficiently, and there's less stress. This not only results in a significantly higher yield, but also significantly fewer losses due to fusarium."

Healthy soil
A prerequisite for keeping fusarium under control is healthy, fertile soil. "Make maximum use of compost, solid manure, and green manures, and ensure good soil drainage. This may seem obvious, but we see that this isn't yet fully implemented everywhere. These kinds of things are essential for successful onion cultivation," says Arens. He also advocates paying close attention to the 'front end' of the cultivation process. "Is the plot suitable, and what is its history? What about the rotation? Have there been any previous problems with soil-borne diseases? Clear answers to all these questions must be provided before anything is planted." In this context, the crop advisor also mentions the new fusarium test from the HLB (Dutch Agricultural Crop Protection Agency). "This test, based on soil analysis, can be a valuable additional tool for gaining insight into the presence of fusarium and weighing the risks. This can prevent potential setbacks."

Uireka 4.0
Den Dekker also points to the new Uireka 4.0 study, which will begin next year. Fusarium research will once again play a prominent role in this project. "Besides researching and testing a large number of cultivation measures, the focus this time will also be on understanding the fungus. How exactly does Fusarium oxysporum work? How does the fungus develop under different practical conditions? And what are its host plants? Once we have a better understanding of these issues, we can take even more targeted measures against the fungus."

Serenade in first-year onions
In addition to targeted cultivation measures against fusarium, new agents (combinations) that could be effective against the fungus are also constantly being investigated. For example, last season, Van Iperen, in consultation with Bayer, conducted several field trials with the biological agent. Serenade in first-year onion sets. These were rolled out to two growers in the Hoekse Waard region and one in Voorne-Putten, all three on one hectare.

The strip trial compared soil application of Rudis (0,6 l/ha), soil application of Serenade (5 l/ha), a combination of both (Rudis + Serenade), and an untreated strip. The products were incorporated shortly before sowing – during the final seedbed preparation. Arens emphasizes that these were regular plots, where there are no specific problems with fusarium. "So it's a true field trial, the results of which are representative for the whole of the Netherlands."

Less tare with Rudis and Serenade
During the growing season, the plant numbers were recorded in each test plot and the length of the crop was measured. According to Arens, this did not result in any significant differences. The differences in yield between the four test plots were also quite small, although Rudis (compared to untreated) there was a clear plus of a few percent in the net yield in all sizes (8-21, 21-28 and ›28) and the combination of Rudis + Serenade again gave a small plus compared to Rudis solo.

"It therefore appears that the products Rudis and Serenade reinforce each other in some way, particularly in the form of reduced losses. We know that Rudis is effective against fusarium, but what does Serenade add to this? Does this product actively help suppress fusarium? Or does it primarily stimulate root growth and protect the roots, making the onion sets more resilient to soil fungi? These are questions we would like to clarify," Arens and Den Dekker describe their preliminary findings.

'Every kilo of additional yield is profit'
In any case, both men see the increased yield (in the form of reduced tare weight) achieved by Rudis + Serenade as a step forward. "Every additional kilogram of onion yield is a gain, and that's especially true for a capital-intensive crop like first-year onion sets. That alone makes it worthwhile to repeat the trial next year," they say.

Another area being explored next season is research into the persistence of Rudis and Serenade in second-year onion sets. The focus will be on whether, as with applications in seed potatoes, there's a "growth-away" effect. "Hopefully, Serenade will also show some promise here, as that would make the product even more interesting for practical use. In any case, we can certainly use any link that has an effect against fusarium," concludes Arens.

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