Brought to you by Bayer Crop Science
'Witrot and Fusarium: broad and structural approach needed'
"A great, insidious and often underestimated danger that makes new victims every year." With these words, Mark Ermers of Bayer Crop Science outlines the ever-increasing problems with white rot and Fusarium in onions. In order to suppress these soil diseases as much as possible, they must be tackled in all possible ways – in an integrated manner, according to the onion specialist.
In his view, the most opportunities for this lie at the 'front end' of cultivation, such as a sufficiently wide rotation, a healthy, resilient soil and the right variety choice. In addition, growers also have the option – prior to sowing – to apply a soil application with Rudis to be carried out.
To start with: how big is the problem of soil-borne diseases in onions?
“We don't know exactly. It is estimated that at least 10 percent of the current onion area is damaged to a greater or lesser extent by soil-borne diseases such as white rot and Fusarium. I personally think that this percentage could be much higher. By no means all infections come to the fore, so to speak, all the more so when it concerns places in a plot – often headlands or spray tracks. In addition, growers are not for sale with these kinds of problems."
White rot is a well-known disease that has been circulating in onions for many decades. And yet the infections are increasing. How is that possible?
“White rot is indeed a known problem in Zeeland – the oldest onion growing area – but white rot has also been occurring on the somewhat lighter soils in the Noordoostpolder for at least 20 years. The biggest drawback is that white rot remains in the ground for a very long time. I know of companies that stopped growing onion 40 years ago because of white rot and where it is now being tried again. So that turns out to be nasty. The sclerotia remain intact for many decades and immediately become active again as soon as an allium root passes by. It is therefore very important to prevent a white rot infection, because once you have it, you almost certainly can't get rid of it."
What about the newer onion growing areas, such as the northeast of the Netherlands; does white rot also occur?
,,The problem of white rot is hardly an issue there. In addition to the fact that onion cultivation is still relatively new here – and white rot has not yet been able to settle in – white rot does not thrive as well at a low pH and can therefore develop less well on the more acidic soils in the Northeast.”
Dan Fusarium. How big is that problem?
“Fusarium has become the most damaging soil disease in onions in recent years. Like white rot, it is very persistent – it survives in the ground for at least 15 years – and shows itself mainly under less favorable growing conditions, often first on headlands and in spraying tracks. A tricky problem is that we don't really know the fungus well yet. At the moment we assume that mainly the strain Fusarium oxysporium f. sp.cepea is the main culprit, but research is now underway within Uireka into the virulence of other strains."
What measures can you take against Fusarium and white rot?
,,First of all, it is important to realize that white rot and Fusarium not only negatively affect the quality and quantity of the next harvest, but that they are also increasingly affecting the continuity of the cultivation as a whole. Measures must therefore extend beyond the next crop and also secure the harvests after three or four rotations.
I therefore see the most important measures against white rot and Fusarium mainly at the 'front end' of cultivation. That is to say: the right choice of field, a wide crop rotation (better 1-to-8 instead of 1-to-6), good industrial hygiene and a smart choice of varieties. As a grower, you have to work with this in any case.”
Onion growers have the option of applying a soil treatment with the product before sowing Rudis to be carried out against white rot and Fusarium. What can they expect from that?
,,The big plus of Rudis is that you can secure an important part of the proceeds. Multi-year trials have shown that with an average Fusarium and/or white rot infection, approximately 30 percent fewer onions are affected after application of the product. In other words: when using Rudis an average of 30 percent more salable product remains. In heavily infected fields, the damage caused by Fusarium and white rot is up to 20 percent lower. It is therefore very soon profitable to Rudis to be used on slightly contaminated plots.
Operates Rudis just as much on white rot as on Fusarium?
“Yes, about the same percentage increase in the number of healthy onions applies to both diseases. In practice, Rudis often referred to as 'Fusarium repellent', but it certainly works just as well against white rot. And maybe good to emphasize: with Rudis you do not eliminate the problems with white rot and Fusarium. Preventive measures to prevent these diseases should really be number one!”
Finally: are aspects that you have to take into account when using Rudis?
“It is important to avoid direct contact with onion seed. The active ingredient – prothioconazole – can be phytotoxic to the seed. It is therefore preferable to carry out the soil application before the last tillage, so that the product is optimally distributed throughout the top layer."
