Downy mildew and fusarium set the agenda

Stefan van Heist is the new onion specialist at Bayer. "In my position as Crop Advisor Outdoor Vegetables and Strawberries, onion cultivation in the South of the Netherlands was already part of my package. That's why I'm already quite familiar with it," he says. Bayer spoke to him about the main problems and challenges in cultivation.

Open field vegetable and strawberry cultivation differ considerably from arable onion cultivation. Can these two worlds be combined within one function?
"Yes, I think that's going well. The themes in onions are largely the same as in leeks, for example. Stemphylium, fusarium and thrips also play a major role in cultivation there. The main difference is that onion cultivation is many times larger and that the impact and interests are therefore also greater. There are also many more parties involved in onion cultivation. That makes it both more complex and more challenging."

What do you currently see as the biggest challenges in onion cultivation?
"These are without a doubt the soil-borne diseases. We are of course talking about fusarium, but certainly also about white rot and pink root. All three diseases are on the rise, especially in areas where a lot of onions have traditionally been grown. For many growers, it is That's a hard blow, especially since onions have been one of the best netting crops for years."

"The most frustrating thing is that we don't actually have a real solution for these diseases. In other words: we do have some puzzle piece, but we can't put a complete puzzle with it. For example, the breeding sector is doing its best to build in tolerances against fusarium, but those developments are still largely in their infancy. Furthermore, the cultivation rotation is being expanded here and there and some are switching to alternative cultivation methods, such as onions on ridges. How successful this is against fusarium will have to be seen in the coming years."

"We also have a few pieces of the puzzle in terms of crop protection, such as a soil application with Rudis. Rudis is known that - with an average contamination - you get about 15% to 20% more healthy onions from storage. That is admittedly a nice and almost always profitable application, but it is not a magic bullet to banish fusarium."

"The same applies to the biological product Serenade, with which we are conducting several practical trials in seed onions this season. We already know from potato cultivation that Serenade offers protection against, among other things, rhizoctonia, silver scurf and streptomyces, and from various cultivations under glass we know that the product is also preventive. works against pythium and fusarium. These are indications to us that it can also work against fusarium and white rot in the onions. This season we mainly want to gain knowledge about the effect and the best way of applying it. We expect that Serenade will once again be a piece of the puzzle in the fight against soil diseases.

For now it remains with puzzle pieces. A total solution for soil-borne diseases is not in sight?
"No, there simply isn't. Soil-borne diseases will continue to set the agenda - especially in the older, intensive cultivation areas. It is all the more important to warn onion growers in the newer cultivation areas not to let it get to that point. crop rotation of at least 1 in 6, pay attention to host crops for fusarium, such as peas, spinach, Japanese oats and also goosefoot, and also check field margins (mixtures) for possible host plants. ever apply!"

What about fungal diseases in onions; what is the biggest bottleneck there?
"Then we are mainly talking about downy mildew. This fungus remains the most elusive of them all, mainly because of its long incubation period. In other words: once the fungus is in the crop, there is still at least fourteen days of misery on its way. Growers who have had to deal with it know how quickly the fungus can rage through the crop and how extensive the damage is. Downy mildew is not called perenospora destructor for nothing."

"We see the biggest problems in areas where, in addition to seed onions, many onion sets and winter onions are also grown. In some regions, onions are grown on the land almost all year round, which poses major risks in the form of early outbreaks of downy mildew."

What about the knowledge about downy mildew; is it complete?
"We know a lot, but by no means everything. What worries me, for example, is that we know almost nothing about strains of downy mildew. What perenospora strains are there? What are the relationships in the field and how are they developing? And which strains are the most aggressive? All matters that have never been researched until now, while in my opinion this can provide a lot of useful information. In potatoes, for example, we already know a great deal about the various phytophthora strains and we adapt crop protection accordingly It would be nice if we could also respond to downy mildew in onions in a much more targeted way."

To what extent can downy mildew be predicted?
"That remains difficult. There are BOS systems available that predict the disease reasonably well, but that mainly applies to the plots on which the weather posts have been placed. If you want to predict the disease more broadly and more accurately, you have to measure the relative humidity on each plot. , leaf wetness period and temperature. That is still very difficult in practice."

What about the availability of downy mildew remedies?
"That package has been thinned out considerably in the meantime. At the moment we only have Fandango, Orondis® Plus + Amistar and Zorvec® Endavia against downy mildew in seed onions, whereby the first two are also partly in the same active group and the Orondis Plus and Zorvec consists of the same active substance. Alternation - based on the FRAC guidelines and adherence to the label dosages - is therefore more important than ever to prevent resistance."

"Specifically for smaller onion crops - including onion sets and shallots - Infinito against downy mildew has been available since this season. This product is best suited at the very beginning of the cultivation - at the place where the first Mancozeb sprays were previously applied. An option is to first spray with Infinito twice and then start with the Fandango block.Although Infinito is not as strong as the aforementioned trio, it does offer an extra preventive option to keep downy mildew out of the crop for longer. We have, among other things, from experiences in spring onions, where the product has been permitted for some time. With Infinito, spring onions are well protected against the disease. In Belgian trials, the use of Infinito in onion sets also shows a small additional yield. All in all, these are good reasons to include Infinito in the spraying schedule for onion sets."

Now about thrips: to what extent can they be controlled with useful insects?
"In recent years, we have done a lot of research into resources that are selective for consumption, such as Batavia and FLiPPER, and into the role of flower strips. Uses can play a role, but it has not yet been possible to have them flown in en masse from the environment to keep the thrips under control. Occasionally we see that the larvae of the lacewing fly are present in large numbers and that they then make a major contribution to the control of thrips.But why this works sometimes and not other times is still a mystery."

"The basis for effective thrips control therefore remains a timely application - usually June - of two sprayings with Batavia. After that, the green agent FLiPPER can prevent any remaining thrips from causing damage. FLiPPER is best used in the early morning on a dew-wet crop or a crop that has recently been irrigated."

_{Orondis® Plus and Amistar® are registered trademarks of Syngenta
Zorvec® Endavia is a registered trademark of Corteva Agriscience}

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