Dutch Crop tour Onions - week 29
Treacherous year for downy mildew in onions
Downy mildew has been a hot topic this past year, and it's been heard more and more lately. Downy mildew was already quite prevalent in onion sets, but the first infestations are now also appearing in seed onions.
You might not expect it in this dry year, but recently there have been increasing opportunities for the fungus to infect. Yesterday morning (Monday, July 14th) was a prime example, with the whole of the Netherlands covered in a thick layer of fog. Last year, downy mildew pressure was incredibly high due to the wet growing season, but most growers will likely remember 2021 more fondly, when the fungus caused significant yield losses in several locations.
Downy mildew is rearing its head
Downy mildew pressure varies by region. In places where, a few weeks ago, downy mildew was primarily found in onion sets, there are now numerous reports of infestation in seed onions. Jesse Torringa, commercial promoter at Groot & Slot, confirms this. "We're seeing downy mildew in several fields. Currently, the infestation is relatively limited to the occasional pipe, but it could worsen rapidly in the near future. The infestation you're seeing now is from two to three weeks ago," he says. "Growers are alert and are well on top of it with crop protection."
Steven Dorrestijn, crop advisor at Syngenta, indicates that downy mildew has already been found this year in the 4-pipe stage. This means that with an incubation period, the infection in some cases already began in the 2-pipe stage. Although the spring was dry and relatively warm, that is precisely what makes it treacherous, Dorrestijn emphasizes. "Such days are often accompanied by large temperature differences between day and night," he explains. According to him, this actually favors downy mildew. "Irrigation, which occurred on a large scale this spring, also contributes to a moist microclimate in the crop, which increases the infection pressure." The variety also makes a difference, Dorrestijn explains. "We are clearly seeing this year that varieties with abundant foliage are more susceptible. They create more biomass and thus a dense, moist crop with a high relative humidity, ideal for the development of downy mildew."
What makes it so difficult is that the available remedies against the fungus are only preventative and no longer have any curative effect. Dorrestijn warns: "The remedies we have are all preventative and, moreover, often belong to the same chemical group. If we continue at this rate, repeatedly applying remedies from the same chemical group, we will encourage resistance."
MH spraying
Many consider MH spraying the onions the most important. Properly executed spraying (at the right time) ensures good dormancy, which largely determines the onions' shelf life. The first sign that MH spraying is imminent is the pruning of the first onions. Jacky Dieleman from Philippine in Zeeland and Hubert Linders from Nederweert in Limburg report that the time for MH spraying is getting closer. Both participants believe that the onions will be ready in about a week. David de Wit from Lepelstraat in Brabant has already sprayed his first plot of seed onions with MH and expects to spray the Gewastour plot this week as well.
As we move further north, we also discover that the onions there aren't quite ready yet. Both Peter Holster from Marknesse (Flevoland) and Reinder Hogenhout from Kimswerd (Friesland) find it difficult to predict when their Gewastour plots are ready for MH spraying.
Jan Willem Veldman
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