Prevent downy mildew by spraying in time

Experiences from the 2024 season show that a timely start with spraying against downy mildew is crucial to keep the fungus under control. Because the onions are ahead in their development compared to previous seasons, there is sufficient reason to pay attention to this now, says Steven Dorrestijn. "Starting early and preventatively is simply the best."

"Downy mildew has already been seen in winter onions. And it will undoubtedly rain again. Then you not only get a huge growth spurt, but the spores of the downy mildew also get a chance to infect again. Because spores are always there. I don't know of a year that there was no mildew. These are just a few risk factors that onion growers have to be alert to." Dorrestijn still remembers exactly how things went wrong in many places last year. "2024 was a very wet spring with extremely early downy mildew infections in onion sets. The fungus quickly spread to the seed onions and we saw the first infestations in early June. But many growers only started spraying against downy mildew in the usual period, around 25 June when the onions are approximately at the 4-tube stage. And then it started to rain and people could not go into the fields for 10 days. Many practical plots were infected during that period. Things also went wrong in our own trials. But we also saw that we kept the fungus under control on the objects that had already been sprayed in the first half of June."

Don't worry unnecessarily
Dorrestijn does not intend to unnecessarily worry onion growers, he swears. "We have had a very dry spring so far with virtually no chance of infection by downy mildew. But the onions are well ahead in their development and if you have risk factors in your area, I advise you to start spraying mildew a bit earlier. Because you don't want to be behind the facts the entire season."

Dorrestijn does not only think of winter onions and future precipitation when it comes to risk factors. "In plant onions and silver onions, the climate is much more humid than in seed onions. The fungus therefore has a better chance there. And in irrigated plots, favourable conditions for downy mildew are also conceivable." If such risk factors do not play a role, growers can, according to the Syngenta advisor, simply adhere to the rule of thumb of starting with the mildew sprays at the 4-pipe stage. "But that stage has been reached earlier this year than we were used to in previous seasons," he warns.

2025 Resource Package
An additional reason to pay sufficient attention to downy mildew is the available package of products. "The good news is that we can still set up a schedule", says Dorrestijn. But we also see that the possibilities shrink a little every year. For example, because a product may be sprayed less often or gets a different combination partner. Fortunately, our Orondis plus Amistar is still available with the same label as last year. And the experiences with the product are undoubtedly positive." According to Dorrestijn, the Orondis Plus Amistar is the best choice for spraying on T2, T5 and T8. In addition to downy mildew, the Orondis Plus Amistar also tackles secondary fungi such as leaf spot disease. The product may be applied a total of 3 times with a minimum interval of 21 days. In the intervening periods, it can be alternated with other products. "That is always sensible from the point of view of resistance management", Dorrestijn continues. For the T1, he advises spraying with azoxystrobin + adjuvant of another brand name. "Then you spray a broad-spectrum fungicide that is also strong on downy mildew." Finally, the Syngenta employee draws growers' attention to the fact that they may spray a product with the active ingredient oxythiopiprolin a maximum of 3 times. "The advisors usually keep an eye on that, but forewarned is forearmed."