Was powdery mildew manageable this year?

After a relatively difficult start, the onions got the taste for it at the end of June and started a growth spurt. Unfortunately, the downy mildew did the same at about the same time. Steven Dorrestijn saw that only growers who had already started spraying in June were able to control the mildew well. Time for a retrospective.

"I expect large variations in yield this year." Steven Dorrestijn immediately fires a warning shot. "I see plots that are still green as grass and have considerable growth potential, but unfortunately there are also plots that are completely covered in downy mildew. If that happens in the important growth period from mid-July to the end of August, you easily miss 15 to 20 tons." 

Early start essential
Looking back on the season, Syngenta's crop protection advisor can reconstruct well how the downy mildew was able to spread so much this season. "Around the longest day, many plots of seed onions were only 10 to 15 cm in size and the onions had 3 to 4 tubes at most. Not the stage at which onion growers are used to starting with fungicide spraying. The onions grew moderately but were already starting to bulb and therefore relatively susceptible. In addition, there must have already been downy mildew at that time, probably from plant onions. And when the crop started its growth spurt at the end of June, beginning of July due to the higher temperatures, many growers were unable to drive due to the heavy rainfall. The growers who did start spraying in the second half of June were able to control the mildew well in most cases. But on many plots they continued to lag behind and the number of onion plots that remained completely free of the fungal disease can be counted on the fingers of one hand, so to speak." Dorrestijn also saw in a connection test of a crop protection company that starting early was essential this year. "In one of the objects they started spraying a week earlier and that object stayed clean. All other objects got mildew, regardless of the spraying schedule."

2025 won't be any easier
The advice to start spraying early and preventively against downy mildew has proven its worth once again this year, Dorrestijn continues. "I hope and expect that growers realize that the premature death of many plots was due to the downy mildew and not to a natural process. And that is why I assume that growers will be more on their guard next year." Combating downy mildew will not be any easier next year, the Syngenta advisor warns. "The exact picture is not yet known, but assume that we will have fewer resources and also fewer sprayings next year. Zorvec Endavia will disappear and Fandango may only be used twice. There is still a new product in the pipeline and when that arrives we will probably have just enough applications for a season of normal length. But it will be a matter of trial and error again."

New positioning Orondis plus Amistar
A proven agent that will remain next year is Orondis plus Amistar. "That is still the strongest combination in trials," Dorrestijn knows. "Because the agent picture will change next year, we have adjusted our advice for Orondis plus Amistar. We are now positioning it at T1, 4 and 7. Then you use the permitted maximum number of sprays. And you benefit at crucial moments from the effect on downy mildew, leaf spot disease and Stemphylium. Because all those fungi are dormant every year, but you never know in advance which one will raise its head."