Dutch Crop tour onions - week 34
Downy mildew explosion in onions continues
The explosion of downy mildew in the seed onions has continued in recent weeks. Multiple plots turn brown from this fungus and hot spots are common. The infestation with downy mildew has meanwhile resulted in a significant loss of yield.
The fungus downy mildew has made a comeback this year's growing season. The month of July and August had numerous chances of infection and that is now clearly visible: almost no plot is free of this fungus. Like phytophthora, downy mildew does not cause any problems with a small infestation, but as soon as the fungus is present, all alarm bells should go off.
Incubation period
A tight spraying schedule is the key to keeping the crop healthy. But in a season like this with a lot of moisture and a long wet leaf period, that is easier said than done. What makes it especially difficult when dealing with downy mildew is that the incubation period of 10 to 16 days is very long. The infection of downy mildew can therefore have taken place some time ago, before the crop shows any symptoms. That, together with a favorable weather type for fungi, makes it difficult.
Loss of revenue
Where the organic fields used to have a hard time, the fungus has also struck hard in conventional fields. Arjan Bos, onion crop specialist at Hazera, sees yield loss on several plots: "There are plots that have had to lose significantly in yield due to downy mildew. But it's not just downy mildew. The early infestation of stemphylium also plays a role in this. In the event of an infestation, the plant is a lot more susceptible and goes into stress. The crop then goes into survival mode and that will cost you kilos at the end of the journey."
The crop tour plots are also not completely free of downy mildew, but it has not suffered from any problems. The growers are keeping a close eye on it, although the first plots are also almost ready for harvest. The growers of De Gewastour do have other onion plots where fireplaces can be found.
Interval too long
In a humid season like this, it once again shows how quickly fungi can develop and what problems it can cause. In spite of a strict spraying schedule, if the plot is carefully checked, the fungus will fly in. The interval of the means therefore seems too long in the seed onions. That could get worse in the coming years, if you realize that this is the last season in which the active ingredient mancozeb is allowed in crop protection products.
Jesse Torringa
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