Downy mildew rears its head in onions

We haven't seen it much in recent years, but downy mildew is clearly present again this season in both sowing and onion onions. The fungus is already found nationally, with both small and large infestations in the field. The rapid growth of foliage in recent times seems to be one of the causes.

There were plenty of opportunities for infection for the fungus in recent weeks with the humid weather conditions. It is not surprising that infections are already being found in the field throughout the Netherlands. It has been a while since the fungus was found so prominently in our country. Three seasons ago is still fresh in the memory of some growers, because the fungus caused yield losses in various places. 
 

Mildew attack on sowing onions. Mildew attack on sowing onions.

Downy mildew pressure varies by region, but small infestations can be found in most areas. This is also confirmed by Tom Langenberg, account manager of de Groot & Slot in Central Netherlands: "We see attacks of mildew in various plots. But it is not yet explosive in the region. Growers are on top of it with crop protection and pay a lot of attention to it." . A lot also depends on the weather conditions for the further development of the fungus, says Langenberg. "I am not yet very concerned about the current weather forecast because it is positive against the fungus," Langenberg refers to the dry conditions and higher temperatures. "But if we get a lot of dew nights, things could turn out wrong."

What makes it so difficult is that the available remedies against the fungus only work preventively and no longer have a curative effect. In addition, the fungus has a long incubation period of 14 days on average, which means that the fungus can already fully invade the crop before any damage becomes visible.

Climatic conditions were perfect for downy mildew
A strict spraying schedule with the right crop protection products does not always appear to be sufficient to protect the seed onions, even with the recent moldy weather. According to Steven Dorrestijn, Crop Advisor at Syngenta, this has to do with a combination of various factors. One of those factors is the rapid growth of foliage under the wet conditions of the past period. This means that new leaves will quickly appear that are unprotected against mildew. "And the climatic conditions were consistently very favorable for the fungus. Then the risk of infection is simply very high." According to Dorrestijn, a plant is quickly susceptible to the fungus, even if the plants are not yet fully developed and small. "In principle, an onion plant becomes sensitive to downy mildew from the moment it grows. Even if they are very small, barely 15 centimeters for example, they are already sensitive to downy mildew. And the point is, those are the most dangerous plants. Those onions make a growth spurt and grow 1 centimeter or slightly more per day, leaving the new unsprayed leaves very susceptible."

Dorrestijn also indicates that curative control is no longer an option. However, you can limit and control it as much as possible. "You can't do anything about it curatively where there is now mildew. However, you have to do everything you can to ensure that it does not develop further in the crop. For example, add Actirob to the OrondisPlus Amistar for better penetration and adhesion."

Jesse Torringa

Jesse is an editor at Boerenbusiness and focuses in particular on the arable farming sector, including grain and onions. He also closely follows the fertilizer market. In addition, Jesse works on an arable farm in Groningen with seed potatoes as the main branch.

j.torringa@boerenbusiness. Nl

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