Yield loss due to downy mildew in onions

After three extremely dry years, pressure from fungal diseases returned in 2021. As a result of a warm and very humid air flow at the beginning of the growing season, many onion growers have had to cope with substantial yield losses. The cause: downy mildew.

Downy mildew is back in 2021. Hardly any onion field remained free of this fungus. Due to the warm and humid weather conditions, the onions were not able to cope with this disease. The long incubation period (10-16 days) of downy mildew does not make control any easier. When the infestation is seen in the field, the infection took place much longer ago and therefore the control with a curative agent is hopeless.

To achieve good results, a tight spraying schedule is crucial. However, even this was not easy due to the weather conditions last season. Due to the large quantities of moisture and long leaf-wetness periods, determining the right spraying moment was a difficult job. The shrinking range of agents did not have a positive influence on the control of downy mildew either. Johnny Remijn, cultivation specialist at Delphy, expresses his concerns about this. He says: “Now that the weather type causes maximum fungal growth early in the season, we are forced to face the facts again. If nothing changes to the possibilities, downy mildew will make profitable onion cultivation in this weather type very difficult in the years to come.”

The outcome of the webinar ‘What does the Green Deal mean for crop protection?’, organised by Nieuwe Oogst and Bayer, was also very clear. Decision support systems will play a very important role in European (onion) cultivation, with the Netherlands being no exception. A BOS system contains models that use weather data, among other things, to calculate the infection pressure of a disease and thus arrive at a crop protection recommendation.

SmartFarm, in combination with the FieldMate, provides insight into the disease pressure of a crop and helps to determine the right spraying moment. The models that run behind the system were developed by the internal expert team, by agrometeorologists Erno Bouma and Joost Nieveen. Among other things, the Disease warning system offers support in preventive crop protection, because a high disease pressure can be seen days ahead. In this way, the legally permitted minimum spraying interval can be better anticipated and plant health can be guaranteed.

By acting immediately when the risk of infection is high, the chance of infestation in the field can be reduced or even prevented. The spread of the fungus, by water and wind through hot spots in the field, will thus be limited as the season progresses. Besides the application of crop protection agents at the moment when the chance of infection is high, the time of application is also of great importance.

The moment a droplet leaves the nozzle and comes into contact with the plant is very important for the effectiveness of a crop protection product. The legally permitted minimum spraying intervals and the ever decreasing range of products make it necessary that the sprayed product does its work as effectively as possible. The Spray planner gives every grower more insight into this. With this component in the SmartFarm app, the optimum spraying moment can be determined, so the onions are better and longer protected against infection by downy mildew. When a spraying does not take place at the optimal moment, it is still very important to know how effective the pesticide has been. If it was not optimal, you know that the chance of infection will be higher and that a following spraying will have to be done sooner or at a better time.

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