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Sulphur + baking soda = Healthy organic apples

The right combination of spraying technique and organic insecticides can significantly reduce the incidence of scab in organic apples.

[Translate to English:] Selvom skurvede æbler kan bruges til saft eller most, er det mere tilfredsstillende, at forbrugerne kan sætte tænderne direkte i æbler, der er avlet for at være sprøde, runde og saftige. Foto: AU Årslev

The right kind of spraying technique combined with sulphur and baking powder is a good recipe for warding off destructive apple scab in organic apple orchards. This is the finding of researchers from Aarhus University who for two consecutive years tested a number of apple varieties in the orchard at AU Aarslev.


Scab is the worst disease of apples in terms of the proportion of eating apples that needs to be discarded. Although scab does not destroy the apple as such, the brown spots look unappetising and reduce the shelf life of the apples. The apples are discarded when scab covers more than 1 cm2 of the apple.


Limiting apple scab is a major challenge not least for organic apple growers since there are very few approved remedies against fungal diseases in organic apples. The use of elemental sulphur is permitted, but the method requires frequent spraying, since sulphur is not very persistent.. Spraying with sulphur usually takes place just before or while it is raining. Apple growers therefore depend on reliable weather forecasts so that they can spray at the right time.


Good effect of spraying with sulphur and baking powder

To test if a more efficient procedure combined with a good efficacy against scab would be possible, the researchers compared a system of normal sulphur spraying with that of a sprinkler system with sulphur.


- The advantage of using sprinklers is that they are a permanent fixture in the orchard. You do not need to get equipment out every time you have to spray. They require only a small window of opportunity to work in, and you can delay the sprinkling until you actually see the rain coming, explains senior researcher Marianne G. Bertelsen from Aarhus University.


In the second year of the study researchers also used baking powder as part of the control strategy. Baking soda has a curative effect on scab. It acts on the fungal spores of the scab when they germinate but before they infect the leaves of the apple tree. Infection occurs mostly from the time of budding to Midsummer. When it rains, the spores – especially from dead leaves – are splashed onto the tree. Apple trees are therefore treated with baking powder immediately after it has rained.


The tests were conducted for two consecutive years on nine varieties of organic apples. One lot of trees was not treated, another was treated in the usual way using a standard sprayer and a third lot was treated from a sprinkler positioned by every third tree in the orchard. The first year treatment consisted only of sulphur while in the second year baking powder was also used.


The results in the second year, when the sprinkling technique had been well incorporated and baking powder was also used, were significant. Of the non-treated apples 45 percent had to be discarded because of scab. The proportions of discarded apples in the lot sprayed using a standard sprayer and the lot sprayed from a sprinkler were, respectively, one percent and three percent.


- Many of the organic apple varieties that have been developed over the years were initially resistant to scab, but over time the resistance has been overcome. It is therefore pleasing to see that the treatment with sulphur and baking soda works effectively. With a sprinkler system the spraying frequency can be reduced, which is an advantage for apple growers in terms of labour and costs, says Marianne G. Bertelsen.


Read also the article "Less scab on apples grown under cover" here.


The study was part of the project Sustainable future for Danish fruit which was funded by the Danish AgriFish Agency and Plan Denmark. Read more about the project here.


Further information: Senior researcher Marianne G. Bertelsen, Department of Food Science, e-mail: marianne.bertelsen@agrsci.dk, telephone: +45 8715 8328