- It started as a crazy idea. What would happen if you put a roof over the apple trees? Would you be able to grow apples without the use of fungicides and without the apples becoming too infected, recounts senior scientist Marianne G. Bertelsen from Aarhus University.

The questions led to an experiment and to everyone’s surprise the roof over the apples gave amazingly good results.

- After only one year of results we can conclude that the crazy idea of putting a roof over the apples was not that crazy after all – and that a small roof makes a big difference to the incidence of all the diseases that attack apples, says Marianne G. Bertelsen.

Drastic fall in apple scab

In the experiment the scientists used Elshof and Rubens apples. Both varieties are very susceptible to apple scab, which is the worst disease that apples can get. Under normal untreated conditions, 95 per cent of the Rubens apples had to be discarded due to serious attacks of scab, the size of the fruit was halved and yields were only a third of what was harvested under a roof. With a roof over the trees, only 15 per cent of the apples had to be thrown out.

The picture was the same for the Elshof apples that are slightly more resistant to apple scab. Without a roof, 70 per cent of the apples had to be discarded and of the remainder only three per cent were without scab. With a roof, only 0-5 per cent of the apples were badly affected and in January after storage 85 per cent had no scab at all.

Other fungal diseases were also reduced or completely absent on the apples grown under roof.

Dry is best

- The explanation for the somewhat unexpected and very positive result is that the modest cover has been able to shorten the periods when leaves and fruit are wet from rain, explains Marianne G. Bertelsen.

Scab and many other fungal diseases require relatively long periods with wet leaves for the fungal spores to germinate and infect leaves. The length of the period depends on the temperature. At 10°C, leaves need to remain wet for 28 hours to become infected, but at 17°C this is shortened to 18 hours.

The roof does to some extent shade the trees, but this had no effect on apple colour, size or sugar content. For both varieties, there was a tendency for the flesh to be a little firmer in January after storage.

The next question is then – how much does it cost?

- Rain covers are often used in the production of sweet cherries where the cover has to stop the berries from splitting when it rains in the summer. Commercial systems are therefore available, and we hope to get an opportunity to test if putting a roof over apple trees can also make it a viable production, says Marianne G. Bertelsen.

The experiment was part of the project FruitGrowth aiming to optimise the Danish production of organic apples. FruitGrowth is an Organic RDD project. 

Organic RDD has been funded by the Ministry of Food, Agriculture and Fisheries and is coordinated by the International Centre for Research in Organic Food Systems (ICROFS).

For further information please contact: Senior scientist Marianne G. Bertelsen, Department of Food Science, e-mail: marianne.bertelsen@agrsci.dk, telephone: +45 8715 8328