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30 September 2021

Covered apple cultivation a practical alternative?

Covered apple cultivation a practical alternative?

There are small-scale experiments with covered apple plots on a few practical farms in Europe. However, there are still too many questions to deploy covers on a large scale. How do diseases and pests develop under a cover?

Author: Matty Polfliet, consultant Fruitconsult.

The article is from Fruitteelt 17 and is published in cooperation with the NFO and Fruitconsult.


In 2019, part of the organic plot was roofed with a combined rainfoil and hail net system. Scab, rain blotch and storage diseases are much less prevalent here, supported only by a sulfur scheme. Mildew is more difficult to control and requires a lot of labor input. Pressure from green aphids was high in the early stages, but this problem is likely to diminish as the trees age. Blood lice, on the other hand, are likely to become a pain point. There is little that can be done about this, other than encouraging natural enemies. How fruit tree canker on Kanzi will evolve in the coming years will be very exciting. If it remains limited, this is a great success.

Covered fruit cultivation

Covered fruit cultivation at Proeftuin Randwijk started on a small scale. In 2015, one row of Jonagold was roofed with an old berry cover. This row was not treated against scab or storage diseases for four years. Despite the fact that the trees did get wet during the rain, no significant fruit scab has ever developed here. Also, the loss due to storage diseases after long storage was minimal. On the other hand, on an adjacent - not roofed - untreated row, considerable fruit scab developed every year. In organic fruit production, late varieties can suffer a lot from rain spot disease in wet years. In conventional cultivation, the side effect of most summer fungicides offers protection against this. In 2018, therefore, one row of Natyra/Elstar on the organic plot was covered with raincloth stretched under the hail net. In 2020, this was replaced by a combined anti-rain cloth / hail net with zipper closure. In addition, the cover was extended to three rows. As soon as the canvas is closed, the treatments stop, with the exception of some sulfur treatments against mildew. In addition to storage diseases and scab, rain stains were then also absent. Also notable was the smoothness of Elstar. Scab, rain blotch and storage diseases are therefore much less prevalent on the covered apple plots.

Diseases and pests

But how do other diseases and pests develop when a larger plot is roofed over several years? It was known from previous studies that pressure from spider mites, rust mites, blood lice and mildew can increase. However, nothing was yet known about the development of fruit tree canker. The next step was therefore to canopy half an acre in the organic plot with a combined rainfly and hail net system (in 2019). The system was largely financed by FruitMasters and is therefore planted with the FruitMasters varieties Kanzi, Tessa, Sunspark, Greenstar and Migo. In addition, there are also organic Elstar and Conference under the canopy. For sufficient light - raincloth blocks more light than hail netting - a wider planting distance of 4 x 1 meter was chosen. Still, this planting should produce about 40 tons per hectare. Underground irrigation hoses should ensure optimal watering. In addition, the drainage requires extra attention, because twice as much rainwater falls in the drive aisle because of the cover system.

Insects and mites

From the 2019 planting year, the pressure of apple grass aphid was very high in the covered plot. This insect cannot be controlled in an organic crop. The first year, the infestation was still acceptable; as the trees continued to grow vigorously. The second year the infestation was severe, especially on Kanzi. on Kanzi. This threatened to damage too many trees. On that occasion A summer oil was used that reasonably suppressed the aphids, but also caused coarsening. The lice also occurred in the other varieties, but to a much lesser extent so that correction was not necessary. This year there was again a lot of pressure from lice, but due to the cool spring their development was much slower. The infestation could be well suppressed with the nettle extract Urtibasic and by several beneficial insects. At the end of the planting year, small colonies of blood lice occurred sporadically in the covered plot. They seemed to have been blown in from the nearby Santana plot, which was heavily infested. In the second year, aphid pressure remained relatively limited, but in the fall several trees were suddenly completely covered in aphids. Strange as it may sound, no earwigs were present on the bio plot. These were imported into the plot from the conventional Conference plot in August 2020 using tonkin canes. Currently, we are investigating whether they have remained. Last winter all trees were checked for blood aphids and if necessary treated with a hand sprayer with oil. This had a good effect until May, but then the pressure increased again slightly. On the organic plot under the rain cover this year there has also been a strong increase in aphids, especially on Elstar. Finally, rust mite also seems to occur a bit more under the covered plot, but these are sufficiently suppressed by the sulphur treatments. In time, the predatory mites should provide peace of mind.

Not scabies but ...

Already in the planting year, powdery mildew emerged strongly; the timely removal of the white plumes in combination with an ample sulfur schedule was clearly insufficient. In 2020, too, despite a tight sulfur schedule, powdery mildew could not be brought down to a low level. Both 2019 and 2020 were warm and dry, which also resulted in more powdery mildew infestation in practice. This year, powdery mildew developed more calmly because of the cool spring. However, despite this, new infestations developed on all varieties. The infestation seems to be strongly related to the absence of rain. The mildew only develops on leaves, not on fruits; these were always very smooth. Strawberry cultivation in tunnels has had good experiences with mildew control using sprinklers. Therefore, a sprinkler system was installed above Tessa. This year, from June onwards, sprinkling was carried out for two days a week (five minutes twice a day, about 2 mm at a time). This is too little sprinkling to cause scab infections, but hopefully enough to reduce mildew infections. As a check, one row of Tessa will not be sprinkled. Next spring will show whether this will suppress mildew sufficiently.

And fruit tree cancer?

The cover above the organic plot is closed before the buds sprout and only opened well after the leaves have fallen. Pruning takes place at the end of leaf fall. After that, the cover remains closed for another two weeks to allow pruning wounds to dry. This ensures that the trees are protected from rain during all periods when wounds occur. For fruit tree canker on Kanzi, there is a zero tolerance: the first two years, all trees with infestation are removed (a total of two trees). Starting in the third growth year (2021), trees with stem cancer are removed and those with shoot cancer are cut back deeply. This year, 14 trees were removed and 29 branches were cut back. In the trees removed, the cancer was all in the same place, about 20 centimeters above the rootstock. In the removed branches, the infestation was frequently at a picking wound that was infected. It was striking that all but one of the removed trees were in the two outer rows of the plot. In total, just under 4 percent of the trees have now been removed.