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Sunburnt Apples

Like humans, apples can suffer from too much sun. In the last few seasons, sunburn has affected more than 15% of the fruit at the packhouse level, with significantly more rejected in orchards. HortResearch scientists are looking to reduce the physical damage and economic loss that such sunburn causes.

As a result, some research and commercial orchards turned ghostly white, as trees were treated with various "sunscreen" mixes of calcium carbonate, lime and clay-based products. The dense whiteness coated the fruit and diffused the light.

HortResearch scientist Jens Wünsche says that despite the coating, the fruit developed very well, in terms of size, and there was no sunburn. Now he and his team need to grade and evaluate the fruit checking blush colour, flesh firmness, soluble solids and starch index.

There was a concern over ripeness and colour retardation, as the fruit on the trial trees, when rubbed free of the coating, were greener than control fruit. Fruit also has to be assessed to see if the coating makes it susceptible to fungal growth; any cracks in the coating could allow moisture to enter and black spot and other diseases could thrive.

A number of different methods have been tried and evaluated. Wünsche says that far from being able to pinpoint a specific catalyst there would appear to be multiple factors. He is also testing the variety of possibilities to control sunburn, from commercially available products to cultivation practices already in place for other crop care measures, such as fruit bagging, fruit orientation, plant stress, overhead shading and overhead irrigation.

"The white-washing appeared to be effective in terms of reducing the incidence of sunburn, whereas blush colour development seemed to be down compared to the control fruit," Wünsche says.

Some cultivars are definitely more susceptible than others are, and Wünsche's team is exploring the reasons for this. Another risk comes from consumers wanting enhanced fruit colour, and sunburn is likely to become even more serious as growers use management practices to obtain that colour.

Central Otago, Hawke's Bay and even the Nelson/Marlborough area, have sunburn problems, especially with the apple cultivars Braeburn, Fuji and Pacific Rose.

Sunburnt Apples Figure A (49KB)
Technical Officer Ross Marshall with a white-washed Prima "ghost tree" in the HortResearch Clyde trial block. Prima is a black spot resistant variety.
Sunburnt Apples Figure B (44KB)
Jens Wünsche in the HortResearch Nelson research orchard.