Finding ways to fight the sun - Apples

Release HortResearch

Like humans, apples can also suffer from too much sun.

In the last few seasons sunburn has affected more than 15 percent of the fruit in some cultivars. The 15 percent are only the wastage at the pack house, that figure would increase dramatically if we knew the amount left in the orchards. Sunburned fruit represents a considerable financial loss to the apple industry since it is not marketable at any level.

However the search for a solution is underway and the ghostly white appearance of some apple trees in some research and commercial orchards last season were due to research to find ways to stop the sunburn.

HortResearch scientist Jens Wünsche said the research was looking at several methods for sunburn protection. The white trees were the result of spraying with several ‘secret’ mixtures; calcium carbonate, lime and clay based products. The dense whiteness coated the fruit and diffused the light.

Dr Wünsche said that the fruit developed very well, in terms of size, and best of all there was no sunburn. Now he and his team need to grade and evaluate fruit from the trial for sunburn and blush colour. Internal fruit quality such as flesh firmness, soluble solids, starch index must also be assessed.

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

This was the second research season into apple sunburn and a number of different methods have been tried and evaluated. Dr Wünsche said that far from being able to pinpoint a specific catalyst there would appear to be multiple factors and testing a number of hypotheses is one end of his research. The other, and possibly more dramatic end, is testing the variety of possibilities to control sunburn; some are commercially available products, others are suggestions and some are practices already in place for other crop care measures.

Other methods trialled have included fruit bagging, fruit orientation, plant stress, overhead shading and overhead irrigation.

"I have not yet analysed data for statistically significant differences among treatments. I can, however, say that the whitewashing 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," Dr Wünsche said.

Some cultivars are definitely more susceptible than others are, and Dr 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'.

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