What you can do to minimise sunburn of fruit:
• Apple trees on dwarfing rootstocks such as M26 and M9, need good protection to avoid large scale sunburning of fruit.
• Sun damaged cells in the skin can be susceptible to infection by pathogens such as fungi that cause summer rots.
• Sunburn can occur in the bins during harvest, when temperatures exceed 30C. Cover the bins in the orchard to avoid overheating of the harvested fruit.
• After storage, flesh breakdown can often be observed under a sunburned spot.
• The skin which has been exposed to high temperatures is very susceptible to storage scald.
• Fruit can look good going into cold storage but develop bitter pit or breakdown even in good cold storage.
Apples can develop a lot of watercore in hot weather, long before the normal time of fruit maturity.
• Netting the orchard can help protect fruit from sunburn but the amount of protection depends on the shade factor of the net.
Net that reduces light levels by up to 17 per cent has reduced sunburn by up to 75 per cent.
On average, netting reduces orchard temperature by about 8C.
There are many types of net available for apple and pear growers—not only to reduce sun damage but also for hail protection and to exclude birds and bats.
Weave density, colour and the support structure (permanent or throw-over) need to be considered.
• Evaporative cooling with overhead sprinklers has been shown to be effective at limiting sunburn damage.
• Sunburn protective sprays such as kaolin clays and calcium products have been developed to help reduce sunburn of fruit. They block the damaging light wavelengths of the sun but allow the light needed for fruit colouring to pass through the sunscreen.
The performance of these products has been difficult to evaluate in the field. They have only been partially successful or come with undesirable side effects such as difficulty in removing visible residue on harvested fruit.
This article is from the Orchard Manual: UNDERSTAND FRUIT TREES
See this article in Tree Fruit Feb 2023
