The increased intensity of solar radiation and temperature causes symptoms of sunburn in fruits of various plants. Symptoms are often caused in the fruits of apple trees.
Varieties of apple grown mainly in lowland areas are particularly susceptible to sunburn and the fruits either sunburn or lose their typical skin colour with a significant reduction in their commercial value and keeping quality. Apple sunburn occurs mainly on trees grafted to dwarf trees as the crown is limited to protect the fruit (1, 2). Anti-sun nets, sprinklers over the crown for water spraying and spraying with various formulations are used mainly to protect the fruit from sunburn.
In summary:
Apples are susceptible to sunburn because they have a much greater thermal mass (the ability of a material to absorb and store heat energy) than leaves and cannot dissipate this heat like leaves.
There are mainly 3 types of sunburn : 1) necrosis 2) brown-copper metachronism 3) photo-oxidative sunburn (bleaching)
The control of sunburns requires a specific approach. Good plant health and proper orchard management are necessary to balance vegetative growth and fruiting. Some factors that increase the risk are :
- Low leaf area index and high ratio of fruit per leaf
- Water stress
- Low nitrogen levels in the leaves
- Age of fruit
- Selective picking of fruit
Management in sunburn mainly involves 3 methods.
- Misting. A method difficult in practice.
- Spraying with various formulations (kaolin formulations, specific calcium formulations and wax-based formulations which we will mention in another article. Depending on the conditions, the formulations have disadvantages such as staining or the need for repeated spraying, costs, etc.)
- The use of anti-fouling nets. The initial cost seems significant but through a reinforcement programme the installation costs are less than that of simply supporting the trees. The netting also protects against wind and birds and in other crops can also create a rain screen. In addition, netting has been shown to significantly reduce the spread of some pests such as stink bugs. Finally (not yet in Greece) the use of different net colours can have a positive effect on shoot growth, colour and fruit quality.