High temperatures and sunlight are the main causes of sunburn on fruit, but there are several indirect factors that also influence its development. One of these is the cultivar planted. Yet even within a cultivar, there may be individual fruit that respond differently to sun exposure, with some simply not suffering from the problem at all. Scientists are still trying to understand the reasons for this individual variation.
Sunburn browning and necrosis are more common in fruit that is at least 45mm in diameter. Photo- oxidative sunburn, however, can afflict fruit of any size.
Fruit is at risk of sunburn when a fruit’s surface temperature rises above a particular threshold. The threshold is higher in fruit that is acclimatised to the sun. Any fruit is capable of acclimatisation given sufficient time. For example, apples that grow in sunlight can tolerate far more light and heat than those that have developed in the shade.
Equally, apples that have experienced a long period of overcast weather may be vulnerable to sunburn if the days suddenly turn bright and hot.
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