—Damaged fruit is always found on the west side of the tree, affecting the side that faces the afternoon sun (Figure 1).
—Picked fruit that is exposed to the sun will experience burn damage in a few hours, while fruit that remains on the tree does not (Figure 2). This speaks of the major cooling effect of the tree on the fruit.
—Bagged fruit that has developed in darkness will burn when it has reached a very high temperature. (Figure 3).
—In laboratory tests, it was possible to induce burn damage to the fruit with a temperature of 45°C (113°F) for at least five hours. Ultraviolet radiation by itself does not have the same effect. It should be noted that in the orchard, the fruit can reach temperatures of 54°C (129°F).
—Placing different UV radiation filters (plastic, polycarbonate, or glass) over exposed fruit does not prevent fruit from sunburn. The lack of UV light resulted in less development of red color in the peel.
—Cooling detached fruit with ventilation prevents them from burning, while unventilated fruit suffer burning. The use of glass or polycarbonate did not slow the rate of damage (Figure 4).
—Burning still occurs in bioassays when solar protectors that filter UV light are applied to attached fruit, while burn does not occur when protectors that filter infrared light, like kaolin, are applied.
—Evaporative cooling is an effective way to reduce sunburn in the orchard.
High temperature and excess solar radiation, particularly in the infrared spectrum, are responsible for sunburn damage in apples.
The fact that the problem has become more acute in Chile in the last 20 years could be due to three factors: the incorporation of more susceptible cultivars; increasing use of dwarfing rootstocks; and more exposed training systems, such as Solaxe. The increase in ambient temperature in recent years is another factor to be considered.
Finally, we recommend the use of nets over the orchard as the best protection against sunburn, because the efficiency of protective sprays is marginal. Growers should start using protective systems when the air temperature has reached 29°C (84°F) for five hours—normally in mid-December.
Contact Dr. Jose Antonio Yuri at email@example.com