Why it happens and how to prevent it. By Anna Mouton.
Sunburn is the damage caused by too much sun exposure — anyone who spends time outdoors knows this. People tend to find a tan attractive in humans but not so much in apples and other fruit. Sunburn is a major reason why fruit is rejected as unsuitable for export and ends up in informal markets or as juice.
“Sunburn is most noticeable in pome fruit,” says Prof. Stephanie Midgley of the Department of Horticultural Science at Stellenbosch University. The worst-affected apple cultivars are Granny Smith, Golden Delicious and Fuji.
The impact of sunburn on sensitive cultivars is significant, according to Midgley. “In my experience an average orchard that is well-managed and not under drought stress will often have 10%–15% sunburn. It can go up to 30%, sometimes even more than that.”
Stone fruit are also susceptible to sunburn. Prof. Karen Theron holds the Chair in Applied Preharvest Horticulture at Stellenbosch University. “There are seasons when there are sunburn problems in certain plum cultivars,” she confirms.
Sunburnt fruit frequently suffer from post-harvest disorders because of the role of heat stress in both sunburn and many post-harvest problems.
The sunburn spectrum
Sunburn is a collective term for three preharvest conditions that differ in ways that are relevant to control. The descriptions below refer to apples, but similar conditions can occur in other fruit and even some vegetables.
Sunburn necrosis is the most dramatic manifestation of sunburn. Affected fruit have dark brown or black areas of dead tissue on the surface. The damage may extend for several millimetres into the flesh and is a perfect substrate for the growth of spoilage organisms. Sunburn necrosis renders fruit unfit for anything other than perhaps juicing.
Research has shown that sunburn necrosis occurs when the fruit surface temperature attains 52°C for as little as ten minutes. Heat — not light — causes the tissue damage and death. Sunburn necrosis can be experimentally induced in complete darkness by raising fruit surface temperature.
Sunburn browning is the equivalent of a tan. The fruit responds to excess light and heat by developing yellow, brown, bronze, or dark discoloration of sun-exposed areas of the peel. The underlying flesh is not damaged. Affected fruit can have higher sugar levels and lower acidity but consumers dislike sunburn browning. Anything more than mild discoloration will lead to rejection by the market.
Both light and heat are necessary for sunburn browning — it will not develop in darkness. Apple cultivars vary in their sensitivity to browning. The dark colours of certain red apple and stone fruit cultivars may also mask browning.
The third type of sunburn is called photooxidative sunburn. It affects fruit that are suddenly exposed to sunlight after having grown in shaded conditions. This can happen due to management practices such as summer pruning or even when newly picked fruit are left lying in full sun. Light exposure — not heat — is the primary driver of photooxidative sunburn.
Fruit affected by photooxidative sunburn develop a white — or photobleached — area. Tissues in the centre of the bleached area may eventually die off and the fruit is left with a necrotic area similar to that seen in sunburn necrosis.
Sometimes fruit only develop signs of sun damage while in storage. Minor sunburn browning at harvest can progress to extensive darkening in Granny Smith apples during storage. This is called sunburn scald or sunscald. Sunscald — like all scalds — is a post-harvest disorder. Sunscald develops on the exposed side of the fruit whereas other scalds tend to develop on the shaded side.
Factors that contribute to sunburn
Temperature and sunlight are the main culprits — as described above — but there are several indirect factors that also influence the development of sunburn. One of these — cultivar — has already been mentioned.
Individual fruit within a cultivar respond differently to sun exposure. Some fruit — like some people — just never seem to burn. Scientists are still trying to work out the reasons for this individual variation. Sunburn browning and necrosis are more common in fruit that are at least 45 millimetres in diameter. Photooxidative sunburn can afflict fruit of any size.
Fruit are at risk of sunburn above a threshold fruit surface temperature. The threshold is higher in fruit that are acclimated to the sun. All fruit are capable of acclimation — given sufficient time. Apples that set and grow in sunlight can tolerate far more light and heat than those that have lived all their life in the cool shade. Apples that have experienced a long period of overcast weather may be vulnerable to sunburn if the days suddenly turn bright and hot.
Fruit surface temperature influences sunburn and therefore so does anything that heats or cools the fruit. Low relative humidity may promote sunburn by increasing fruit surface temperature. Air movement can retard sunburn by decreasing fruit surface temperature. Even modest air circulation can cool fruit by 4–5°C.
Strategies to reduce sunburn
“The main thing that farmers want to do is canopy management,” says Midgley. Fruit that were exposed to sunlight from a small size have had time to acclimate and build up their defences against strong light and high temperatures. Fruit that have been shaded are not acclimated and they will burn if suddenly exposed during summer pruning. For this reason summer pruning is best done as soon as possible after final fruit set or when doing final hand-thinning.
The trend toward high-density plantings of smaller trees on dwarfing rootstocks increases the risk of sunburn due to increased light exposure. Dwarfing rootstocks may also predispose trees to drought stress if irrigation is not well-managed because their root systems are smaller. Optimal irrigation is important in mitigating the risk of sunburn but there is no benefit in over-irrigating the trees.
Correct row orientation will help reduce sunburn. “In South Africa the standard is north-south for that reason,” explains Midgley. “It’s actually a little off north-south. There are still orchards out there that are east-west and when you walk through them there’s no sunburn on the south. But there’s sometimes serious sunburn on the north.”
Researchers have tested a number of interventions for sunburn. Theron recalls experiments with kaolin as a sunblock. “Yes, it reduces sunburn, but if you don’t wash it off one hundred percent, it looks as though there’s spray residues on the fruit, which is most definitely not acceptable to the market.”
Midgley adds that see-through sunscreen products were also trialled but never caught on.
Evaporative cooling is effective at reducing sunburn but it’s not a viable solution for South Africa. “We don’t have enough water to use this,” says Theron. “And in plums you have the problem that the fruit often burst if you apply evaporative cooling close to harvest.”
Shade net has proved the most practical option for combatting sunburn. “When we did the trials almost ten years ago, it was absolutely clear that there’s almost no sunburn under shade net,” confirms Midgley. “The intensity is reduced, and the numbers are really low. So that’s what people are using.”
Shade net is likely to become even more widespread with the trend toward high-density plantings. “What I saw in Australia was that every single one of those orchards where they’re doing two-dimensional plantings has shade net,” states Midgley.
“There would have to be research done here where we have a very two-dimensional planting to see how that changes the risk for sunburn. To be absolutely safe one would advise farmers to put up a net if they want a system like that.”
The inexorable rise in global temperatures is leading us to a hotter and drier future where sunburn and related disorders can only become more troublesome. Canopy management and shade nets will play an important role in helping growers to safeguard their production in years to come.
Racsko J. and Schrader L.E. 2012. Sunburn of apple fruit: historical background, recent advances and future perspectives. Critical Reviews in Plant Sciences 31(6):455–504.
Cover image supplied by Stephanie Midgley | Stellenbosch University.