Sunburn on fruit is a very real risk, particularly in the type of extended heatwaves that are becoming increasingly normal due to climate change.
It’s different from the type of burn damage that trees and fruit suffer in an actual fire (which is also becoming more common) but is still very destructive. If you live in a hot climate (like we do) sunburn is an inevitable part of fruit growing.
Sunburn can also happen in temperate fruit growing areas during heat waves.
There are three types of sunburn damage you may see on your fruit:
1. Sunburn necrosis
2. Sunburn browning
3. Photo-oxidative sunburn
How hot does it have to be to cause sunburn on fruit?
This is a difficult question to answer, as it depends on quite a few factors. As a rough guide, you may expect to see some browning damage at temperatures over 35C, and once the thermometer climbs over 40C necrosis also becomes a risk.
This is the damage we experienced when a blistering day of 44C hit the Pizzaz plums just before they were quite ripe enough to pick.
You can see the spots and shrivelling on the skin – that’s a version of sunburn browning. Most of the plums are still perfectly usable for jam, or cooking, or even for eating, but it definitely downgrades them.
What type of fruit can be burned?
All of them!
Plums are very vulnerable, which is why we’ve included so many photos of plums in this blog. Apples are also very vulnerable, and apple trees are often pruned with this in mind.
The sad truth is that pretty much any fruit (or vegetables for that matter) can experience sunburn.
Is sunburn preventable?
Preventing sunburn can be incredibly difficult in cases like this.
There was probably only a very brief window of a day or two when the plums were ripe enough to pick (with the confidence that they would continue to ripen off the tree) before the heatwave hit.
If you have fruit that’s almost ripe, it’s definitely worth paying careful attention to both your trees and the weather forecast. When a heatwave is predicted, it may be possible to harvest the fruit (or at least some of it) in time.
There’s a number of other things you can do to prevent sunburn damage.
Our top 10 tricks and techniques for preventing sunburn
- Give your trees extra water on extremely hot days
- Encourage good leaf growth
- When fruit thinning aim to provide all fruit with leaf cover
- Avoid bare ground under the tree
- Encourage good air movement through the tree
- Try kaolin clay spray
- Shade netting
- Overhead sprinklers can reduce the risk of sunburn to apples (but may have other consequences)
- Make sure your fruit is not calcium depleted, as this can increase the risk of sunburn
- Don’t leave fruit sitting in the sun after it’s picked
We include more detail on each of these techniques in “What’s that spot? Common diseases of deciduous fruit trees” (even though sunburn is not actually a disease, but an environmental impact).
You can’t control the weather, but there’s still a lot you can do.