When the sun is too strong (intense sunlight, high temperatures, low humidity), or when wet fruits get hit by the sun (do not water in the heat of the day and water the base of plants rather than spraying all over them), they can get sunburnt. Sunburn is a disorder, that is: a physiological reaction to less than ideal environmental circumstances.
It happens quite regularly with blackberries, sometimes raspberries. It had happened to me before with the odd gooseberry that found itself short of shelter. But it was last year that I first witnessed it on my apples, luckily just a couple of them. It was July. This year, in another first, gooseberries got sunburnt at the end of April during a heatwave, when they were still just forming, their leaves were not full size yet, and the trees above them were still not in leaf. Some of flowers on a couple of apples got sunburnt too (either dew that caught particularly strong morning sun, or maybe my mistake of spraying the flowers while watering); luckily they were not irretrivably damaged. We are getting a lot of firsts, aren’t we, and in very quick succession now: we’ll have to start seriously adapting.
To avoid sunburn, you can make sure that your plants are properly watered during a drought (I have written about the art of watering before, but – in essence – don’t water little and often, but take your time to let the water seep through the various layers of the soil), but the fruits themselves remain dry. Make sure that your soil retains moisture and nutrients (with proper feeding, mulching and cover) as buffer against extremes . You can also let your bushes grow in a sheltered place, and/or cover them in a heatwave. As far as fruit trees are concerned, I guess the best cover comes from their own canopy, something you can have an influence on with proper pruning.
How does one identify sunburn? It may resemble a burn, a (sunken) bruise or blanching/discolouration of the skin.
Sunburn can weaken the skin, leading to further damage by pests and diseases. It can also exhacerbate a number of other disorders, thus shortening the lifespan of produce.
Sunburn in apples
With regards specifically to apples, the three levels in which sunburn can appear are generally described as necrosis, browning and photooxydative sunburn.
Disorders that are associated to sunburn are “lenticel marking, cracking, splitting, russetting, watercore, and bitter pit”, according to the Washington State website. The US extension websites are meant for commercial growers, so the website provides quite detailed prevention information, in terms of climate-ameliorating techniques and/or chemicals: some even use sunscreen!
WUR, my alma mater, has a useful leaflet on how some of these sunburn-related disorders manifest post-harvest and how to deal with them in a professional context.
All amateurs can really do when prevention fails is: consume the produce, store it separately, and/or monitor it more carefully than usual.
Because of the enormous amount of fruit produced last year in my orchard, I had the opportunity to store and eat my own apples and pears well into April. I did notice a considerable increase in watercore* in my “Gloster” apples compared to previous years, but that did not appear to affect the quality until the very bottom of the last crate (maybe February?) when the watercore areas became darker and softer, unpleasant to eat.
*Watercore is an accumulation of sugars that gives the fruit flesh a glassy, watersoaked appearance. More information on the why and how in this article from Michigan State Extension from 2020, and a couple of curious notes on appreciation of watercore on my old post).
M.Paola, that’s me
