Sunscald: Tree, Leaf, And Fruit Sunburn

Increasingly, growers deal with sunscald in their gardens. While sunscald is often attributed to either trees or solanaceous plants, the term applies to both situations. Within those two topics are more nuanced incidences largely dependent on plant conditions and the weather. 

Sunscald affects fruit trees, younger trees, vegetables, fruit, vines, and shrubs. Sunscald occurs in extremes: sunny winter days, freezing nights in winter months, and summer. It originates from direct sunlight that causes injury in plants. The resulting damage needs dealing with to prevent issues going forward. 

So, just what is sunscald? Which plants are susceptible, and how can it be prevented? This piece is designed to address those topics. With a little understanding of the condition and the factors involved, you’ll prevent sun scald from entering your garden. 

What is Sunscald?

On pomegranate, sunscald shows as darkened, blotchy patches. Source: joncutrer

Sunscald usually refers to a few different scenarios. All of these scenarios are caused by intensely hot or cold weather. In recent years, the area I live in has experienced sub-zero freezes and extremely hot summers, making sunscald of greater concern among local gardeners. While a quick search on the topic in an internet browser will suggest sunscald is only present on trees, the fact of the matter is many other plants and fruits are susceptible. 

In the northern hemisphere, the condition is called southwest injury because it often occurs on the southwest side of a tree trunk. Tomato gardeners are also well-aware of sun scald damage on their tomato fruits – a condition that could render them inedible. But before we get too far, let’s examine all the scenarios people deem “sunscald” within each of their contexts. 

Sunscald on Trees

Lower trunks painted to prevent sunscaldLower trunks of mature trees can be painted to reduce heat on unshaded bark. Source: fullres

Sunscald occurs on the bark of trees. Often referred to as southwest injury, the disease presents itself on the southwest side of the tree’s trunk. The resulting damage is a section of dead or discolored bark on the tree trunk. It also appears as a sunken area with thin bark that is cracked. Dead bark due to sunscald is due to dead tissue as a result of intense temperature fluctuations that stress the tree. 

There are two types of sunscald on trees. One occurs in winter, and the other occurs in summer. Let’s take a look at those, and the conditions in which they arise. 

Winter Sunscald

Most common in trees with thin bark, like the birch paper tree, winter sunscald is sometimes referred to as frost cracking. Often this occurs when winter dormancy is broken in trees when temperatures are unseasonably warm. If a snap freeze follows, the tree doesn’t have time to return to dormancy and the cells on the southwest side of the trunk are struck by plummeting temperatures. Sometimes the damage is minimal, and the dead tissue is only seen in slightly cracked bark. Other times, (especially in thin-barked trees) sunscald damage can cause an open wound that exposes the tree’s interior and leaves it vulnerable to infestations by insects, animals, and diseases. 

Particularly, younger trees are prone to sunscald damage. As they grow and their bark thickens, the likelihood of sunscald affecting the tree lessens. Older trees that are nearing the end of their lifecycle also experience sunscald more so than a tree in its prime.  

Summer Sunscald

Summer sunscald affects trees in the opposite manner. When active cells in the trunk of a tree are thriving in the summer, and extremely high temperatures strike, the bark gets too hot, gets a sunburn, and dies. 

Just like winter sunscald, thin-barked trees and young trees are most likely to take on damage from summer sunscald. In this case, the sunscald is a sunburn of sorts that results in cracking bark pieces that expose the sensitive parts of the tree. A lack of soil moisture is sometimes the culprit for summer sunscald. 

Leaf and Fruit Scald

Fruit scald on tomatoesOn tomatoes, mild sunscald shows as a fruit discoloration. Source: kimberlysteinmann

People use the term “sunscald” to refer to sun damage on plants and fruits, rather than solely deciduous trees. In this case, the cause of the damaged area is the same as summer sunscald. Temperatures in the summer rise rapidly, damage plant and fruit cells, and discolor fruit and leaf tissue. 

The damaged plant or fruit takes on white discoloration that indicates the plant or fruit’s cells have died as a result of exposure. Sometimes the cause of this is a lack of adequate water or soil moisture which can originate in early spring. Sometimes it’s current weather conditions. Hot weather climates experience more leaf and fruit scald than temperate or cold climates. 

Leaf Scald

Plants growing in high heat experience leaf scald as active cells become overworked by extreme heat conditions. Sun scald damage in leaves occurs on the top part of the plant, and the parts most exposed to the elements. Often the top leaves of vines and shrubs turn white, while lower growth remains unscathed in the shade of dead leaves. Plants with sunscald damage are more susceptible to attacks by pests, and they’re more likely to contract diseases. 

Fruit Scald

Most common in solanaceous plants like tomatoes, fruits experience a form of sunscald from a rapid rise in temperature in the heat of the summertime. Damaged areas of sunscald-affected fruit and vegetables turn white. They become inedible in the process. Berries, grapes, tomatoes, bell peppers, and all other vegetables can experience sunscald in select conditions. 

Because fruit results from plants, it’s often the case that sunscald occurs first on leaves, and then spreads to fruit. When fruit takes on sunscald, they are more susceptible to injury from insects, and other pests. The damage also makes them more likely to experience diseases. 

How to Prevent Sunscald

Now that we’ve pinpointed how sunscald occurs on trees, plants, and fruit, in winter or summer, let’s discuss what particular traits make a plant more susceptible to sunscald. 

Then, we’ll touch on ways to prevent sunscald so you can keep your vegetable plants and fruit trees healthy all year round. 

Susceptibility in Trees

Orchard row with sunscald preventionAny portion of the trunk that isn’t shaded by the tree’s canopy can be painted. Source: oh contraire

As mentioned before, young trees or thin-barked trees are more susceptible to both summer and winter sunscald. Deciduous trees that lack leaves in winter are especially susceptible. The leaves on evergreens provide shade that shields bark from sunscald, though this doesn’t eliminate it. Trees subject to intense ground freezes are more likely to take on damage, especially in times with less than normal precipitation. 

A freezing temperature coupled with dry conditions is no good for tree roots and trunks. The same goes for dry, hot weather. Sunscalds occur on trees that have been planted in the garden at the wrong time when it’s too hot. Trees in hot weather with dry soil are also more likely to suffer injury from intense sunlight. 

Prevention in Trees

If you like planting trees in your garden or on your property there are several things you can do to protect trees exposed to intense temperature fluctuations. One surefire way to protect a young tree in freezing weather is by wrapping it with a tree wrap. These keep bark temperatures lower than they would be if they were fully open to the elements. 

If you use a wrap, put it on in fall up to the first branch of the tree. Remove it as soon as possible when the last frost has passed in spring. If the wrap isn’t removed promptly the area between the wrap and the tree trunks can become a harbor for insects. You don’t want to make the effort to wrap the bark, only to have your protection turn on you. 

Another way to wrap the tree is to provide a frost cover. While this is great for smaller and young trees, it’s not an option for large ones. For larger trees where a wrap is not possible, painting the bottom trunks and lower limbs (especially on deciduous trees) with white latex paint suffices. The paint acts as a wrap of sorts and reflects sunlight from trunks. 

When it comes to hot weather, the most effective way to prevent sunscald is by attempting to conserve soil moisture as much as possible. Add sufficient mulch at the base of the tree to keep the soil temperature warm and prevent injury from sunscald. Mulch is a great option for maintaining soil temperature and moisture even in the intense light and heat of the active hot seasons. Leaf mold, wood chips, wood shavings, straw, and grass clippings will all protect root growth and save a tree from injury by sunscald. 

A tree wrap or painted trunks can also be used to keep trunks safe in scorching hot weather. Don’t wrap the tree tightly, and check it often for insects that might want to congregate out of sunlight. Painting the tree trunk will also shelter it from too much light, and you only have to go up to the point where the tree is shaded by its own canopy. The paint can be part of your integrated pest management strategy as well when paired with sticky traps for insects.

Proper pruning is important in trees, too. In the winter pruning season, don’t remove too many branches, and prune for the overall attractiveness of the tree taking no more than 25% of the total branches. 

Susceptibility in Plants

Severe sunscald on bell pepperSevere sunscald can cause some types of fruit to collapse inward and begin to rot. Source: John and Anni

The most common conditions in which leaf scorch occurs are hot, dry, and involve low moisture retention. Especially in windy conditions, in the warm, active seasons, leaf scorch is more likely to occur. Leaf scorch doesn’t just occur on plants. It also affects the leaves of trees and bushes. A plant that isn’t suited to a hot, dry climate is more susceptible to sunscald damage on leaves. 

Plants that have undergone excess pruning won’t have the capability of protecting themselves from the intense sun. Light isn’t the only culprit. Plants that have been planted in the garden improperly will have moisture uptake problems and are prone to scorching by sunlight. Improper applications of pesticides can also leave plants exposed to sun injury. 

Prevention in Plants

To prevent sunscald on leaves, do not remove damaged foliage. While it may seem best to remove leaves that have taken on sun injury, instead allow those dead leaves to block out sunlight while you figure out a good solution. If you can move a plant out of the sun, do so immediately.

A row cover is a great and effective way to wrap your plants and shield them from the searing sun with shade cloth. This especially is good for young garden plants with tender leaves that haven’t quite acclimated to conditions outside of temperate fall or spring. If you acquire a row cloth, remove the damaged foliage, and cover the plant to save it from the sun. 

Another thing that helps prevent sunscald is watering less in the summer to promote deep roots that uptake moisture more easily. Fertilizers applied in the proper manner strengthen your plants and help them withstand more drastic conditions. Phosphorous helps plants in cold, while potassium helps plants build thicker cell walls that keep them safe from disease. Avoid too much nitrogen late in the spring or fall growing season. Then plants continuously grow soft, green leaves more prone to frost injury. 

Prevention in Fruit

Because fruit sunscald results from the same conditions as leaf scorch, or leaf scald, use the same prevention methods listed in the last section. Accompany those with planting varieties of fruits and vegetables that produce a lot of leaves, shading the resulting fruit. Use integrated pest management to ensure defoliation doesn’t occur from infestations or disease. A protective layer of kaolin clay shields tender plants and prevents pests at the same time. Proper irrigation will also help fruits maintain health amidst a sudden climate shift. 

Frequently Asked Questions

Q: How do you fix sunscald?

A: On tree bark, leafy plants, and fruit producers, there is no treatment for existing sunscald. Prevention is vital. 

Q: What does sunscald look like?

A: On the trunk of a tree, it’s discolored bark. On plants, it’s white, dead leaves. On fruit, it’s white or pale discoloration that can lead to rot of the fruit once advanced. 

Q: Is tree wrap good for trees?

A: A tree wrap is especially great for young trees in freezing weather until their bark thickens. Use a tree wrap in summer, placed less tightly around the trunk. 

Q: Can you save a sunburned plant?

A: If you have the option to move it, remove the damaged leaves and place the plant out of the sun. If it can’t be moved, cover it with shade cloth as soon as possible and leave damaged leaves intact for extra shading.

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