Few things are more disappointing than watching a beautiful tomato reach the home stretch only to develop a large, pale patch a few days before harvest.
The damage often begins as a yellow or white area on the side of the fruit exposed to direct sunlight. Before long, the tissue becomes leathery, papery, and susceptible to secondary infections. Gardeners know this condition as sunscald, and the explanation seems obvious enough: the tomato simply got too much sun.
Case closed.
Or is it?
As it turns out, the biology behind sunscald is far more interesting than simple overheating. The same sunlight that helps tomatoes produce sugars, pigments, and many of the compounds responsible for their flavor can also damage the very machinery that makes those processes possible.
In this week's post, we'll explore what causes sunscald, why some varieties are more susceptible than others, and the surprising connection between this common garden problem and the exceptional flavor of many heirloom tomatoes.
Tomato sunscald can be very frustrating for gardeners—resulting in fruit that have yellowed, leathery skins. Photo courtesy of USDA GRIN.
What Causes Sunscald?
To understand sunscald, we first need to talk about chlorophyll.
Most of us think of chlorophyll as the green pigment that allows leaves to capture sunlight and produce energy through photosynthesis. And that's true. But the end products of photosynthesis aren't energy in the abstract—they're sugars and starches that the plant uses to build everything from roots and stems to flowers and fruit.
Tomatoes, it turns out, are photosynthetic too.
While leaves do most of the heavy lifting, immature tomato fruits contain chloroplasts of their own. These tiny solar panels capture sunlight and contribute sugars that ultimately influence the flavor of the ripe fruit. Research has shown that tomatoes with greater numbers of chloroplasts accumulate more starch and sugar during development and later produce higher concentrations of carotenoids like lycopene.
In other words, the fruits with the greatest capacity for producing exceptional flavor are often the ones doing the most work while they're still green.
Unfortunately, that productivity comes with a risk.
Sunscald happens when chloroplasts—the plant's light-harvesting factories—receive more light than they can safely process, resulting in excess heat and photosynthetic byproducts which can damage cells.
When Good Chloroplasts Go Bad
Under normal conditions, chloroplasts are remarkable little machines.
Using sunlight, water, and carbon dioxide, they manufacture the sugars, pigments, and aroma precursors that make a tomato worth eating. But like any machine, they have limits.
Plant scientists refer to the breakdown that occurs under excessive light as photooxidative stress. One review defines it this way: "Photooxidative stress occurs when light exceeds the coping capacity of the plethora of mechanisms that plants have developed to manage excess absorbed energy."
In simpler terms, the tiny solar panels begin taking in more energy than they can safely process.
As a byproduct of photosynthesis, chloroplasts produce oxygen. Leaves can readily exchange gases through thousands of stomata, but tomato fruits possess relatively few. When the fruit is exposed to intense sunlight and high temperatures, harmful reactive oxygen species begin to accumulate, damaging cell membranes, pigments, proteins, and the photosynthetic machinery itself.
The result is the pale, leathery tissue we recognize as sunscald.
Far from being a simple sunburn, sunscald is better understood as a photosynthetic system pushed beyond its limits—a machine designed to make sugars and flavor compounds that has, quite literally, begun to spin out of control.
And that creates an interesting tradeoff: the tomatoes with the greatest capacity for producing exceptional flavor may also be the ones that have the farthest to fall when conditions become extreme.
Sunscald is most common in tomatoes that have sparse or wispy foliage and is exacerbated when diseases cause defoliation. In our experience, sunscald is most common in determinate cultivars, which slow or discontinue leaf production once the fruit are set.
Do Some Tomato Varieties Resist Sunscald Better?
The short answer is yes—but that protection often comes with tradeoffs.
Over the past century, tomato breeders have intentionally selected for varieties that produce more uniform, marketable fruit. In the process, many modern tomatoes have become less susceptible to problems like green shoulders, environmentally induced cracking, and sunscald.
One of the best examples is the uniform ripening (u) mutation that we discussed in our article on green shoulders. By reducing chloroplast development in the fruit, the mutation creates tomatoes that ripen more evenly and tolerate intense sunlight better than many heirloom varieties.
That insurance, however, comes at a cost.
Researchers have shown that tomatoes carrying the ancestral version of the uniform ripening gene accumulate more sugars, more carotenoids, and ultimately produce a richer array of flavor compounds. The same chloroplasts that increase susceptibility to sunscald also help create the sweetness, color, and aroma that make heirloom tomatoes so memorable.
Of course, this doesn't mean that every modern variety is bland or that every heirloom is destined to suffer from sunscald. But it does remind us that plant breeding is often an exercise in compromise. The traits that make tomatoes easier to grow, harvest, and market are not always the same ones that maximize eating quality.
The best way to prevent sunscald without sacrificing flavor is to select varieties that have a good supply of foliage to shade out the ripening fruit. Here Perestroika tomatoes sit comfortably cool beneath an umbrella of healthy foliage.
How to Prevent Sunscald on Tomatoes
Fortunately, reducing sunscald doesn't necessarily mean sacrificing flavor.
While some modern varieties achieve greater resistance through reduced chloroplast development, many heirlooms solve the problem in a much simpler way: they keep their fruit covered.
Choose Varieties with Dense Foliage
Not all tomatoes provide the same degree of natural protection from the sun.
Varieties with dense, leafy canopies tend to shield their fruits from intense afternoon sunlight, greatly reducing the risk of sunscald without compromising the sugars, pigments, and flavor compounds that make homegrown tomatoes so special. Potato-leaved varieties like Hillbilly and Pink Brandywine are excellent choices because their large leaves provide abundant coverage throughout the growing season.
By contrast, varieties with sparse, wispy growth habits leave much of the fruit exposed. These plants may benefit from additional support, lighter pruning, or even temporary shade cloth during periods of extreme heat.
When selecting varieties for hot, sunny climates, it's worth considering not only flavor and productivity, but also the plant's natural ability to protect its own fruit. After all, a healthy canopy is the tomato's first line of defense against sunscald.
Support Your Plants
Proper staking and caging help keep that protective canopy intact throughout the season. Plants that collapse under their own weight often expose previously shaded fruit to sudden, intense sunlight—a common recipe for sunscald.
A sturdy cage or trellis helps maintain leaf cover where it's needed most.
Be Careful with Pruning
The best protection against sunscald is a healthy canopy of leaves. For this reason, heavy pruning can sometimes do more harm than good. Removing too many leaves may improve airflow, but it also exposes developing fruit to conditions they were never meant to endure.
Use Shade Cloth During Extreme Heat
When temperatures climb into the upper nineties or beyond, even healthy plants can struggle. Temporary shade cloth can reduce light intensity enough to help fruits survive particularly harsh conditions without significantly affecting growth or yield.
Many commercial growers now employ this strategy during heat waves, and home gardeners can benefit from it as well.
Healthy red Marglobe tomatoes safely buried inside carefully trellised plants. Diligent staking or caging is one of the best tools a gardener has for protecting tomatoes from sunscald.
Conclusion
If you've found yourself staring at a sunscalded tomato this summer, take heart: the problem isn't necessarily a sign that you've done anything wrong, nor is it a reason to give up on a favorite variety.
As we've seen, the same chloroplasts that help tomatoes produce more sugars, richer colors, and more complex flavors are also vulnerable to intense heat and sunlight. In many cases, the qualities that make a tomato worth growing are the very ones that require a little extra care from the gardener.
Fortunately, protecting your crop doesn't mean sacrificing flavor. Choosing varieties with good foliage cover, avoiding excessive pruning, staking plants to maintain a healthy canopy, and using shade cloth during extreme heat can dramatically reduce the risk of sunscald while preserving everything that makes heirloom tomatoes special.
The story of sunscald is a useful reminder that appearance isn't always the best measure of quality. Sometimes the most flavorful tomatoes are also the most delicate, and sometimes a few cosmetic imperfections are simply the price of growing fruit that was bred first and foremost for eating quality.
In the end, most gardeners would agree that's a trade worth making.
Ready to get growing? Check out our extensive selection of heirloom tomato seeds each carefully selected for productivity, ease of growth, and most important of all—flavor.
