Sunlight, while critical for plant growth, can sometimes provide too much of a good thing. Crops experience solar stress from exposure to the ultraviolet (UV) and infrared (IR) radiation that is produced by the Sun along with valuable photosynthetic light. Neither UV nor IR wavelengths are visible to the human eye, but both can be harmful.
- Ultraviolet light has a wavelength range of 220 to 400 nanometers (nm). Exposure to this spectrum can cause various skin disorders on produce, similar to how a human sunburns from staying in the sun too long.
- Infrared light (wavelength >1000 nm) is the spectrum that creates heat, and it’s the wavelength that can make the skin surface of produce reach a temperature 20°F warmer than the surrounding air temperature.
The Impact of Solar Stress
Too much UV or too much IR can lead to sunburn, sunscald, and heat stress in plants—all of which compromise plant health. Depending on the crop, the negative effects can range from surface cell damage and discoloring, to dehydration, to a reduction in internal fruit quality (i.e., firmness, color, juice). The following examples show how solar stress impacts various crops.
Apples
Solar stress can cause a general yellowing of the apple’s skin, and in some varieties, such as the Gala, dark brown spots can appear on the fruit’s exposed surface. These symptoms can occur even on a typical summer day when UV levels are high, and IR levels can increase the surface temperature of the fruit by as much as 20°F higher than the air temperature.
Tomatoes & Peppers
Tomatoes and peppers generally thrive in hot climates, but their fruiting structures can be very sensitive to solar stress when temperatures climb above 86°F (30°C). In tomatoes, moderate sunscald can yellow the shoulders of the fruit, and if temperatures rise above 104°F (40°C), lethal damage can cause the fruit’s tissue to turn white, die, and become infested with fungi and bacteria.
Peppers experience similar solar-related problems, such as irregular coloring, sunken or wrinkled tissue, and infection from fungi and bacteria. In addition, if peppers experience heat stress 10 to 30 days after anthesis (flowering), the fruit’s width and weight can be negatively affected. Growers in any region around the globe run the risk of 20%–40% of their crop being destroyed or, at a minimum, downgraded as a result of solar stress.
Tree Nuts
Solar stress can also impact tree nut crops. Walnuts, for example, are particularly sensitive to solar stress during the early stages of maturation, when the kernel is in liquid or gel form. If the nuts are continually exposed to sunlight, the husk surface may show yellowish-brown spots, which will develop into dark leathery lesions by mid or late summer. Sunburned nuts will have shriveled and darkened kernels, which can make the meat unmarketable or, at a minimum, reduce its value.
Solar Stress Is a Costly Problem That Can Be Avoided
Solar stress is now recognized by researchers as a very serious threat to the value of crops. Studies conducted all around the globe on various crops—such as apples, tomatoes, peppers, melons, walnuts, and wine grapes—have shown average losses in marketable yield ranging from 8% to as high as 50%. When these losses are converted to a monetary value, they add up to hundreds of millions of dollars in lost return on an annual basis.
The good news? With advancements in science, particularly with the introduction of Purshade® plant protectants, solar stress can now be managed without cutting into a grower’s bottom line. Better yet, reducing the incidence of solar stress has proven to increase a grower’s total return on investment. It is important to remember, however, that while Purshade products offer effective solar protection, they can be effective only if they are used on a consistent and regular basis. Humans who suffer from sunburn can heal, but crops that are sunburned are irreversibly damaged, so prevention is the key to success in mitigating solar damage in crops.
