Larry Schrader and his team have identified and characterized three types of sunburn in apples: (1) sunburn browning, (2) photooxidative sunburn, and (3) sunburn necrosis.
Sunburn browning is the most prevalent type of sunburn, and is caused by high fruit surface temperatures (FST) and ultraviolet radiation. The FST required to induce sunburn browning varies by cultivar, but is in the range of 113 to 120 °F (45 to 49 °C).
In contrast, photooxidative sunburn occurs in apples that have been shaded prior to sudden exposure to full sun (e.g. apples exposed during thinning or pruning, or when a limb shifts with heavy crop load). This sunburn type is independent of temperature, and is caused by visible light. The first symptom of photooxidative sunburn is whitening of the sun-exposed side. Later necrosis may form on these apples in the center of the bleached area.
Sunburn necrosis results from thermal death that occurs at an FST of ~126 °F (52 °C). If no protective practices are used, Washington State apple growers lose about 10%, on average, of their crop to sunburn cullage.
Sunburn Browning Model for Apple
A model was developed by Jianguang Zhang and Larry Schrader (WSU TFREC) to predict the incidence of sunburn browning of apples. This model is available on DAS (using automatically updated NOAA weather forecast data) as well as on Larry Schrader’s web page (for user-entered weather data; http://hort.tfrec.wsu.edu/pages/Sunburn). Note that this predictive model pertains to sunburn browning ONLY.
Factors contributing to sunburn browning
Sunburn browning usually occurs during the afternoon on fruits acclimated to high light (i.e. previously exposed to sunlight for several days before the sunburn event). However, sunburn browning symptoms may not appear for a few days after the sunburn event occurs. High fruit surface temperature (FST) and ultraviolet-B radiation are the two major factors that cause sunburn browning. Several factors contribute to FST, and some of the factors interact with each other making it more difficult to precisely predict whether sunburn browning will occur. Factors that contribute to FST include intensity of solar radiation intercepted by a fruit, air temperature, wind velocity, relative humidity, fruit size, cultivar or variety, canopy density, and cultural practices. Our research indicates that the FST must be maintained at 113°F (45°C) or higher for 1 hour to induce sunburn browning. In a 2-year study, we found that maximum air temperatures exceeded 86°F (30°C) on 55 days one year and on 60 days the second year, but the FST exceeded 113°F (45°C) on only 22 or 23 days, respectively. Hence, air temperature alone is not a suitable criterion for predicting sunburn browning. Sunburn browning does not occur in fruit smaller than 1.8 inches in diameter (golf ball diameter). Wind speeds above 3 mph and high relative humidity reduce the risk of sunburn browning. Also, some cultivars (e.g. Cameo and Honeycrisp) are more prone to sunburn browning than others (e.g. Cripps Pink).
Fruit surface temperature vs. air temperature
The graph shows that air temperature continued to increase beyond 6 PM PDT on a typical day in July, whereas the FST of a ‘Fuji’ apple exposed to afternoon sun began to decline about 4 PM PDT. This indicates that less solar energy was absorbed by the apple after 4 PM due to the increased solar zenith distance (or angle of incidence) later in the day. Hence, the FST declined before air temperature decreased.
"A comparison of fruit surface temperature (FST) vs. air temperature for a typical day in July in Wenatchee, WA. Note that FST declines before air temperature in late afternoon. (L. Schrader)"
Internal fruit quality
Internal fruit quality of apples with sunburn browning is affected and declines further during cold storage. This is another reason to protect your apples from sunburn.
For further information about sunburn and it’s effects on internal fruit quality, as well as the sunburn browning predictive model, visit Larry Schrader's web page http://hort.tfrec.wsu.edu/pages/Stress_Physiology.(Larry Schrader, WSU-TFREC)