Three Types of Sunburn
Schrader and his team have identified and characterized three types of sunburn in apples (see Fig. 1 below). (Schrader et al., 2001, Schrader et al., 2008). 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). Sunburn necrosis results from thermal death that occurs at an FST of ~126 °F (52 °C). Photooxidative sunburn occurs on shaded (non-acclimated) apples that are suddenly exposed to full sunlight (e.g. during thinning or pruning, or if a branch shifts with heavy crop load). It is independent of temperature, and is caused by visible light. If no protective practices are used, Washington State apple growers lose about 10%, on average, of their crop to sunburn cullage.
Fig. 1. Three types of sunburn (shown left to right): Sunburn Necrosis, Sunburn Browning, Photo-oxidative Sunburn
Protection from Sunburn
RAYNOX® treated 'Fuji' apples
Schrader invented a carnauba-wax based sunburn protectant called RAYNOX® (U.S. Patent No. 6,857,224). RAYNOX® was successfully commercialized during 2003 in Washington State, and its commercialization was later expanded to other states and countries. It is now the leading sunburn protectant for apples, and reduces sunburn losses by 50%, on average. The reduction of sunburn due to Dr. Schrader's research has the potential to save Washington State apple growers over $50 million annually.
Schrader's team has also conducted research on evaporative cooling (EC) as another method for sunburn protection. EC is the most effective means of decreasing FST of apples, but EC does not filter out damaging ultraviolet rays. RAYNOX® has been shown to be compatible with EC, and the two tactics used together are the best management practice to reduce fruit losses due to sunburn or other heat-induced disorders. The combined use of EC and RAYNOX® improves fruit quality at harvest.
Because water usage is excessive with conventional EC systems, his research team invented an FST sensor that can be used to control EC systems and reduce water usage. The sensor was successfully Beta-tested in commercial orchards, but a manufacturer/distributor for the sensor has not been identified.
Pigment changes in apples with sunburn browning
We have identified the pigment changes that occur in apple skin with different degrees of sunburn browning (See Fig. 2 below). The green chlorophyll pigments and red anthocyanin pigments decrease or disappear as severity of sunburn browning increases from SB-1 (slight sunburn) to SB-4 (severe sunburn browning), but the yellow carotenoids increase concomitantly. This explains why sunburned apples have a yellow or brown spot on the damaged side of the fruit. Felicetti & Schrader, 2008, 2009a and 2009b.
Fig. 2. Classes of Sunburn Browning. NB=No Sunburn;
SB-1 to SB-4 = increasing from slight to severe sunburn browning.
Changes in internal fruit quality in apples with sunburn browning
As severity of sunburn browning increased from SB-1 (slight sunburn) to SB-4 (severe sunburn browning) (See Fig. 2 above), flesh firmness and soluble solids increased, but titratable acidity decreased sharply. These trends persisted over several months and became more apparent as time in cold storage increased. Thus internal fruit quality declined more rapidly in sunburned fruit.(Schrader et al., JASHS (2009) & IJFS (2009). This finding provides another incentive for growers to protect apples from sunburn damage.
Larry Schrader, Horticulturist/Plant Physiologist
Tree Fruit Research & Extension Center, Wenatchee
Office Ph: 509-663-8181 x265
Office Hours: by appointment