ISHS Acta Horticulturae 618: XXVI International Horticultural Congress: Environmental Stress and Horticulture Crops
ENVIRONMENTAL STRESSES THAT CAUSE SUNBURN OF APPLE |
| Authors: | L. Schrader, J. Zhang, J. Sun |
| Keywords: | [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.], Malus x domestica, thermal death, fruit surface temperature, UV-B radiation, air temperature, solar irradiation, wind velocity, relative humidity |
| DOI: | 10.17660/ActaHortic.2003.618.47 |
| Abstract:
Sunburn of apple costs growers millions of dollars in economic losses annually. We have identified two types of sunburn in apple and their causes. One type (sunburn necrosis) is caused by thermal death of epidermal and subepidermal cells when the peel reaches 52 ± 1°C. A necrotic spot appears on the sun-exposed side of the fruit. The second type (sunburn browning) is sublethal and results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. When we experimentally induced sunburn in attached fruit of several cultivars, the fruit surface temperatures (FST) at which sunburn browning occurred varied between 46°C and 49°C depending on cultivar. Maximum FST exceeded 45°C on 22 days in 2000 and on 23 days in 2001 in our orchard. Sunlight was required for sunburn browning. Protecting fruit from UV-B radiation usually prevented sunburn browning. The FST reached a maximum between 1430 and 1645 hours between June 15 and September 30, and was affected by several meteorological parameters. Maximum FST was highly correlated with maximum daily air temperature (r = 0.90**). Maximum FST also was highly correlated (r = 0.88**)with the mean of maximum hourly air temperatures between 1100 and 1700 hours. Mean solar radiation between 1100 and 1700 hours also was highly correlated (r = 0.65**) with maximum FST. Mean wind velocity and mean relative humidity (RH) between 1100 and 1700 hours were inversely correlated (r = -0.24** and -0.66**, respectively) with maximum FST. During 2000 and 2001, we found that FST never exceeded 45°C on days when maximum air temperature was below 30°C. When maximum air temperature exceeded 35°C, maximum FST almost always exceeded 45°C. When maximum air temperature was between 30 and 35°C, solar radiation, wind velocity and RH also were important determinants of maximum FST. | |
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