Apple Sunburn Risk Detection\u2014A\xa0Simple Model for Agricultural Decision Making and Some Fruit Temperature Measurements

Klaus-Peter Wittich

doi:10.1007/s10341-021-00544-y

Abstract

As a response to strong radiative heating, the fruit temperature of apples may rise significantly above the air temperature. This may result in damage to the skin tissue and the epidermal and hypodermal cell layers. To avoid economic losses induced by sunburn symptoms on the skin, apple growers need forecasts of the fruit temperature, which will allow them to organize sun-protection measures in time.The temperatures of detached apples, which had been exposed to incident radiation and wind, were measured in order to quantify the extent of fruit heating under the climate conditions of northern Germany. On average, the daily maximum skin temperatures measured on the sunlit south side of the fruit during selected sunny and warm days were about 7 °C above the daily maximum air temperature. However, skin temperatures of more than 40 °C (the predefined warning threshold) were rarely detected, indicating that sunburn is still an uncommon phenomenon in northern Germany. Maximum apple-core temperatures were on average 4 °C higher than the maximum temperatures of ambient air.Based on energy balance principles, a simple model has been developed to predict the diurnal cycle of the sky- and ground-facing hemispherical apple temperatures. Focusing on fruit growers’ needs, daily temperature maxima of the modeled sky-facing hemisphere were compared with skin temperatures measured on the sunlit south side, resulting in a mean absolute error (MAE) of 1.7 °C. Comparison between daily maxima of total sphere temperatures and apple core temperatures provided better correlations (MAE = 0.8 °C).

Highlights

During the last decade, fruit growers in Germany have become increasingly concerned about the growing apple yield losses caused by a physiological disorder of the fruit referred to as sunburn (Lebe and Schulte 2008; Steffens 2008, 2013; Wagner and Böttcher 2010; Wiebusch 2019)
Apple sunburn is induced by excessive radiative heating of the fruits during the summer when the air temperature and light intensity are high and cooling wind is lacking. This environmental impact causes skin discolorations on the southwest facing side of the fruits, ranging from bleaching or yellowing through to browning and blackening (Schrader et al 2001, 2003a). These pigment changes, which are partly associated with sub-surface cell damage, are classified into three different types of sunburn: Sunburn browning, which appears as a yellow to dark brown patch, is caused by skin temperatures over approx. 45 °C and bright sunshine with high UV-B impact
At DWD’s Agrometeorological Research Centre in Braunschweig, northern Germany, apple temperatures were measured on fair summer and fall days as a marker for sunburn disorder

Summary

Introduction

Fruit growers in Germany have become increasingly concerned about the growing apple yield losses caused by a physiological disorder of the fruit referred to as sunburn (Lebe and Schulte 2008; Steffens 2008, 2013; Wagner and Böttcher 2010; Wiebusch 2019). Apple sunburn is induced by excessive radiative heating of the fruits during the summer when the air temperature and light intensity are high and cooling wind is lacking. This environmental impact causes skin discolorations on the southwest facing side of the fruits, ranging from bleaching or yellowing through to browning and blackening (Schrader et al 2001, 2003a). The third type, photo-oxidative sunburn, is independent of any significant temperature trigger and occurs at far below the skin-temperature thresholds mentioned It is caused largely by irradiation leaving a bleached patch. The sudden change from shade to direct sunlight may happen during excessive summer pruning

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Conclusion