Scientia Horticulturae, 2(1974)265--272 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands INDUCTION OF SUNSCALD DAMAGE IN TOMATOES UNDER NATURAL AND CONTROLLED CONDITIONS H.D. RABINOWITCH, N. KEDAR and P. BUDOWSKI Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot (Israel) (Received January 31, 1974) ABSTRACT Rabinowitch, H.D., Kedar, N. and Budowski, P., 1974. Induction of sunscald damage in tomatoes under natural and controlled conditions. Scientia Hort., 2: 265--272. Typical sunscald injury was induced in mature-green tomato fruit under controlled conditions. During the initial phase, the induction period, exposure to temperatures of 40°C or higher was necessary. Symptoms of sunscald became visible during the subsequent incubation period at room temperature. A relatively short induction period at high temperatures, e.g. 18 h at 45.1°C, was equivalent to more prolonged induction at more moderate temperatures, e.g. 28 h at 40.8°C. In addition to heat, visible light was necessary for the development of typical sunscald symptoms. The light treatment was most effective when applied both during the induction and the subsequent incubation period, resulting in 100% of fruit with sunscald. When illumination was restricted to the induction period, 13% of the fruit escaped injury, as compared to 66% of healthy fruits when light was applied during incubation only. It was concluded that sunscald is caused by the concurrent action of two external factors, heat and light, suggesting a mechanism of diversion of energy from the photosynthetic pathway into abnormal photodynamic reactions. INTRODUCTION Sunscald is a physiological disorder of frequent occurrence which severely affects the marketing quality of many kinds of fruits, flowers and vegetables grown in warm climates (Barber and Sharpe, 1971). In tomato fruit, the symptoms are usually white necrotic areas surrounded by a yellow halo or, in mild cases, a permanent yellow discoloration, resulting in a high rate of rejection (Retig and Kedar, 1967). Several studies had been made on the relative susceptibility of tomato fruits to sunscald at different developmental stages. Very young green tomatoes are not affected by the disorder, whereas fruit at the "mature-green" and "breaker" stages are highly susceptible (Harvey, 1924; Aharoni, 1967; Retig and Kedar, 1967). During the subsequent "pink" and "red" stages, the susceptibility to sunscald decreases and disappears (Harvey, 1924; Ramsey and Link, 1932; Retig and Kedar, 1967), but at these stages previously induced