Bloom, Maturity, and Fruit Set of Pistachio in Response to Early Season Application of Ethephon Ehsan Askari, Sayareh Irani, and Khorshid Razmjoo * Department of Agronomy, Isfahan University of Technology, Isfahan, Iran *Corresponding author: Krazmjoo@cc.iut.ac.ir Received February 2, 2010 / Accepted September 27, 2010 Korean Society for Horticultural Science and Springer 2011 Abstract. Spring frost damage has been a problem in pistachio producing areas. This experiment was conducted to evaluate the effect of ethephon application rate and date on bloom and maturity delay, fruit set, and yield of ’Kalleh-guchi’ pistachio in Rafsanjan, Iran. Fourteen-year-old whole trees were sprayed with 0, 250, 500, and 750 mg L -1 ethephon on February 10 or March 1 for single application and both dates for two applications. With increasing the application rate, timings of the initial and full bloom and the maturity were delayed by 11.2, 12.6, and 7.6 days in 2007 and 11.3, 10.7, and 5.4 days in 2008, respectively. The double spray was the most effective in delaying bloom and maturity, and in increasing yield. The double spray with 750 mg L -1 ethephon delayed initial and full bloom, and maturity by 14, 15, and 10 days in 2007 and 15, 15, and 8 days in 2008, respectively. This treatment increased the yield by about 64% in both years. It was concluded that ethephon may be applied to pistachio in late winter to delay bloom in spring without any apparent side effects, and thus to prevent spring frost damage and increase the yield. Additional key words: flowering, frost, Pistacia vera, plant growth regulator Hort. Environ. Biotechnol. 52(1):29-34. 2011. DOI 10.1007/s13580-011-0029-4 Research Report Iran is the largest pistachio producing country in the world (Rahemi and Ramezanian, 2007). However, spring freeze damage has been a major problem in pistachio pro- ducing areas of the country. Pistachio trees bloom early in the year when the risk of frost damage to blossoms is still high. Prolonged dormancy and delayed flowering may reduce frost damage and increase yield. Plant growth regulators have been used to delay blossom- ing and improve yield. Ethephon application in autumn delayed blossoming in almond (Browne et al., 1978), sweet and sour cherry (Dennis, 1976), and plum (Webster, 1984). Bloom delay was reported in sweet cherry when ethephon was applied at 100 to 500 mg L -1 in the fall (Proebsting and Mills, 1976). Coston et al. (1986) have also shown that fall application of 120 to 250 mg L -1 ethephon delayed peach bloom by 3-5 days, while 500 mg L -1 resulted in tree injury. Durner and Gianfagna (1988) and Gianfagna (1991) reported that fall applications of ethephon delayed flowering by 7 days. Krewer et al. (2005) also reported up to 14 days of delay in blueberry blooming with the application of 400 mg L -1 ethephon in the fall. They also found that two appli- cations were more effective than single application. Ethephon treatments (150-200 mg L -1 ) in October was also reported to delay blooming of apricot and peach trees in the following spring, resulting in 50% yield increase (Buban and Turi, 1986). Irving (1987) observed that autumn applied ethephon delayed full bloom 6-15 days for the first and 14- 16 days for the second spray, thereby increased the fruit diameter and yield of nectarine. Durner and Gianfagna (1988) obtained yield enhancement of peach by fall appli- cation of ethephon. The above studies only showed that ethephon was applied mainly in the summer or fall in order to delay blossoming in the following spring. Meanwhile, information on the effects of ethephon on pistachio yield and blossoming delay is not yet available. Therefore, this study was conducted to evaluate the effects of ethephon application rate and date on ’Kalleh-guchi’ pistachio flowering, maturity, fruit set, and yield. This research was conducted in Rafsanjan (30° 25´ N latitude, 55° 54´ E longitude, and 1,581 m elevation), Iran in 2007 and 2008, using a 14-year-old ‘Kalleh-guchi’ pistachio