THERMAL TIME REQUIREMENT AND HARVEST TIME FORECAST FOR PEACH CULTIVARS WITH DIFFERENT FRUIT DEVELOPMENT PERIODS

Abstract

Non-linear models using growing degree hours (GDH), based on the choice of base, critical and optimum temperatures, have been successfully applied to calculate thermal time required for spring bud burst in deciduous fruit trees. The flexibility of the model can fit the wide range of temperatures that occur during the peach fruit development period (FDP), which takes place from early spring to late summer. In this experiment, fruit growth was studied in relation to thermal time accumulated from bloom to fruit harvest for peach and nectarine cultivars whose fruit development period range from 70 to 150 days. Thermal time was calculated in terms of degree days (DD) (base temperature 7 °C, and critical temperature 35 °C) and GDH (base temperature 7.5 °C, optimum temperature 26 °C and critical temperature 38.5 °C). Climatic and phenological data (bloom and harvest dates) were available for a minimum of three to a maximum of nine years. GDH showed a lower coefficient of variation and a higher predictive capacity, in terms of days, than degree days for all of the cultivars tested. Taking into account the whole FDP, the accuracy of the GDH model in predicting harvest time ranged from 1 day, in the early ripening peach cultivar ‘Maycrest,’ to 4 days in late ripening peach cultivar ‘O’Henry.’ An accurate early forecast of fruit harvest time was obtained using the GDH accumulated during the first 25-52 days of FDP, depending on the cultivar. INTRODUCTION The development period of peach fruits is genetically controlled (Vileila-Morales et al., 1981) but ranges greatly with season (Blake, 1930) and environment (Weinberger, 1948; Topp and Sherman, 1989; Caruso et al., 1993). Time of ripening depends on bloom time and on the length of the fruit development period (FDP), which is regulated chiefly by the temperature range from bloom to ripening and by the cultivar response to temperature (Boonprakob et al., 1992); as a matter of fact, temperature has been used for building models to predict harvest time (Munoz et al., 1986; Boonprakob et al., 1992; Caruso et al., 1993; Motisi et al., 1992; Motisi et al., 1998). FDP has been associated to the average daily mean temperature (Munoz et al., 1986) or to mean monthly temperature (Topp and Sherman, 1989). Anyhow, prevailing temperatures during the first 2 months after full bloom vary the most and are the best correlated to the FDP, serving as the best predictor for harvest time (Lilleland, 1965; Boonprakob et al., 1992; Caruso et al., 1993). Motisi et al. (in press), re-examining FDP data by Munoz et al. (1986), showed that the coefficient of variation (c.v.) of thermal time accumulated from bloom to harvest, for five early ripening peach cultivars grown in four different environments, is much lower than the c.v. relative to the same period and expressed in term of days. As a matter of fact, thermal time requirement, during fruit growth, is cultivar dependent and its c.v. between years or environments, should be close to zero. Based on this hypothesis the Proc. 5 IS on Peach Eds. R.S. Johnson & C.H. Chrisosto Acta Hort. 592, ISHS 2002