Thermal time requirement and harvest time forecast for peach cultivars with different fruit development periods

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