Use of bioclimatic indexes to characterize phenological phases of apple varieties in Northern Italy.

The research was designed to characterize the phenological behaviour of different apple varieties and to compare different bioclimatic indexes in order to evaluate their adaptability in describing the phenological phases of fruit species. A field study on the requirement for chilling units (winter chilling requirement) and the accumulation of growing degree hours of 15 native apple cultivars was carried out in a fruit-growing area in North West Italy (Cuneo Province, Piedmont). From 1991 to 1993, climatic data were collected at meteorological stations installed in an experimental orchard (Verzuolo, Cuneo). Four methods were compared to determine the winter chilling requirement: Hutchins, Weinberger-Eggert, Utah and North Carolina. The Utah method was applied to determine the time when the chilling units accumulated become effective in meeting the rest requirements. A comparison of the different methods indicated that the Weinberger-Eggert method is the best: as it showed the lowest statistical variability during the 3 years of observations. The growing degree hour requirement (GDH) was estimated by the North Carolina method with two different base temperatures: 4.4 degrees C and 6.1 degrees C. More difficulties were met when the date of rest completion and the beginning of GDH accumulation was determined. The best base temperature for the estimation of GDH is 4.4 degrees C. Phenological and climatic characterizations are two basic tools for giving farmers and agricultural advisors important information about which varieties to choose and which are the best and the most correct cultivation practices to follow.