AN INVESTIGATION OF CHILLING REQUIREMENT OF SOME SCAB RESISTANT APPLE CULTIVARS IN PO VALLEY

Abstract

In order to improve the quality of fresh produce, in terms of food security, reduction of pesticide usage, preserving the environment and health of farmers, many scab resistant apple cultivars were released from several institutions, mainly located in temperate zones during the last years. The cultivation of temperate fruit trees in tropical and subtropical areas, where scab is one of the main diseases, could benefit from introduction of these cultivars, for the reasons cited above. Unfortunately, the indication of chilling requirements has been often missed in the pomology characteristics of the cultivars, although blossom and harvesting times are indicated. In order to provide more information on this matter some scab resistant apple cultivars planted in 2002 were tested. During the winter season 30 one-year-old twigs and 30 shoots with spurs were taken from the trees every week and kept with their bases in water and in a growth chamber for 20 days at 21°C. Apical and lateral buds that grew were both counted and evaluated as to percentage of the total and as to bud development stage. Temperatures during the winters were recorded and both the Richardson and Dynamic Model formulae were tested. The onset of bud break and the end of rest in the one-year-old shoots and in the spurs were assessed by means of the ratio between the chilling units (CU), the chill portions (CP), and by the above mentioned indexes, applying two parameters (% bud break or burst and bud development stage). Significant regressions were observed between the indexes and CU and CP. Throughout the years the prediction of end of the rest was generally lightly better when Dynamic Model was used. The two different buds varied in length of endo-dormancy within cultivars; this can modify the value of their chilling requirement. In the Po Valley the evaluation of the chilling requirement of some apple cultivars resistant to scab showed that low and mid-low cultivars are available and could be grown in tropical and subtropical zones.