To what extent does pre-harvest variability affect chilling injury susceptibility of citrus postharvest?

Chilling injury (CI) is a physiological rind disorder that commonly occurs during the long-time cold storage of citrus. The occurrence of CI on citrus reduces fruit quality and shelf-life, leading to market rejection. The susceptibility of citrus to CI during long cold storage period depends on the variability of several pre-harvest factors. However, the impact of pre-harvest variability on the postharvest occurrence of CI for citrus remains largely unknown. To address this, the impact of the variability in production area, cultivar, orchards and fruit harvest maturity on CI of citrus was investigated over two growing seasons. Four main citrus production regions in South Africa were considered, namely, Citrusdal, Nelspruit, Letsitele and Sundays River Valley (SRV). Five cultivars representing the main citrus subgroups were selected; 'Turkey' and 'Midknight' Valencia oranges, 'Nova' and 'Nadorcott' mandarins, and 'Star Ruby' grapefruit. For the variability in harvest maturity, fruit were harvested with two week harvest intervals from 8 weeks before commercial harvest until 6 weeks after. Our result revealed that differences in production area, cultivar, orchards, and fruit maturity have a significant impact on CI susceptibility. However, among these factors, orchard variability had the lowest impact on CI susceptibility, while the regional weather variability between production areas had the most significant impact. Fruit from the SRV, a coastal region, showed the highest risk of CI for all cultivars. They were up to 70 % more susceptible than those from Citrusdal, Nelspruit, and Letsitele. The CI susceptibility of 'Turkey' and 'Midknight' Valencias is ~60 % more than those of mandarins and grapefruit. Fruit harvested early in the season (e.g., 8 weeks before commercial harvest) have a very high risk of CI, while those at the commercial maturity stage will have the highest CI tolerance. This study provides insight into which major pre-harvest factors affect the quality of citrus postharvest.