ASHS 2025 Conference

The citrus industry experiences considerable losses due to a physiological rind disorder that compromises mandarin fruit quality and marketability—particularly in cultivars like Satsuma Owari. This disorder, characterized by water-soaked areas that progress to brown lesions, is likely triggered by multiple factors, including environmental conditions such as high relative humidity and rainfall. It typically appears after fall rain events and is more prevalent in fruit from the outer canopy. Our study, conducted in a commercial orchard in California’s San Joaquin Valley, explored the role of environmental conditions, rind water status, and potential management strategies. We found that treatments applied at the color break stage, 2,4-dichlorophenoxyacetic acid (2,4-D, 16 mg/L), gibberellic acid (GA₃, 20 mg/L), and Vapor Gard® (0.5% v/v)—significantly reduced rind damage. However, GA₃ delayed external color development by approximately four weeks. Susceptibility to rind disorder varied by cultivar, with Kishu being the most susceptible, followed by Satsuma Owari and Gold Nugget, while Page, Tango, and W. Murcott exhibited greater tolerance. We also investigated biochemical, hormonal, and molecular changes associated with the disorder. Damaged tissues showed reduced cellulose concentrations but similar levels of pectin and hemicellulose compared to healthy tissue. Additionally, damaged rinds had higher hydrogen peroxide (H₂O₂) content and increased peroxidase (POD) and polyphenol oxidase (PPO) activities. Metabolomic and hormonal profiling identified 76 metabolites, with 43 differing significantly between healthy and damaged tissues, highlighting the involvement of jasmonic acid and related compounds in the disorder’s development. Transcriptomic analysis further revealed differential expression of genes involved in stress responses, cell wall degradation, and hormone signaling between healthy and affected tissues. These findings suggest that delaying rind senescence using targeted treatments may reduce the incidence of this disorder. This research provides new insights into the physiological, biochemical, and molecular basis of mandarin rind disorder and offers practical management strategies to mitigate fruit losses across different varieties.