Occurrence of Pericarp Damage and Mitigation Methods in the New Citrus Cultivar, ‘Mizuki’

The purpose of this study was to clarify the characteristics and potential preventative measures for yellow spots, a disorder that affects the pericarp of the new citrus cultivar, ‘Mizuki’. Yellow spots were found to be the primary disease affecting the pericarp in ‘Mizuki’ and exhibited different symptoms to sunburn. Yellow spots were first observed in mid–late July, and increased in frequency in mid–late August. New instances of yellow spots were observed as late as December. Observation of pericarp sections at the site of yellow spot occurrence revealed browning in some of the oil glands, which was absent in healthy sections. More than 80% of the pericarp sections exhibited one browning oil gland. There were fewer occurrences of yellow spots in the inner canopy than in the outer canopy. In the outer canopy, yellow spots more commonly occurred in fruits exposed to sunlight than to shade. Coating the fruit with black shade cloth suppressed the development of yellow spots. This approach was particularly effective when the coating was applied prior to late July, but ineffective when the coating was applied after late August. This study also showed that higher solar radiation resulted in increased yellow spot occurrence. This study therefore suggests that yellow spot occurrence in ‘Mizuki’ is affected by both the stage of fruit development and solar radiation conditions, and that yellow spot occurrence may be suppressed by controlling the amount of solar radiation received by the fruits.