Citrus black spot severity related to premature fruit drop in sweet orange orchards

Citrus black spot (CBS) may cause substantial yield loss in sweet orange orchards. Severity and distribution of CBS symptoms on fruit may be variable and not all affected fruit may fall. However, the relationship between CBS symptoms and yield loss has not yet been investigated. This study aimed to determine CBS characteristics associated with premature fruit drop. A set of fruit was removed from the tree to measure the detachment force and another set of dropped fruit was collected from Pera and Valencia sweet orange orchards in three municipalities in Sao Paulo state, Brazil. CBS severity, distance of the nearest lesion from the peduncle, and fruit peel colour were assessed in both fruit sets. CBS severity was consistently related with the force to remove fruit and hence associated to the premature fruit drop, irrespective of the cultivar and location. The segmented linear regression showed that the fruit detachment force was reduced abruptly, at a rate of 7% for every 1% increase in disease severity up to 4.2 +/- 1.0% CBS severity and, above this breakpoint, the force decreased slightly at a rate of 1.5%. Taking into account all orchards, the proportion of dropped fruit with CBS severity higher than the breakpoint was approximately 90%. This study provides a better understanding of the relationship between CBS severity and premature fruit drop of sweet orange. CBS severity breakpoint may be used as a reference to anticipate harvest and reduce CBS-related yield loss in orchards intended for juice processing.