One symptom of citrus huanglongbing (HLB) is excessive preharvest fruit drop. Recently, a higher incidence of Lasiodiplodia theobromae (Diplodia) was found in HLB-symptomatic orange calyx abscission zones (AZ-C) than in non-symptomatic fruit. The infection was positively correlated with the reduction in fruit detachment force (FDF), suggesting that Diplodia infection may be involved in HLB-related preharvest fruit drop. To verify the direct correlation of HLB-related fruit drop with the fungal infection, HLB-affected sweet orange trees (Hamlin) were shaken manually during the harvest season. The fruit that dropped from trees upon shaking were collected (D), and the fruit that remained (R) on trees after shaking were harvested (R). Fruit ethylene production was measured, and the titers of Diplodia and CLas in AZ-C of D and R fruit were analyzed. Gene expression levels related to signaling and biosynthesis of phytohormone (jasmonate, ethylene, and abscission) were evaluated by qRT-PCR. The results indicate significantly higher Diplodia titers in D than in R fruit; more than half of D fruit but none of R fruit exhibited ethylene production, which was positively correlated with Diplodia titer in D fruit. Genes related to synthesis and signaling of ethylene (ET) and jasmonates (JA) were up-regulated, while abscisic acid (ABA) was down-regulated. The results suggest that different from water stress or carbon shortage induced abscission (which is characterized by induced abscisic acid), HLB-associated preharvest fruit abscission is mediated by JA/ET signaling, which was triggered by secondary fungal infections at the calyx abscission zone.
