Factors associated with citrus fruit abscission and management strategies developed so far: a review

Citrus is one of the most important and widely grown fruit crops around the world. Citrus fruit drop is due to the influence of various biotic and abiotic factors. Major biotic factors include insects like fruit flies and diseases such as Huanglongbing (HLB) and postbloom fruit drop (PFD) due to Colletotrichum acutatum. The associated abiotic factors are temperature, rainfall, hailstorms, water stress, nutritional imbalance, etc. Studies have been conducted to understand fruit drop and its relation to various plant growth regulators. Plant growth regulators such as auxin and gibberellins have been categorised as abscission inhibitors, while ethylene, abscisic acid (ABA) and methyl jasmonate (MJ) are abscission promoters. Here, in this review, we have emphasised on the different phases of fruit drops, factors causing fruit drop, the role of plant growth regulators, fruit detachment force (FDF), rootstocks, water relation, nutrient status and different chemical, biological, botanical and hormonal strategies applied for controlling citrus fruit drop. Moreover, we have also discussed the molecular basis of citrus fruit drop and have reviewed recent studies involving biosynthesis/signalling pathway genes and phytohormone genes affecting fruit drop. This review offers comprehensive information about citrus fruit drop and shall provide the direction of future research for students, horticulturists, researchers and other stakeholders.