Under Florida conditions, sweet orange (Citrus sinensis) affected by Huanghongbing {HLB [Candidatus Liberibacter asiaticus (CLas)]} frequently exhibits irregular flowering patterns, including off-season flowering and prolonged bloom period. Such patterns can increase the opportunity for temporal and spatial proliferation of pathogens that infect flower petals, including the fungal causal agent for postbloom fruit drop (PFD) Colletotrichum acutatum J.H. Simmonds. For the development of strategies to manipulate flowering, the effects of floral inhibitor gibberellic acid (GA(3)) sprayed monthly at full- and half-strength rates (49 and 25 g.ha(-1), or 33 and 17 mg.L-1, respectively) with different regimens, starting from September and ending in November, December, or January, on the pattern of spring bloom were evaluated in field-grown HLB-affected 'Valencia' sweet orange at two locations in subsequent February through April for two separate years in this study. To further examine whether GA(3) effects on flowering patterns vary in different cultivars, early-maturing 'Navel' sweet orange trees receiving no GA(3) or full-strength GA(3) monthly in September through January were included. Overall, for 'Valencia' sweet orange, monthly applications of GA(3) at 49 g-ha(-1) from September to December not only minimized the incidence of scattered emergence of flower buds and open flowers before the major bloom but also shortened the duration of flowering, compared with the untreated control trees. In addition, exogenous GA(3) led to decreased leaf flowering locus t (FT) expression starting in December, as well as reduced expression of its downstream flower genes in buds during later months. When applied monthly from September through January at 49 g.ha(-1), similar influences of exogenous GA(3) on repressing flower bud formation and compressing bloom period were observed in 'Navel' sweet orange. These results suggest that by effectively manipulating flowering in HLBaffected sweet orange trees under the Florida climate conditions, exogenous GA(3) may be used to reduce early sporadic flowering and thereby shorten the window of C. acutatum infection that causes loss in fruit production.
