Effects of exogenous gibberellic acid in huanglongbing-affected sweet orange trees under Florida conditions - I. flower bud emergence and flower formation

Authors: Tang, Lisa; SINGH, GARIMA - University Of Florida; DEWDNEY, MEGAN - University Of Florida; VASHSISTH, TRIPTI - University Of Florida

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2021
Publication Date: 11/19/2021
Citation: Tang, L., Singh, G., Dewdney, M., Vashsisth, T. 2021. Effects of exogenous gibberellic acid in huanglongbing-affected sweet orange trees under Florida conditions - I. flower bud emergence and flower formation. HortScience. https://doi.org/10.21273/HORTSCI16080-21.
DOI: https://doi.org/10.21273/HORTSCI16080-21

Interpretive Summary: In Florida, Citrus trees affected by Huanglongbing often flower in multiple waves, resulting in an extended bloom period in spring. In the event of rain, this irregular flowering pattern increases the growth of a fungus that infects flower petals, leading to fruit drop shortly after bloom. To limit fungus buildup on citrus trees, we attempted to shorten the bloom period by foliar spraying flower-inhibiting gibberellic acid at different rates and times. Our results demonstrated that gibberellic acid at 20 gram per acre applied monthly from September to December compressed flowering time of sweet orange the following March. The results of this study provide a field approach to synchronize citrus spring bloom, which allows precise and reduced usage of pesticide for fungus-related fruit drop control.

Technical Abstract: Under Florida conditions, sweet orange (Citrus sinensis) affected by HLB [Candidatus Liberibacter asiaticus (CLas)] frequently exhibits irregular flowering patterns, including off-season flowering and prolonged period of return bloom. Such patterns can exacerbate postbloom fruit drop (PFD) of citrus by increasing the opportunity for temporal and spatial proliferation of the fungal causal agent Colletotrichum acutatum J. H Simmonds that infects flower petals. For the development of strategies to manipulate flowering for PFD control, the effects of floral inhibitor gibberellic acid (GA3) sprayed monthly at full- and half-strength rates (49 or 25 g·ha-1) with different regimes (starting from September and terminated in November, December, or January) on the pattern of spring bloom were evaluated in field-grown HLB-affected ‘Valencia’ sweet orange at two locations for two consecutive years in this study. To further examine whether GA3 effects on flowering patterns vary in different cultivars, early-maturing ‘Navel’ sweet orange trees receiving no GA3 or full-strength GA3 monthly in September through January were included. Overall, for ‘Valencia’ sweet orange, monthly application of full-strength GA3 from September to December not only minimized the incidence of scattered emergence of flower buds and open flowers prior to the major bloom but also shortened the duration of return flowering, compared to the untreated control trees. Additionally, exogenous GA3 led to decreased leaf FLOWERING LOCUS T expression starting in December as well as reduced expression of its downstream flower genes in buds during later months. Similar influences of GA3 applications on repressing flower bud formation and compressing bloom period were observed in ‘Navel’ sweet orange. The results suggested GA3 treatments can be potentially used in HLB-affected sweet orange trees to minimize the buildup of C. acutatum inoculum on flowers early in the season that allows precise applications of fungicides with reduced total usage for PFD prevention.