The US third largest citrus industry located in Lower Rio Grande Valley (LRGV), South Texas is renowned for producing premium quality fresh fruit. However, the fresh-fruit industry dynamics have changed significantly as devastating Huanglongbing (HLB) disease become endemic in LRGV citrus groves. The primary reasons that affected the Texas citrus industry are unhealthy crop load, and unnatural and excessive fruit drops in HLB affected trees. Therefore, this study investigates HLB effects on citrus fruit development in the region's subtropical semi-arid climate, focusing on the period from bloom to harvest. Research was conducted on mature 'Rio Red' grapefruit and 'Marrs' sweet oranges grafted onto Sour orange rootstock, all exposed to HLB for over a decade. In both citruses multilocation trial and replicated number of trees were pre-screened for blotchy mottled symptoms on foliage and presence of presumptive causal bacterium of HLB, Candidatus Liberibacter asiaticus (CLas) using qPCR assay. The CLas ve trees were further evaluated for sectorial presence of HLB symptoms in 10 branches and divided into two sub-categories of HLB affected symptomatic (Symp) and apparently healthy, asymptomatic (Asymp) branches. In year-2024, monthly fruitlet retention analysis from April to July reveled significantly higher fruitlets drop in ‘Symp’ branches compared to ‘Asymp’ ones across both citrus varieties. In late-May, the ‘Symp’ grapefruit branches showed 28.5-35.3% drop, while sweet orange experienced 23.3-36.3% losses. While ‘Asymp’ branches had significantly lesser fruit drop ranged in 23.1-27.5% in grapefruit and 22.7-29.4% in sweet orange. Although late-May fruit drop is a natural phenomenon in citrus trees, the excessive fruit drop under HLB disease pressure highlights the negative effect of HLB in fruit retention and possibly in fruit size expansion. Ongoing research continues to identify vulnerable stages of fruit growth and expansion under HLB stress, aiming to develop targeted treatments for maintaining high-quality fruit production under HLB prevalence in LRGV citrus groves with region’s unique environmental and edaphic conditions.
