Field Performance of Novel Citrus Rootstocks Grafted with ‘Valencia’ Orange and Their Response to Systemic Delivery of Oxytetracycline

The global citrus industry faces unprecedented challenges due to Huanglongbing (HLB), which is associated with the bacterial pathogen <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las). This study evaluates the field performance of 11 rootstocks, grafted with 'Valencia' orange (<i>Citrus sinensis</i>), under Florida's HLB-endemic production conditions, while also examining the impact of systemic applications of oxytetracycline (OTC) via trunk injection. Mature trees received annual OTC injections and were assessed over two production seasons. In year 1, OTC-treated trees exhibited significant improvements regardless of the rootstock, including a 36% increase in yield, an 11% increase in juice TSS, and reduced leaf bacterial titers. During year 2, the positive effects of OTC were sustained, or even enhanced. <i>C</i>Las titers were reduced in both leaves and roots; yield increased by 70%; and fruit weight, juice color, and TSS also improved significantly. Moreover, OTC-injected trees exhibited a larger percentage of finer roots compared to non-injected trees. US-1688 and US-1672, both hybrids of <i>C. maxima</i> 'Hirado' and <i>C. reticulata</i> 'Cleopatra', emerged as the most productive rootstocks. These results demonstrate the importance of rootstock selection for sustainable citrus cultivation while highlighting the benefits of integrating the systemic delivery of OTC to manage HLB and maximize the resilience of citrus.