S16 - Session O1 - Enhancing yield efficiency and fruit quality in citrus through use of tetraploid rootstocks

Authors: Ute Albrecht *, Sudip Kunwar, Jude Grosser, Fred G. Gmitter Jr., William B. Castle

Citrus is Florida's signature crop with over 160,000 hectare of commercial production and supporting a multi-billion-dollar industry. In 2005, the devastating citrus disease huanglongbing (HLB), also known as citrus greening, was discovered in the USA and has since reduced citrus production by more than 50%. HLB is associated with phloem-limited, insect-vectored bacteria, and no cure exists at present. Ninety percent of citrus production in Florida is for juice processing and sweet orange ( Citrus sinensis ) is the most used species. Sweet oranges are among the most HLB-susceptible species and, if not managed properly, trees will decline and become unproductive within several years after infection. This has increased the demand for HLB-tolerant rootstocks to retain tree health and maximize productivity through higher-density plantings. Polyploidy in citrus has been associated with reduced tree size and enhanced biotic and abiotic stress tolerance. In this study we compared the potential of tetraploid rootstocks with diploid rootstocks for inducing dwarfing of the grafted sweet orange scion and enhancing productivity and tolerance to HLB. Two field trials were established in 2015 in different locations in Florida, USA. The soil in one location is sandy and well-drained, whilst in the other it is poorly drained requiring raised beds and furrows for drainage. Tree size, yield, yield efficiency, fruit quality, and tree health were assessed over three production seasons from 2019 to 2021. The different rootstocks varied significantly in their effects on tree size, yield, and yield efficiency. With few exceptions, diploid rootstocks induced a larger tree size than tetraploid rootstocks and produced higher cumulative yields. However, most of the tetraploid rootstocks induced higher yield efficiency and better fruit quality. These rootstocks have therefore good potential for use in high-density plantings to increase revenue in an HLB-endemic production environment such as Florida.