Field performance of ‘Valencia’ orange trees on diploid and tetraploid rootstocks in different huanglongbing-endemic growing environments

Abstract

Huanglongbing (HLB, a.k.a. citrus greening disease) has reduced Florida citrus production and acreage substantially since its discovery in 2005. Most commercially important citrus scion cultivars such as sweet oranges are susceptible to the disease and no cure is available at present. 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. This study compared the potential of tetraploid rootstocks with diploid rootstocks for inducing dwarfing of the grafted ‘Valencia’ orange scion and for enhancing productivity and tolerance to HLB. Two trials, one at a poorly drained site in south-east Florida and another at a well-drained sandy site in central Florida, were established in 2015. Tree size, yield, yield efficiency, fruit quality, leaf nutrients, and canopy health were assessed over three production seasons from 2018/19 to 2020/21. The different rootstocks varied significantly in their effects on tree size, yield, and yield efficiency. Tree size induced by the diploid rootstocks, which included the Florida industry standards Swingle, sour orange, X-639, and US-897, was 55% larger on average compared to the tetraploid rootstocks. Cumulative yields were 32% higher on diploid rootstocks compared to tetraploid ones. However, tetraploid rootstocks induced a 27% higher yield efficiency on average. A significant rootstock effect on canopy health and nutrient status was found, but this effect was not consistent across locations and production years. Tetraploid rootstocks did not display higher HLB tolerance than diploid rootstocks, including the industry standards. However, effective tree size control, high yield efficiency, and higher fruit quality induced by many of the tetraploid rootstocks revealed their potential to be used in high-density plantings.