The Development of Improved Tetraploid Citrus Rootstocks to Facilitate Advanced Production Systems and Sustainable Citriculture in Florida

The Florida citrus industry is now under siege by the insect vectored bacterial disease Huanglongbing (HLB) (ex greening). HLB affects all cultivated citrus varieties and is extremely difficult to control. Genetic resistance to HLB is not present in commercial scion or rootstock cultivars. A possible mid-term solution to this problem is to shorten the grove rotation and time to profitability by adapting open hydroponics systems (OHS) or other similar evolving advanced citrus production systems (ACPS) to Florida conditions. These methods have been shown to bring new groves into production 2-3 years earlier than with standard practices. Such new citrus production systems that feature high density plantings being developed and adapted for Florida growing conditions will require improved tree-size controlling rootstocks that bear early and produce good yields of fruit with juice of high quality. We have been exploring somatic hybridization of complementary diploid rootstocks via protoplast fusion to generate allotetraploid rootstock candidates, and more recently we have been conducting rootstock breeding at the tetraploid level by crossing superior somatic hybrids to produce ` tetrazyg' rootstocks. Preliminary greenhouse and field testing suggests that selected allotetraploid rootstocks generally exhibit good soil adaptation, disease resistance, nursery and young tree growth, and also a strong tendency to reduce tree size. A major rootstock trial is underway that features high planting densities and a production system that mimics the principles of OHS, with a goal of identifying superior rootstocks for use in ACPS. This trial includes selected somatic hybrids, 'tetrazygs', diploid hybrids and commercial controls, all with mid-season processing sweet orange scions 'Valquarius' and 'Vernia'. We are observing significant rootstock effects on both yield and fruit quality, with good performance from several somatic hybrid and 'tetrazyg' rootstocks. We have also observed an unexpected reduced impact of HLB in the trial. Our working hypothesis is that there is a rootstock/nutrition interaction that is affecting HLB disease frequency and severity.