The Development of Improved Citrus Rootstocks to Facilitate Advanced Production Systems In Florida

The 10 billion dollar/year Florida citrus industry is now under siege by an insect vectored bacterial disease citrus greening, also known as huanglongbing (HLB).  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, although a few show some limited tolerance.  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 production systems to Florida conditions.  These methods have been shown to bring new groves into production 2-3 years earlier than with standard Florida cultivation practices.  Such systems also provide better control over tree flushing and more efficient vector (psyllid) control spray programs. 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. We have coined the term ‘tetrazyg’ to describe zygotic allotetraploid rootstock candidates that are progeny from crosses of somatic hybrids.  Preliminary testing suggests that tetraploid rootstock candidates from both of these sources have potential to facilitate the success of new production systems. A major rootstock trial (The St. Helena project) 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 advanced production systems.  This trial includes selected somatic hybrids, tetrazygs, diploid hybrids and commercial controls.  Rather than unavailable fertigation, we are using a modified Harrell’s slow release fertilizer product and daily short-duration microjet irrigation, interrupted only when soil moisture probes indicate high water content. Yield and fruit quality data from trees just under 3 years of age will be presented, as there were significant rootstock affects on both parameters. Higher performing rootstocks will be identified.  The very low incidence of HLB in the trial will also be discusses.  Fast growing yet precocious bearing rootstocks that produce higher quality fruit are also candidates for use with transgenic citrus produced from juvenile explants, as necessary to reduce juvenility – this will also be discussed.