Breeding citrus rootstocks to mitigate Huanglongbing (HLB, or citrus greening disease)

The often-repeated dogma regarding HLB (Huanglongbing or citrus greening disease) and commercial citrus germplasm is that adequate tolerance or resistance does not exist. However, the UF/CREC citrus improvement broad-based germplasm collection is now undergoing a massive natural HLB screen because of the HLB epidemic currently underway in Florida, and potentially tolerant/resistant scions and rootstocks, some with commercial potential, are being identified. Complex diploid and tetraploid rootstock candidates, not pre-selected for HLB tolerance/resistance, are showing a differential response to HLB in field trials compared with highly susceptible commercial rootstocks, with several showing significantly lower HLB infection rates, and less severe symptoms once infected. Based on this evolving information, we have established a robust rootstock HLB screening program, and we are identifying new rootstock candidates showing potential to protect susceptible scions from this destructive disease. This greenhouse/field screening procedure (referred to as the 'gauntlet') has been underway for the past 3 years, and we will report on promising results obtained so far. Crosses of superior rootstock parents are made at diploid and tetraploid levels. Seed harvested from crosses is planted in bins of calcareous soil (pH 8), inoculated with Phytophthora nicotianae and P. palmivora. Robust seedlings are selected on the basis of growth rate, health and color, and are then transferred to 4x4 pots in commercial potting soil. The top of each new selected tree goes for seed source tree production, and the remaining liner to the HLB screen. Each hybrid liner is grafted with an HLB-infected budstick of Valencia sweet orange; with the remaining rootstock top removed to force flushing from the HLB-infected sweet orange budstick. Trees are monitored for HLB symptoms, and apparently healthy trees are entered into a 'hot psyllid' house for approximately 8 weeks, followed by field planting at an approved site. The ultimate goal is to develop rootstocks that can be used to establish sustainable, productive groves without the current requirement for efficient psyllid (the HLB vector) control, which is costly and leads to pesticide buildup.