2013 ASHS Annual Conference

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 due to the HLB epidemic currently underway in Florida, and potentially tolerant/resistant scions and rootstocks, some with commercial potential, are being identified. In addition, a robust rootstock HLB screening program is also identifying candidate rootstocks that may be able to protect susceptible scions. A few scion genotypes, including mandarins and pummelos, are still thriving in blocks that are devastated by HLB, with nearly 100% infection rates and most trees showing severe symptoms. Of particular interest is a sweet orange-like triploid hybrid that contains approximately 8% trifoliate orange parentage, produced via embryo rescue from an interploid cross. Remarkably, this hybrid produces seedless fruit that is not easily distinguished from true sweet orange, and its pasteurized NFC (not from concentrate) juice has a flavor profile in the range of ‘Hamlin’ and ‘Valencia’, the two most important processing oranges in Florida. The tree is productive and has yet to show any HLB symptoms, whereas adjacent trees have been devastated by HLB. Additional information on this selection and others showing tolerance/resistance will be presented, as well as their utilization in the CREC breeding program. Complex diploid and tetraploid rootstock candidates, not pre-selected for HLB tolerance/resistance, are also showing a differential response to HLB in field trials as compared to highly susceptible commercial rootstocks, with several showing significantly lower HLB infection rates, and less severe symptoms once infected. Field trial data will be provided. This suggests great potential for HLB pre-screening of complex rootstock germplasm, in hopes to identify rootstock candidates that can protect susceptible scions. We have developed a robust greenhouse/field screening procedure that has been underway for the past two years, with promising results. The ultimate goal is to develop rootstocks that can be used to establish sustainable, productive groves without the current requirement of efficient psyllid (the HLB vector) control. Progress in such rootstock development will be presented.