Citrus Scion Breeding in Florida Affected by Endemic Huanglongbing Disease

The citrus disease Huanglongbing (HLB), recently established in Florida, greatly weakens and often kills trees. HLB in Florida is associated with CandidatusLiberibacter asiaticus (CLas), vectored by the Asian citrus psyllid (ACP), and infections typically remain latent for many months.   Citrus breeding by the USDA has been ongoing for 120 years and developing cultivars with HLB-resistance has become a priority.  Unfortunately this disease has also pervasively influenced and impeded many aspects of citrus improvement.  Soon after HLB was first identified in Florida during 2005, one of our two farms quickly became infected with HLB.  HLB/ACP is largely uncontrolled on this site to facilitate resistance screening and testing of control measures.  However, since most recent segregating hybrids are on this farm, we must accelerate identification of promising selections before they are too weak to assess.  Extensive efforts have been made to rescue priority material field-infected with HLB.  Marked reduction in CLas is achieved, but systemic infection and latency make it uncertain when rescued budwood can be declared clean. Antibiotic and other treatments are being used in an effort to prolong life of infected trees, to permit more thorough assessment and permit use as parents. HLB-infected seed-parent trees drop many fruit, reducing program efficiency. CLas in anthers from field-exposed trees counter-indicate transfer of pollen from infected to HLB-free areas. In addition, several studies indicate CLas may sometimes be transferred in seeds, restricting options for growing hybrids elsewhere.  Prudence and Florida law requires that citrus nursery production now occur in ACP-proof structures, and retrofitting greenhouses cost our program > $400,000 (US). At the second USDA farm, aggressive ACP control began in 2008, requiring new equipment and many expensive sprays.  However, in November 2011, HLB-infected trees were found here also.  At this farm, aggressive spraying continues along with annual scouting and removal of infected trees, all at substantial expense.  ACP-proof screenhouses are being established to house ground-planted parent trees and to protect high-potential new hybrids in high-density plantings, where horticultural manipulations are used to rapidly overcome juvenility.  It is anticipated that citrus production and breeding will continue in Florida, but in a much more challenging environment.