2019 ASHS Annual Conference

Huanglongbing (HLB) is ubiquitous in FL and severely impacting citrus, is becoming widespread in TX, and has been verified in over a thousand backyard trees in CA. The FL industry has embraced more intensive production practices to sustain production on HLB-affected trees. Growers report that these costly practices permit ‘Valencia’ production at ~60% of healthy trees, but are less effective on ‘Hamlin’ (and other early and mid-season processing oranges), and ineffective on grapefruit. So far, the TX industry reports that HLB-affected trees show little deleterious response, with no decline in production. Citrus researchers are immersed in extensive and broad-ranging efforts to identify solutions to HLB. Previous research indicates susceptibility to HLB throughout cultivated citrus: in FL none are immune and many are extremely adversely affected. Small scale trials are producing high yields and quality of susceptible types under protective structures, but this is likely only feasible for very high value fresh fruit. High juice prices, combined with acceptable ‘Valencia’ production currently sustains FL citrus, but transition to resistant or tolerant planting material is likely critical for maintaining the industry, and would benefit citrus industries wherever HLB threatens. Some rootstocks are reported to confer greater productivity, and new FL plantings emphasize their use. Potentially useful HLB- tolerance is apparent in several mandarin hybrids, including some released cultivars. Work is underway exploring use of HLB-tolerant hybrids to supplement orange juice. HLB-tolerance is reported in some advanced hybrids that include Poncirus in their pedigrees, and analyses suggest strong potential for inclusion in the juice stream, as well as for fresh fruit. Biotechnology offers the hope of HLB-tolerance/resistance in cultivars with established high market demand. Numerous transgenic strategies are being tested and show promise, but commercial implementation will require extensive effort and expense to achieve deregulation and replant. Genome editing should provide invaluable solutions, but clear targets to induce HLB-tolerance currently remain obscure. Implementation of effective therapies will be critical, at least in the near-term, to sustain production in existing infected trees. Use of CTV-vectors and transgenics to express anti-bacterial peptides has entered testing on substantial acreage.