1a. Objectives (from AD-416):
1. Create new genetic combinations of citrus germplasm via conventional breeding, mutation, and transformation. a. Improve and enhance methods to evaluate and screen new priority commercial citrus scions and rootstock cultivars for resistance and/or tolerance to Huanglongbing and the Asian citrus psyllid. 2. Screen germplasm for important traits and select superior individuals. 3. Evaluate selections for field performance and other traits. 4. Release new scion and rootstock varieties for commercial use. 5. Develop new, more effective testing methods (especially RE-PCR) for screening and identifying host-plant resistance to Huanglongbing and citrus bacterial canker, and apply these testing methods to discover novel resistance genes from elite citrus germplasm, unstudied citrus wild species, traditional dooryard varieties, etc.
1b. Approach (from AD-416):
New citrus selections will be created by sexual hybridization, mutation, and genetic transformation from existing cultivars and species. Hybrids and other new types will be tested for important traits using molecular markers, greenhouse and laboratory assays. Promising selections from these assays will be entered into long-term field trials at multiple locations and data collected on tree health, size, fruit yield and quality. Selections that appear to have desirable combinations of traits will be released for commercial or dooryard use.
3. Progress Report:
About 1200 seedlings were produced from crosses made in 2011. In 2012, made 49 types of crosses directed at major market types emphasizing genetic seedlessness and huanglongbing-tolerance. Top-quality selections were identified: a fine-flavored large mandarin; a late ripening mandarin; and four sweet-orange- and grapefruit-like trifoliate hybrids. To free them of virus, 21 priority selections were shoot-tip grafted by a collaborator including a peelable orange-type. Progress has been made to characterize apparent tolerance to huanglongbing in some conventional citrus scion cultivars. Several rootstock selections were identified with significant tolerance or resistance to huanglongbing disease. Hybrids with trifoliate orange continue to test as the most huanglongbing-resistant citrus, and a replicated trial of 114 hybrids and standards has been established and genotyped with cooperators. Tree health, size, and cropping information were collected from numerous established field trials. More than one thousand transgenic citrus selections were produced, including numerous gene constructs with potential to induce resistance to huanglongbing disease and other useful traits. Factors potentially important for efficient genetic transformation of juvenile and mature citrus tissue were investigated and included types, proportions and concentrations of plant growth regulators, mineral nutrients, carbon sources, explant source, preconditioning treatments, culture matrix, and genotype. Many transgenic selections have been propagated for further testing, including a select group that was planted into a field trial. Several transgenic sweet oranges grow significantly better than controls under severe huanglongbing pressure. Metabolomic analysis of juice from fruit affected by huanglongbing disease revealed significant differences in the concentrations of sugars, amino and organic acids, limonin glucoside, and limonin. With the exception of phenylalanine, limonin glucoside, and limonin, oranges from huanglongbing infected trees had lower concentrations of metabolites compared to fruit from uninfected trees. Experiments were conducted to determine pollination requirements of the recently released cultivar, US Early Pride mandarin. US Early Pride was hand pollinated using eleven different pollen sources. Results indicate that each pollen parent induced fruit set and could serve as a pollenizer for US Early Pride. In a second experiment US Early Pride trees growing in the field were enclosed in screen cages to exclude alternate sources of pollen; bees were placed in the cages to ensure movement of US Early Pride pollen; results suggest that US Early Pride is self-fertile, but cropping may be improved with the use of pollenizers.
1. Some rootstocks provide improved sweet orange tree tolerance to huanglongbing. Huanglongbing disease is widespread in Florida and severely affects health in infected citrus trees. A study was completed by ARS researchers in Ft. Pierce, Florida comparing tree health and fruit cropping on sweet orange trees grafted on numerous rootstocks in four field trials growing in an area severely affected by huanglongbing. The study demonstrated significant rootstock differences in ability to tolerate huanglongbing disease. The results indicate that some rootstocks enable citrus trees to better tolerate huanglongbing disease, and suggest that use of a tolerant rootstock will be one component of successful citrus production management in the presence of that disease.
2. Effects of nutritional treatments on huanglongbing (HLB). Nutritional treatments are being widely used in Florida to sustain production in citrus groves affected by huanglongbing disease. In one study, ARS researchers at Fort Pierce, Florida found that enhanced nutritional treatments of citrus trees affected by huanglongbing disease did not sustain tree health, yield, or fruit quality of Las-infected HLB-symptomatic trees. Moreover, since the nutritional supplements had no effect on Las titer, a major concern is that existing enhanced nutritional strategies have promoted area-wide buildup of inoculum and increased disease spread within and between citrus orchards. The findings of this research will have significant impact on management decisions for citrus growers dealing with huanglongbing affected orchards.
3. Two grapefruit-like cultivars perform better than standard grapefruit under disease pressure. Two diseases, huanglongbing and citrus canker, severely affect standard grapefruit varieties and threaten fruit production in Florida. ARS researchers at Ft. Pierce, Florida have identified grapefruit-like cultivars that perform far better than true grapefruit under severe pressure from huanglongbing and citrus canker. Tree health and productivity of Triumph and Jackson were greater than standard cultivars Marsh and Flame in a replicated field trial assessed for 3 years. In fruit quality assessments Triumph and Jackson fruit always met commercial maturity standards whereas Marsh and Flame fruit usually did not due to low total soluble solids and low Brix/acid ratios. These results suggest that Triumph and Jackson or other grapefruit-like cultivars may be viable alternatives to standard grapefruit cultivars in the presence of severe huanglongbing and citrus canker.
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