1. Rootstocks that significantly improved fruit production under severe huanglongbing disease pressure were identified in field trials. Huanglongbing disease is widespread in Florida and severely affects health in infected citrus trees. Numerous studies were completed by ARS researchers in Ft. Pierce, Florida comparing tree health and fruit cropping of sweet orange and other cultivars grafted on numerous rootstocks in field trials growing in areas severely affected by huanglongbing. The studies demonstrated significant rootstock differences in fruit production under conditions of severe huanglongbing. 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. Identified the stage of the transformation process where there is a significant difference between recalcitrant and amenable citrus types. Citrus types vary widely in their transformability. For example, Poncirus and Poncirus hybrids (e.g., Carrizo citrange) are amenable to genetic transformation and routinely have a transformation efficiency > 50%. Conversely, scion types such as sweet orange grapefruit have efficiencies of 1% or less. Improving transformation of the scion types by an order of magnitude would greatly reduce the resources required to widely utilize transgenic technology in citrus. Numerous factors and stages were examined and compared between sweet orange, grapefruit, and Carrizo. Though Carrizo has a higher shoot regeneration capacity, this property is not sufficient to explain the difference in transformability. These results suggest that these citrus types respond differently at the early stages of transformation. Thus, improving the transformation efficiency of the scion types probably requires modified treatment at these early stages.
3. Four new sweet orange-like hybrids with high quality fruit were selected. ARS citrus breeders at Ft. Pierce, Florida have identified four hybrids with high-quality fruit similar to sweet orange in appearance and flavor. All established sweet oranges are selected mutations of the original progenitor orange, and are almost identical genetically. Sweet oranges are among the most highly susceptible citrus to the devastating disease huanglongbing, and the new sweet-orange-like hybrids have entered tests to determine whether they have greater tolerance/resistance. Working with other ARS Researchers, these hybrids have been shown to have aroma volatiles very similar to existing sweet oranges, making it likely they can be officially classified as sweet orange and marketed commercially as sweet oranges. One hybrid is easily peeled by hand. Clean budwood was developed and trees are being produced for testing in growers’ orchards.
Zhao, H., Sun, R., Albrecht, U., Padmanabhan, C., Wang, A., Coffey, M.D., Girke, T., Wang, Z., Close, T.J., Roose, M., Yokomi, R.K., Folimonova, S., Vidalakis, G., Rouse, R., Bowman, K.D., Jin, H. 2013. Small RNA profiling reveals phosphorus deficiency as a contributing factor in symptom expression for Citrus Huanglongbing Disease. Molecular Plant. 6(2):301-310.