DEVELOPMENT OF PROMISING NEW ROOTSTOCKS AND SCIONS FOR FLORIDA CITRUS

This project is related to inhouse project Objective 1: Create new genetic combinations of citrus, Objective 2: Screen germplasm for important traits and select superior individuals, and Objective 3: Evaluate selections for field performance and other traits. Results from four sweet orange rootstock field trials exposed to huanglongbing were published. A detailed study comparing tolerance of rootstocks to huanglongbing in the greenhouse was completed and results are being prepared for publication. The summation of these studies indicates there are significant differences in the tolerance of different rootstocks to huanglongbing, and that, under some conditions, this may have a significant effect on tree growth, health, and performance. Depending on the conditions, rootstocks that sometimes showed increased tolerance to huanglongbing included Volkamer, US-897, US-802, US-942, US- 812, and Carrizo. Fruit quality, yield, and tree size data were collected from five late-season rootstock field trials. Detailed fruit quality data collection continued from a large grapefruit rootstock trial in Indian River County at multiple harvest times to assess the influence of Sour orange, Swingle, US-812, US-942, US-897, US-852, and X-639 on grapefruit quality early, middle, and late in the season. Significant differences were observed among the rootstocks. Cuttings were made of 100 supersour selections in preparation for testing with Diaprepes/Phytophthora and huanglongbing, and also for field trials with commercial scions. A supersour rootstock trial with Hamlin scion and 500 trees was prepared for field planting. Cooperative work was continued with a commercial nursery to multiply 250 advanced supersour selections for placement of trees into cooperative field trials with growers at multiple locations. Work continued to assess supersour tolerance of Citrus tristeza virus and high pH soils, using carefully controlled tests in the greenhouse. Studies continue to assess citrus germplasm tolerance to Liberibacter - Huanglongbing in the greenhouse and under field conditions. Studies to identify the metabolic changes associated with huanglongbing disease development have been completed and a manuscript is being prepared for publication. Recognizing the clear tolerance of some citrus germplasm to huanglongbing, the second half of a study is underway to define the interaction of rootstock tolerance/susceptibility with scion tolerance/susceptibility. Another study has begun to assess the additional benefit of expanding the amount of a tree that is the huanglongbing -tolerant rootstock to include the trunk and scaffold branches. Collaborative work continues to study gene expression and metabolic changes associated with susceptible and tolerant plant responses to huanglongbing, and to define genetic characteristics needed to prevent infection or avoid the damaging effects of the disease. Greenhouse and field studies are continuing to determine the most efficient methods to evaluate new citrus germplasm from crosses and transformation for resistance or tolerance to huanglongbing. Selected anti-microbial and citrus plant resistance genes were inserted into outstanding rootstock and scion cultivars to develop new cultivars with increased resistance to huanglongbing. Research is continuing to use huanglongbing responsive citrus genes and promoters identified in the gene expression study published last year for inducing or engineering resistance in citrus. Fifteen new transgenic rootstocks with selected antimicrobial genes were propagated and entered into controlled greenhouse tests to assess tolerance to huanglongbing. A field trial continued with selected transgenic rootstocks. Collaborative work continued to assess rootstock interaction with scion, nutrition, and management factors in determining tree tolerance to huanglongbing.