Create new candidate hybrids. Sexual hybridization is completed between selected elite parents during spring flowering and seed collected in the fall. During this quarter, seed were collected from crosses of elite rootstock hybrids US-942 and US-897 with pummelos. Selected hybrids will be grown-out for propagation, testing, and establishment of seed trees. Emphasis of hybridization in the USDA rootstock program is among parents with superior tolerance to HLB, CTV, and Phytophthora, along with showing favorable effects on grafted tree yield, fruit quality, and tree size. Propagate and plant new field trials. Replicated multi-year field trials with commercial scions are essential to evaluate performance of rootstocks, both to determine whether each new rootstock should be released for commercial use, and to develop comparative performance information among new and existing rootstocks for a diversity of scions, soils, and management conditions. Most of the rootstock field trials are planted with a single scion representing a common commercial type on each of 40-60 different rootstocks. Adequate replication is considered a critical factor in the USDA rootstock trials, with 6-7 replications the minimum and 12 replications the optimum to provide an acceptable level of reliability for results. Three new rootstock trials with sweet orange scion on promising rootstocks were planted in this quarter. Nursery trees for three additional rootstock trials are being prepared in the greenhouse for planting in spring 2020.Collect data from field trials. Information on tree performance is collected from established field trials, and includes measurement of tree size, fruit crop, fruit quality, and pathogen titer, HLB symptoms, and assessments of tree health. Measurements related to cropping are on an annual cycle based on the scion, while measurements of health and tree size are on a schedule determined by the specific conditions and goals of the trial. Cropping data was collected from 6 trials with early-maturing scions during Nov 2019-Jan 2020. Assessments of tree health and measurements of tree size were completed on nine trials during this quarter.Evaluate effectiveness for seed propagation of new rootstocks and develop seed sources. Some of the newest hybrid rootstocks can be uniformly propagated by seed, but others cannot. As the best rootstocks are identified through testing, seed sources are established and used to determine trueness-to-type from seed. Studies were initiated this quarter to evaluate seed propagation for 25 of the most promising SuperSour hybrid rootstocks.Posting field trial results for grower access. The USDA rootstock trials produce large amounts of information that is useful to identify the most promising of the new hybrids, as well as comparative information on the relative performance of many commercially available rootstocks. I recognize that this information is of great value to citrus growers and nurseries, and am working to create summaries from 38 trials that will be posted onto a website for easy access by Florida growers. It is anticipated that the first stage of that information will be posted to https://citrusrootstocks.org/ soon. Release of superior new rootstocks for commercial use. Several of the 300 advanced Supersour rootstock hybrids in field trials are exhibiting outstanding performance in comparison with the commercial standard rootstocks. Perforance data continues to be collected, but it is anticipated that 2-3 of the most outstanding of these will be officially released in 2021-22.