Development of Next-Generation SuperSour rootstocks with tolerance to HLB

Development of Next-Generation SuperSour rootstocks with tolerance to HLB

Report Date: 12/15/2022
Project: 21-008   Year: 2022
Category: Plant Improvement
Author: Kim Bowman
Sponsor: Citrus Research and Development Foundation

Objective 1. Collect field performance data from Stage 1 and 2 replicated rootstock field trials and release new rootstock cultivars as justified by superior performance in multiyear field trials.

Collect field performance data. Since the beginning of the project, three of the 31 replicated trials listed on the original proposal were removed, and three new replicated trials have been established. So the current list of active replicated rootstock trials for the project remains at 31. During this quarter, canopy health data was collected from all trees in the trials. For two trials with an early mandarin scion, fruit yield and fruit quality data was collection during this quarter. Preparations began for the collection of yield and fruit quality data from trials with Hamlin and lemon scions, which will be completed during the next quarter.

Supplemental testing of promising selections. When rootstock selections appear promising in field trials and are being considered for commercial release, additional information about stress tolerance and other traits is highly desired. This will help inform appropriate sites and management for commercial use. During this quarter, rootstock plants were prepared for a study to evaluate salinity tolerance among selected promising new rootstocks. During this quarter, seedlings of the most promising advanced rootstock selections were grown for assessment of trueness-to- type or uniformity from seed, and will be evaluated by morphology and molecular markers.

Preparation for release. The most promising USDA rootstock selections are provided to the FDACS-DPI program for shoot tip grafting and disease testing. Those that have completed the process are held as trees at FDACS and USDA, Ft. Pierce, for use as seed trees and sources of clean budwood. The introduction of new plant material to this FDACS program was temporarily suspended in 2022, but is expected to begin again soon. The two new USDA rootstocks tentatively planned for release in 2023 have already gone through the FDACS program, and are available as clean budwood.
Release of new rootstocks. New rootstocks will be officially released by USDA for commercial use when justified by multi-year performance in the early-stage replicated field trials. Based on outstanding performance in field trials, the proposed release documentation is being prepared for two new USDA rootstocks, coded as US-1688 and US-1709. It is anticipated that these two rootstocks will be released during 2023. New USDA clones are assigned official names at release, and the new names for these have not yet been approved.

Objective 2. Create hybrid rootstocks that combine germplasm from parental material with good rootstock traits and HLB tolerance, propagate the most promising of these hybrids, and establish replicated field trials with commercial scions.

Create hybrids. Current focus of USDA rootstock work is primarily on testing of hybrids previously created. Selected new crosses are planned for spring 2023, with parental combinations based on new information and breeding values of particular clones assembled over the past year.

Propagate hybrids. New hybrids to be used in trials will be propagated by uniform nucellar seed or stem cuttings. Trees were budded in the nursery with two new superior scion cultivars on 20 rootstocks, in preparation for field planting in 2023. Plant material was prepared in the nursery for one new Stage 2 trial with Valencia scion including the important commercially available rootstocks and the most promising of the new rootstocks. It is anticipated that these trees will be ready for field planting in spring 2024.

Establish replicated field trials. No new field trials were planted for this reporting quarter.

Tree care in trials. Periodic care was applied in the 31 field trials to maintain tree health and productivity, and manage weeds.

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