Breeding new rootstocks for the Australian citrus industry (CT18004)

Eight consecutive seasons of data have been used to identify 38 individual rootstock hybrids that are consistently high yielding and produce good fruit quality. These are being recovered for semi-commercial performance confirmation.

Tree performances varies widely from year to year making it impossible to find superior germplasm in short-term trials, hence almost 900 individual trees were assessed over the eight year period in order to discover these promising 38 individuals.

Breeding efficiency has been improved through the deployment of two molecular markers, thus enabling large numbers of hybrids to be quickly screened ahead of field testing. This is the first time that molecular markers have been used so early in the rootstock breeding process and is possible because of validation research conducted on Australian bred germplasm.

Young seedlings can now be quickly screened for virus resistance and the ability to produce clonal seeds (polyembryony).

A new field trial with ‘Premier’ is cropping for the first time and has already demonstrated promising signs in terms of rootstocks that control vigour and productivity.

Rootstocks from the University of Florida and US Department of Agriculture have been budded with ‘IrM2’ and will be established within a commercial planting during 2023.

New rootstock selections from the program have been transferred to the Elizabeth Macarthur Agricultural Institute (EMAI) for disease confirmation and national distribution via AusCitrus.

Since the last reporting period, four major field trials were assessed, along with the continuation of nursery-based screening activity and the generation of new hybrids. Knowledge from the trials proved fundamental for guiding parent choice in the team’s hybridisation work. New international discoveries, such as the efficacy of a stable anti-microbial peptide (SAMP) and the publication of the Poncirus trifoliata genome, also guided the crossing program.

Results from field trials located on commercial properties confirmed:

• The usefulness of ‘Barkley’ rootstock for Imperial mandarin
• The need to assess tree performance over multiple seasons
• Opportunities for genetic reduction of tree vigour
• A possible new rootstock release.

Even the youngest field trial (Jamesons, planted November 2020) generated useful information on tree establishment and replanting decisions at the site. The oldest existing field trial (Presslers, planted August 2011) is due for completion next season with opportunities to explore genetic aspects of rootstock effects on scion phenotype.

Unique germplasm capturing new species and virus resistance demonstrated commercial merit and supported the breeding strategy of tackling major faults (under simple genetic control) that mask otherwise useful genetics.

In November 2021, a major new field trial was planted within a commercial orchard at Gayndah, representing a shift in emphasis toward export-style mandarins in direct response to industry consultation and advice.

Containing 154 different rootstocks, most of which have never been assessed under field conditions, this is also the first large field trial to test rootstocks for the new scion variety ‘11C017’ resulting from project Building a genetic foundation for Australia’s citrus future through targeted breeding (CT15017).

Data generated from field trials and nursery-based research has helped guide crossing decisions in the annual pollinating season (Aug-Sep). In 2020, emphasis was placed on finding robust sources of resistance to phytophthora diseases, and then including these as parents.

The continued strong performance and wider scale testing of one genotype in the program is pointing to a likely commercial release and distribution. 

Interstate distribution is progressing well, with confirmation of virus freedom in all 55 accessions sent to NSW.

Three commercial field sites were chosen, prepared and maintained in Gayndah, Emerald and Wallaville to conduct assessments of the new germplasm on fruit quality, productivity and tree growth/health.

Emerald

A week was spent in March 2020 to assess the 891 trees that make up this Imperial rootstock experiment (151 rootstocks). The project team found that granulation levels were high and crop load was somewhat reduced compared to last season. Wind breaks were found to reduce granulation, however they also reduced crop size. The impact of rootstock strains was predictable on tree size and trunk structure, however not on fruit quality which varied between seasons and replicates. Key insights suggest the true value of the rootstock can only be determined by averaging across multiple seasons.

Gayndah

Fruit quality testing of 245 rootstocks and 490 trees was conducted in April 2020. Crop load was very light and generally the amount of granulation was high. Extensive effort was put toward finding rootstock hybrids in this trial that may be of value, however, assessment over consecutive seasons will provide more accurate averages and present sensible choices of the hybrids that show the most promise.

Wallaville

Trees at this field site presented granulation levels lower than expected finding only a handful of trees stripped because of excess granulation. Additionally, only a few trees needed to be stripped because they were too small to carry the crop. Assessments also found that large vigorous rootstocks (US812, Benton, Barkley, D77) were not producing fruit, whereas many of the small or intermediate size trees had lots of fruit.

Overall

Some progress has been made and new rootstocks are beginning to emerge that have only moderate granulation while also offering benefits in other important traits such as fruit size, and tree productivity. Most notable being rootstock 05C009 and its hybrids with P.trifoliata similarly have amongst the lowest granulation. 

Results from field trials are guiding the choice of parents in the breeding work and this information will be used in the August to September pollinating season.

Testing has been conducted on C. wakonai x P. trifoliata hybrids, which have been field planted at Bundaberg Research Station and are being observed for seed structure as they fruit.