National papaya breeding and evaluation program (PP18000)

Significant progress has been made on the breeding, selection, and ongoing assessment of advanced red and yellow papaya lines. New elite, genetically stable cultivars that meet the needs of growers and the preferences of consumers will be released that are adapted to the Tableland and/or Coastal growing regions of north Queensland. Recombinant inbred F7 lines will be assessed in semi-commercial block trial for comparison of performance of desirable tree agronomic and fruit quality traits against the industry standard cultivars. New F1 hybrid cultivars will be produced through crossing with preferred current parental cultivars.

The best performing red papaya lines bred for the Coastal region set fruit significantly lower to the ground, have 17 per cent thicker trunks, produce 20 per cent more marketable and 12 per cent sweeter fruit than RB1. These lines also appear to have some resistance to Phytophthora fruit disease compared to RB1 based on limited disease assessments. The best lines selected for the Tablelands also set fruit significantly lower to the ground for ease of picking, have almost 30 per cent thicker trunks, 30 per cent more marketable and 13 per cent sweeter fruit than RB1. Further selections are underway for advanced yellow lines with significantly better performance than 1B.

Since the project team’s last update, work has progressed in the breeding, selection, and ongoing assessment of advanced red and yellow papaya lines and F1 hybrids.

A total of 240 seedling of F6 red generations were field planted on two trial sites in Innisfail (Zappala Farm and RMC Farming). Evaluations were completed for both sites in October 2021 and February 2022. Outcomes demonstrated that:

• The advanced breeding lines grew fruit significantly closer to the ground than RB1.
• The trunk circumference (cm) of the advanced breeding lines (F6 generation) was not significantly different to RB1.
• The Zappala site produced lower numbers and smaller sizes of side shoots than RB1, however the same was not observed for the RMC site.

The F5 advanced breeding lines of yellow papaya were field planted at two trial sites in the Tableland (Lecker Farming and Rocky Top Farm). The first evaluation was completed at both trial sites showing the Lecker Farming site set fruit significantly lower than the commercial cultivar 1B, and the Rocky Top site set fruit significantly lower to the ground than all of the breeding lines (except Lecker Farms). Another 120 are to be planted at Lecker Farms in March-April 2022.

Progress has also been made towards the award of Plant Breeder Rights (PBR) protection of the advanced breeding lines and new hybrids.

Progress continues on the international germplasm collection, and activities relating to potential trait introgression.

The team reported the following progress toward providing genetically stable and superior papaya varieties:

• Production of F6 generation advanced breeding lines of red and F5 generation of yellow papaya continued. To make breeding line selections, the weighted selection index previously developed was applied and an updated Papaya Evaluation Handbook was developed.
• Overall, the selected lines performed better in the key traits assessed than the industry standard varieties RB1 and 1B. The F6 seedlings of red papaya were sex determined and field planted in May and August 2021 at the Innisfail trial sites, with the Tableland trials to be planted in October 2021. They will be evaluated, selected, and self- and crosspollinated to produce F7 and F1 generations by mid-2022.
• The RNA of RB1 and 1B were sequenced to identify differentially expressed sequences related to potential genetic components detected within the two flavour profiles that had previously been characterised. A total of 180,368 transcribed sequences were annotated of which approximately 50% were differentially expressed among genotypes and samples. From these, 15 candidate genes associated with preferred flavour were selected for validation of differential expression through qPCR.
• To protect the newly developed and genetically stable varieties, Plant Breeders’ Rights (PBR) will be applied for selected F7 lines.

Since the project team’s last update, work has progressed towards providing genetically stable and superior varieties (F7 and F1 hybrids) to the industry.

• Evaluation of fruit quality and tree productivity on F5 red papaya was completed at the two Innisfail trial sites. Overall, the advanced red breeding lines performed better than the standard red variety (RB1) with fruit set lower to the ground, higher ºBrix, higher yield, more cylindrical shape, and fruit size of 800-1100g.
• The production of F6 generation advanced breeding lines of red papaya was achieved. These will be field planted at Innisfail in April-May 2021 and at Tablelands in August 2021.
• Yellow papaya F5 seedlings were sex determined and will be field planted in March 2021. Planting was delayed due to poor weather conditions.
• The team plan to assess pre- and post-harvest disease reactions of the advanced breeding lines for black spot (Asperisporium caricae), brown spot (Corynespora cassiicola), Phytophthora fruit and root rot (Phytophthora palmivora), Phytoplasma disease (Candidatus Phytoplasma), anthracnose (Colletotrichum gloeosporioides), stem end rot (Lasiodiplodia theobromae), Phomopsis caricae-papayae, Mycosphaerella spp. and Phytophthora palmivora), and papaya sticky disease (Umbraviruses, papaya meleira virus 2).
• To broaden the genetic base of Australian papaya, seed from diverse sources was obtained under material transfer agreements and biosecurity protocols from Malaysia, Taiwan and Thailand. Eleven varieties were field planted during 2020 at a Tablelands trial site for evaluation. Further germplasm from Philippines and varieties from Thailand have been identified and import procedures commenced.
• The team provided regular reports on research progress to industry through bulletined updates and industry meetings.

The project team are progressing their efforts in key areas of this long-term investment.

Advanced breeding of red and yellow papayas

A questionnaire was delivered to industry to gain a better understanding of production costs, from field preparation to transport to wholesale. Interviews with fruit agents were conducted at Brisbane market to also understand marketers’ perspectives on the preferred characteristics to increase fruit sale. This information was then consolidated to formulate a weighted selection index for papaya breeding targeting fruit quality.

Breeding lines were selected from trees at two trial sites where the trees exhibited fruit set lower to the ground, a trait required for ease of harvest and higher yield.

The selected trees were subsequently self-pollinated to produce F5 generations. These will be evaluated, selected, and self-pollinated to produce F6 generations by the end of 2020, towards F7 seed being produced by 2021.

The project team is collaborating with the Queensland Department of Agriculture and Fisheries (QDAFF) to evaluate common major papaya field diseases for the new advanced breeding lines.

Flavour and sensory analysis by trained panel

The project team has partnered with the Centre for Nutrition and Food Science within Queensland Alliance for Agriculture and Food Innovation (QAAFI) at the University of Queensland to identify flavour profiles of the main commercial varieties and the new breeding lines.

Two sensory studies were successfully carried out over two harvest periods in September 2019 (winter/spring fruit) and January 2020 (summer fruit) with the key findings to inform the breeding program.

To protect the newly developed genetically stable varieties, Plant Breeders’ Rights (PBR) will be applied for selected F7 lines.

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Read more about this program in the project updates published in the Papaya Press magazine:

• Discovering sweet genes in papaya on page 5 of Issue 2, November 2018
• New varieties part of the long game for papaya industry, on page 8 of Issue 3, May 2019
• Papaya breeding program update: Putting flavour front and centre, on page 4 of Issue 4, November 2019.

To date, the first stage of the Plant Breeders Rights process has commenced for three new red papaya varieties with significant and stably inherited trait advantages over existing commercial RB1. These are ‘Sunlight 1’ and ‘Sunlight 2’ for the Coastal region and ‘Sunlight 3’ for the Tableland region. These set fruit closer to the ground (30-40 per cent), had more marketable fruit (~10 per cent), thicker trunks (four per cent to 10 per cent), and more and sweeter fruit (~20 per cent) than RB1. The weighted breeding index indicated an overall 27 per cent performance improvement over RB1. These are currently being trialled in semi-commercial block trials, with initial data to be assessed by growers and industry representatives by end of 2023. 

The second stage of the Plant Breeders Rights application will be submitted for these in October 2023. Meanwhile, two new yellow papaya varieties were also selected for PBR, with significant and stable trait gains compared to existing commercial 1B. These are ‘Moonlight 1’ and ‘Moonlight 2’ with fruit setting up to 31 per cent lower to the ground, up to 36 per cent thicker trunk circumference and 16 per cent more marketable fruit. Also, in response to consumer demand, the fruit were up to 55 per cent smaller with reduced 16 per cent smaller cavities and 11 per cent sweeter than 1B. New F1 hybrids of red and yellow papaya have been developed and PBR Part 2 application will proceed shortly. 

Further work is underway to create new hybrid combinations with advanced breeding lines that are selected for trait performance within specific location (Tableland or Coastal region). New bio-economic valuing and ranking tools are under development to provide an understanding on potential additional financial return to the industry from the new varieties with elite traits, above return from existing commercial varieties.