Incorporating host plant resistance traits to pests and diseases in the cotton breeding program (HPR V)
Project Leader: Warwick Stiller
Key Researchers: Lucy Egan
Brief Summary of Project Objectives:
The overarching theme of this project is to build upon knowledge and previously identified sources of resistance against Verticillium Wilt (VW), Two-Spotted Spider Mites (TSSM) and Silverleaf Whitefly (SLW) and continue the improvement of resistant germplasm.The project aims to develop advanced germplasm resistant to the target pest or disease in collaboration with the core breeding program.The specific project themes are categorised into four areas.Firstly, the identification and introgression of new sources of resistance to each of the two causal pathotypes of VW (non-defoliating (ND) and defoliating (D)).Secondly, the continual improvement of the early generation D VW and advanced ND VW resistant germplasm.Thirdly, the continued development of okra-glabrous breeding populations for improved resistance to SLW.
Finally, the continuation of the introgression of resistance to TSSM and advancement of germplasm through the core breeding program, while investigating increasing the durability by incorporating new genomic regions providing resistance.
Market / end-user: Cotton Growers through the Core Breeding Program.
Estimated year to uptake by end-user:
Currently, the most advanced material that has been selected for resistance to TSSM and SLW has been moved into the core breeding program and were in progeny rows in the 2021/22 season.However, the timeframe for the development of new cultivars will depend on how far away the source of resistance is from elite varieties and the agronomic performance of the breeding lines. Whilst markers will accelerate the introgression of the material, the timeframe is at least 6 years for the development of TSSM resistant lines and 8 years for the SLW resistant lines.The VW lines with increased resistance to the ND pathotype aim to be incorporated into the core breeding program in the 2022/23 season. However, the VW lines with increased resistance to the D pathotype are still in the early generation phase and will continue to be incorporated and assessed for performance in field trials in the VW disease nurseries.
Executive Summary
Progress has been made in continuing the improvement of germplasm identified for resistance to Two-Spotted Spider Mites (TSSM), Silverleaf Whitefly (SLW) and Verticillium Wilt (VW) from the last project.Resistance to VW is separated by the target causal pathotype; non-defoliating (ND) or defoliating (D).The previous host plant resistance (HPR) project demonstrated that resistance to one pathotype does not necessarily confer resistance to the other. Therefore, breeding targets are aimed at each individual pathotype. Within our current germplasm, the ND resistant material is at a more advanced stage than the D material, so the main aim for our field trials in the 21/22 season was to continue the development and assessment of breeding lines selected for resistance to the ND pathotype of VW from the Sicot F-1 (SF1) source of resistance. However, this season was the first time that lines from a Synthetic source of resistance were trialled in the field, and they are specifically targeted at the D pathotype. The VW resistant material has been evaluated in field trials and selections will be made. The level of resistance of several advanced and precommercial B3XF lines from the core breeding program has been evaluated and the data will be used to inform selections for next season.
Figure 3: The phenotypic progress of the advancement non-defoliating resistant material compared to Sicot 714B3F in a verticillium wilt disease nursery (a and b).
Collaboration with Crown Analytical Services and SARDI has identified the pathotype and quantified the level of V. dahliae inoculum present in the disease nurseries at four field sites: Myall Vale, CSD Farms, North Star and Maules Creek. These results will be corroborated with our ‘standard’ pathotyping technique for validation of the SARDI method.
The development of new screening and evaluation methods to discriminate the level of resistance to both D and ND VW have been encouraging although more development is needed to replicate field results. The aim of these methods is to develop a controlled environment assay which is more reflective of what is observed under field conditions so that more material can be screened and quantified for VW resistance. The VW isolates used in the development of the new methods have undergone continual regeneration through subculturing off the plant to ensure that pathogenicity remains as high as possible.
The most advanced okra-glabrous B3XF material, selected for morphological resistance to SLW, has been incorporated into the core breeding program. We have made selections from the 21/22 season results and 40 lines have been selected to move forward to replicated field trials in the core breeding program. All of the material was imaged and scored for leaf hair density, and we have confirmed that the material has maintained the appropriate morphological characteristics for resistance. A whitefly experimental field trial in the 22/23 season will confirm the resistance of the selected lines.
Figure 4: The leaf hair density pattern of a silverleaf whitefly resistant line on the midrib (a) and leaf blade (b), compared to a control variety midrib (c) and leaf blade (d).
Figure 5: Silverleaf whitefly on a cotton leaf.
Several lines from the most advanced TSSM resistant breeding material from the ‘Pima A8’ and ‘TX111’ sources of resistance are now incorporated into the core breeding program. We have selected 31 lines to move forward to replicated field trials in the 22/23 season.
During the previous project, the relative contribution of the two genomic regions (D07) and (D11) identified for TSSM resistance was determined. D07 had the larger contribution. However, to ensure the robustness of resistance, we have commenced a crossing plan to investigate the D11 region singularly.
Due to the weather conditions, there were delays in harvesting field trials and subsequent processing. Therefore, some of the HPR breeding lines have not been fibre quality tested yet.