Improving Host Plant Resistance to pests and diseases (HPR IV) Final Report

22 November, 2020

Project Leader: Carlos Trapero

Key Researchers: Carlos Trapero, Warwick Stiller, Lewis Wilson

Brief Summary of Project Objectives:
The aim of this project is to use the knowledge and methodology previously generated to advance into a more applied phase of the development of germplasm with improved resistance to Two-Spotted Spider Mites (TSSM) and Silverleaf Whitefly (SLW), while also adding Verticillium Wilt (VW) resistance into the Host Plant Resistance (HPR) package. This project focuses on evaluating a range of germplasm for VW resistance and progressing the development of breeding lines for resistance to SLW and TSSM. It also aims to accelerate the breeding process through the validation of molecular markers associated with resistance to VW and TSSM and the development of rapid screening techniques to predict the level of resistance to VW in the field.

Executive Summary 

After identifying fields suitable to evaluate material for resistance to Verticillium, a range of Gossypium germplasm (G. hirsutum, G. barbadense and G. arboreum) were evaluated and a few genotypes selected for introgression of VW resistance into elite material. This material has been advanced to BC2 and is being evaluated under controlled conditions. Phenotypic data has been collected to perform a preliminary validation of markers associated with VW resistance. Populations have been generated from a highly resistant line from this material for further introgression. 

The level of VW resistance of advanced and precommercial lines from the core breeding program have been assessed, and particularly for the first time in lines containing B3XF. This data has been very valuable for selection in the core breeding program as well as for updating the VRank values of a number of commercial cultivars. Regarding the VW resistance evaluation under controlled conditions, we have selected a Defoliating and a Non-Defoliating pathotype and they have been used in a range of inoculations methods. A bioassay involving watering with V. dahliae inoculum, but without having to uproot the plants, has been successfully developed with the aim of selecting highly resistant individuals among a big population. Another method involving the use of artificially produced microsclerotia has also been developed and could be very useful in experiments where it is necessary to determine the specific level of resistance of a genotype.

One Okra-Glabrous B3F breeding line has been tested in large field plots over several years and proven to harbour a lower population of SLW compared to a commercial cultivar under the same conditions, suggesting that commercial fields planted with a variety having the Okra and Glabrous traits will still maintain their high level of resistance to this pest and therefore could be an alternative option if the SLW threat became a more serious problem. We have also found that the amount of honeydew contamination on the lint is also lower in the okra-glabrous line compared to a commercial variety, although the amount of this reduction does not appear to be in proportion to the reduction in the size of the adult and nymph SLW populations in the crop. 

Breeding lines resistant to TSSM have been advanced to the BC7 generation from the synthetic tetraploid source of resistance. The ‘Pima A8’ and ‘TX111’ sources of resistance have already been incorporated into the core breeding program, and current data from several sites where plants grew without mite pressure under normal growing practices shows promising yield performance for the most advanced lines. Additionally, these newest backcross breeding lines have shown a similar susceptibility to SLW under field conditions compared with current commercial varieties so resistance to TSSM did not increase susceptibility to SLW. The relative contribution of the two genomic regions (D07 and D11) previously identified for spider mite resistance has been determined under controlled and field conditions. It seems that both are required for a high level of resistance, however a single region seems enough to provide an acceptable level of field resistance. We also found that the D07 region has a slightly larger contribution than that on D11. Validation of the use of these markers under glasshouse conditions is therefore considered completed, only pending results on the field validation. The data has shown the feasibility of using these markers as a routine screening aid to speed up the progress of integrating these traits in the breeding program. 

Project Research Achievements

  • The virulence and pathotype of many V. dahliae isolates have been characterized. 
  • Phenotypic data has been collected for the preliminary validation of markers associated with VW resistance. Resistant selections have been performed within these populations using those markers. 
  • Backcross breeding lines from three sources of resistance to TSSM have been progressed in controlled conditions and some of them have started introgression in the core breeding program using a parent containing B3XF.
  • It has been demonstrated that the most advanced mite resistant backcross breeding lines maintain a high level of resistance under field conditions while increasing the yield compared with earlier generations. The newest lines are close to having an acceptable agronomic performance.
  • Backcross breeding lines resistant to TSSM have also been demonstrated to be resistant against strawberry and bean spider mites.