- Vegetables: tomatoes
- Pest Management: genetic resistance
We propose that the critical solution to mitigate VW damage may be host resistance, and on-farm trials in Vd2 infested fields are the key component of the efforts to successfully identify new sources of VW resistance tomato germplasm.
Conventional breeding for race 2 VW resistance has been difficult due to the lack of novel genetic sources with durable resistance against V. dahliae non-race 1 strains; furthermore, complex genetic control and pathogen’s heterogeneous population have impeded the efforts to identify any tomato genotypes with consistent resistance to non-race 1 isolates of the pathogen. In an effort to find a single strong resistance gene, researchers have used an aggressive disease screening technique, seedling root-dip test, over the years for VW resistance evaluation, resulting in the inoculation test being too severe to identify moderate horizontal resistance levels. Furthermore, this method can be affected by external factors, such as method of inoculation, inoculum concentration and age, and disturbance of the roots, therefore, appear to influence the eventual fate of tomato plants infected at different growth stages (Visser and Hattingh, 1981). Based on the above, the use of the seedling root dip screen may not be a way to identify potentially useful horizontal resistance that may not be present in the young seedlings at a level strong enough to withstand this extreme screening technique. Therefore, other techniques for screening as proposed in this project may be needed to identify useful resistance to multiple strains of the fungus, if that does exist.
In recent years, Dr. Randy Gardner, Professor Emeritus at NC State, has identified two sources of useful, moderate resistance to verticillium wilt that have held up in WNC grower fields with high disease incidences over the last six years. The Vd2 resistant breeding lines, designated as ‘NC 6GEM’ and ‘NC 7GEM’, are derived from self-pollinated ‘Mountain Fresh Plus’ and the wild tomato line, ‘Hawaii 7998’, respectively. Mountain Fresh Plus is the F1 hybrid of NC 84173 crossed with NC 109. In replicated grower field trials with and without fumigation, NC 109, but not NC 84173, showed similar resistance against Vd2, and we assume that the moderate resistance in NC 6GEM originated from NC 109. We believe investigation of the genetic background of these resistant breeding lines will be one of the most important contributions to gain deeper insights into the genetic foundation of Vd2 resistance in tomatoes and develop new resistant tomato germplasm.
On-farm trials will also help us to evaluate and develop new material derived from VW resistant parental lines (‘NC 109’, ‘NC 6GEM’, and ‘NC 7GEM’) with improved horticultural characteristics and additional disease resistances.
Project objectives from proposal:
Funding from this proposal will be used to perform two replicated on-farm trials in Henderson and Haywood Counties, NC in Vd2 infested fields with no fumigation. A randomized complete block design (RCBD) with two blocks and four-plant plots will be used to evaluate 500 tomato genotypes in two different locations in the summer of 2021 and 2022. VW incident and severity will be evaluated based on Horsfall-Barratt ratings and recorded (Horsfall and Barratt, 1945). A five-scale subjective rating system will be used to evaluate horticultural performances of the lines including fruit set (yield), size, and shape as well as earliness and fruit’s internal and external color. To identify the most resistant lines with the best horticultural characteristics, all data will be subjected to analysis of variance. We will employ the linear mixed-model (MIXED) procedure of SAS software (version 9.2; SAS Institute Inc., Cary, NC) to perform the analysis of variance based on the RCBD with 4 replications (2 reps per location). The list below gives a breakdown of the key objectives and descriptions of the samples included in the on-farm trials.
1- Confirming VW resistance in new fresh market tomato breeding lines and hybrids. To increase the durability of the resistance to non-race 1 isolates, Dr. Randy Gardner crossed ‘NC 7GEM’ and ‘NC 6GEM’ to combine VW resistance from different parental sources. Interestingly, the F2 population was segregating for resistance confirming the differences in the genetic structures of the resistance sources in the two resistant breeding lines. To improve the horticultural performances, he also crossed ‘NC 6GEM’ to ‘NC 9BS’, a susceptible breeding line with more horticulturally desirable traits. He selected the most resistant 31 F2 individuals and advanced them to F6 progeny to be used in new testcrosses and hybrid development. New hybrids are being developed from testcrosses in fall 2020 and spring 2021. Funding from this proposal will be used to test those lines and hybrids to confirm their Vd2 resistance and evaluate horticultural performances in the on-farm trials.
2- Advancing and evaluating two mapping populations for future DNA marker development and Vd2 resistant gene/s Discovery. In an effort to develop mapping populations for novel Vd2 resistant gene/s discovery, we grew 215 new F2 plants from the cross, (NC 7GEM x NC 6GEM), and evaluated the VW disease incidence and severity under field conditions in summer 2019 and 2020. A five-scale disease severity index (DSI) was used to evaluate verticillium resistance in this study. All 215 F2 individuals were advanced to F4 generation by the single seed descent method. We applied the same scheme to develop a second mapping population with 214 individuals from a cross between ‘NC 6GEM’ and a susceptible breeding line, ‘NC 9BS’. The field evaluation was conducted under natural VW pressure and disease severity index (DSI) was assigned subjectively to individual F2 and F3 plants in summer 2019 and 2020, respectively. Segregation ratios of DSIs were very close to a 1:2:1 ratio in both populations, about 25% of the F3 progenies showed resistance equal or better than both resistant parents (DSI=1), 25% were highly susceptible (DSI=4-5), and 50% showed moderate tolerance (DSI=2-3). Considering phenotypes of the 3 parental lines, resistant (‘NC 6GEM’ and ‘NC 7GEM’) and susceptible (‘NC 9BS’), and the observed segregation ratios, presence of a single dominant or semi-dominant gene in the resistant parental lines ‘NC 6GEM’ and ‘NC 7GEM’ is possible, but replicated and comprehensive experiments will be necessary to confirm the phenotypes of the advanced progenies under field conditions for genetic mapping. In summer 2021, 429 F4 recombinant inbred lines (RILs) from the two populations will be included in the replicated on-farm trials for VW disease incidence and severity evaluation. All F4 individuals will be advanced to F5 generation by the single seed descent method. The phenotypic data will be confirmed again by using progenies at F5 generation, and tissues will be collected for gene fine mapping and expression profiling. Fine mapping and expression profiling of the samples to develop DNA markers and identify the Vd2 resistant gene/s will be beyond this project and other sources of enteral and external funding will be secured to do the work. However, the most tolerant recombinant inbred lines (RILs) in the populations with potential horticultural performances will be identified and used in new testcrosses and hybrid development in the fall of 2022.
3- Confirming Vd2 resistance in NCSU tomato breeding lines and hybrids. In summer 2019, three on-farm trials in verticillium-infested grower fields, two non-fumigated and one fumigated, revealed additional NCSU breeding lines and commercial tomato hybrids with moderate tolerance to verticillium. Of 160 fresh market tomato breeding lines and F1 hybrids tested, only six breeding lines from NCSU and nine F1 hybrids showed moderate tolerance (DSI≤3) to verticillium infection consistently through all three trials. Sixty fresh market tomato breeding lines from NCSU (developed from 1987 to 2019), thirty-three breeding lines from the University of Florida (UF), and 71 new and commercial F1 hybrids from NCSU, UF, and private seed companies were tested and no strong source of VW resistance was identified (data not shown). To confirm the effectiveness, the identified lines with moderate resistance to VW will be included in the on-farm trials supported by funding for this proposal.