Single dominant genes conferring resistance to Fusarium wilt (Fusarium oxysporum f.sp. lycopersici) race 3 has been reported in Lycopersicon pennellii accessions PI 414773 (McGrath et al. 1987) and LA 716 (Scott and Jones, 1989). All L. pennellii accessions recently were reported (Scott and Jones, 1990) resistant to races 1, 2, and 3. The race 3 resistant gene, I-3, from LA 716 is linked to Got-2 with about 2.5% crossing over (Bournival et al. 1989) and Got-2 could be used to select for race 1 and race 2 resistance as well (Bournival et al. 1990). However, it is not clear whether I-3 confers resistance to all three races or if there are other genes linked to I-3 conferring resistance to race 1 and 2. A cross between a Fusarium wilt susceptible tomato such as 'Bonny Best' with LA 716 with hundreds of backcross progeny evaluated for the three races of Fusarium wilt would be ideal to detect crossovers and estimate map distances. Unfortunately, this was not done, in part because of fertility problems associated with many of these interspecific backcrosses. Instead we crossed a tomato line, E427, with Bonny Best. E427 had been derived from LA 716 crossed 2 times with tomato lines with the I-2 gene and presumably the I gene. The probable presence of I-2 and I precluded a detailed linkage study. Nevertheless, several hundred plants of the backcross plants to Bonny Best were self-pollinated and progeny were screened for all 3 Fusarium races. Those which showed segregation patterns indicative of crossing over were transplanted to the field and selfed seed was saved from several plants per line. This was done until homozygosity was obtained. Data presented are from the BC1F4 generation. Lines susceptible to race 3 but resistant to races 1 and 2 were ignored due to the confounding of I-2. Crossover types obtained were: 6 lines resistant to race 1 but susceptible to races 2 and 3, 5 lines resistant to race 2 but susceptible to races 1 and 3, and 4 lines resistant to races 2 and 3 but susceptible to race 1. All lines of the former two types have the wild type Got-2 allele while all the lines of the latter type have the Got-2 allele linked to Fusarium resistance. Crossover types not obtained as yet are; 1) race 3 resistance alone and 2) race 1 plus race 3 resistance alone without resistance to race 2. Race 1 plus race 2 resistance alone was not pursued as mentioned earlier. The race 2 plus race 3 alone type must indicate a break on chromosome 7. The race 1 alone and race 2 alone crossover types obtained are due to either breaks on chromosome 7 or between I-2 and I on chromosome 11. Allelism tests are presently underway to determine which is the case. If the breaks are on chromosome 7, the race 1 alone and race 2 alone genotypes provide evidence for new resistance genes on chromosome 7. Putting our limited information together, the following working model is proposed. Since resistance to race 1 alone, and race 2 alone have been obtained, but race 3 alone has not, the race 3 locus is between the loci for 1 and 2. Race 1 resistance alone has been the most frequent type obtained (data not presented) and is the furthest away from the race 3 locus which seems to be more tightly linked to the race 2 locus. Got-2 is on either side of the race 3 locus and crossovers of race 1 and race 2 resistance alone have been distil to it. Obviously, more work is needed to verify this model. In the meantime, breeders should not assume that screening for race 3 resistance automatically will insure resistance to races 1 and 2.
Literature cited:
Bournival, B. L. et al. 1989. TAG 78:489-494.
Bournival, B. L. et al. 1990. TAG 79:641-645.
McGrath, D. J. et al. 1987. Aust. J. Agric. Res. 38:729-733.
Scott, J. W. and J. P. Jones. 1989. Euphytica 40:49-53.
Scott, J. W. and J. P. Jones. 1990. HortScience 25(9):68. (Abstr.).