Abstract
The genetics of resistance to root-knot nematode (M. hapla Chitwood) was studied in crosses of three carrot inbred genotypes, two resistant genotypes (R1 and R2) and one susceptible genotype (S1) identified in previous screening tests. Seedlings of three parental genotypes, six F1 crosses including three reciprocal crosses, two BC1 populations, and three F2 populations were evaluated for their resistance and susceptibility to infestation of M. hapla Chitwood based on gall number per root, gall rating per root, and root rating per root in a greenhouse experiment carried out in 1994. All six F1 plants were susceptible, which indicated a lack of heterosis for resistance in these F1s. The R1 × S1 cross segregated 3 susceptible: 1 resistant in the F2, 1 susceptible: 1 resistant in the BC1R1, and did not segregate in the BC1S1. The R1 × R2 cross yielded 44 susceptible: 36 resistant seedlings in the F2 (R1R2), and 48 susceptible: 32 resistant in the reciprocal cross of R1 and R2, both of which closely fit a 9: 7 ratio (P ≤ 0.001). These results indicate these two resistant genotypes carry two different homozygous recessive genes conditioning root-knot nematode resistance. We propose a model of duplicate recessive epistasis control the reactions of host plants and nematode in these crosses.
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