Resistance to root knot nematode (Meloidogyne hapla Chitwood) in carrot is controlled by two recessive genes

Carrot is highly susceptible to attack by the root-knot nematode (Meloidogyne hapla Chitwood), and severe yield reductions occur each year as a result of nematode infection. Despite the importance of this pest, there is at present conflicting evidence regarding the inheritance of resistance to the root-knot nematode. The objective of this work was investigation of the inheritance of resistance to the root-knot nematode in adapted inbred lines of processing carrot. Two resistant inbred lines (R1 and R2) and one susceptible inbred line (S1) were identified via green-house screening tests in the presence of nematode infection. Seedlings of three parental genotypes, six F1 populations 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, gall rating and root rating in a greenhouse experiment. All six F1 populations were susceptible which indicated a lack of dominance for resistance. The R1 X S1 cross-segregated 57 susceptible:23 resistant in the F2, 1 susceptible:1 resistant in the BC1R1, did not segregate in the BC1S1. The R1 X R2 cross yield 44 susceptible:36 resist seedlings in the F2 (R1R2), and 48 susceptible:32 resistant in the F2 of reciprocal cross, both of which closely fit a 9:7 ratio. These results indicate two different homozygous recessive genes condition root-knot nematode resistance. We propose a model of duplicate recessive epistasis explains the reactions of host plant and nematode in these crosses.