Heritability and gain from selection for field resistance against multiple root-knot nematode species (Meloidogyne incognita race 1 and M-java']javanica) in carrot

Heritability and gain from selection of traits associated with field resistance to multiple root-knot nematode species were estimated in carrot (Daucus carota L.). Experimental plots were uniformly and simultaneously infested with Meloidogyne incognita race 1 and M. javanica. Forty-seven half-sib families derived from the cv. 'Brasilia' were evaluated for: (1) percentage of commercial roots with non-galling symptoms (%RNG); (2) percentage of commercial roots with gall symptoms (%RGS) and (3) percentage of non-commercial roots (%NCR). The cultivars 'Brasilia' (resistant), 'Caranda' (intermediate) and 'Nova Kuroda' (susceptible) were employed as standards. Broad-sense heritability estimates were 61.9% for %RNG; 30.6% for %RGS and 67.9% for %NCR. However, the gains from selection were very small for all traits. The genotypic correlation between %RGS and %NCR was -0.38 and the correlation between %RNG and %RGS was 0.13. Selection on increasing %RNG resulted in a negative correlated response on %NCR (genotypic correlation = -0.99) indicating repulsion phase linkage(s) between the genetic factor( s) controlling these traits. The phenotypic correlation of -0.88 between %RNG and %NCR suggested that incomplete penetrance and dosage effects might be involved in the expression of these traits. Selection of superior genotypes (i.e. high %RNG, low %RGS, and low %NCR) can be achieved. 'Brasilia' was among the genotypes selected for the following cycle of recombination reinforcing the notion that this cultivar is one of the most promising sources of stable, wide-spectrum field resistance to Meloidogyne species in D. carota.