Evidence for simply inherited dominant resistance to Meloidogyne java']javanica in carrot

Root-knot nematodes (Meloidogyne spp.) are serious pests of carrot (Daucus cal ota L.) worldwide. While soil treatment with nematicides is the primary means for managing nematodes in carrot, there is a need to identify and introduce host plant resistance for crop improvement. This study was conducted to determine the inheritance of resistance to root-galling and reproduction by nr. javanica (Treub) Chitwood in a selection (BR-1252) of carrot variety Brasilia. F-2, F-3, F-4, and BC1 progenies from the cross BR-1252xB6274 (a susceptible inbred line) were screened in pot tests for reaction to M. javanica. The observed reactions based on galling and egg production on fibrous roots gave segregation patterns in all tests that were consistent with relatively simply inherited dominant resistance. Field testing in progress indicates that this resistance is very effective against both M. javanica and hi. incognita. A single gene model fits the observed data acceptably well in F-3 generations. However, the range of 3% to 51% susceptible plants in segregating Fg families and 1% to 47% in segregating F-4 families is much wider than the 25% expect ed with a single-gene model, and linked duplicate factors in the coupling phase could also explain the observed segregation patterns. The variation in percentage susceptibility among these families did not clearly cluster into three expected categories (25% S, 20.25% S, and 0.25% S for a 10-cM linkage distance, or 25% S, 16% S and 1% S for 20 cM), but it did tend to occur over the same range. Thus a 10-cM to 20-cM-linked duplicate factor model cannot be dismissed at this time. Egg production data in the F-2, F-3, and F-4 families provided evidence for slightly lower resistance expression in the heterozygous condition. Thus, while overall expressed in a dominant fashion, the resistance does exhibit some allelic dosage response.