Reaction of Trifolium semipilosum to four species of root-knot nematode (Meloidogyne spp)

The clover root-knot nematode, Meloidogyne trifoliophila (Mt), debilitates white clover (Trifolium repens) in New Zealand pastures. Genetic resistance (R) to Mt in white clover is complexly inherited and difficult to utilise in breeding programmes. Single-gene, dominant, and complete R to Mt has been identified at the TRKR locus in T. semipilosum. The current study aimed to characterise the T. semipilosum reaction to three additional root-knot nematode species, viz. M. hapla, M. javanica, and M. incognita and if resistant, to determine if TRKR was the R source. Tests were conducted in peat-based potting mix in a temperature-controlled glasshouse using a subset of Mt-resistant and Mt-susceptible plants from the full-sib T. semipilosum population used to map TRKR. Mt resistant and susceptible white clover plants were challenged with Mt, M. hapla, M. incognita, and M. javanica for comparison. Experiments confirmed that T. semipilosum was able to host the four Meloidogyne species. Some T. semipilosum plants exhibited R to M. javanica and M. incognita whereas all were uniformly susceptible to M. hapla. There were no correlated reactions among the full-sib plants to challenge by the four nematode species, suggesting the genetic factor(s) conferring R to M. javanica and M. incognita are independent of the TRKR locus. Furthermore, the R to Mt is complete, with no galls formed on challenged plants whereas the observed R to M. javanica and M. incognita is apparently partial. White clover plants either resistant or susceptible to Mt were uniformly susceptible to M. hapla and M. javanica. These experiments suggest that T. semipilosum is a source of multiple nematode R loci. We plan to characterise further nematode resistances at loci in Trifolium genomes, and to investigate transfer of R alleles from T. semipilosum to white clover and other economic species.