Assessing the residual effects of the “defeated” tomato Mi-1.2 gene against a Meloidogyne enterolobii (guava race) population via comparative assays with contrasting near-isogenic lines

Meloidogyne enterolobii represents a major threat to the global tomato ( Solanum lycopersicum L.) production due to its ability to “break-down” the resistance conferred by the dominant Mi -1.2 gene. However, a subgroup of “defeated” resistance genes in various pathosystems exhibits residual effects characterized by an enduring interference in quantitative levels of disease expression induced by novel virulent pathogens. Thus far, residual effects of the “defeated” Mi -1.2 gene to M. enterolobii have not been properly investigated. Herein, two comparative assays using contrasting near-isogenic lines (NILs) for presence/absence of the Mi -1.2 locus were carried out using a guava race population of M. enterolobii . Seedlings of two pairs of contrasting NILs ‘Nemadoro’ (homozygous dominant; Mi -1.2/ Mi -1.2) / ‘Rio Grande’ (homozygous recessive, mi -1.2/ mi -1.2) and ‘Del Rey’ ( Mi -1.2/ Mi -1.2) / ‘Calipso’ ( mi -1.2/ mi -1.2) were inoculated with ≈ 2,000 M. enterolobii eggs. The homozygous dominant ( Mi -1.2/ Mi -1.2) NILs displayed values for the quantitative parameter NEGR (number of eggs + occasional J2 per gram of root tissue) similar or even superior to their corresponding recessive ( mi -1.2/ mi -1.2) NILs. A slight positive impact of the resistance gene in the reproduction factor (RF) value was observed only for one pair of contrasting NILs (‘Del Rey’ / ‘Calipso’), which was restricted to one bioassay. The employment of NILs in our bioassays allowed us to hypothesize that the Mi -1.2 gene, although extremely effective against populations of at least 13 Meloidogyne species, does not confer significant residual effects against M. enterolobii race from guava.