A novel virulent Brazilian pathotype of Meloidogyne javanica towards the tomato Mi-1.2 gene and pathogenicity to resistant rootstock

Very effective genetic control of some root-knot nematode (RKN) species (Meloidogyne spp.) is obtained by the employment of the tomato (Solanum lycopersicum L.) dominant Mi-1.2 gene. However, the phenotypic expression conferred by the Mi-1.2 gene may be impaired by high soil temperatures (above 28 °C) and by previously reported virulent RKN species and/or populations. Here, a putative Mi-1.2 gene-virulent RKN population was found inducing severe gall symptoms in roots of the tomato rootstock ‘Guardião’ (a homozygous Mi-1.2 hybrid of S. lycopersicum × S. peruvianum) under protected crop conditions in Southern Brazil. Females were extracted from severely infected roots displaying large galls and used for biochemical characterization of α-esterase (EST) and confirmed by SCAR markers. Additional pathogenicity assays were carried out in order to confirm the virulence (= resistance-breaking) feature of this RKN population by employing other or the same Mi-1.2 gene-carrying rootstocks. This virulent RKN population was identified as M. javanica (EST J3 and J2). The pathogenicity tests confirmed that this M. javanica population can overcome the Mi-1.2 resistance gene, and it is able to induce severe root-gall symptoms and to reproduce in two dominant resistant rootstocks (‘Muralha’ and ‘Guardião’) under greenhouse conditions. In addition, an initial inoculum of 2000–7000 eggs was considered ideal for future studies with the virulent M. javanica population. It is the first report of a Mi-1.2 gene-virulent M. javanica population in Brazil, which may represent a potential threat to the tomato agribusiness sector.