A novel virulent Brazilian pathotype of Meloidogyne java']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 degrees 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 'Guardiao' (a homozygous Mi-1.2 hybrid of S. lycopersicum x 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 alpha-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 'Guardiao') 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.