Screening for Resistance in Selected Tomato Varieties against the Root-Knot Nematode Meloidogyne incognita in the Philippines Using a Molecular Marker and Biochemical Analysis

Tomato is a high-value vegetable crop widely cultivated in the Philippines, and its production is threatened by various stresses including infection by the root-knot nematode M. incognita. In this study, we checked for resistance to M. incognita in selected tomato germplasm collections and commercially available varieties using a bioassay method, the molecular marker Mi23 and biochemical analysis. Among the eight varieties tested, none showed a resistant reaction against M. incognita. Use of the molecular marker Mi23 yielded 430 bp in all the tomato varieties screened. Phylogenetic analysis using the neighbor-joining method revealed the clustering of consensus sequence of the varieties tested with the susceptible variety S. lycopersicum cv. M82-1-8 and a wild relative, S. pimpinellifolium isolate LA2184. The biochemical analysis showed varying responses among the varieties when they were inoculated with M. incognita. Increased levels of total antioxidant activity were observed in Diamante Max F1, Ilocos Red and Tm 2016 11-1, while total phenolic content was found to be elevated in Athena, Avatar TY F1 and Rosanna. Increased levels of ascorbic acid were observed in Athena and Avatar TY F1 even at 45 dpi. Even though these varieties showed elevated levels of the abovementioned biochemical parameters related to a resistance reaction, all of them showed highly susceptible reactions. Hence, this study showed that these tomato varieties have no resistance against M. incognita and that there is a need to identify other sources of resistance against M. incongita and produce resistant tomato cultivars adapted to local conditions.