Grafting tomatoes for production in the hot-wet season
Flooding and soil-borne diseases cause severe yield losses in tomato during the hot and wet summer months in the lowland tropics. The high market price during the summer period motivates farmers to adopt additional cultural practices to overcome production constraints, such as raised beds, rain shelters, and grafting. Grafting tomato scions onto rootstocks with disease resistance and flooding tolerance could increase tomato yield and provide good economic returns to farmers. Soil-borne diseases, such as bacterial wilt caused by Ralstonia solanacearum, fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici, and root-knot nematodes (Meloidogyne spp.), could devastate tomato crops, especially under continuous cropping systems. Use of resistant planting materials is the foundation for managing these diseases. Compared with the long period of time required to breed resistant cultivars, resistant rootstocks could offer a more rapid means to provide resistant planting materials to farmers. Rootstock resistance traits should be evaluated against multiple strains or isolates of the target pathogen and preferably under multiple environments to ensure the durability of the rootstocks. For example, tomato rootstock Hawaii 7996 and eggplant rootstock EG203 were selected after exposure to diverse R. solanacearum strains. Although the resistant rootstocks are not immune to the bacterial pathogen, studies showed that the pathogen could be confined mostly below the grafting joint. Although the overall resistance level of the grafted tomato is largely determined by the rootstock, there was an additive effect when using a scion cultivar with tolerance to bacterial wilt. Adventitious roots tend to develop from the base of the scion when grafted to eggplant rootstocks. The adventitious roots should not have contact with the soil to avoid introducing soil-borne pathogens into the scion, which could result in loss of resistance in the grafted plant. Flooding can cause substantial growth reduction, and in some cases, death of tomato. The formation of adventitious roots and aerenchyma tissue is a key flood-adaptation mechanism of tomatoes. Eggplant roots have more aerenchyma, thus favoring survival under flooded conditions. Grafting tomato cultivar FMTT22 onto flood-tolerant tomato or eggplant rootstocks resulted in yields more than twice as high as for the non-grafted control. Experiments showed tomato fruit yield in tomato-eggplant grafts was lower than that from tomato-tomato grafts. Tomato fruit quality, such as pH, total soluble solids, titratable acids, color, and water content varies, depending on the scion and rootstock combination. Surveys conducted in 2012 found 48% adoption of tomato grafting among farmers in the Red River Delta of Vietnam. Eggplant variety EG203 was used as the rootstock, as it is resistant to bacterial wilt and tolerates waterlogging. Results showed yield of grafted tomato (81.4 t/ha) in the intensive production area in the Red River Delta was significantly greater than non-grafted tomato (56.5 t/ha). Although the total costs, particularly labor costs, were significantly higher for grafted tomato, the difference in revenue was large enough to make grafted tomato significantly more profitable. The benefit-cost ratio for grafted tomato was 4.6 in comparison to 3.5 for non-grafted tomato. Similar results were found in North Carolina, where grafting was used as an alternative to methyl bromide to manage soil-borne diseases. Other findings indicate grafting could enhance tolerance to other abiotic stresses, such as heat or salinity. Application of grafting in hot-wet season production of tomato could be expanded into more agro-ecosystems.