Grafting Vegetable Seedlings for Soil-borne Disease Resistance

Solanaceous vegetables are widely grown in tropical and subtropical regions of the world where pressure from soilborne diseases is often high.  For smallholder farmers in developing countries, these crops, particularly tomato and eggplant, provide income and also improve family nutrition.  Bacterial wilt, caused by Ralstonia solanacearum, and root knot disease, caused by the root knot nematode  Meloidogyne spp. (RKN), are arguably the most important soilborne diseases of solanaceous vegetables in Africa and Asia.  Ralstonia solanacearum is a complex of strains that are adapted to hot, high moisture conditions, with the exception of a group adapted to temperate climates.  Our earlier work in the Philippines indicated considerable homogeneity among strains collected throughout the main island of Luzon from eggplant, with all strains in phylotype I and biovars 3 or 4.  The majority of the strains could be assigned to a single sequevar.   Similarly, in a recent survey of tomato and eggplant in Nepal and Bangladesh, all strains recovered were phylotype I, distributed among biovars 3 and 4.  All of the strains recovered from tomatoes in seven growing regions in Uganda were biovar 3.  Disease resistance is the most effective means of mitigating damage caused by R. solanacearum and RKN.   Several rootstocks have been identified that confer resistance in grafted plants to one or both of these diseases, including Solanum melongena EG203 (AVRDC), S. lycopersicum MT56 (IPM CRSP), and wild Solanum species S. torvum (turkey berry) and S. sisymbriifolium (sticky nightshade).  In Bangladesh, the local eggplant variety ‘Chega’ grafted onto S. sisymbriifolium rootstock survived 22–29 days longer, produced 250% to 280% more fruit and provided 3–4 times higher income than non-grafted ‘Chega’ in bacterial wilt-infested fields in two locations.  In Kenya, bacterial wilt incidence in the local variety ‘Onyx’ grafted onto MT56 was significantly lower and tomato yields significantly higher than in non-grafted ‘Onyx’.  The potential for, and challenges to, widespread adoption of grafting technology among smallholder farmers will be presented.