Bacterial wilt, caused by strains belonging to the Ralstonia solanacearum species complex, inflicts severe economic losses in many crops worldwide. Host resistance remains the most effective control strategy against this disease. However, wilt resistance is often overcome due to the considerable variation among pathogen strains. To help breeders circumvent this problem, we assembled a worldwide collection of 30 accessions of tomato, eggplant and pepper (Core-TEP), most of which are commonly used as sources of resistance to R. solanacearum or for mapping quantitative trait loci. The Core-TEP lines were challenged with a core collection of 12 pathogen strains (Core-Rs2) representing the phylogenetic diversity of R. solanacearum. We observed six interaction phenotypes, from highly susceptible to highly resistant. Intermediate phenotypes resulted from the plants' ability to tolerate latent infections (i.e., bacterial colonization of vascular elements with limited or no wilting). The Core-Rs2 strains partitioned into three pathotypes on pepper accessions, five on tomato, and six on eggplant. A “pathoprofile” concept was developed to characterize the strain clusters, which displayed six virulence patterns on the whole set of Core-TEP host accessions. Neither pathotypes nor pathoprofiles were phylotype specific. Pathoprofiles with high aggressiveness were mainly found in strains from phylotypes I, IIB, and III. One pathoprofile included a strain that overcame almost all resistance sources.

We report here the complete genome sequences of two Ralstonia pseu-dosolanacearum strains, isolated from the warm northeast region of Brazil. They display divergent (compatible versus incompatible) interactions with the resistant tomato line Hawaii 7996. Polymorphisms were detected in a subset of effector genes that might be associated with these contrasting phenotypes. R alstonia pseudosolanacearum is a soilborne pathogen and one of the main causal agents of the bacterial wilt (BW) disease of tomato (Solanum lycopersicum L.) and other crops (1). R. pseudosolanacearum is currently classified as a distinct species within the R. solanacearum complex, which comprises strains of phylotypes I and III (2-5). Although they are of putative exotic origin, R. pseudosolanacearum phylotype I isolates are currently disseminated in Brazil (north, northeast, and central regions) and infect mainly Solanaceae crops (tomato, peppers, eggplant, and scarlet eggplant) (6, 7). In this study, two tomato-infecting R. pseudosolanacearum strains from the warm Brazilian northeast region were sequenced in order to analyze candidate genes associated with their divergent (compatible versus incompatible) interactions with the tomato line Hawaii 7996, which is the main breeding source of BW resistance in this vegetable crop (8, 9). Strain RS 476 (sequevar I-18 from Maranhão state) is characterized by its ability to induce severe BW symptoms on Hawaii 7996 (60% incidence), whereas strain CRMRs218 (also sequevar I-18 from Pernambuco state) is able to induce severe BW symptoms in a wide range of tomato cultivars, but it is avirulent to Hawaii 7996.

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.