Application of a Preliminary Screen to Select Locally Adapted Resistant Rootstock and Soil Amendment for Integrated Management of Tomato Bacterial Wilt in Taiwan

Quantitative Trait Loci Mapping for Bacterial Wilt Resistance and Plant Height in Tomatoes

Bacterial wilt (BW) of tomatoes, caused by Ralstonia solanacearum, is a devastating disease that results in large annual yield losses worldwide. Management of BW of tomatoes is difficult due to the soil-borne nature of the pathogen. One of the best ways to mitigate the losses is through breeding for disease resistance. Moreover, plant height (PH) is a crucial element related to plant architecture, which determines nutrient management and mechanical harvesting in tomatoes. An intraspecific F2 segregating population (NC 11212) of tomatoes was developed by crossing NC 84173 (tall, BW susceptible) × CLN1466EA (short, BW resistant). We performed quantitative trait loci (QTL) mapping using single nucleotide polymorphic (SNP) markers and the NC 11212 F2 segregating population. The QTL analysis for BW resistance revealed a total of three QTLs on chromosomes 1, 2, and 3, explaining phenotypic variation (R2) ranging from 3.6% to 14.9%, whereas the QTL analysis for PH also detected three QTLs on chromosomes 1, 8, and 11, explaining R2 ranging from 7.1% to 11%. This work thus provides information to improve BW resistance and plant architecture-related traits in tomatoes.

Genetic Structure of Ralstonia solanacearum and Ralstonia pseudosolanacearum in Brazil

Bacterial wilt-causing Ralstonia threaten numerous crops throughout the world. We studied the population structure of 196 isolates of Ralstonia solanacearum and 39 isolates of Ralstonia pseudosolanacearum, which were collected from potato- and tomato-growing areas in 19 states of Brazil. Regardless of the species, three groups of isolates were identified. One group encompassed R. pseudosolanacearum isolates. The other two groups comprise isolates of R. solanacearum (phylotype II) split according to geographic regions, one made of isolates from the North and Northeast and the other made of isolates from the Central, Southeast, and South regions (CSS). Among the isolates collected in CSS, those from tomato were genetically distinct from the potato isolates. The genetic variability in the population of R. pseudosolanacearum was lower than that of R. solanacearum, suggesting that the former was introduced in Brazil. Conversely, the high genetic variability of R. solanacearum in all regions, hosts, and times supports the hypothesis that this species is autochthonous in South America, more precisely in Brazil and Peru. For R. solanacearum, higher variability and lower migration rates were observed when tomato isolates were analyzed, indicating that the variability is caused mainly by the differences of the local, native soil population. The North subpopulation was distinct from all others, possibly because of differences in environmental features of this region. The proximity of some geographic regions and the movement of potato tubers could have facilitated migration and therefore low genetic differentiation between geographic regions. Finally, geography, which also influences host distribution, affects the structure of the population of R. solanacearum in Brazil. Despite quarantine procedures in Brazil, increasing levels of trade are a threat to biosecurity, and these results emphasize the need for improving our regional efforts to prevent the dispersal of pathogens.

Draft genome sequence of eggplant (Solanum melongena L.): the representative solanum species indigenous to the old world

Unlike other important Solanaceae crops such as tomato, potato, chili pepper, and tobacco, all of which originated in South America and are cultivated worldwide, eggplant (Solanum melongena L.) is indigenous to the Old World and in this respect it is phylogenetically unique. To broaden our knowledge of the genomic nature of solanaceous plants further, we dissected the eggplant genome and built a draft genome dataset with 33,873 scaffolds termed SME_r2.5.1 that covers 833.1 Mb, ca. 74% of the eggplant genome. Approximately 90% of the gene space was estimated to be covered by SME_r2.5.1 and 85,446 genes were predicted in the genome. Clustering analysis of the predicted genes of eggplant along with the genes of three other solanaceous plants as well as Arabidopsis thaliana revealed that, of the 35,000 clusters generated, 4,018 were exclusively composed of eggplant genes that would perhaps confer eggplant-specific traits. Between eggplant and tomato, 16,573 pairs of genes were deduced to be orthologous, and 9,489 eggplant scaffolds could be mapped onto the tomato genome. Furthermore, 56 conserved synteny blocks were identified between the two species. The detailed comparative analysis of the eggplant and tomato genomes will facilitate our understanding of the genomic architecture of solanaceous plants, which will contribute to cultivation and further utilization of these crops.

Inhibitory Effects of Chinese Medicinal Herbs on Plant-Pathogenic Bacteria and Identification of the Active Components from Gallnuts of Chinese Sumac

The aqueous extracts of 30 out of 67 Chinese medicinal herbs were shown to have inhibitory effects on growth of Xanthomonas euvesicatoria by a paper disc diffusion assay. The inhibitory substances with the strongest antibacterial activity were extracted from Chinese sumac gallnut and black myrobalan. The aqueous extract of gallnut inhibited the growth of eight of the tested plant-pathogenic bacteria, and that of black myrobalan inhibited five. The gallnut extract produced at least an 8-mm inhibition zone against Acidovorax citrulli, Ralstonia solanacearum, X. citri pv. citri, and X. euvesicatoria at a 10-fold dilution, and it was still active at 800- to 1,600-fold dilutions. The aqueous extract of gallnut was more inhibitory than the acetone-water extract. To identify the inhibitory compounds in the gallnut aqueous extract, the crude extract was chromatographed over a silica column, and the primary compounds in fractions 3 and 8 were identified by nuclear magnetic resonance as gallic acid and methyl gallate, respectively. The inhibitory effect of methyl gallate on the growth of four plant-pathogenic bacteria was 10 to 80 times that of gallic acid. The minimum inhibition and minimum bactericidal concentration tests showed that the inhibition effect of the original aqueous was higher than that of methyl gallate. These results indicate that methyl gallate in gallnut is an important compound that is inhibitory to plant-pathogenic bacterial growth, and there are other unidentified compounds that are also responsible for the antibacterial effects. This is the first report regarding the antibacterial effects of gallnut extract and its chemical components on plant-pathogenic bacteria.

Grafting Tomato to Manage Bacterial Wilt Caused by Ralstonia solanacearum in the Southeastern United States

Bacterial wilt, caused by Ralstonia solanacearum, can result in severe losses to tomato (Solanum lycopersicum) growers in the southeastern United States, and grafting with resistant rootstocks may be an effective strategy for managing this disease. However, R. solanacearum populations maintain considerable diversity, and little information is known regarding the efficacy of commercially available rootstocks to reduce bacterial wilt incidence and subsequent crop loss in the United States. In this study, tomato plants grafted onto 'Dai Honmei' and 'RST-04-105-T' rootstocks had significantly lower area under the disease progress curve (AUDPC) values compared with nongrafted plants (P < 0.05). Across three locations in North Carolina, final bacterial wilt incidence for non- and self-grafted plants was 82 ± 14 to 100%. In contrast, bacterial wilt incidence for the grafted plants was 0 to 65 ± 21%. Final bacterial wilt incidence of plants grafted with Dai Honmei rootstock was 0 and 13 ± 3% at two locations in western North Carolina but 50 ± 3% at a third site in eastern North Carolina. Similarly, grafting onto RST-04-105-T rootstock significantly reduced AUDPC values at two of the three locations (P < 0.05) compared with that of the nongrafted plants, but performed poorly at the third site. Total fruit yields were significantly increased by grafting onto resistant rootstocks at all three sites (P < 0.05). Regression analyses indicated that yield was significantly negatively correlated with bacterial wilt AUDPC values (R² was 0.4048 to 0.8034), and the use of resistant rootstocks enabled economically viable tomato production in soils naturally infested with R. solanacearum.