ABSTRACT
The introduction of plant pathogens can quickly reshape disease dynamics in island agro-ecologies, representing a continuous challenge for local crop management strategies. Xanthomonas pathogens causing tomato bacterial spot were probably introduced in Taiwan several decades ago, creating a unique opportunity to study the genetic makeup and adaptive response of this alien population. We examined the phenotypic and genotypic identity of 669 pathogen entries collected across different regions of Taiwan in the last three decades. The analysis detected a major population shift, where X. euvesicatoria and X. vesicatoria races T1 and T2 were replaced by new races of X. perforans. After its introduction, race T4 quickly became dominant in all tomato-growing areas of the island. The genomic analysis of 317 global genomes indicates that the Xanthomonas population in Taiwan has a narrow genetic background, most likely resulting from a small number of colonization events. However, despite the apparent genetic uniformity, X. perforans race T4 shows multiple phenotypic responses in tomato lines. Additionally, an in-depth analysis of effector composition suggests diversification in response to local adaptation. These include unique mutations on avrXv3 which might allow the pathogen to overcome Xv3/Rx4 resistance gene. The findings underscore the dynamic evolution of a pathogen when introduced in a semi-isolated environment and provide insights into the potential management strategies for this important disease of tomato.
ABSTRACT
Bacterial spot caused by Xanthomonas spp. is an economically important disease of pepper causing significant yield losses in Taiwan. Monitoring the pathogen population on a continuous basis is necessary for developing disease management strategies. We analyzed a collection of xanthomonad strains isolated from pepper in Taiwan between 1989 and 2019. Among the sequenced genomes, sixty-five were identified as Xanthomonas euvesicatoria and ten were X. perforans. Thirty-five X. euvesicatoria and ten X. perforans strains were copper tolerant, whereas only five X. euvesicatoria and none of the X. perforans strains were tolerant to streptomycin. Nine X. euvesicatoria strains were amylolytic, which is considered an unusual characteristic for X. euvesicatoria. Bayesian analysis of the population structure based on core gene SNPs clustered the strains into five clusters for X. euvesicatoria and three clusters for X. perforans. One X. perforans cluster, designated as TP-2019, appears to be a novel genetic cluster based on core genes, accessory gene content, and effector profile. This knowledge of pathogen diversity with whole genomic information will be useful in future comparative studies and in improving breeding programs to develop disease-resistant cultivars and other disease management options.
ABSTRACT
Bacterial wilt (BW) is one of the most economically important diseases of tomato and eggplant in the tropics and subtropics, and grafting onto resistant rootstocks can provide an alternative and effective solution to manage soil-borne bacterial in these crops. This study was conducted to evaluate the BW resistance and agronomic potential of newly identified eggplant accessions as rootstocks for tomato grafting. Five BW resistant eggplant accessions (VI041809A, VI041943, VI041945, VI041979A, and VI041984) from the World Vegetable Center were evaluated as rootstocks for grafting with two different fresh market tomato cultivars (Victoria and TStarE) as scion under open field conditions in Taiwan. Graft compatibility using the tube grafting method as well as BW wilting percentage, disease index, fruit yield and quality parameters were assessed. All the rootstocks showed good graft compatibility (93% and above) and grafted plants showed low wilting percentage (0.0-20.0%) and disease index (0.0-20.8%) following inoculation with BW. Yield for the eggplant rootstock grafted tomato plants was higher compared to the non-grafted tomatoes and self-grafted tomato. Fruit quality was not affected by grafting, although some differences in antioxidant activities were observed. The new eggplant rootstocks can be considered as alternatives to the rootstocks currently used for commercial production of tomatoes during the hot-wet season.
ABSTRACT
Bacterial wilt caused by Ralstonia solanacerum is one of the most economically and destructive eggplant diseases in many tropical and subtropical areas of the world. The objectives of this study were to develop interspecific hybrids, as potential rootstocks, between the eggplant (Solanum melongena) bacterial wilt resistant line EG203 and four wild accessions (S. incanum UPV1, S. insanum UPV2, S.anguivi UPV3, and S. sisymbriifolium UPV4), and to evaluate interspecific hybrids along with parents for resistance to bacterial wilt strains Pss97 and Pss2016. EG203 was crossed successfully with wild accessions UPV2 and UPV3 and produced viable seeds that germinated when wild accessions were used as a maternal parent in the crosses. In addition, viable interspecific hybrids between EG203 and UPV1 were obtained in both directions of the hybridization, although embryo rescue had to be used. Hybridity was confirmed in the four developed interspecific hybrid combinations with three SSR markers. EG203 was resistant to both strains Pss97 and Pss2016, while UPV1 and UPV3 were, respectively, resistant and moderately resistant to Pss2016. The four interspecific hybrids with UPV2, UPV3, and UPV1 were susceptible to both bacterial wilt strains, indicating that the resistance of EG203, UPV1, and UPV3 behaves as recessive in interspecific crosses. However, given the vigor of interspecific hybrids between eggplant and the three cultivated wild species, these hybrids may be of interest as rootstocks. However, the development of interspecific hybrid rootstocks resistant to bacterial wilt will probably require the identification of new sources of dominant resistance to this pathogen in the eggplant wild relatives.
ABSTRACT
Bacterial spot caused by Xanthomonas spp. is the second most important bacterial disease after bacterial wilt of tomato and pepper in Taiwan. To determine the species composition of the Xanthomonas population over 27 years (1989 to 2016) across the country, a large collections of strains from tomato (n = 292) and pepper (n = 198) were examined. In the 1989 to 1999 population, all strains (n = 147) from pepper and 95% strains (n = 198) from tomato were Xanthomonas euvesicatoria. The remaining 5% of strains from tomato were X. vesicatoria. In a 2000 to 2009 population from tomato (n = 36), 22% of the strains were X. perforans and the remaining 78% strains were X. euvesicatoria. In the 2010 to 2016 population, 92% of the strains (n = 50) from pepper were still X. euvesicatoria and the remaining 8% of the strains were X. perforans; however, 99% (n = 58) of the strains from tomato were X. perforans. All of the evaluated (n = 25) strains of X. euvesicatoria collected during 1990 to 2006 were tomato race T1. Four pepper races (P1, P2, P7, and P8) were identified in the X. euvesicatoria population. The strains of X. vesicatoria collected during 1989 to 1999 (n = 8) were tomato race T2 and strains of X. perforans from tomato collected during 2010 to 2016 (n = 12) were race T4 (83%) and race T3 (17%). Four strains of X. perforans from pepper were race T4. All of the strains of X. vesicatoria and X. perforans caused a hypersensitive response in all pepper differentials. Biochemical characterization of representative strains (n = 48) showed that strains of X. euvesicatoria were negative on and amylolytic test and positive on lipase and oxidative-fermentative (OF) tests. The strains of X. vesicatoria were positive on amylolytic and OF tests and were negative on the lipase test. All X. perforans strains showed positive reactions on three tests. Evaluation of the same 48 strains for the sensitivity to copper sulfate (50, 100, 200, 300, and 400 mg liter-1) revealed that the majority of X. euvesicatoria (86%) and X. perforans (94%) strains in the 2010 to 2016 population were tolerant to copper sulfate. The findings suggest that management strategies and breeding programs should consider the new X. perforans species and their new races. The increased number of copper-sulfate-tolerant strains in the 2010 to 2016 population further shows the need for alternative options to copper for managing bacterial spot of tomato and pepper.
