Abstract:
Tomato production in tropical and subtropical regions, including Thailand, is often affected by bacterial wilt, caused by Ralstonia solanacearum. The development and selection of disease-resistant tomato varieties with commercial traits begins by evaluating resistant germplasm and selecting segregating tomato populations for disease resistance, yield, and yield components. Therefore, this research aims to study the improvement of processing tomato populations resistant to bacterial wilt disease (Ralstonia solanacearum). The experiment was divided into two sub experiments, was aimed to assess resistance to bacterial wilt incited by Ralstonia solanacearum with fruit yield and its components of tomato germplasms. The experiment began by assessing the suitable strain and volume of pathogens used a 3x2x2 factorial design in a completely randomized design (CRD). The evaluation included three Ralstonia solanacearum strains: To1 (biovar 4, phylotype I, sequevar 47), To3 (biovar 2T, phylotype I, sequevar 13), and Ch1 (biovar 2T, phylotype I, sequevar 17) as a factor A. Factor B involved two different volumes, namely 5 ml and 10 ml (The concentration of bacteria is 10⁸ CFU/ml). Lastly, Factor C is comprised of two tomato cultivars: a resistant check (Hawaii7996) and a susceptible check (VF134-1-2). The results revealed that Hawaii7996 exhibited a high resistance, with a disease index (DI) of 0%. While VF134-1-2 displayed extreme susceptibility, with DI of 100% when inoculated with To1 (biovar 4) using a volume of 10 ml. Therefore, this specific condition is recommended for accurately evaluating the bacterial wilt resistance of tomato germplasms. Following this, a total of twenty-six tomato germplasms, along with six commercial cultivars, Hawaii7996, and VF134-1-2, were assessed for bacterial wilt resistance under greenhouse conditions. A CRD with three replications and fifteen plants per replication was used in this experiment. In addition to the bacterial wilt resistance evaluation, the yield and yield components were analyzed in open-field conditions using a Randomized Complete Block Design (RCBD) with three replications, each with ten plants. After eight days of inoculation, Hawaii7996 exhibited DI of 11.13%, while VF134-1-2 with a DI of 91.99%. Moreover, KKU-T73025, KKU-T24020, KKU-T73021, KKU-T73020, CK6, CK4, and VFT were classified as highly resistant (HR) to Moderately resistant (MR). Furthermore, KKU-T73021 and KKU-T24020, producing 1,631.08 and 2,083.60 grams per plant, respectively, and yield components outperformed CK4 and CK6. Consequently, KKU-T73021 and KKU-T24020 should be considered valuable genetic resources for breeding programs to enhance bacterial wilt resistance and maintain high productivity.In Experiment 2, the improvement of processing tomato populations was focused on resistance to bacterial wilt disease, yield, and yield components. Specifically, Experiment 2.1 investigated methods for assessing resistance by comparing uninjured and injured root inoculation methods under greenhouse conditions in August 2022. This experiment utilized a 6x2 factorial design in a Completely Randomized Design (CRD) with three replications, each consisting of 15 plants. Factor A included six tomato varieties: KKU-T24020, three commercial cultivars (CK1 - CK3), Hawaii 7996, and VF134-1-2. Factor B comprised the inoculation method, which involved two treatments: uninjured roots and injured roots. The pathogen concentration used was 10⁸ CFU/ml, with a 5 ml inoculum volume per plant. As a result, the 5 ml volume assessment method, when combined with the injured root inoculation technique, can effectively differentiate between resistant and susceptible varieties. This makes it a reliable and suitable approach for evaluating resistance traits in subsequent experiments.In Experiment 2.2, the evaluation and selection of populations were conducted to identify maternal lines for creating tomato segregating populations. Seven tomato populations (BW01 - BW07) from Experiment 2.1 were assessed along with three commercial cultivars (CK1- CK3), a resistant check (Hawaii 7996), and a susceptible check (VF134-1-2). The experiment was conducted using a randomized complete block design (RCBD) with three replications, each consisting of twenty plants, under open field conditions between December 2022 and March 2023. The study found that 90 days after transplanting, Hawaii 7996 exhibited no dead plants, while VF134-1-2 showed a high dead plant of 96.7%. The populations had dead plants ranging from 8.3% to 100%. When considering the variance and mean of disease resistance, yield, and fruit quality, it was found that populations BW01, BW02, BW03, BW05, and BW06 exhibited a distribution of disease resistance, with average yields in a favorable range (2,548.2, 2,535.8, 2,416.0, 2,848.4 and 2,411.8 g/plant, respectively). The yield components of these populations were comparable to those of the commercial cultivar CK3.In Experiment 2.3, disease resistance in tomato populations was evaluated under greenhouse conditions to identify lines with improved resistance to bacterial wilt. The tomato lines selected through pedigree selection from Experiment 2.2 were tested for disease resistance alongside two commercial cultivars (CK1 and CK2), the resistant check Hawaii 7996, and the susceptible check VF134-1-2. The experiment was conducted in June 2023, following a Completely Randomized Design (CRD) with three replications of fifteen plants each. The results revealed that P119-4-4 exhibited moderate resistance, comparable to KKU-T24020, while P117-2-4 demonstrated moderate susceptibility. Based on these findings, KKU-T24020, P117-2-4, and P119-4-4 are considered suitable parental lines for developing base populations in future breeding programs aimed at enhancing bacterial wilt resistance.In Experiment 2.4, the focus was on evaluating six generations of segregating tomato populations for resistance to bacterial wilt and desirable commercial traits. Two tomato populations were developed from resistant selections made in Experiment. These populations were evaluated for their resistance to bacterial wilt as well as for key commercial traits such as fruit yield, size, and quality. The experiment also included three commercial cultivars (CK1, CK2, and CK3), a resistant check (Hawaii 7996), and a susceptible check (VF134-1-2). A completely randomized design (CRD) was implemented under greenhouse conditions in April 2024. The plants were inoculated using the soil-drenching method, with a pathogen concentration of 10 CFU/ml, as described in Experiment 2.1. The results showed that Hawaii 7996 had a disease index of 77.62% 8 days after inoculation, with no plants exhibiting resistance by the end of the experiment. The P (KKU-T24020) and all populations studied displayed a high disease index, indicating a range from very susceptible to extremely susceptible. Meanwhile, the BC117 had a disease index of 54.35% after 14 days of inoculation, which was lower than that of Hawaii 7996. When considering, the distribution of the disease index in the generation populations, F119, exhibited a resistant to susceptible ratio of 1:15, controlled by two recessive genes. While, Backcross generation populations, exhibited a resistant to susceptible ratio of 1:1, controlled by monogenic recessive genes. The evaluation of yield and yield components was conducted under open field conditions from January to April 2024. The experiment was designed as a randomized complete block design (RCBD) with three replications. The study found that the population, population, and backcross population produced yields comparable to those of commercial cultivars (CK2 and CK3) and exhibited higher fruit quality than the resistant check, Hawaii 7996. Therefore, the BC1PR117 population is suitable for variety development through backcrossing to enhance the gene frequency of both resistance traits and commercial traits for future tomato breeding programs focused on resistance to bacterial wilt.