Status of Bacterial Wilt of Tomato in Kajiado and Kirinyaga Counties and Its Management by Host Resistance and Biocontrol Agent

Abstract

Bacterial wilt, caused by Ralstonia solanacearum, is responsible for yield losses of up to 64 to 100% in tomato grown under open field and greenhouse conditions, respectively. Additionally, the disease has resulted in a decrease in farmer’s income. Therefore, the study determined (i) the current status of bacterial wilt of tomato in Kajiado and Kirinyaga counties, (ii) the susceptibility and resistance of newly introduced tomato cultivars to bacterial wilt, and (iii) determined the efficacy of Bacillus subtilis in managing bacterial wilt in tomato. A survey was conducted in tomato growing farms in Oloitoktok and Mwea in Kajiado and Kirinyaga counties in May and June 2021, using a semi-structured questionnaire. Symptomatic tomato plants and soil samples were collected from the farmers’ fields and taken to Kenya Agricultural and Livestock Research Organization, National Agricultural Research Laboratory for isolation, identification, and quantification of R. solanacearum. Data was collected on the size of land under tomato production, duration of tomato production, diseases, and pest levels, and sources of water for irrigation. Susceptibility of newly introduced tomato cultivars to bacterial wilt were evaluated in the field in Oloitoktok, Mwea, and greenhouse at the Field Station, Kabete campus. Experimental treatments were laid out in a Randomized Complete Block Design (RCBD) in the field and a complete randomized design (CRD) in the greenhouse with three replications. In the greenhouse, the root system of each cultivar was clipped and dipped into an inoculum concentration of 1 x 107cfu/ml before transplanting. Data was collected on both the number of wilted and dead plants from the emergence up to the maturity stage. The efficacy of Bacillus subtilis on bacterial wilt was conducted in the greenhouse at the Field station, Kabete campus, and open field in Oloitoktok and Mwea regions. Experimental treatments were laid out in a Randomized Complete Block Design (RCBD) in the field and the greenhouse with three replications. Before transplanting in the field and greenhouse, the root systems of each cultivar were dipped into 200 ml of Bacillus subtilis and after one week, the cultivars in the greenhouse were inoculated with an inoculum concentration of 1 x 107cfu/ml. Data was collected on the number of wilted plants, plant height, root length, stem discoloration, biomass, and yield data was recorded during harvesting of mature fruits. The survey indicated that more than 90 % of the farmers in both regions were able to identify bacterial wilt of tomatoes in their farms. In Mwea, over 70 % of the famers considered bacterial xii wilt to be highly significant while in Oloitoktok, 90% of the respondents and 40% in Mwea reported experiencing bacterial wilt experienced in the past two years of cropping seasons. The management practices for bacterial wilt varied between the two regions, with more than 50% in Mwea and 60% in Oloitoktok employing rouging as a control method. Field experiments revealed that tomato cultivars Cal J and Rio Grande were highly susceptible to bacterial wilt, each exhibiting 100% incidence score. On the other hand, Big rock variety displayed tolerance with a 30% incidence score eight weeks after inoculation. Furthermore, Cal J variety had the highest severity score, exceeding 4.9 on the disease index. The least affected varieties were Big rock, TO 135, and Crown, with incidence scores of 1.0, 3.0, and 3.1 respectively days after inoculation. Significant difference (P< 0.05) were observed among the eighteen varieties in the management of bacterial wilt in the green house in Kabete. In Mwea and Kajiado the disease levels were lower compared to those observed during the screening process. Therefore, the study provided insights into the current status of bacterial wilt in Kajiado and Kirinyaga regions, tolerant varieties, and the efficacy of Bacillus subtilis to bacterial wilt