Abundance of Trichoderma Species in Different Habitats and Their Efficacy in the Management of Bacterial Wilt of Tomato

Abstract

Tomato (Lycopersicon esculentum L.) is a key vegetable in Kenya, listed as second most economically important in the horticultural industry. The production of tomatoes has greatly been affected by bacterial wilt caused by Ralstonia solanacearum. Losses to 100% have been reported in both greenhouse and open fields growing conditions. Most of the bacterial wilt management strategies in place have not provided effective, safe and sustainable solution. Therefore, this study contributes, to sustainable tomato production by the use of Trichoderma species as an alternative method of managing bacterial wilt. The study determined antagonistic activity of Trichoderma species from different habitats against Ralstonia solanacearum in vitro and evaluated their efficacy in managing bacterial wilt of tomato at field level. Trichoderma species were isolated and identified from different soil habitats of Karura forest, compost, manure, coffee, and tomato fields. The dominant Trichoderma species were Trichoderma harzianum and Trichoderma asperellum and antagonistic check performed using dual plate technique against Ralstonia solanacearum. The antagonistic ability of the Trichoderma species was determined by measuring the growth radius as a percentage. The field experiments were further conducted in a randomized complete design (RCBD) replicated four times in three greenhouses at Naivasha, Mirera area. The treatments included; isolated Trichoderma asperellum, isolated Trichoderma harzianum, combination of isolated Trichoderma asperellum and Trichoderma harzianum, plots with no applications, commercial Trichoderma harzianum, commercial Trichoderma asperellum, combination of commercial Trichoderma harzianum and commercial Trichoderma asperellum. The isolated Trichoderma species were mass multiplied by growing in sterilized sorghum grains. The already infested greenhouse soil was re-inoculated with isolated Ralstonia solanacearum to ensure uniform pathogen levels. This was isolated from infected tomato plants and introduced one week earlier at 35 ml per pot and properly mixed to ensure uniformity. Trichoderma application was done at the transplanting stage of a greenhouse tomato variety Anna F1, and two more applications after every two weeks. The bacterial wilt incidence and severity assessment was then done weekly and yield data recorded based on physiological maturity of the tomato crops. The laboratory in vitro work indicated that the habitats with high organic matter and fewer disturbances in terms of cultivation had high Trichoderma presence. The habitats had a xii total of 42 Trichoderma harzianum isolates and nine Trichoderma asperellum. Trichoderma harzianum were 15 and four Trichoderma asperellum from forest habitat while three Trichoderma asperellum and 10 Trichoderma harzianum from compost habitat. The other habitats also had similar Trichoderma isolates with low frequency. Trichoderma asperellum and Trichoderma harzianum from the forest and compost habitats had the highest percentage inhibition in vitro. In greenhouse conditions, treatments with Trichoderma asperellum or Trichoderma harzianum at P ≤ 0.05 had significant reduction of bacterial wilt incidence and severity as compared to the plots with no applications done. The Trichoderma species combinations treatments had no significant difference from the single Trichoderma species applications at P ≤ 0.05. The incidence and severity of Ralstonia solanacearum were greatly reduced hence better yields in the Trichoderma treated plots. The results indicated that Trichoderma harzianum and Trichoderma asperellum were efficient in managing bacterial wilt in tomatoes an adoptable alternative management solution to bacterial wilt in tomatoes.