Application of Biocontrol Agents for Management of Fusarium Wilt in Watermelon Production in Ogun State, South West Nigeria

Fusarium wilt caused by Fusarium oxysporum f. sp. niveum remains a persistent soilborne disease constraint on watermelon production in Ogun State, with soil fumigation offering limited long-term efficacy and raising environmental concerns. Biological control using antagonistic microorganisms presents an environmentally acceptable alternative with demonstrated potential in comparable production systems. This study evaluated the efficacy of Trichoderma harzianum, Bacillus subtilis, and Pseudomonas fluorescens, applied individually and in combination, for managing Fusarium wilt on watermelon under Ogun State field conditions. Trials conducted over two seasons in Abeokuta compared wilt incidence, plant vigour indices, and marketable fruit yield across treatment combinations using a randomised complete block design. The T. harzianum and B. subtilis combination treatment reduced wilt incidence by 61 percent compared to the untreated control and produced yields comparable to the chemical fungicide standard at significantly lower input cost. Soil enzyme activity analyses showed elevated chitinase and beta-glucanase activity in biocontrol-treated plots, consistent with enhanced microbial antagonism. Single-agent biocontrol treatments showed moderate but consistent disease suppression across both seasons. Biocontrol treatments were less effective during the second season, which experienced higher rainfall and elevated soil temperatures, suggesting the need for adjusted application frequencies under specific seasonal conditions. The study recommends a combination biocontrol programme as a viable, cost-effective alternative to synthetic fungicide-based Fusarium wilt management in Ogun State watermelon systems. Keywords: Fusarium wilt, biocontrol, Trichoderma harzianum, watermelon, Ogun State.