Biocontrol potential of indigenous Trichoderma isolates against Fusarium wilt of cotton in Tanzania: In vitro screening and screen-house validation

Abstract

Fusarium wilt constrains cotton production in Tanzania. This study evaluated indigenous cotton-rhizosphere microorganisms as biological control candidates through sequential in vitro and screenhouse assays. Thirty-six rhizosphere soil samples collected from cotton farms in Magu, Kwimba and Misungwi districts yielded 27 microbial isolates. Preliminary screening selected five candidates: two putative members of the Trichoderma harzianum species complex (T1 and T2), Bacillus subtilis (T3), Bacillus amyloliquefaciens (T4) and a T3+T4 consortium (T5). In dual-culture assays against Fusarium oxysporum EF1, T2 and T1 produced the greatest Day-7 inhibition of radial growth at 65.9% and 61.2%, respectively, compared with 29.5–36.4% for the Bacillus-based treatments. T1 and T2 were subsequently evaluated in a six-week screenhouse trial using cotton cultivar UKM 08. Relative to the pathogen control, T1 and T2 reduced mean area under the disease progress curve by 74.6% and 43.2%, respectively. At Week 6, both treatments significantly reduced normalized disease severity, although Holm-adjusted comparisons of disease incidence were not significant. T2 showed slightly greater inhibition in vitro, whereas T1 produced a larger numerical reduction in cumulative disease, but the isolates did not differ significantly in direct comparisons. These findings demonstrate that in vitro antagonism may not fully predict plant-associated performance and support sequential laboratory and host-based screening. Both isolates require field validation before practical recommendation.