Biological control of Sclerotium rolfsii Sacc. in Stevia rebaudiana using native isolates of Trichoderma spp. from Paraguay

Stevia, Stevia rebaudiana [(Bertoni) Bertoni], is cultivated in Paraguay as a natural, non-nutritive sweetener, but yields are reduced because of wilting and plant death caused by the fungal pathogen Sclerotium rolfsii. This study aimed to evaluate the native fungi Trichoderma spp. isolates, individually and in mixtures, for controlling S. rolfsii. As a first step, ten Trichoderma isolates from agricultural soils of Paraguay were screened in vitro using dual culture tests against S. rolfsii to identify the most effective isolates. This preliminary phase allowed screening to select the most promising candidates before conducting in planta experiments under more realistic conditions. A greenhouse experiment with seven treatments and four replicates was carried out to assess their efficacy in controlling S. rolfsii in stevia. Treatments included commercial Trichoderma viride, three native isolates (3KH and TCAS, T. asperellum; MS28, T. erinaceum), a mixture of these isolates, a fungicide (azoxystrobin + cyproconazole), and a control. Stevia plants of the variety “Katupyry” were preventively treated with Trichoderma (1×10⁷ spores ml-1) twice before S. rolfsii inoculation. Fungicide and Trichoderma treatments were applied to the plant base and substrate after inoculation, followed by two weekly applications. Disease incidence and the area under the disease progress curve (AUDPC) were evaluated. The AUDPC for TCAS (7.67) was significantly lower than that in the control (13.27), the mixture (13.77), and the commercial T. viride (15.58), but not significantly different from the 3KH (9.15) or MS28 (10.47). Fungicide-treated plants had the lowest AUDPC (1.25). These results suggest that the Trichoderma isolate TCAS effectively manages S. rolfsii under greenhouse conditions and has potential for use in both organic and conventional stevia production.