Evaluation of Tulasnella and Ceratobasidium as Biocontrol Agents of Fusarium Wilt on Vanilla planifolia

Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. vanillae (Fov), is a disease that results in significant losses in commercial vanilla production. The genera Ceratobasidium (Ceratobasidiaceae) and Tulasnella (Tulasnellaceae), which are often reported as mutualistic symbionts in orchids, belong to the form genus Rhizoctonia, a paraphyletic group of fungi with potential for pathogen biocontrol. We assayed the antagonistic properties of the form genus Rhizoctonia from the roots of neotropical orchids: two Tulasnella spp. isolates (Bv3 and Er1) and one Ceratobasidium sp. (Er19). In a dual culture, we found that form genus Rhizoctonia isolates can generate a biocontrol effect against Fusarium through the mechanisms of antibiosis and competition for space and nutrients. On histological observations, orchid root endophytes also demonstrated potential for mutualistic symbiosis development by establishing themselves on the surface and within the root tissue of Vanilla planifolia accessions multiplied in vitro (NSF021 and NSF092). However, in plant assays, the form genus Rhizoctonia isolates did not reduce symptom expression or disease development due to infection by Fov in the host. These results contribute to the knowledge of the interactions between tropical orchids and their microbiota and demonstrate the need for multidisciplinary studies for the implementation of integrated management strategies for Fusarium disease in commercial systems.