Atividade antifúngica de cepas de Bacillus no manejo biológico do mal do Panamá

Abstract

Banana cultivation has great economic and social importance to Brazil, with a production of 6.8 million tons in 2023. However, the crop is affected by Fusarium wilt, caused by the species Fusarium oxysporum f. sp. cubense (FOC), and one of the alternatives to reduce the impacts is the use of biological control agents, such as Bacillus. The objective of this study was to investigate the antifungal activity of bacteria from the Bacillus genus in controlling the disease, as well as to evaluate the lytic enzymatic activity in inoculated banana plants. For this purpose, micropropagated banana seedlings of the 'Prata Catarina' cultivar, four strains of Bacillus (LPPC242, LPPC259, LPPC262, and LPPC283), and one FOC isolate (RFB65) were used, divided into five treatments: T1: bacterial inoculation, T2: fungal inoculation, T3: bacterial and fungal inoculation, T4: fungal inoculation and thiabendazole fungicide, and T6: control (without inoculation). Bacterial inoculation (concentration of 10⁷ CFU mL⁻¹) was applied at the base of the seedlings seven days before fungal inoculation and repeated every 15 days. The fungal inoculation was performed by cutting and immersing the roots in a spore suspension (concentration of 10⁶ spores mL⁻¹) for 30 minutes, while control plants were immersed in sterile distilled water. The seedlings were kept in a greenhouse for 60 days, and in the final collection, symptoms such as wilting, leaf yellowing, and pseudostem cracks (external) and reddish spots or rhizome necrosis (internal) were evaluated using a descriptive scoring scale and calculation of the disease severity index (DSI). Additionally, seedlings from treatments T1B2, T2, T3B2, T4, and T6 were inactivated with liquid nitrogen, and a protein extract was obtained for each, for 10 different harvesting times. Total protein assays and enzymatic assays for chitinase and β-1,3-glucanase were conducted. Treatments T1 and T6 showed no symptoms of Fusarium wilt, while treatment T2 had a 53.3 % DSI for external symptoms and 33.2 % for internal symptoms, and seedlings treated with fungicide and fungus (treatment T4) had a DSI of 30 % for external symptoms and 28.5 % for internal symptoms. Finally, seedlings subjected to resistance induction with Bacillus strains had the following DSI values: 0 % for external symptoms and 14.2 % for internal symptoms (LPPC259), 6.6 % for external symptoms and 14.2 % for internal symptoms (LPPC283), 13.3 % for external symptoms and 18.9 % for internal symptoms (LPPC242), and 26.6 % for external symptoms and 28.5 % for internal symptoms (LPPC262). Enzymatic analysis indicated that the presence of Bacillus stimulated greater activity of β-1,3-glucanase and chitinase. The biological treatments showed an earlier and more consistent induction of enzymatic activity compared to plants with only fungal inoculum and those treated with fungicide, which showed fluctuations in the defense response. The results of this study reinforce the use of Bacillus as an effective and sustainable strategy for managing Panama disease, promoting greater resistance of plants to the pathogen.