Chitosan-Based Nanocarriers Loaded with Natural Plant Extracts Against Fusarium Wilt of Banana: In Vitro Activity Against Foc STR4 and TR4 and Preliminary in Planta Assessment Against TR4

Fusarium wilt of banana, caused by Fusarium oxysporum f. sp. cubense (Foc), threatens global production, and although several fungicides inhibit Foc under controlled conditions, none provides reliable field control. We evaluated two complete chitosan oligosaccharide (COS)-based nanocarrier formulations, loaded with Rubia tinctorum or Uncaria tomentosa extracts, against Foc subtropical and tropical race 4 (STR4, TR4) in vitro, and against TR4 in a preliminary plant assay. Both formulations inhibited both races, with EC50 values of 320.0–571.7 µg·mL−1 and no difference in potency between them. In a descriptive time-kill assay with two tubes per condition, no colonies were recovered from 48 h onwards with the R. tinctorum formulation, whereas the U. tomentosa formulation still yielded 3–7 CFU·mL−1 at 72 h. Applied as a single root dip to TR4-challenged ‘Grand Nain’ plants, in one controlled-environment run of nine plants per treatment, with one tray and one application batch per treatment, the R. tinctorum formulation gave a standardized area under the disease progress curve 34.9% lower than the inoculated control, whereas the 7.8% difference for U. tomentosa was not distinguishable from that control; incidence remained 100% in every inoculated group. The plant comparison is an observation to be confirmed rather than a replicated estimate of efficacy, and cannot be attributed to any single formulation component; it identifies the R. tinctorum formulation as the candidate for independently replicated testing.