During curcumin production in Vietnam, curcumin-removed turmeric oleoresin (CRTO) has been considered as a by-product. It costs to treat the by-product to prevent environmental pollution. In this study, the by-product was utilized as an active ingredient for preparing a botanical fungicide-based nano-emulsion and evaluated for its in vitro and in vivo control efficacy against Colletotrichum gloeosporioides, a causal agent of anthracnose of litchi, in the laboratory as well as a field trial. The nano-emulsion is colloidally stable and uniform with particle sizes of 95–250 nm. CRTO nano-emulsion significantly affected various Colletotrichum species. Notably, this nano-emulsion showed potent inhibition for the mycelial growth of C. gloeosporioides and solidly suppressed the development of anthracnose on litchi fruits. In the in vitro inhibition test, the equivalent half-maximal inhibitory concentration of CRTO in nano-formulation was 0.11 mg·mL−1, which was 3.0× and 6.1× lower than IC50 values of CRTO alone (0.33 mg·mL−1) and a mixture of curcuminoids (0.48 mg·mL−1), respectively. In the field trial, the litchi anthracnose infection was effectively controlled by nano-formulation. These results suggest that CRTO nano-emulsion could be used as an alternative to harmful synthetic fungicides to control anthracnose on litchi fruits.