Produksi cabai merah (Capsicum annuum L.) mengalami penurunan akibat serangan penyakit antraknosa yang disebabkan oleh jamur Colletotrichum sp. Sehingga untuk mengatasi penyakit antraknosa tersebut diperlukan pengendalian menggunakan fungisida nabati. Daun duku diketahui memiliki kandungan senyawa aktif seperti alkaloid, flavonoid, fenol, tanin, saponin, dan terpenoid yang diketahui memiliki potensi sebagai antifungi. Penelitian ini bertujuan untuk mengetahui pengaruh ekstrak daun duku terhadap jamur Colletotrichum sp. serta menentukan konsentrasi terbaik ekstrak daun duku terhadap pertumbuhan jamur Colletotrichum sp. Penelitian ini menggunakan Rancangan Acak Lengkap (RAL) dengan tujuh perlakuan konsentrasi ekstrak daun duku (0%, 0,5%, 1%, 1,5%, 2%, 2,5%, dan 3%) masing-masing perlakuan dilakukan empat pengulangan. Pengamatan dilakukan terhadap masa inkubasi, kejadian dan keparahan penyakit setiap hari selama 16 hari setelah inokulasi. Data dianalisis menggunakan uji ANNOVA dan apabila berbeda nyata dilanjutkan dengan uji Beda Nyata Jujur (BNJ) taraf 5%. Hasil penelitian didapatkan bahwa ekstrak daun duku tidak berpengaruh pada masa inkubasi tetapi berpengaruh dalam menekan pertumbuhan jamur Colletotrichum sp. yang ditunjukkan oleh persentase kejadian penyakit, keparahan penyakit, serta didapatkan konsentrasi terbaik pada perlakuan F (2,5%) dalam menekan kejadian dan keparahan penyakit dan perlakuan B(0,5%) adalah konsentrasi terbaik pada tinggi dan jumlah daun tanaman.The production of red chili peppers (Capsicum annuum L.) has declined due to anthracnose disease caused by the fungus Colletotrichum sp. Therefore, to control this disease, the use of botanical fungicides is necessary. Duku leaves are known to contain active compounds such as alkaloids, flavonoids, phenols, tannins, saponins, and terpenoids, which are known to have antifungal potential. This research aims to determine the effect of duku leaf extract on the fungus Colletotrichum sp. and to identify the optimal concentration of duku leaf extract for inhibiting the growth of Colletotrichum sp. This study employed a Completely Randomized Design (CRD) with seven treatments of duku leaf extract concentrations (0%, 0.5%, 1%, 1.5%, 2%, 2.5%, and 3%), with each treatment repeated four times. Observations were made regarding the incubation period, disease incidence, and severity daily for 16 days following inoculation. Data were analyzed using ANOVA, and where significant differences were found, the analysis was continued with the Honest Significant Differences (HSD) test at the 5% level. The results showed that duku leaf extract had no effect on the incubation period but was effective in suppressing the growth of Colletotrichum sp., as indicated by the percentage of disease incidence and severity. The optimal concentration for suppressing disease incidence and severity was found in treatment F (2.5%) and treatment B (0.5%) yielded the best results in terms of plant height and the number of leaves.