Melanin biosynthesis in fungal pathogens such as Colletotrichum lagenarium is essential for appressorial penetration into host plants, and blocking this pathway is a target for antifungal drug development. The first step in 1,8-dihydroxynaphthalene (1,8-DHN) melanin biosynthesis involves polyketide synthase PKS1, but no specific PKS1 inhibitors were available. An in vitro PKS1 reaction system using Aspergillus oryzae-expressed C. lagenarium PKS1 was developed to screen for inhibitors. Screening of microbial metabolites identified Streptomyces sp. No. 4a as a producer of PKS1 inhibitors, leading to the isolation of abikoviromycin (1) and dihydroabikoviromycin (2) as active principles. Their structures were confirmed by mass spectrometry (MS) and nuclear magnetic resonance (NMR) analysis. PKS1 inhibitory activity (IC50: 1 = 5 µg/ml, 2 = 30 µg/ml) and antifungal activity (MIC: 1 = 63 µg/ml, 2 > 200 µg/ml) were tested, with 1 showing stronger PKS1 inhibition. Compound 1 dose-dependently inhibited melanin production in C. lagenarium in vivo, achieving 88% inhibition at 12 µg/ml. This study demonstrates for the first time that PKS1 inhibitors like abikoviromycin and dihydroabikoviromycin are potential candidates for selective inhibition of melanin production in pathogenic fungi.