Coffee Anthracnose-Associated Fungi from Wanning, Hainan: Fungicide Sensitivity and Broad-Spectrum Antagonism by Mangrove-Derived Streptomyces sp. CD103

Fungi associated with coffee leaf lesions provide a diverse target panel for evaluating fungicide sensitivity and microbial antagonism. This study evaluated the antagonistic spectrum of mangrove-derived Streptomyces sp. CD103 and fungicide sensitivity in selected lesion-associated fungi. Of 49 isolates recovered from symptomatic Coffea arabica leaves in Wanning, Hainan, 39 induced necrosis following wound inoculation. These isolates were assigned to eight genera based on internal transcribed spacer (ITS) sequence comparisons and phylogenetic placement; Colletotrichum accounted for 64.1% of the necrosis-inducing isolates. Six fungicides were tested against seven isolates spanning four genera. Sensitivity varied among isolates and compounds; concentrations causing 50% inhibition of mycelial growth (EC50) for thiophanate-methyl ranged from 0.57 to 22.53 µg mL−1. Among 45 mangrove-derived actinomycetes screened against Colletotrichum gloeosporioides CCG3, CD103 showed the highest mean inhibition (99.21 ± 0.40%). In subsequent dual-culture assays, CD103 completely inhibited 19 of the 21 selected lesion-associated isolates spanning eight genera and all six reference plant pathogens. The two exceptions, a Fusarium isolate and an Alternaria isolate, showed 85.04% and 97.10% inhibition, respectively. These findings identify CD103 as a broad-spectrum in vitro antagonist against the tested fungi and warrant further evaluation of its disease-suppressive potential on coffee plants.