We introduced a novel method to prepare silver core-shell nanoclusters (NCs) in which 3,4-dihydroxyphenyl acetic-conjugated oligochitosan (DHPAC) reduced silver salt and subsequently protected the produced nanosilver via mussel adhesion mechanism. Results indicated that the degree of conjugation was 14 dihydroxyphenyl acetamide moieties over 100 glucosamine units of oligochitosan. We used chitosan-catechol derivative to prepare the well-defined silver core-shell NCs and applied UV-visible spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques to characterize the NCs. The core-shell NCs exhibited strong growth inhibition of plant-pathogenic fungi such as Phytophthora capsici, Phytophthora nicotianae, and Phytophthora colocasiae. These positive results may offer great potential to produce silver core-DHPAC shell NCs for several biomedical applications.