<jats:p> This study investigates the potential of natural products derived from a mangrove rhizosphere bacterium in tomato early blight management. A Streptomyces puniceus strain L75 was isolated from the rhizosphere of Acanthus ilicifolius Linn in the Mai Po Reserve, Hong Kong. The crude ethyl acetate (EA) extract of L75 fermentation cultures has broad-spectrum antifungal bioactivities. L75 EA extract was significantly more effective in Alternaria solani growth inhibition at 25 μg/ml or lower compared with Mancozeb, with no observable negative impacts on tomato leaves or root development. Furthermore, L75 EA extract had significantly lower aquatic toxicity than Mancozeb at the same concentrations. L75 EA extract targets germ tube elongation of A. solani conidia, with a fungistatic mode of action. Liquid chromatography-quadrupole time-of-flight mass spectrometry analysis identified two possible antifungal compounds, Alteramide A and the Heat-Stable Antifungal Factor, which together contribute partially to the bioactivity of L75 EA extract. On detached tomato leaves, coinoculation of A. solani with L75 EA extract of 50, 25, or 5 μg/ml reduced diseased areas by ∼98, ∼90, and ∼48%, respectively, relative to the control after 5 days. This study demonstrates the potential of natural products from mangrove rhizosphere bacteria in agricultural applications.