Abstract
Introduction Marine macroalgae are increasingly recognized as promising sources of bioactive compounds for sustainable plant disease management. This study evaluated the anti-phytopathogenic potential of organic extracts from three intertidal macroalgal species, Ulva fasciata , Gracilaria khanjanapajiae , and Sargassum wightii , collected from Thalpe Reef, Sri Lanka. Methods Chloroform, ethyl acetate, and methanol extracts were screened against phytopathogenic fungi Aspergillus niger , Colletotrichum lindemuthianum , Colletotrichum fructicola , Sclerotinia sclerotiorum , and Fusarium oxysporum using the poisoned plate method, and against phytopathogenic bacteria Xanthomonas cucurbitae , Pectobacterium carotovorum , Pseudomonas syringae , Dickeya chrysanthemi , and Ralstonia sp. using the well diffusion assay. Bioassay-guided fractionation was performed on selected active extracts, and the resulting fractions were analyzed by gas chromatography–mass spectrometry. Results Antimicrobial activity varied significantly among macroalgal species, extraction solvents, concentrations, and target pathogens (ANOVA, p < 0.05). The ethyl acetate extract of G. khanjanapajiae exhibited the highest antifungal activity with 71.52 ± 1.60% inhibition against C. lindemuthianum . In contrast, the chloroform extract of U. fasciata showed the strongest antibacterial activity, producing a 24.2 ± 0.3 mm inhibition zone against D. chrysanthemi . Bioassay-guided fractionation further enhanced antimicrobial efficacy, with selected fractions demonstrating higher inhibition than crude extracts. GC–MS analysis of the most active fractions revealed diverse metabolites, including lipophilic phenolics, fatty acids, fatty amides, terpenes and terpenoid derivatives, long-chain alcohols, aldehydes, ketones, and long-chain hydrocarbons such as alkanes, alkenes, and α-olefins, many of which are known for antimicrobial properties. Discussion The findings necessitate further investigation of Sri Lankan marine macroalgae species and solvent-dependent variation of their antimicrobial potential, as candidates for environmentally compatible plant disease control strategies.