Antifungal activity of extracts from Ulva, Sargassum, and Gracilaria against three fungal pathogens and GC-MS analysis of the most effective extracts

Coastal marine macroalgae are highly diverse and rich in bioactive compounds, though only a few studies have explored their antifungal potential against plant pathogens in Sri Lanka. This study investigated the antifungal activity of Ulva sp., Gracilaria sp., and Sargassum sp. from Thalpe Reef, Galle, Sri Lanka, against the fungal pathogens Lasiodiplodia theobromae, Pseudopestalotiopsis theae, and Diaporthe eugeniae. These pathogens cause leaf necrosis, leaf chlorosis, and leaf blight, respectively, in Solanum melongena plants. To evaluate the antifungal activity of each species, sequential crude extraction was performed using ethyl acetate and methanol. The poisoned food technique was used to screen the antifungal activity and extracts showing the highest antifungal activity were further analyzed using Gas Chromatography-Mass spectrometry (GC-MS). The best inhibition against D. eugeniae and P. theae was exhibited by Ulva-ethyl acetate (UE) at 2000 ppm with inhibition percentages of 79.29% and 56.68%, respectively. Ulva-methanol (UM) at 2000 ppm showed the highest inhibition against L. theobromae, with an inhibition percentage of 43.09%. These results revealed that UE and UM extracts effectively controlled tested fungal pathogens. GC-MS analysis revealed the presence of three compounds in UE, nine in UM, and seven in Gracilaria-ethyl acetate (GE) extracts. Notably, the most abundant compounds with potential antifungal activity included Dihydroactinidiolide (30.02%), 4-Hydroxy-2-butanone (37.37%), and 6,10,14-Trimethylpentadecan-2-one (58.86%).