Growth inhibition of three fungal pathogens by marine macroalgal extracts: An in vitro study

Abstract

Increased reliance on synthetic fungicides to control crop fungal diseases has led to numerous environmental and health issues. Thus, this study aimed to investigate the antifungal activity of marine macroalgal species of Ulva, Gracilaria, and Sargassum from Thalpe reef against fungal pathogens, Colletotrichum sp., Fusarium sp., and Sclerotinia sclerotiorum. Algal extracts were obtained using ethyl acetate, chloroform and methanol. Antifungal activity was tested in vitro at four concentrations using poisoned plate method. Captan and dimethylsulfoxide were used as positive and negative controls, respectively and all treatments were triplicated. Fungal growth inhibitory percentages were calculated, and their significance was assessed by Kruskal-Wallis test followed by Dunn’s post hoc test for comparison. Results revealed that most of the extracts were effective at 2000 ppm concentrations. However, the highest inhibition (75%) was exhibited by Gracilaria–chloroform extract at 1000 ppm against S. scleritiorum, which was significantly different from all other macroalgal x solvent x concentration (MxSxC) combinations except for 2000 ppm Sargassum–chloroform extract (72%). Similarly, Gracilaria–ethyl acetate extract at 2000 ppm demonstrated significantly higher inhibition (71.5%) against Colletotrichum sp. compared to all other MxSxC interactions (P<0.05). None of the MxSxC combinations inhibited the growth of Fusarium sp. more than 50%, with 2000 ppm Ulva-methanol extract showing the highest inhibition of 49.4%. Methanol extracts were most effective against Fusarium sp. Ulva extracts consistently increased the inhibition against tested pathogens with concentration increments. Altogether, 15 MxSxC combinations exhibited more than 50% inhibition. Overall, Gracilaria and Ulva extracts could be identified to have a significantly higher antifungal potential compared to Sargassum. Thus, 2000 ppm Ulva–methanol and Gracilaria–chloroform extracts can be concluded to possess the highest antifungal potential, though the bioactive compounds in these have to be identified. Further, the efficacy and feasibility of these extracts in field applications need to be investigated.