Marine macroalgae mainly include different plant species of red (Rhodomelacea), brown (Phaeophyceae) and green (Chlorophyceae) algae. These macroalgae have not been often regarded as an underutilized bioresource. In agriculture, these plants have been utilized as amendments as bio fertilizers, plant growth regulators and biopesticides. Literature survey reveals that some of the renowned activities are reported from the brown algae for tomato pathogenic fungi, tobacco mosaic virus, cotton wilt and angular leaf spot. Our results showed crude extracts, chlorophyceae (Caulerpa scalpelliformis, Caulerpa veravalensis, Ulva fasciata and Ulva lactuca) species were found to inhibit Cotton wilt causing phytopathogen Fusarium oxysporum f.sp. vasinfectum (FOV) in dose-dependent manner whereas ochrophyta (Padina pavonica and Sargassum wightii) species did not show any fungicidal activity against FOV. All extracts of caulerpa veravalensis were active among them the chloroform extract was highly active at 0.4% whereas 0.8% of hexane and methanol extracts were active against xanthomonas axonopodis pv. malvacearum (XAM). The chloroform extracts of C. scalpelliformis and Padina pavonica; methanol extracts of U. fasciata and U. lactuca were effective against XAM but their activity was not significantly pronounced, as compared to commercial product, bacterimycin. The gracilaria corticata tannin extract significantly suppressed the growth of Fu. oxysporum (< 90%) than turbinaria ornata, stoechospermum marginatum (=90%), sargassum wightii, gracilaria fergusonii, chaetomorpha medica, ulva lactuca and Spatoglossum asperum at 8% concentration. Stoechospermum marginatum showed higher growth inhibition activity to Xanthomonas malvacearum than chaetomorpha medica, sargassum wightii, gracilaria corticata, spatoglossum asperum, ulva lactuca, turbinaria ornata, and gracilaria fergusonii. Emulsifiable concentrations (ECs) were prepared and utilized for cotton phytopathogens in Tamil Nadu, India.