C.J. Straker

Short note on the effects of ethanolic extracts of selected South African seaweeds on the growth of commercially important plant pathogens, Rhizoctonia solani Kühn and Verticillium sp.

This paper describes the biological activity of ethanolic extracts from some commonly abundant seaweeds from KwaZulu-Natal, South Africa. The pour-plate method was used to determine the effects of selected seaweed extracts on the growth of two phytopathogens; Verticillium sp. and Rhizoctonia solani. Extracts from Caulerpa filiformis and Ulva rigida (Chlorophyta); Zonaria tournefortii (Phaeophyta); and Hypnea spicifera, Gelidium abottiorum and Osmundaria serrata (Rhodophyta) inhibited fungal growth by more than 50%. The extracts from the rhodophytes, Spyridia cupressina and Beckerella pinnatifida showed the weakest antifungal activity of the seaweeds tested. The pour plate method demonstrated both fungal growth promotion and inhibition, due to the seaweed extracts. At low concentrations (1:16 and 1:32 dilutions) the H spicifera extract promoted the growth of R. solani.

Extracts from seaweeds can promote fungal growth

Hormesis is the stimulation of a biological response at low concentrations of an inhibitor. Ethanolic extracts were made using Osmundaria serrata (Suhr) R. E. Norris and Stypopodium zonale (Lamouroux) Papenfuss from the East coast of South Africa. Two plant pathogens (Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. and Rhizoctonia solani Kühn) were used as test organisms in bioassays. Serial dilutions of macroalgal extracts were tested by the pour plate technique. Both growth inhibitory and promotory responses were observed. The hormetic response was observed in both the fungi when grown on low dilutions of ethanol and the O. serrata extract, and when R. solani was grown on the S. zonale extract. This study provides more evidence of hormesis in macroalgal products and the phenomenon is discussed in relation to its possible cause and significance in the application of seaweed extracts.

Hormesis in Bioassyas of Macroalgal Fungal Propagules

A range of dilutions (1:1 to 1:320) of ethanolic extracts of dried tissues of the macroalgae, Osmundaria serrata and Zonaria subarticulata were made. These dilutions were incorporated into fungal growth media and their influence on the germination of spores of Aspergillus niger and Alternaria sp., and the colony growth of Rhizoctonia solani after breaking of scerotial dormancy, observed. Pure ethanol inhibited spore germination and colony growth by up to 100 %, but the toxicity of ethanol decreased with increasing dilution and showed hormetic trends. The presence of macroalgal factors ameliorated the toxicity of pure ethanol to different percentages but showed both amelioratory and inhibitory influences on the action of the different dilutions of ethanol on spore germination or colony growth. The data are discussed in the light of other examples of hormesis and the need to isolate the active macroalgal factors.