Phytoelicitor activity of three Caribbean seaweed species on suppression of pathogenic infections in tomato plants

Seaweed extracts are commonly used as phytoelicitors to promote crop growth and yield, worldwide. However, extracts of some seaweed species are known to induce defence processes in plants and may have potential for use as plant protection agents in the management of crop diseases. Most of the research on seaweed extracts as elicitors of plant defences have been on temperate seaweed species. Caribbean islands have an abundance of tropical seaweed species along their coasts, but their potential for use in agriculture has not been explored yet. The aim of this study was to determine the phytoelicitor potential of extracts of three seaweeds abundantly found in Trinidad and the southern Caribbean. The efficacy of the seaweed extracts in suppressing Alternaria solani and Xanthomonas campestris pv vesicatoria infections in tomato was analysed. Alkaline extracts of Ulva lactuca, Sargassum filipendula and Gelidium serrulatum were applied to tomato plants which were inoculated with the pathogens. The plants were then monitored over time for development of disease, activity of defence enzymes and expression levels of key marker genes of various plant defence pathways. Plants treated with the seaweed extracts showed reduced disease severity which was coupled with elevated rates of activities of defence enzymes. Seaweed extract-treated plants also had increased expression levels of marker genes for defence signalling pathways. The extracts of U. lactuca and S. filipendula induced jasmonate signalling defence systems. However, G. serrulatum extract was able to sequentially induce both salicylic acid and jasmonate signalling pathways and, consequently found to be very effective in controlling the intensity of both diseases. The results of this study indicate that extracts made from tropical seaweeds found locally in the southern Caribbean could be used in tomato as phytoelicitors. Local seaweed extracts therefore present a good, potentially environmentally friendly, alternative for farmers of the Southern Caribbean to manage diseases caused by A. solani and X. vesicatoria in tomato.