Effect and mechanism of action of extracts from algae and cyanobacteria in the control of fungal plant pathogens

Fungal pathogens can attack plants in all cultivation systems and cause fruit decay in post-harvest. Their control was based on the use of synthetic products. Upon the EC regulation No. 1107/2009, concerning the placing of plant protection products on the market and repealing the Council Directives 91/414/EEC, many synthetic pesticides are currently banned for their non-target effects and environmental and health hazards. The use of many others has been restricted. In agriculture, algae and cyanobacteria extracts are used for their stimulant effects on plant vigour and productivity but little is known on their effect against fungal pathogens. The objectives of this thesis were to study: (i) the antifungal activity of water extracts from the cyanobacterium Anabaena sp., and from the algae Ecklonia sp., Halopithys sp. Jania sp. and Chlorella sp. against the fungal plant pathogens Rhizoctonia solani, Fusarium oxysporum f. sp. lycopersici and Podosphaera xanthii, in vitro and in vivo assays; (ii) quantification of compounds in the extracts; (iii) the antifungal activity against B. cinerea of polysaccharides obtained from the extracts (iiii) the plant defence responses by bioassay, by the increase of enzymatic activities related to the plant defence responses and by the accumulation of PR proteins; (iiiii) phytotoxic and biostimulant effects of extracts. The treatment with extracts from Anabaena sp., Ecklonia sp., Jania sp. showed activity against all fungal pathogens by working directly to the pathogen and indirectly through the involvement of plant defence responses. Moreover, polysaccharides derived from the extracts played a role in B. cinerea control by working directly against the pathogen and indirectly by increasing plant defence responses. Considering these results, algae and cyanobacteria extracts may provide a useful tool for the disease management in sustainable agriculture, once their antifungal effects will be verified on plants in a larger scale experiment.