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Algal compounds exhibit great potential to enhance plant growth and resistance to abiotic and biotic stresses. This review focuses on aspects concerning the physical-chemical properties, function and biological activity of macroalgae polysaccharides. Updated results of the main poly-and oligosaccharides studied for the control of plant diseases are discussed and summarized. The carrageenans from red algae have a well-established obtaining system, but its high market value discourages its use in plant protection. The fucans found in the cell walls of brown algae are present in several fertilizers and accounted for the benefits of such commercial products on plant physiology. The laminarans, from brown algae mainly Laminaria digitata, are currently the main algal polysaccharides on the phytosanitary market. The ulvans, from cell the walls of Ulva spp., open new ways to obtain polysaccharides able to induce resistance due to its abundance worldwide. All these algal polysaccharides show ability to activate multiple plant defense mechanisms against a broad spectrum of plant pathogens. Taking into account the promising results reported in the literature and the enormous biochemical diversity of these biopolymers, it is likely that they will provide new types of resistance inducers in a near future.

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Colletotrichum fructicola is the main species causing apple bitter rot (ABR) and Glomerella leaf spot (GLS) in southern Brazil, and ABR in Uruguay where GLS remains unnoticed. Thus, this work aimed to determine the genetic structure of C. fructicola isolates of both the countries. A total of 28 out of 31 Brazilian isolates (90.3%) caused typical symptoms of GLS, while only 6 of 25 Uruguayan isolates (24.0%) originating from fruits were able to infect leaves, but causing atypical symptoms. Both populations showed similar levels of Nei's gene diversity (h = 0.088 and 0.079, for Brazilian and Uruguayan populations, respectively), and Bayesian cluster analysis inferred two genetic clusters correlated with the geographical origin of isolates. A principal coordinates analysis scatter plot and an unweighted pair group method with arithmetic mean-based dendrogram also grouped Brazilian and Uruguayan isolates into two groups. By pairwise comparison of nitrate-nonutilizing (nit) mutants with a proposed set of testers, all Uruguayan isolates were grouped into a unique vegetative compatibility group (namely VCG 1), while Brazilian isolates were grouped into four VCGs (VCG 1 to 4). Brazilian and Uruguayan populations of C. fructicola were found to be genetically distinct. Our results suggest that isolates of C. fructicola from Brazil capable of causing GLS and ABR arose independently of those from Uruguay. Possible causes leading to the evolutionary differences between populations are discussed.

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The protective activity of a crude extract prepared from the green macroalga, Ulva armoricana, previously shown to induce plant defence responses, was evaluated on three plant species, common bean, grapevine and cucumber, cultivated in the greenhouse and inoculated with three powdery mildew pathogens Erysiphe polygoni, E. necator and Sphareotheca fuliginea respectively. Chemical analyses showed that the extract was enriched in ulvans, which are green algae polysaccharides essentially composed of uronic acid and sulphated rhamnose. Weekly applications were performed by spraying of the green algal extract at various dilutions on bean, grapevine and cucumber leaves. A significant effect (50% protection) was observed using a dilution corresponding to about 3 gl −1 dry matter and up to 90% reduction of symptom severity was obtained for the highest concentration (1/9 dilution, 6 gl −1 dry matter) for the three plant species. To study the natural variability of the protective activity, five extracts prepared from algae batches harvested at different year periods were evaluated. Although polysaccharide composition varied among batches, all extracts elicit a reporter gene regulated by a defence-gene promoter in a transgenic tobacco line, and protect cucumber plants against powdery mildew infection. Together, these data demonstrate that U. armoricana is a reproducible source of active compounds which can be used to efficiently protect crop plants against powdery mildew diseases.