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.

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.

Glomerella leaf spot (GLS) is one of the most important diseases of apple, affecting a wide range of economically important cultivars, particularly Golden Delicious and its descendants. Caused mainly by species of the Colletotrichum gloeosporioides species complex (CGSC), C. fructicola has been described as the most prevalent and aggressive species associated with GLS and apple bitter rot (ABR) in Brazil and Uruguay. Recently, new CGSC species, closely related to C. fructicola, have been identified causing ABR. To verify the accuracy of species identification within the CGSC, we aimed to reevaluate the identity of representative GLS-causing isolates from Brazilian and Uruguayan populations, previously identified as C. fructicola. Multilocus phylogenetic analysis based on APN2, ApMAT, CAL, GAPDH, GS, ITS, and TUB2 allocated these isolates in a monophyletic clade with C. chrysophilum. This species was first described as the causal agent of anthracnose in banana fruits in Brazil, and recent reports indicate its association with ABR in the United States. This is the first report of C. chrysophilum causing GLS disease on apple worldwide.