Sweet Chestnut Fractions from a Sustainable Circular Process for the Control of Phytopathogenic Oomycetes

One natural, sustainable extract of Castanea sativa Mill. was tested for in vitro antimicrobial activity against three phytopathogenic Oomycetes of agronomic interest: Pythium dissotocum, P. sylvaticum, and P. ultimum. The extract is obtained from chestnut culture waste by hot water extraction and membrane fractionation/concentration. The chemical characterization of bioactive secondary metabolites highlighted the presence of gallic and ellagic hydrolysable tannins (26.0% vs weight of powder), with a prevalence of vescalagin and castalagin (4.8% and 9.8%). The extract showed good inhibition of mycelial growth, particularly for P. sylvaticum, for which an inhibition of 79.4% with 0.1% w/V extract was observed. For P. dissotocum and P. ultimum, the inhibition was 57.1% and 56.5%, respectively, at 0.1% extract. Furthermore, in all tests, a different phenotype (laxer mycelium) was observed. This evidence could open perspectives for using sweet chestnut tannins for the contrast of phytopathogenic Pythium, e.g. in sowing substrates or for crop biodefence, or even in substrates for hydroponic crops, given the dependence of Pythium spp. in aquatic environments. This study is in line with the growing interest in eco-sustainable solutions for reducing chemical inputs in agriculture.