Effects of Silicon Supplementation in the Cucumber–Anthracnose (Colletotrichum orbiculare) Pathosystem

Many monocot crops actively take up silicic acid from the soil, which then polymerizes during evapotranspiration into subcuticular deposits that act as physical barriers against fungal and arthropod penetration. This "mechanical barrier" is one hypothesis explaining the protective effect of silicon supplementation in monocots, most notably sugarcane and rice. Conversely, cucurbits experience similar mediated resistance with Si supplementation, but without forming a physical Si barrier in leaf tissue. Previous researchers have found increased phytoalexin accumulation in Si-treated plants in a cucumber-powdery mildew (Sphaerotheca fuliginea) pathosystem.  We investigated another pathosystem (cucumber-anthracnose, Colletotrichum orbiculare) to determine whether Si-mediated resistance also occurs via similar phytoalexin production. Si-treated plants reduced anthracnose severity by 20-60% in comparison to non-treated plants.  MS-HPLC data of water-soluble extracts of leaves showed UV-positive peaks that differed significantly between Si-treated and non-treated plants between 65 and 80 minutes elution time. This region is consistent with flavonoids, phenolic compounds, and other known defense compounds.  We discuss how these results compare to previous studies on Si-mediated resistance, including research on the cucumber-powdery mildew pathosystem, to elucidate plant responses to pathogen attack.