Antifungal Properties of Urtica dioca against Six Phytopathogenic Fungi ^†

Abstract

Several phytopathogenic fungi greatly affect the production of important crops across the globe, as they lead to huge losses. To control diseases caused by fungal phytopathogens, a wide range of synthetic fungicides are applied in the fields. However, these agrochemicals are harmful for ecosystems (aerial, aquatic and terrestrial), non-target organisms and humans. In addition, since these antifungals have one specific cellular target, fungi can acquire resistance to them via the accumulation of mutations. Plant extracts provide natural alternatives to the use of synthetic fungicides in agriculture. Several plants are rich in secondary metabolites, including alkaloids, coumarins, flavonoids, terpenoids and saponins, which confer antifungal activity. This sustainable option is biodegradable, environmentally friendly and proves to be safer, and it is less prone to development resistance since they often have several cellular targets. This study was conducted to investigate the antifungal activity of Urtica dioica extract against Colletotrichum acutatum, Colletotrichum gloeosporioides, Colletotrichum godetiae, Colletotrichum nymphaeae, Diplodia corticola and Phytophthora cinnamomi. Urtica dioica extract was prepared with 50% (v/v) ethanol, the solvent was evaporated at a low pressure, and the residue was dissolved in water. The extract was incorporated into PDA medium at different concentrations (100, 500, 1000 and 2000 µg/mL) and mycelial discs were placed in the center of each Petri dish. Growth was measured in terms of radial mycelial growth in the third, sixth and ninth days of incubation, at 25 °C and in the dark. Urtica dioica extract was able to inhibit the growth of all strains except C. nymphaeae. Growth inhibition was around 20% at 2000 µg/mL for the remaining Colletotrichum species. An inhibition of growth was also observed with D. corticola in a concentration-dependent manner, from 100 µg/mL to 2000 µg/mL and revealed statistically significant differences (p < 0.05) between these concentrations. Regarding the growth of P. cinnamomi, significant differences were observed between the 100 µg/mL and 2000 µg/mL extract (p < 0.0001 and p < 0.05 on day 3 and 6, respectively). The most pronounced mycelial growth reduction (39.9%) was observed on day 3, an effect that is significantly different from (24.9%; p < 0.05) the result observed on the sixth day of incubation. Overall, the results of this work suggest U. dioca as a potential, ecologically sustainable alternative to conventional fungicides to protect crops from damage caused by phytopathogenic fungi.