Evaluation of Antifungal Effects of Various Extracts of Indigo (<i>Indigofera tinctoria</i>) on <i>Phytophthora</i> Fungus In Vitro

Background: The damage to agricultural products caused by fungal diseases accounts for approximately 12% of global production, with a higher impact in developing countries. One particularly destructive disease that inflicts significant damage on vegetable crops and greenhouses annually is seedling death and bush dieback disease, caused by the Phytophthora fungus. Currently, producers often rely heavily on chemical fertilizers and pesticides to enhance productivity and product quality. However, the use of some synthetic pesticides has recently been banned due to their high toxicity and long preharvest interval (PHI) periods. Biological control of plant diseases and pests is the optimal solution to mitigate the damages caused by synthetic pesticides. Objectives: This study aimed to evaluate the in vitro antifungal efficacy of different extracts derived from the aerial shoot of Indigofera tinctoria against plant pathogenic fungi. Methods: The aerial shoots of the Indigo plant were collected during the flowering stage from the medicinal plants collection of the Institute of Agricultural Research, Agriculture Institute Research, Institute of Zabol, Zabol, Iran. Ethanolic, methanolic, n-hexane, and acetone extracts were prepared by cold maceration with a ratio of 1:10. The antifungal effects of these extracts were investigated by measuring the diameter of halo growth inhibition under in vitro conditions. The LSD test was used to compare data means. Results: Our results showed that the best growth-inhibiting effect was observed with the ethanolic extract at a concentration of 300 μg/mL (2 mm) after 24 hours of incubation, whereas the diameter of colonies in the control was 27 mm, confirming a 92.5% inhibition of fungal growth. After 48 hours of incubation, the growth of Phytophthora drechsleri mycelium was clearly evident. In fact, the growth rate nearly doubled within 48 hours, but then at 73 hours, the mycelium grew at a slower rate. The inhibitory effect of the indigo extract was dose-dependent; as the extract concentration increased, the inhibitory effect became stronger. Conclusions: According to these results, the in vitro treatments were more significant when the plant extract concentration was higher. These findings could be promising for the production of natural fungicides.