Effect of extract and nano extract of Bottle brush (Callistemon citrinus (Curtis) Skeels) and Denak (Oliveria decumbens Vent) on the control of citrus green mold (Penicillium digitatum (Pers.) Sacc.)

Background and Objectives

Green mold caused by Penicillium digitatum (Pers.) Sacc. is a major fungal pathogen responsible for postharvest rot in citrus fruits, leading to substantial economic losses sometimes as high as 90% in horticultural crops.The fungus spreads rapidly through asexual spores and primarily infects fruits through wounds caused by insects, branches, or mechanical damage during handling and transport.Due to the adverse effects and residual toxicity of chemical fungicides, plant extracts and nanoextracts have emerged as promising alternatives for disease management.Callistemon citrinus an ornamental plant from the Myrtaceae family and Oliveria decumbens of the Apiaceae family are known to contain bioactive secondary metabolites, including phenolic compounds with antifungal properties. Additionally, green nanotechnology provides a sustainable strategy to enhance the effectiveness of natural antifungal agents while reducing environmental risks.

Materials and Methods

Two experiments were conducted in 2024 at the agricultural laboratories of Shahid Chamran University of Ahvaz, using a completely randomized design with three replications. The first experiment assessed the antifungal efficacy of ethanolic, methanolic and aqueous extracts of C.citrinus and O.decumbens at 50% and 100% concentrations against P.digitatum, alongside a commercial fungicide (carbendazim) and a control group. Antioxidant activity (DPPH%), total phenol content, and total flavonoid content of both the crude extracts and nano-extract were measured using standard laboratory methods.Tangerine fruits infected with green mold were collected from storage facilities in Ahvaz. The pathogen was isolated, purified and identified morphologically and molecularly. In the second stage, the most effective extract was selected, and its nano-extract was prepared using the sonication method.The inhibitory effects of the extract and its nanoform were compared against green mold.The morphology, size, and distribution of the nanoparticles were analyzed using Transmission Electron Microscope (TEM) and Dynamic Light Scattering (DLS).

Results

The aqueous extract of C.citrinus at 100% concentration exhibited the highest inhibitory effect among the common extracts, with a growth inhibition of 5 mm.In contrast, the methanolic extracts of C. citrinus and O.decumbens at 50% concentration showed no inhibitory activity, with a growth zone of 32 mm, indicating no suppression of fungal growth.The commercial fungicide carbendazim exhibited a stronger inhibitory effect producing a growth inhibition zone of 28 mm, while the control group showed a zone of 32 mm. Results from the nano-extract synthesis indicated that 100% of the nanoparticles were spherical, had a uniform structure, and a diameter of less than 20 nanometers. The second experiment confirmed the superior efficacy of nano-extracts over conventional extracts.The nanoextract of C.citrinus at 100% concentration produced the most significant inhibitory effect, completely preventing fungal growth. In contrast, the weakest effects were observed with the common extracts of O. decumbens at both 50% and 100% concentrations. Additionally, the highest antioxidant activity (DPPH) was recorded in the nano-extracts of O.decumbens (74.35%) and C. citrinus (73.87%) at 100% concentration. The nanoextract of C.citrinus also recorded the highest total phenolic (283 mg g-1 gallic acid) and flavonoid (198.53 mg g-1 quercetin) content at the same concentration. Overall, the nanoextracts of both plants were more effective against P.digitatum than their common extract counterparts. Based on these findings, both the aqueous extract and nano-extract of C. citrinus demonstrated notable potential for the laboratoryscale control of green mold.

Discussion

In this study the extract of C. citrinus demonstrated greater antifungal efficacy compared to that of O. decumbens. Previous research has emphasized that variations in plant genus and species, the nature of active compounds, the type of extract, and the method used for extracting bioactive components all influence the antimicrobial effectiveness of plant-based treatments.The superior performance of aqueous extracts over alcoholic ones extracts may be attributed to the higher sensitivity of certain fungal strains to water-soluble compounds. Furthermore, some antifungal compounds, such as glycosides are more effectively dissolved in water.In contrast, alcohol-based extraction processes may alter or inactivate structurally sensitive compounds. Nano extracts of C. citrinus and O.decumbens demonstrated significantly enhanced antifungal activity against green mold compared to their conventional extract forms. One key factor contribution to this improvement is particle size.In nano-extracts, reduced particle size increases the surface area of plant material exposed to the solvent, thereby accelerating mass transfer. In addition, nano-extracts exhibited higher antioxidant activity as well as greater total phenolic and flavonoid content than common extracts, further contributing to their superior bioactivity.