Green Synthesis of Silver Nanoparticles Using Tagetes erecta Flower Extract: Structural Characterization and Dose-Dependent Antifungal Efficacy Against Fusarium oxysporum and Rhizoctonia solani of chickpea

In the semi-arid tropics, where the chickpea is a vital source of protein for millions, the crop faces a silent, persistent enemy beneath the soil. Two fungal pathogens, one causing a wilt that drains the plant's life from the roots up and another that rots the root system itself, can devastate entire fields, wiping out yields that farmers rely on for their livelihood. For decades, the solution has been chemical fungicides, powerful synthetic compounds that kill these invaders but carry their own heavy costs: they can leave residues in the environment, harm non-target organisms, and eventually lose their power as the fungi evolve resistance to them. This has driven scientists to look for a different kind of shield, one that works with nature rather than against it. The field of green synthesis offers a promising path, using the natural chemistry found in plants to build tiny, powerful particles called nanoparticles. These are not the large grains of sand or dust we see with the naked eye, but structures so small that thousands could fit on the head of a pin. When made from silver, these particles have a unique ability to disrupt the cell walls of fungi, stopping them from growing and reproducing. The question researchers have been asking is whether a simple, common flower could be used to manufacture these microscopic defenders effectively enough to protect a major food crop.

A team of scientists at Jawaharlal Nehru Krishi Vishwa Vidyalaya in India set out to answer this by turning to the marigold, a bright orange flower known locally as Tagetes erecta. They did not use complex machinery or harsh industrial chemicals to create their silver nanoparticles. Instead, they took fresh marigold flowers, chopped them up, and boiled them in water to create a simple tea-like extract. This liquid, rich in natural compounds like flavonoids and phenols, was then mixed with a solution of silver nitrate. Almost immediately, the clear mixture began to change color, shifting from transparent to a deep brown. This visual shift was the first sign that the plant extract was doing its work, acting as both a reducing agent to turn silver ions into solid metal and a capping agent to keep the new particles from clumping together. The result was a suspension of silver nanoparticles, tiny spheres of metal suspended in water, ready to be tested.

To understand exactly what they had created, the researchers examined the particles under powerful microscopes and with X-ray diffraction, a technique that reveals the internal arrangement of atoms. They found that the particles were indeed crystalline, with a structure typical of metallic silver, and measured an average size of about 19 nanometers. While the majority of the particles were spherical, they were not perfectly uniform; some were slightly oval, and they tended to cluster together, wrapped in a thin layer of the organic plant material that had helped create them. This coating is crucial, as it stabilizes the particles and likely helps them interact with fungal cells. The analysis also revealed a small amount of silver chloride mixed in with the silver, a common occurrence when plant extracts containing natural salts are used in the synthesis, but one that did not hinder the particles' effectiveness.

With the nanoparticles characterized, the team moved to the real test: could they stop the fungi that plague chickpea fields? They began with a controlled laboratory experiment, placing the fungi on petri dishes containing a nutrient gel mixed with different amounts of the silver nanoparticles. They tested concentrations of 50, 100, and 200 parts per million. The results were clear and consistent: the more nanoparticles they added, the less the fungi grew. At the lowest dose, the growth of the wilt-causing fungus was reduced by about 32 percent, while the root-rot fungus was held back by nearly 42 percent. As the concentration increased to 200 parts per million, the inhibition became much stronger, stopping nearly 70 percent of the wilt fungus and over 77 percent of the root rot fungus. For comparison, they also tested a standard commercial fungicide called azoxystrobin, which is widely used by farmers. While the chemical fungicide was slightly more effective at the highest doses, the silver nanoparticles made from marigold flowers performed remarkably well, achieving results that were statistically very close to the chemical standard.

The researchers then took the experiment out of the lab and into a greenhouse, simulating the conditions a farmer would face. They planted chickpea seeds in soil that had been infected with the two fungi and treated the seeds with the silver nanoparticle solution before planting. They also included a healthy group of plants and a group treated with the standard chemical fungicide. After sixty days, they measured the health of the plants. The untreated, infected plants suffered severe disease, with most of the plants showing signs of wilting or root rot. However, the seeds treated with the 200 parts per million silver nanoparticle solution showed a dramatic improvement. The severity of the wilt disease was reduced by nearly 65 percent, and the root rot was reduced by over 72 percent. These reductions were nearly identical to those achieved by the chemical fungicide at a lower dose, proving that the plant-made nanoparticles could protect the crop in a living environment, not just in a petri dish.

One of the most interesting findings was that the two fungi did not react in exactly the same way. The root rot fungus, Rhizoctonia solani, was consistently more sensitive to the silver nanoparticles than the wilt fungus, Fusarium oxysporum. At every concentration tested, the root rot fungus was held back more effectively. This suggests that the structure of the fungal cell walls might differ in a way that makes one easier for the nanoparticles to penetrate or disrupt than the other. While the chemical fungicide remained the single most effective treatment overall, the silver nanoparticles offered a compelling alternative that approached its performance without the same risks of environmental residue or the potential for the fungi to develop resistance. The study concludes that using marigold flowers to synthesize silver nanoparticles is a viable, sustainable strategy for managing these soil-borne diseases. It offers a way to harness the power of nanotechnology using materials that are easy to grow and process, providing a potential tool for farmers to protect their chickpea crops while reducing their reliance on synthetic chemicals.