*2.4.1 Effect of nickel nanoparticles on fungal mycelial growth*

Both the concentrations of nickel nanoparticles (50 and 100 ppm) inhibited the fungal mycelial growth on solid media, and the inhibitions were significant (p ≤ 0.05) over control (**Figure 1**). Nickel nanoparticles at 100 ppm concentration inhibited the mycelial growth of *F*. *oxysporum* f. sp. *lactucae* and *F*. *oxysporum* f. sp. *lycopersici* by 60.23 and 59.77%, respectively, over control.

The inhibitory effects of nickel nanoparticles were also assessed in liquid medium, and the results were similar with solid media. In the liquid media, the fresh mycelial weight of the tested fungal pathogens decreased significantly and more than 50% reduction was recorded with the use of nickel nanoparticles at the concentration of 100 ppm. The results revealed that mycelial growth of tested pathogens was inhibited in a concentration dependent manner. These results

**Figure 1.**

*Petri dishes showing inhibition of Fusarium wilt causing pathogens: first row, Fusarium oxysporum f. sp. lycopersici [(A) control; (B) 50 ppm nickel nanoparticles; (C) 100 ppm nickel nanoparticles]; second row, Fusarium oxysporum f. sp. lactucae [(D) control; (E) 50 ppm nickel nanoparticles; (F) 100 ppm nickel nanoparticles] [45].*

suggest that using Ni nanoparticle solution can significantly increase the surface areas acting on the mycelia of *Fusarium* and mycelial growth.

Nickel nanoparticles at the concentration of 100 ppm decreased the number of spore development by 81.40 and 74.60% in *F*. *oxysporum* f. sp. *lactucae* and *F*. *oxysporum* f. sp. *lycopersici*, respectively. The conidial germination was negatively affected by nickel nanoparticles (**Figure 2**).

The inhibitory effect of Ni nanoparticles on spore germination could be due to their fungicidal effect. These results agreed with the results obtained from other workers on antifungal effects of different metal nanoparticles against some pathogenic fungi such as silver nanoparticles and zinc nanoparticles against copper nanoparticles [46]. Inhibitory effect of Ni nanoparticles could be due to producing of extracellular enzymes from fungi as survival agents caused by stress of toxic materials [47] or could be due to large surface areas (**Figure 3**) and small sizes to penetrate into the cell membrane of pathogen and work in the cytosols [48].
