# Antifungal Activity of Quaternary Pyridinium Salts Against _Fusarium culmorum_ in Wheat Seedlings
## 4 Discussion  
This study aimed to evaluate the in vivo antifungal efficacy of nicotinamide and isonicotinamide derivatives in reducing _F. culmorum_ infections and their effects on the growth parameters of wheat seedlings. The results show that the tested compounds have varying degrees of efficacy, with some showing similar or better performance than the positive control. However, further research is needed to understand their applicability and long-term effects on plant development and health. The disease index is an important indicator of disease control effectiveness, quantifying symptom intensity on individual plants. Its relevance is highlighted in studies on fungicide efficacy, where a high disease index indicates low treatment effectiveness [54]. These results confirm substantial variation in antifungal efficacy among nicotinamide and isonicotinamide derivatives, even within the same structural group, suggesting that specific molecular features strongly influence their  
Figure 9: The effect of isonicotinamide derivatives on the shoot and root length. The columns (shoot length) and lines (root length) represent the mean of three replicates per treatment. Error bars represent the standard error (SE) of the mean. Differences between treatment means are indicated by different letters according to the Tukey HSD test (\(p\leq 0.05\)). Labels: **(6), (7), (8), (9)—**compounds at a 100 \(\upmu\)g/mL concentration; **(6a), (9a)—**compounds at a 10 \(\upmu\)g/mL concentration. Small letters indicate differences in shoot length, and capital letters indicate differences in root length. biological activity. Compounds **(2)** and **(3)** contain phenacyl substituents, likely improving interaction with fungal enzymes such as succinate dehydrogenase through \(\pi\)-\(\pi\) stacking and hydrogen bonding. Carboxamide-containing compounds inhibit succinate dehydrogenase by forming hydrogen bonds with histidine and interacting with aromatic residues through \(\pi\)-\(\pi\) and hydrophobic forces [55; 56]. Structural features, such as substituent type and flexibility, influence binding: phenacyl groups enhance inhibition, while rigid or hydrophobic groups like methyl or bromopropyl reduce activity by limiting proper alignment within the active site. Some nicotinamide and isonicotinamide derivatives with different natures of substituents preferentially bind to the active site, increasing fungicidal efficiency, while others, due to their specific substituents, favor binding to peripheral sites, ultimately reducing their inhibitory effect. These structural and interaction-based differences likely explain the variations in antifungal activity observed in this study. According to the data, the control group (sterile grain) exhibited the highest disease index (14.2), which was expected due to the absence of any protective treatment. In contrast, the positive control (fungicide) demonstrated strong efficacy, with a significantly lower disease index of 2.2 (Tables 3 and 4). Most nicotinamide and isonicotinamide derivatives also resulted in a notable reduction in disease severity compared to the untreated control, indicating their potential to reduce _F. culmorum_ infection in wheat seedlings. Compounds **(2)** and **(3)** at 10 \(\upmu\)g/mL showed the strongest antifungal effect, with a disease index of 0.9, even lower than the positive control (Table 3). These results are consistent with previous studies on vitamin B3 derivatives. For example, nicotinamide mononucleotide and nicotinamide reduced late blight incidence in wheat, indicating their ability to suppress fungal infections [57]. Similarly, the application of nicotinic acid to fenugreek plants infected with _Agroathela rofsii_ (Sacc.) Redhead & Mullineux significantly reduced disease symptoms [58]. These studies, in line with our results, support that B3-based compounds act as elicitors of early plant defense responses, potentially enhancing both pathogen resistance and physiological resilience under stress.