In the present investigation, three plant species identified as
Ocimum gratissimum,
Kopsia fruticosa and Hibiscus rosa-sinensis were selected to determine the antifungal potential of their extracts against plant pathogens. The results of the antifungal assay revealed that oil extract obtained from
O. gratissimum exhibited antifungal activity against the test fungal pathogens in varying degrees. The extract showed an inhibitory effect on the radial growth of all phytopathogens and the highest inhibitory effect was observed in
C. capsici KMPS 12423 (81.4±1.5%) followed by
A. alternata KMPS 12623 (73.7±1.3%) and
F. oxysporum KMPS 12023 (68.7±1.7%). The extract showed the lowest inhibitory effect against
C. acutatum KMPS 13423 (Fig 1). Similarly, organic leaf extracts of
K. fruticosa and
H. rosa-sinensis also exhibited notable inhibition of radial growth against all the phytopathogens. It was observed that the hexane extract of
K. fruticosa and the methanol extract of
H. rosa-sinensis displayed remarkable antifungal activities against
C. acutatum KMPS 13423, with an inhibition percentage of 75.9±1.1% and 82.4±1.0%, respectively, while moderate antifungal activity was observed in the ethanol extracts (Fig 2 A). Effective inhibition of
C. capsici KMPS 12423 was observed in the ethanol leaf extract of
K. fruticosa and methanol extract of
H. rosa-sinensis while less activity was observed in hexane extracts (Fig 2 B). A similar result was observed against
A. alternata KMPS 12623 in which ethanol leaf extract of
K. fruticosa and methanol extract of
H. rosa-sinensis effectively inhibited the pathogen significantly (Fig 2 C). It was again observed that ethanol leaf extract of
K. Fruticosa and hexane extract of
H. rosa-sinensis effectively inhibited the growth of
F. oxysporum KMPS 12023 (Fig.2 D). From the above results, it was noted that ethanol extract of
K. fruticosa and methanol extract of
H. rosa-sinensis effectively inhibited the growth of all the test fungal pathogens.
Considering the inhibitory effects shown by the oil and plant extracts, Minimum Inhibitory Concentration (MIC) was determined for all the test fungal pathogens. The result indicated that oil extract of
O. gratissimum could able to inhibit
C. capsici KMPS 12423,
A. alternata KMPS 12623 and
F. oxysporum KMPS 12023 at MIC values of 50, 100 and 200 µg/mL, respectively. The oil was less effective against
C. acutatum KMPS 1342 and showed inhibition at MIC value of 500 µg/mL (Table 1). Furthermore, the susceptibility of various organic leaf extracts of
K. fruticosa and
H. rosa-sinensis was also assessed. Notably, the ethanol extract of
K. fruticosa and the methanol extract of
H. rosa-sinensis displayed higher susceptibility compared to the other extracts against the tested fungi, as presented in Table 1. The ethanol extract of
K. fruticosa, showed MIC values ranging between 100–350 µg/mL against the phytopathogens,
C. acutatum KMPS 13423,
C. capsici KMPS 12423,
A. alternata KMPS 12623 and
F. oxysporum KMPS 12023. Further, the hexane and methanol extracts demonstrated antifungal activity against the same fungi with MIC values ranging from 200-500 µg/mL each. In the case of
H. rosa-sinensis, the methanol extract exhibited MIC values ranging from 50-200 µg/mL against
C. acutatum KMPS 13423,
C. capsici KMPS 12423,
A. alternata KMPS 12623 and
F. oxysporum KMPS 12023. Conversely, the hexane and ethanol extracts displayed antifungal potential against the aforementioned fungi, each displaying MIC values of 100-350 µg/mL.
In the present investigation, it was found that oil extract obtained from
O. gratissimum was effective in inhibiting the fungal pathogens. Therefore, in order to observe the further efficacy of the oil extract in growth reduction potential spore germination and growth kinetics assay was undertaken for all the test fungal pathogens. Results obtained for the oil extract on the spore germination assay of each of the test fungi are shown in Fig 3. DMSO (0.5%, v/v) was used as a negative control and it did not inhibit the spore germination of any of the plant pathogens tested. There was a significant inhibition of fungal spore germination shown by the oil extract at different concentrations. Complete inhibition (100%) of fungal spore germination was observed in
C. capsici KMPS 12423 at 100 µg/mL concentrations while in
A. alternata KMPS 12623 inhibition was shown at 200 µg/mL concentrations. The oil also exhibited a potent inhibitory effect on the spore germination of
F. oxysporum KMPS 12023 and
C. acutatum KMPS 13423 in the range of 50–80% at concentrations ranging from 200 to 500 µg/mL.
The antifungal kinetics assay was determined only against
C. acutatum KMPS 13423 and the result of the oil extract against the pathogen is shown in Fig 4. Exposure of
C. acutatum KMPS 13423 spores to different concentrations of the essential oil for 30-150 min caused varying degrees of inhibition of spore germination. As the exposure time and concentration increased, there was an observed increase in fungicidal activity. The oil extract at 50 µg/mL showed antifungal activity but did not inhibit completely and about 50% inhibition was observed at an exposure time of 120 min. However, there was a marked increase in the killing rate at 100 and 200 µg/mL after 30 min of exposure and 95 to 100% inhibition of spore germination was observed on 150 min exposure, respectively. At low concentrations, the significant rate of inhibition was the characteristic feature of the oil extract.
As the global population continues to grow, ensuring food security becomes a prominent issue, especially in developing countries. Nonetheless, these food supplies entail risks from a variety of pathogens that not only lessen the usefulness and economic value of food products but also make them unfit for consumption, thus negatively impacting human health and well-being. Over time, a wide range of synthetic chemicals have been employed as antifungal agents to curb the proliferation of plant pathogenic fungi. However, these chemicals are well-known for causing adverse effects on living organisms and the environment. Thus, at present attention is shifting towards other alternatives that consumers perceive as natural and eco-friendly. This has resulted in research activities dedicated to formulating safer antifungal solutions, including those utilizing plant derived oils and their extracts to counteract plant pathogens in agriculture. Studies have demonstrated that the antifungal activity of plant extracts and compounds against phytopathogenic fungi could be good alternatives for controlling plant diseases (
Deresa and Diriba, 2023).
In our present study, oil and organic solvent extracts obtained from three plant species namely
O. gratissimum,
K. fruticosa and H. rosa-sinensis have shown antifungal activities against plant pathogens causing diseases in solanaceous crops. The oil of
O. gratissimum successfully inhibited mycelia growth and spore germination of
C. capsici,
A. alternata and F. oxysporum. Similar studies made by
Faria et al., (2006) reported that essential oil obtained from aerial parts of
O. gratissimum inhibited the growth of several fungi including
Botryosphaeria rhodina,
Rhizoctonia and Alternaria sp. Previous investigations have noted that essential oils obtained from
O. gratissimum exhibit antimicrobial characteristics. It is documented that the volatile oil extracted from this plant is primarily composed of phenolic compounds, with special emphasis on thymol (
Oliver, 1960;
Sainsbury and Sofowora, 1971). These phenolic constituents have been attributed for the documented antimicrobial properties to the oil. The organic solvent extracts obtained from
K. fruticosa and
H. rosa-sinensis also showed inhibitory effects on the mycelia growth of
C. capsici,
A. alternata and F. oxysporum in different concentrations. Similarly,
Debjani et al., (2017) observed that plant extracts obtained from Ginger,
Polyalthia and
Clerodendrum showed a good inhibitory effect on
Rhizoctonia solani and also inhibited the growth of
Colletotrichum capsici at different concentrations
. Ethanol extract obtained from
K. fruticosa has been reported to exhibit antifungal, antimicrobial and cytotoxic activities
(Long et al., 2018). However, there is no report on leaf extract obtained from this plant against phytopathogenic fungi to date. The extract obtained from this plant revealed novel indole alkaloids, specifically kopsifolines G-K, alongside a recognized alkaloid, kopsifoline A. Therefore, the antifungal efficacy can be ascribed to the presence of these alkaloids.
Hibiscus rosa-sinensis, commonly referred to as the Chinese hibiscus, is a widely recognized medicinal plant employed in addressing diverse health conditions (Magdalita and San Pascual, 2022). There are reports of antimicrobial activity of this plant extract against pathogenic bacteria (
Ruban and Gajalakshmi, 2012;
Patel et al., 2012). However, information on this plant extract against pathogenic fungi is still lacking. Our study showed that organic solvent extract obtained from
H. rosa-sinensis leaf showed effective inhibition against plant pathogens namely
F. oxysporum, C. acutatum,
C. Capsici and A. alternata. The organic leaf extract of this plant has been reported to contain various phytochemicals like flavonoids, tannins and phenols
(Farasayu et al., 2021). In many instances, these phytochemicals demonstrated antibacterial effects by inhibiting the synthesis of nucleic acid, biofilm formation, energy for bacterial metabolism and the function of the cytoplasmic membrane. Therefore, the antifungal effect of this plant may also be due to the presence of these secondary metabolites. Our studies have demonstrated that extracts derived from medicinally important plants could be successfully used for the control and management of plant pathogens in eco-friendly and more economical ways as compared to synthetic fungicides.
