Chemical composition, antifungal and antioxidant activity of essential oils from Baccharis dracunculifolia and Pogostemon cablin against Fusarium graminearum

Abstract Fusarium graminearum Schw. is associated with diseases in seeds and seedlings, mainly in maize, and causes damage to crops and food production. We determined the chemical compositions of essential oils from Pogostemon cablin (Blanco) Benth and Baccharis dracunculifolia DC., and evaluated their antioxidant and antifungal activity against F. graminearum. The oils were applied to the fungus using disc diffusion technique at concentrations of 8.0, 4.0, 2.0 and 1.0 μL mL−1. Fungistatic effects of P. cablin oil were detectable at 8.0 μL mL−1, with 80.0% inhibition of fungal growth. This oil contains mono and sesquiterpenes that may be toxic to fungal cell structures. P. cablin oil also had antioxidant activity in free radical sequestration experiments, where as oil from B. dracunculifolia had limited effects. We conclude that essential oil from P. cablin has greater anti-phytopathogenic and antioxidant activity than that from B. dracunculifolia. Graphical Abstract


Introduction
Several species of the Fusarium genus, including Fusarium graminearum Schw. [Gibberella zeae (Schw.) Petch.], are causative agents of serious plant diseases and cause significant destruction of grains during storage (Perczak et al. 2019). One of the alternatives could be essential oils that have fungicidal activity, limiting the development of phytopathogenic fungi and are safe for people and the environment (Hashem et al. 2010). Among the essential oils widely used in industries, with antifungal and antibacterial activities (Salazar et al. 2018), compounds of the Lamiaceae family such as P. cablin and family Asteraceae such as B dracunculifolia, species that are of great commercial, medicinal and industrial importance (Paroul et al. 2016).
Given the resistance of fungi to chemicals and the demand for natural products that are safe for people and the environment, further researchs into the use of essential oils as effective biofungicides are needed. Thus, we determined the chemical compositions of P. cablin and B. dracunculifolia essential oils and evaluated their antioxidant and antifungal activity against F. graminearum.

Chemical composition of essential oil
The average yield of essential oil P.cablin was 2.1% and is in accordance with the average yield of 1.5% to 3.5 cited by Blank et al. (2011) for this species. The major components were patchoulol (31.5%), seichelene (13.6%) and a-bulnesene (15.6%; Table S1, Supplementary material) and were in accordance with the International Organization for Standardization (ISO 3757, 2002).
The average yield of the essential oil of B. dracunculifolia was 0.32% and is in accordance with Santos et al. (2011) who obtained 0.35% yield for this species. The majority components were cis-and trans-nerolidol at 17.5%, c-elemene at 15.0% and D-limonene at 10.5% (Table S2, Supplementary material). The concentration of 8.49% spathulenol was adequate with that mentioned by Fabiane et al. (2008). This component is of great commercial importance, with antibacterial and antifungal properties (Paroul et al. 2016).

Anti-phytopathogenic activity
We observed that the treatment with essential oil of B. dracunculifolia led to a significant inhibition of the growth of fungus (Table S3, Supplementary material). At concentrations of 8.0 and 4.0 mL mL À1 , B. dracunculifolia essential oil inhibited fungal growth by 57.1% and 49.5%, respectively, after 96 h. These results can be justified by the chemical composition of the oil, such as monoterpenes and sesquiterpenes (Table S2, Supplementary material), components with antifungal activity (Donelian et al. 2009).The antifungal activity verified for the essential oil of B. dracunculifolia is in accordance with the results obtained by Souza et al. (2011).
Significant differences in fungal growth were observed between control and P. cablin oil treated cells at the 48 h time point, 8.0 lL mL À1 P. cablin oil inhibited fungal growth significantly (Table S4). After 72, 96 and 120 h incubation, P. cablin oil showed antifungal activity at all concentrations. At higher concentrations (4.0 and 8.0 lL mL À1 ), the anti-phytopathogenic effects of P. cablin oil against F. graminearum were similar to those of clotrimazole at 12 mg mL À1 , indicating promising fungistatic effects at >4.0 lL mL À1 . In addition, phytopathogen growth was inhibited by 80% after 96 h treatments with 8.0 lL mL À1 P. cablin oil, again differing little from the effects of 4.0 lL mL À1 treatments (79.2% growth inhibition). The results obtained in the study corroborate those of Krzy sko-Łupicka et al. (2019). The major component of P. cablin essential oil was patchoulol (monoterpene) that may have contribute to antifungal activity (Dechayont et al. 2017). However, the compounds of lower concentrations also contributed to the antifungal action (Krzy sko-Łupicka et al. 2019).

Antioxidant activity
The results of DPPH antioxidant assays of P. cablin and B. dracunculifolia essential oils showed activity of 3.41 and 12.08 mmol trolox mL À1 , respectively. The antioxidant activity of the P. cablin oil was 71.8% greater than that of B. dracunculifolia. The significant antioxidant activity of the P. cablin could be attributed to the major components of essential oil, mainly monoterpenes (patchoulol and seychellene) and sesquiterpenes (a-bulnesene and a-guaiene). The result of the antioxidant activity of P. cablin oil corroborates with Krzy sko-Łupicka et al. (2019).
The result of the antioxidant activity of B. dracunculifolia oil could be due to the sum or the synergistic effect of each compound present in the essential oil, mainly sesquiterpenes (Donelian et al. 2009). This result is in accordance with the previously published antioxidant activity for B. dracunculifolia (Paroul et al. 2016).

Conclusions
The major components of B. dracunculifolia oil are cis and trans-nerolidol, c-elemene, D-limonene, caryophyllene and b-pinene. In contrast, P. cablin oil mainly comprises patchoulol, seichelene, a-bulnesene and a-guaiene. Essential oils from P. cablin and B. dracunculifolia could be considered sources of compounds with significant in vitro anti-phytopathogenic activities, and a good alternative to chemicals. The antioxidant capacity of P. cablin oil was, however, greater than that of B. dracunculifolia oil.