Antifungal evaluation of extracts from selected medicinal plants against rhizoctonia solani.

Abstract

Maize production consumes about 40 million hectares of land in Sub-Saharan Africa (SSA). Approximately 50% of SSA nations cultivate maize as their primary cereal crop, and in about 75% of these nations, maize is one of the top two cereal crops. Over 100 grams of maize are consumed daily in more than half of the region's nations. Like other countries in sub-Saharan Africa, South Africa produces approximately 60% white maize serving as staple food for many South Africans and grows nearly 40% yellow maize used for animal feed. Maize is one of the cereal crops susceptible to various pathogens including Rhizoctonia solani, Pythium spp, Sclerotium rolfsii and Macrophomina phaseolina. Cultural, physical and chemical methods have been used in combating maize root rot caused by Rhizoctonia solani, However, the pathogen has developed resistance to existing treatment regiments, hence the need to introduce alternative environmentally friendly and sustainable treatment regiments such as medicinal plant extracts is inevitable. This study aimed to investigate the in vitro and in vivo antifungal potential of four selected medicinal plants against R. solani causing root rot in maize and to profile the phytochemistry of the selected medicinal plants.To attain the aim of this study, acetone and ethanol extracts of the medicinal plants were screened in vitro for their antifungal activity against R. solani using two methods; the agar disc diffusion assay and minimum inhibitory concentration (MIC) using a modified version of the micro titre plate methods. The phytochemical profiling was achieved through Ultra Performance Liquid Chromatography-MS/MS (UPLC-QToF/MS/MS). The medicinal plant extracts were further applied as seed treatment in the green house. Plugs containing the pathogen were inoculated into the vicinity of the seeds except in the control. Treatments and parameters including plant height, chlorophyll content, root length, number of root lesions, fresh and dry root weight, fresh and dry shoot weight were recorded weekly. All medicinal plant extracts, except T. ametica showed antifungal activity against the test pathogen. The mean inhibition of the plants against R. solani ranged from 3.9mm (Cascabela thevetia ethanol extracts) to 12.2mm (A. arctotoides acetone extracts). The highest inhibition was induced by both acetone and ethanol extracts of A. arctotoides (12.2 and 10.4 mm respectively), followed by acetone extracts of V. amygdalina (8.0mm) and acetone extracts of C. thevetia (7.5mm). Ethanol extract of C. thevetia exhibited the least antifungal activity with the mean inhibition of 3.8mm, while on the other hand both acetone and ethanol extracts of Trichilia ametica showed no activity against the pathogen. Acetone extracts had higher potency compared to ethanol, with an average of 71.77mm as iii opposed to 55.99mm inhibition diameter. Based on the MIC results, acetone extracts from A. arctotoides exhibited the MIC value (0.2 mg/ml) among the tested medicinal plant extracts. Arctotis arctotoides ethanol (AER1), Vernonia amygdalina acetone (VAaR1), Vernonia amygdalina ethanol (VAER1), Cascabela thevetia acetone (CTaR1) and Cascabela thevetia ethanol (CTER1) recorded the best five treatments with the highest plant height, chlorophyll content, root hairs and the lowest root lesions under greenhouse conditions. Both acetone and ethanolic extracts of Trichilia ametica (TAER1 and TAaR1) showed no activity against R. solani, which confirms the results obtained in the in vitro studies. The pathogen reduced plant height and chlorophyll content and had an increased number of root lesions compared to other treatments including the uninoculated control, which was expected as a confirmation that R. solani was pathogenic to maize. Acetone had high volatility, miscibility with polar and non-polar solvents, and minimal toxicity to test organisms, hence the results of the present study show high activity of acetone extracts in all measured parameters. The A. arctotoides and V. amygdalina plant extracts were regarded as the best treatments as they were among the top treatments in most of the measured parameters. The performance of these extracts indicates that they possess potential as biocontrol agents for the control of Rhizoctonia root rot pathogens of maize. The UPLC analysis identified numerous compounds archived through the comparison of the obtained mass spectra data to literature. Fifteen and eighteen bioactive compounds were identified in acetone and ethanolic extracts of T. ametica respectively, with terpenoid at 27.78% from the ethanolic extract. In C. thevetia, 17 compounds from acetone and 10 compounds from ethanolic extract were identified with the dominant one being terpenoid at 27.76%. A total of17 and 11 compounds were identified respectively from acetone and ethanolic extracts of V. amygdalina the dominant class being fatty acids from the ethanolic extract at 36,36%. In A. arctotoides 19 and 13 compounds were identified from acetone and ethanolic extracts respectively the dominant one being terpenoid at 53.85% from the ethanolic extract. The results of this study showed that medicinal plants harbour phytochemicals responsible for their reported potent antifungal activities that could be further explored against fungal pathogens.