In vitro assay of Bidens pilosa Linn. aqueous extract against postharvest fungal

Introduction

Corn and peanut seeds are vulnerable to pathogenic fungal species before and after harvesting. Their association with the stored seeds may eventually result in the deterioration of seed quality. Although stored seeds apparently look healthy because of the absence of physical damage, however, these may be contaminated with high levels of mycotoxins produced by certain species of fungal pathogens. Mycotoxins are fungal metabolites that cause grain quality deterioration, poor germination potential, and reduced vigor. To prevent these problems, different control strategies should be employed such as irradiation, chemical treatment, and biological control. However, irradiation of seeds before storage is costly while the application of chemical treatments poses hazards to humans and animals. With these issues at hand, one of the promising options is the application of botanical pesticides.

Several studies had been conducted on the use of weed extract to control the growth of plant pathogenic organisms. The water extracts from the weed species (A. conyzoides, Oxalis corniculata, Phyllanthus debilis, Vernonia cinerea, and Desmodium trifolium) were assayed for their antifungal activity against some plant pathogenic fungi (Iqbal et al., 2001).

The extract from A. conyzoides inhibited the mycelial growth of Rhizoctonia solani, Aspergillus niger, and Phomopsis theae. In another study, the extract from O. corniculata was active against A. niger while P. debilis suppressed the growth of P. theae. The activity generally declined after three days of incubation, while A. conyzoides remained active for nine days after incubation. Ethanolic extract of Datura stramonium also contains significant antifungal potential against some important plant pathogenic fungi and thus, could be used as an alternative to chemical fungicides for the management of fungal infection in plants (Sharma et al., 2014). 

There were previous studies on Bidens pilosa, a common weed species in the tropics, focusing on its antibacterial effects against human pathogens. Silva et al. (2014) evaluated nine extracts from B. pilosa (root, stem, flower, and leaves) and Annona crassiflora (rind fruit, stem, leaves, seed, and pulp) against 60 oxacillin resistant Staphylococcus aureus (ORSA) and S. aureus ATCC6538. They found that extracts from the leaves of B. pilosa had significantly wider inhibition zone diameters than chlorexidine against ORSA, and the extracts were more active against S. aureus ATCC. The presence of variable alkaloids, flavonoids, tannins, and saponins was observed which may be responsible for its antibacterial activities.

The antifungal properties of B. pilosa were documented in some studies involving plant pathogenic fungal species. Deba et al. (2007) first evaluated the antifungal potential of this plant against Corticium rolfsii, Fusarium solani, and Fusarium oxysporum using the hot water extracts from the roots, stem, and leaves. They found that C. rolfsii was most suppressed as its growth was reduced almost all the tested doses followed by F. oxysporum and F. solani. Extracts from stems and roots exhibited greater fungicidal action than the extracts from the leaves. In another experiment, the team also demonstrated the antifungal effects of the essential oils and aqueous extracts from the flowers and leaves of B. pilosa using the three fungal species. They again concluded that the extracts and oils had antifungal activity on the fungal pathogens (Deba et al., 2008). 

Polyacetylenes, polyacetylene glycosides, flavonoids, flavone glycosides, aurones, chalcones, okanin glycosides, phenolic acids, terpenes, pheophytins, fatty acids, and phytosterols are among the chemical ingredients identified or isolated from the various portions of B. pilosa (Xuan & Khanh, 2016). Many of these have been identified as bioactive chemicals with pharmacological potential.

According to Silva et al. (2011), as cited by Bartolome et al. (2013), 201 compounds have been identified from this plant as compiled previously, comprising of 70 aliphatics, 60 flavonoids, 25 terpenoids, 19 phenylpropanoids, 13 aromatics, 8 porphyrins, and 6 other compounds. 

This study focused on the assay of B. pilosa aqueous extract against common fungal pathogens associated with corn and peanut seeds. The antifungal effect of the extract was determined under in vitro conditions.