In vitro Antifungal Efficacy of Nanomaterials against Plant Pathogenic Fungi and Oomycetes

Nanoparticulate (NP) substances have widely documented antimicrobial properties, yet their utilisation in the biocides and pesticides industries has yet to be fully exploited. This is particularly so in the pesticides industry, where their potential has not yet been realised. This mini review identifies the emerging trends identified in research characterising the in vitro antimicrobial properties of NP substances against fungal and oomycete phytopathogens. Nanoparticulate substances for which there was a sufficient depth of published studies on activity against fungal and oomycete phytopathogens are covered in this review, these include chitosan, copper, magnesium, silver and zinc. All substances displayed significant activity against a range of phytopathogens, though silver and copper-based NPs appear to be the most potent at relativity low (<50 ppm) concentrations. However, as particle size and shape affect the level of exhibited toxicity, direct comparisons of activity between studies are often difficult due to the different types of NP examined. One particularly promising NP substance is the organic biodegradable substance chitosan which is considered environmentally friendly. Chitosan has also been shown to stimulate plant growth and defence in addition to possessing antifungal activity. The lack of toxicological properties marks chitosan as having particular potential for fulfilling the regulatory requirements for environmental fate and ecotoxicology necessary for gaining approval as an authorised pesticide. Another distinct problem in comparing studies is the lack of a recognised standardised growth medium/media for determining nanomaterial toxicity. A growing body of evidence suggests that the in vitro toxicity of certain nanoparticles is highly influenced by the properties of the growth medium, such as its pH, salinity and components. These confounding factors will be discussed and their implications for comparing nanomaterial efficacy highlighted while also providing suggestions for improving characterisation of nanomaterial efficacy. Characterisation of nanomaterial efficacy in vitro is a critical step in determining which nanomaterials should be progressed for further testing in higher tier tests such as simulated use trials and field trials. The aim of this chapter is to draw attention to the limitations of in vitro characterisation and highlight how these techniques can be improved.

Keywords: Chitosan, Copper, Fungi, Growth medium, In vitro, Magnesium, Nanoparticles, Oomycetes, Phytopathogens, Silver, Zinc.