Natural compounds: an alternative in postharvest disease control

Plant organs produce an amazing diversity of secondary metabolites having a wide range of biological activities. In particular, recent studies have shown how they can play important roles as 'infochemicals' in plant-pathogen interactions, being associated with the defence system and function as fungal inhibitors. A large number of studies on the use of natural compounds have demonstrated their role in the control of fruit postharvest diseases. Natural compounds can be grouped in different categories such as flavour compounds, essential oils (EOs), plant extracts, jasmonates, etc., most of them inhibit pathogens directly; others can enhance the fruit defence system. In the review, advantages and disadvantages in the use of EOs and ITCs will be discussed. The high volatility of some natural compounds makes them suitable for fruit biofumigation, although their use could confer off-odours or off-flavours in fruit, since many natural compounds have strong or unpleasant odours. In in vivo trials, the most fungicidal activity was found with isothiocyanates (ITCs), trans-2-exenal, carvacrol, thymol, citral, and trans-cinnamaldehyde, etc. Against the main postharvest fruit pathogens [Botrytis cinerea, Penicillium spp., Monilinia spp., etc.); however, different levels of sensitivity to treatments were found among fruit species and cultivars. EOs are concentrated mixtures of volatile compounds and most of their components have no specific cellular targets, preventing the appearance of resistant pathogen strains. Nevertheless, their composition can be influenced by many factors, such as climatic and seasonal conditions or harvested period as observed in thyme. In addition, the antifungal activity of ITCs can change according to the species, thus requiring a preliminary thoughtful evaluation of their efficacy. Although there is considerable evidence supporting the use of natural compounds in fruit postharvest disease control, more studies are required before their routine application.