Bacterial pathogens pose a serious threat to either plant health or to food safety. This has the consequence of financial impacts on crop production for pathogens that causes disease on plants, or human health impacts for food-borne pathogens transmitted into the food chain by plants. One of the main challenges to producers is that there are few control options available for reducing or removing bacterial pathogens from plants, and growers have to resort to a limited set of biocides that can cause environmental damage, such as the active ingredient copper oxychloride. This means that a pre-emptive approach is a more sustainable option, e.g. by reducing the risk of pathogens introduction from seeds or transplants and in irrigation water, or for endemic pathogens, using alternative strategies to keep their numbers at manageable, non-harmful levels. These approaches require a degree of monitoring to ensure that the control levels are adequate, since the aim is to keep pathogen levels sufficiently low that they do not cause visible damage to crop plants, or for the food-borne pathogens, that they do not pose a food safety issue.
Approaches to minimise the numbers of pathogens, whether endemic or introduced are aimed at enhancing the natural immunity of the plant, or the natural protective nature of the plant microbiome (the microbial community that exists in association with the plant). Applications that work on plant immunity can be either chemical, whether synthetic or natural, or biological in nature, and they work on the same principle of targeting specific immune-response pathways that help to protect the plant against pathogens. This ‘induced resistance’ acts by increasing the plants response to microbial pathogens by strengthen cell wall to reduce potential damage and through the release of compounds such as reactive oxygen species that have antimicrobial activity. These pathways are normally induced on the first response to pathogens, when the numbers may be quite low, and have the effect of helping the plant to overcome higher numbers that can occur subsequently. Application of chemical or biological active ingredients that target these pathways is simply a mechanism of triggering a natural response in a controlled manner.
The plant microbiome is a large and diverse community of microbes that exists in an intimate relationship with the plant host. The functions of the microbiome are varied, and in many instances are beneficial for the plant, e.g. in facilitating nutrient acquisition and uptake from the bulk soil. One of the well-characterised functions of the microbiome is protection for the plant host, via active competition against other microbes including many of the important plant pathogens, or by inducing natural resistance in the host plant. Therefore, applications that can enhance function or supplement the natural microbiome offer another potential beneficial strategy.
Our research has worked on different approaches that aim to induce natural plant host resistance, with the goal of reducing disease caused by pathogenic bacteria. This has included the application of chemical and biological compounds that either mimic plant hormones or bacterial proteins, respectively, to control phytopathogens on horticultural vegetables. In addition, we also carry out research on bacteria that exist in the root microbiome and can promote induced resistance, again with the ultimate goal of reducing harmful bacteria. One of the major findings of the research is that plant-microbe interactions are dynamic and there is a degree of plant species-dependent specificity.
This is an exciting area with a great potential, although it is still in relative infancy. As with any of the individual approaches in IPM, the results by themselves cannot equal highly potent fungicides or other antimicrobials that have been used historically. Instead, these approaches are designed to work in an entirely different manner, on specific systems and within a package, where relatively small benefits add up to significant changes.
