The basic requirement for the onset of plant disease is well defined as the plant disease triangle. For a plant disease to manifest it requires the presence of a pathogen, a susceptible host and favourable environmental conditions. Conversely, plant disease is prevented upon elimination of or change in any one of these three parameters. For plant bacterial diseases the onset of disease is further complicated by plant commensal and mutualistic bacterial species and the capacity for bacterial species to exchange DNA via horizontal gene transfer (HGT). Plants are frequently colonised and/or attacked and challenged by an array of beneficial and pathogenic microorganisms.
In the modern, global world microbes are provided with even more opportunity to spread and evolve into pathogens of significance than in previous eras. There is increased movement of people and plant products around the globe. The requirement to maximise crop yields to feed increasing populations inevitably results in modifying host crop germplasm and farming practices are continuously evolving, often in response to changing climates. It is inevitable that new pathogens will emerge that will impact significantly on agricultural production systems.
The emergence of plant diseases caused by phytopathogenic bacteria will be discussed. Four case studies (fire blight caused by Erwinia amylovora ; Annual rye grass toxicity (ARGT) caused by Rathayibacter toxicus; Bacterial canker of kiwifruit caused by Pseudomonas syringae pathovar actinidiae; and ratoon stunting disease (RSD) of sugarcane caused by Leifsonia xyli subsp. Xyli)will highlight potential changes in the pathogen, plant host and/or environment that has favoured the onset of disease. The challenge for plant pathologists is to learn from past disease outbreaks and predict the emergence of new pathogens and prevent their spread.