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Opportunistic plant pathogenic bacteria: unravelling meaning and significance

Teresa Coutinho: University of Pretoria

<div>In medicine, the concept of opportunistic pathogens has been well established and defined. Opportunistic pathogens are those bacteria and fungi that usually do not invade healthy hosts, but may invade individuals having underlying conditions that result in a weakened or immunocompromised state. The concept of opportunistic pathogens was later adopted by the field of plant bacteriology pathology in the early 1970’s to refer to bacteria that were considered largely soil-dwelling, but could infect the plant when conditions were favourable. Since then, the term “opportunistic” has been used to refer to those bacteria that are commonly associated with asymptomatic plants, but which periodically cause disease, or those bacteria that are pathogenic only when their plant host is compromised and thus predisposed to infection. The absence of any strict definition has resulted in almost all of the members of the “Big Ten” most important phytopathogenic bacteria being described as opportunistic. In this presentation, we argue that they are those bacteria not typically regarded as plant pathogens, but which periodically cause significant losses to various agricultural crops. We also argue that opportunists exhibit distinct characteristics and meet three specific criteria that separate them from the <em>de facto</em> phytopathogenic bacteria that have evolved to exploit plant hosts.</div>