How Do Plant Diseases Caused byXylella fastidiosaEmerge?

Abstract

Emerging plant diseases frequently have significant economic, environmental, cultural, and social impacts. The prediction of new disease emergence, associated with new pathogens or not, remains a difficult and controversial topic. The main factors driving epidemics are often only identified several years after outbreaks, generally revealing that a limited number of factors are associated with the emergence of specific groups of pathogens. This pattern is illustrated in the insect-borne xylem-limited bacterium Xylella fastidiosa, an organism associated with several new plant diseases in different regions of the globe. Research during the last decade focusing on several severe disease outbreaks has led to substantial changes in our understanding of X. fastidiosa biology, ecology, and evolution. This new information has not only led to new insights into aspects of the biology of this bacterium and its interactions with plant and insect hosts, but also made available a phylogenetic framework that has allowed for better inferences concerning factors leading to the emergence of diseases. Here we identify and discuss these main pathways leading to epidemics caused by X. fastidiosa. Our ultimate goal was to raise critical questions and issues for academics and regulatory agencies alike, since the information generated during the last decade has both raised new questions but also clarified old ones.

Intercepted isolates of Xylella fastidiosa in Europe reveal novel genetic diversity

“…Virtually nothing is known about ST72 and ST76. In addition, these data raise important questions about our knowledge of X. fastidiosa diversity, an important but ignored issue that has been previously discussed (Almeida and Nunney 2015).…”

“…The current scheme, which is generally accepted by the community, is based on MLST data (Almeida and Nunney 2015). The MLST approach provided a necessary framework for X. fastidiosa diversity to be studied in the presence of high rates of homologous recombination, but formal boundaries among subspecies were never proposed.…”

“…More recently, detection of X. fastidiosa throughout Corsica highlighted some of the challenges involved in detection and surveillance of a pathogen previously considered absent from the EU. Accordingly, many X. fastidiosa diseases are the result of long-distance introductions, most likely via contaminated plant material (Almeida and Nunney 2015). Examples include a North American genotype causing plum leaf scald in South America (Nunes et al 2003), a Pierce's disease of grapevines genotype from the United States in Taiwan (Su et al 2013), with the United States Pierce's disease clade itself having been introduced from Central America .…”

Spatial Genetic Structure of Coffee-Associated Xylella fastidiosa Populations Indicates that Cross Infection Does Not Occur with Sympatric Citrus Orchards

“…Although until recently reported only in the Americas, there have been recent reports from Italy and France (Almeida 2016) and Iran (Amanifar et al 2014). Tentatively subdivided in five subspecies, this pathogen causes disease in a variety of crops such as citrus, grapevine, almond, and coffee; trees such as elm, oak, mulberry, and sycamore; as well as ornamental species including oleander (Almeida and Nunney 2015). X. fastidiosa subsp.…”

“…X. fastidiosa subsp. pauca, which was thought to be limited in distribution to South America, has more recently been reported in Central America as well as Europe (Almeida and Nunney 2015). In Brazil, there are three main crops affected by X. fastidiosa: citrus, coffee, and plum.…”