AN EVOLUTIONARY PERSPECTIVE OF PIERCE'S DISEASE OF GRAPEVINE, CITRUS VARIEGATED CHLOROSIS, AND MULBERRY LEAF SCORCH DISEASES

Authors: CHEN, J - FLORIDA A&M UNIVERSITY; Hartung, John; CHANG, C - UNIVERSITY OF GEORGIA; VIDAVER, A - UNIVERSITY OF NEBRASKA

Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2002
Publication Date: 8/20/2002
Citation: Chen, J., Hartung, J.S., Chang, C.J., Vidaver, A. 2002. An evolutionary perspective of pierce's disease of grapevine, citrus variegated chlorosis, and mulberry leaf scorch diseases. Current Microbiology.

Interpretive Summary: Xylella fastidiosa causes Pierce's Disease of grapevine, which was first described from Southern California in the 1880's. In the past decade, new strains of this pathogen were identified in the Americas, and shown to cause important diseases of coffee, citrus, avocado and oleander. As the list of important diseases caused by X. fastidiosa continues to expand, this question arises: what is causing this pathogen to emerge to cause diseases in a rapidly expanding list of hosts? We have developed and compared genetic data on strains of the pathogen isolated from grapevines and mulberry in North America and citrus in South America. With this data, we separate the strains of the pathogen into three distinct groups that are correlated with both host and region of origin. We argue that a single strain of the pathogen is neither spreading nor adapting to new hosts. Instead we suggest that diverse strains of the pathogen are endemic in the New World in native vegetation. Old World crops, such as grapevine, citrus and coffee, have no natural resistance to these strains since the pathogen does not occur in the Old World. Thus when large-scale cultivation of such crops occurs in the New World, insects naturally transmit X. fastidiosa into the new crop from native plants. Inevitably, a strain of X. fastidiosa is selected from the native flora, and then causes an important disease on the introduced crop. In ecological terms, the pathogen acts to limit the 'invasion' of the ecosystem by the introduced agricultural species. This work will be of value to scientists interested in the evolutionary relationships between hosts and pathogens.

Technical Abstract: Xylella fastidiosa causes diseases on a growing list of economically important plants. An understanding of how xylellae diseases originated and evolved is important for disease prevention and management. In this study, we evaluated the phylogenetic relationships of X. fastidiosa strains from citrus, grapevine, and mulberry through the analyses of random amplified polymorphic DNAs (RAPDs) and conserved 16S rDNA genes. RAPD analysis emphasizes the vigorous genome-wide divergence of X. fastidiosa and detected three clonal groups of strains that cause Pierce's disease (PD) of grapevine, citrus variegated chlorosis (CVC), and mulberry leaf scorch (MLS). Analysis of 16S rDNA sequences also identified the PD and CVC groups, but with a less stable evolutionary tree. MLS strains were included in the PD group by the 16S rDNA analysis. The Asiatic origins of the major commercial grape and citrus cultivars suggest the recent evolution of both PD and CVC disease in North and South America, respectively, since X. fastidiosa is a New World organism. In order to prevent the development of new diseases caused by X. fastidiosa, it is important to understand the diversity of X. fastidiosa strains, how strains of X. fastidiosa select their hosts, and their ecological roles in the native vegetation.