Causal Role of Xylella fastidiosa in Oleander Leaf Scorch Disease

136

Strains of Xylella fastidiosa isolated from grape, almond, maple, and oleander were characterized by enterobacterial repetitive intergenic consensus sequence-, repetitive extragenic palindromic element (REP)-, and random amplified polymorphic DNA (RAPD)-PCR; contour-clamped homogeneous electric field (CHEF) gel electrophoresis; plasmid content; and sequencing of the 16S-23S rRNA spacer region. Combining methods gave greater resolution of strain groupings than any single method. Strains isolated from grape with Pierce's disease (PD) from California, Florida, and Georgia showed greater than previously reported genetic variability, including plasmid contents, but formed a cluster based on analysis of RAPD-PCR products, NotI and SpeI genomic DNA fingerprints, and 16S-23S rRNA spacer region sequence. Two groupings of almond leaf scorch (ALS) strains were distinguished by RAPD-PCR and CHEF gel electrophoresis, but some ALS isolates were clustered within the PD group. RAPD-PCR, CHEF gel electrophoresis, and 16S-23S rRNA sequence analysis produced the same groupings of strains, with RAPD-PCR resolving the greatest genetic differences. Oleander strains, phony peach disease (PP), and oak leaf scorch (OLS) strains were distinct from other strains. DNA profiles constructed by REP-PCR analysis were the same or very similar among all grape strains and most almond strains but different among some almond strains and all other strains tested. Eight of 12 ALS strains and 4 of 14 PD strains of X. fastidiosa isolated in California contained plasmids. All oleander strains carried the same-sized plasmid; all OLS strains carried the same-sized plasmid. A plum leaf scald strain contained three plasmids, two of which were the same sizes as those found in PP strains. These findings support a division of X. fastidiosa at the subspecies or pathovar level.Xylella fastidiosa (Wells et al.) (35) is a gram-negative, xylem-inhabiting bacterium that causes Pierce's disease (PD) of grape, phony peach disease (PP), periwinkle wilt, citrus variegated chlorosis, and leaf scorch diseases of almond (almond leaf scorch [ALS]), plum (PLS), elm, maple, oak (OLS), and sycamore (16, 27). All strains of X. fastidiosa are currently classified as one species but differ in important respects in plant host range and pathogenicity. The geographic isolation of the plant diseases caused by X. fastidiosa restricts the easy comparison of the pathological characteristics of the various strains. Previous studies differentiated X. fastidiosa strains on the basis of pathogenicity, nutritional requirements (16), DNA homology (19), structural protein analysis (3), restriction fragment length polymorphisms (RFLPs) (4), and random amplified polymorphic DNA (RAPD)-PCR (1, 6, 9, 25, 27). RFLP and DNA-DNA hybridization studies revealed distinct differences between PD strains and strains that cause PP, PLS, and periwinkle wilt (4, 19). Pooler et al. (25) distinguished five groups of X. fastidiosa using RAPD-PCR: the citrus group, plum-elm group, grape-ragweed group, almond gro...

Section: Resultsmentioning

confidence: 99%

Genetic Diversity of Pierce's Disease Strains and Other Pathotypes of Xylella fastidiosa

Hendson

Purcell

Chen

et al. 2001

136

“…1), but it has a high degree of synonymous-site differentiation from PRC (2.6%) and has very little internal variability (0.05%). In addition, the OLS and PRC strains cannot infect each other's major host plant, oleander and grapevine, respectively (29). For these reasons, placing the old and genetically distinct OLS clade within the X. fastidiosa subsp.…”

Section: Discussionmentioning

confidence: 99%

“…Xylem blockage by X. fastidiosa causes the plant scorch diseases of several economically important plants, such as grapevine, almond, and citrus (28). It also infects ornamentals, such as oleander (29). The known host range continues to expand, with over 100 host species identified (35) and with the most symptomatic hosts being plants nonnative to North or South America (23).…”

mentioning

confidence: 99%

A Multigene Phylogenetic Study of Clonal Diversity and Divergence in North American Strains of the Plant Pathogen Xylella fastidiosa

Schuenzel¹,

Scally²,

Stouthamer

et al. 2005

134

136

Xylella fastidiosa is a pathogen that causes leaf scorch and related diseases in over 100 plant species, including Pierce's disease in grapevines (PD), phony peach disease (PP), plum leaf scald (PLS), and leaf scorch in almond (ALS), oak (OAK), and oleander (OLS). We used a high-resolution DNA sequence approach to investigate the evolutionary relationships, geographic variation, and divergence times among the X. fastidiosa isolates causing these diseases in North America. Using a large data set of 10 coding loci and 26 isolates, the phylogeny of X. fastidiosa defined three major clades. Two of these clades correspond to the recently identified X. fastidiosa subspecies piercei (PD and some ALS isolates) and X. fastidiosa subsp. multiplex (OAK, PP, PLS, and some ALS isolates). The third clade grouped all of the OLS isolates into a genetically distinct group, named X. fastidiosa subsp. sandyi. These well-differentiated clades indicate that, historically, X. fastidiosa has been a clonal organism. Based on their synonymous-site divergence (ϳ3%), these three clades probably originated more than 15,000 years ago, long before the introduction of the nonnative plants that characterize most infections. The sister clades of X. fastidiosa subsp. sandyi and X. fastidiosa subsp. piercei have synonymoussite evolutionary rates 2.9 times faster than X. fastidiosa subsp. multiplex, possibly due to generation time differences. Within X. fastidiosa subsp. multiplex, a low level (ϳ0.1%) of genetic differentiation indicates the recent divergence of ALS isolates from the PP, PLS, and OAK isolates due to host plant adaptation and/or allopatry. The low level of variation within the X. fastidiosa subsp. piercei and X. fastidiosa subsp. sandyi clades, despite their antiquity, suggests strong selection, possibly driven by host plant adaptation.Bacterial systematics is based on identifying species that reflect historical phenotypic clusters, while recognizing that clear species groups can be obscured by the action of genetic processes uncommon among eukaryotes that facilitate horizontal gene transfer from unrelated sources (11). Named bacterial species are often further subdivided by the recognition of distinct pathovars and strains that typically infect different hosts. This strain recognition has been facilitated by the availability of whole-genome DNA sequence data for the phylogenetic comparison of an increasing number of species. Within species, isolates can be compared across multiple loci, enabling strain diversity and genetic diversity to be quantified.Many species exhibit a high degree of genetic diversity among strains that infect the same host, and this pattern is typical of the gamma subdivision proteobacteria, e.g., Escherichia coli (17, 27), Vibrio cholerae (5), Pseudomonas stutzeri (36), Xanthomonas axonopodis pv. manihotis (30), and Xanthomonas fragariae (31). Xylella fastidiosa, another gamma subdivision proteobacterium, offers an opportunity to study genetic variability both between and within plant-host strains (pathovars...

“…For example, almond plants are susceptible to isolates from grape plants but not vice versa (3). Oleander and grape X. fastidiosa clades are monophyletic (47), but neither causes disease in cross-inoculation experiments (41). Therefore, biological relationships must be tested and cannot be directly inferred from evolutionary history alone.…”

mentioning

confidence: 99%

Genetic Structure and Biology of Xylella fastidiosa Strains Causing Disease in Citrus and Coffee in Brazil

Almeida

Nascimento²,

Chau

et al. 2008

121

Xylella fastidiosa is a vector-borne, plant-pathogenic bacterium that causes disease in citrus (citrus variegated chlorosis [CVC]) and coffee (coffee leaf scorch [CLS]) plants in Brazil. CVC and CLS occur sympatrically and share leafhopper vectors; thus, determining whether X. fastidiosa isolates can be dispersed from one crop to another and cause disease is of epidemiological importance. We sought to clarify the genetic and biological relationships between CVC-and CLS-causing X. fastidiosa isolates. We used cross-inoculation bioassays and microsatellite and multilocus sequence typing (MLST) approaches to determine the host range and genetic structure of 26 CVC and 20 CLS isolates collected from different regions in Brazil. Our results show that citrus and coffee X. fastidiosa isolates are biologically distinct. Cross-inoculation tests showed that isolates causing CVC and CLS in the field were able to colonize citrus and coffee plants, respectively, but not the other host, indicating biological isolation between the strains. The microsatellite analysis separated most X. fastidiosa populations tested on the basis of the host plant from which they were isolated. However, recombination among isolates was detected and a lack of congruency among phylogenetic trees was observed for the loci used in the MLST scheme. Altogether, our study indicates that CVC and CLS are caused by two biologically distinct strains of X. fastidiosa that have diverged but are genetically homogenized by frequent recombination.