Leaf Scorch Research Articles

Evidence that the Zebra Chip Disease and the Putative Causal Agent Can be Maintained in Potatoes by Grafting and In Vitro

In the last several years, a disorder of chipping potatoes that causes internal browning of raw tubers and very dark chip color when the tubers are fried has been described from the southwestern United States. The discoloration often shows as rays or stripes and the common name “zebra chip” (ZC) has been used to describe the disorder. Foliar symptoms include chlorosis, purpling of shoot tips, and leaf scorch. Aerial tubers are sometimes produced on diseased plants. The disease has been associated with the potato psyllid, Bactericera cockerelli, but a suspected pathogen that may be associated with the ZC disorder has only very recently been described. In our efforts to identify the causal agent we have been maintaining the disease in the greenhouse by tip grafting symptomatic potato shoots onto healthy plants of several commercially important cultivars of potatoes, including Atlantic, Russet Burbank, and Russet Norkotah. Most of the grafted plants subsequently developed ZC symptoms. Many of the grafted plants produced tubers with symptoms typical of the disease. In cultivar Atlantic, 104 of 138 grafted plants developed foliar symptoms of ZC and 72 of the grafted plants produced ZC-symptomatic daughter tubers. Grafted plants without foliar symptoms did not produce symptomatic tubers. Herein we report the results of these grafting experiments and describe the symptoms produced in grafted plants and daughter tubers. We have also maintained the disease by placing surface-sterilized symptomatic shoots onto growth medium in vitro. In addition, the putative ZC pathogen, a new Candidatus Liberibacter spp., has been detected in symptomatic grafted plants, symptomatic in vitro plants, and potato psyllids by PCR.

Photo-oxidative stress in coconut seedlings: early events to leaf scorching and seedling death

Experiments were conducted on coconut seedlings to delineate events of photo-oxidative stress damage. Studies on chlorophyll fluorescence indicated a clear case of excess light energy under high light conditions causing stress to coconut seedlings raised under coconut palms. Quantum yield of photo-chemistry of leaflets exposed to high light was significantly less than those under shade. Seedlings exposed to high light and then shifted to shade have shown significant improvement in quantum yield. Excess light energy harvested by chlorophyll antenna caused high non-photochemical quenching resulting in production of biologically toxic super oxide, hydrogen peroxide and hydroxyl radicals. It is apparent that photoinhibition of photosynthesis takes place due to i) PSII down regulation and ii) damage to PS II system in initial stages of exposure to excess light and under prolonged exposures inhibition is caused due to iii) chlorophyll bleaching and iv) damage to chloroplast and cell membrane integrity, followed by reduction in photosynthetically active leaf area because of scorching thus reducing canopy photosynthesis. Protein concentration in leaf tissue was higher in seedlings in high light conditions. Three distinct low molecular weight proteins with pI of 4.9, 8.4 and 10.15 having Mr less than 20,000 were found in seedlings exposed to high light intensities. Results clearly demonstrate the events that take place at early stage to subsequent cascading effects leading to the scorching and death of leaf and even seedling death under severe conditions.

Specific Detection and Identification of Xylella fastidiosa Strains Causing Oleander Leaf Scorch Using Polymerase Chain Reaction

A pair of PCR primers, QH-OLS05/QH-OLS08 specific for strains of Xylella fastidiosa causing oleander leaf scorch was developed. The primers were designed according to a DNA sequence of a randomly amplified polymorphic DNA (RAPD)-polymerase chain reaction (PCR) product unique to oleander strains. The PCR assay using primer pair QH-OLS05/QH-OLS08 allowed quick and simple detection and identification of oleander strains in cultured bacterium and infected plant samples. The assay also can be applied to insect samples. Specific detection and identification of oleander strains of X. fastidiosa by PCR is useful for epidemiologic and etiologic studies of oleander leaf scorch by identifying what plants and insect vectors harbor or carry this particular strain of X. fastidiosa, especially in locations where mixed natural infections by oleander and other strains of X. fastidiosa occur.

Evaluation of a Simple Method to Screen Soybean Genotypes for Salt Tolerance

Excessive salt can reduce soybean yield [Glycine max (L.) Merr.] in grower fields. Salt‐tolerant cultivars are needed to prevent field yield losses where excess salt is a problem. Soybean genotypes have primarily been evaluated for reaction to salt in the greenhouse using a labor‐intensive and costly hydroponics method. A reliable, inexpensive method to screen soybean lines for salt tolerance would be useful in breeding programs. A simple, inexpensive method of exposing V2 to V3 plants growing in a sandy soil in plastic cone‐tainers (PC method) was compared to the hydroponics method to evaluate soybean genotypes for salt tolerance in the greenhouse. Fourteen soybean genotypes including checks ‘Hutcheson’ (sensitive), and ‘S‐100’ and ‘Forrest’ (tolerant) were exposed to 100 mM salt solution at the V2 to V3 growth stage. Genotypes responded similarly to the screening methods except leaf scorch, an indicator of salt damage, appeared up to 4 d sooner in the PC method. The PC method was highly correlated with the hydroponics method for leaf scorch score and chloride content in soybean leaves among the 14 genotypes evaluated. Exposure of roots at the V2 to V3 growth stage to salt solution using the PC method was less labor intensive, consumed less time, was less costly, and gave similar results compared with the hydroponics method. Thus, the PC method is an easy, reliable method to screen soybean genotypes for salt tolerance.

First Report of Xylella fastidiosa in Nerium oleander in Costa Rica.

Oleander (Nerium oleander L.) shrubs presenting mottling, leaf tip and margin scorch, short internodes, defoliation, and branch dieback were observed at different localities in the Central Valley in Costa Rica. Severity of the symptoms ranged widely, and most plants showed both diseased and healthy branches. In severe cases, entire sections of the plant were defoliated. Symptoms resembled those described for oleander leaf scorch (OLS) caused by the bacterium Xylella fastidiosa in the United States (3). This bacterium has been reported in coffee and citrus plants in Costa Rica. Sixty plants from five different places were sampled and tested using ELISA (Agdia Inc., Elkhart, IN) against X. fastidiosa. Thirty-five plants showed absorbance mean value of duplicate wells greater than the mean of control wells plus three times the standard deviation, and therefore were considered positive. Thirty-three of the sixty samples were processed for an immunofluorescence assay modified from Carbajal et al. (1) with antibody to X. fastidiosa (Agdia Inc.). Thirteen samples showed fluorescent rod-shaped bacilli with morphology similar to those observed from a pure culture of X. fastidiosa obtained from coffee. Ten of these thirteen samples were positive by ELISA. DNA extracts (2) from three of the oleander plants with high ELISA absorbance values were tested by nested PCR with primer pair 272-1/272-2 followed by the pair 272-1 int/272-2 int (4). Two of the samples were positive for the bacterium and one of the PCR products was cloned and sequenced in both directions (GenBank Accession No. EU009615). The negative (PCR mix) and positive (pure culture of X. fastidiosa isolated from grapevine) controls for nested-PCR were indeed negative and positive, respectively. The BLAST program was used to compare the sequence to the nucleotide collection (nr/nt) and Microbe Assembled Genomes databases in GenBank. All matches corresponded to X. fastidiosa sequences. The sequence showed 97% similarity with strains Found-4 (coffee strain from Brazil) and Found-5 (citrus strain from Brazil) and 96% similarity with strain Ann-1 from oleander in California. On the basis of serological, microscopic, and molecular detection of X. fastidiosa from oleander exhibiting symptoms of OLS similar to those reported in the literature, this pathogen likely is causing the symptoms we observed in Costa Rica.

Transmission Efficiency of <I>Xylella fastidiosa</I> by Sharpshooters (Hemiptera: Cicadellidae) in Coffee and Citrus

Xylella fastidiosa (Wells, Raju, Hung, Weisburg, Mandelco-Paul, and Brenner) is a bacterial pathogen transmitted by several sharpshooters in two tribes of Cicadellinae (Proconiini and Cicadellini). Here, we compared the transmission efficiency of X. fastidiosa in coffee (Coffea arabica L.) and citrus [Citrus sinensis (L.) Osbeck] by Cicadellini [Bucephalogonia xanthophis (Berg) and Dilobopterus costalimai Young] and Proconiini [Homalodisca ignorata Melichar and Oncometopia facialis (Signoret)] sharpshooters that occur in both crops. At different seasons, healthy adults of each species were submitted to a 48-h acquisition access period on citrus or coffee source plants infected with X. fastidiosa isolates that cause Citrus variegated chlorosis (CVC) and Coffee leaf scorch (CLS), respectively, and then confined on healthy seedlings of the corresponding host plant for a 48-h inoculation access period. No significant effect of inoculation season was observed when comparing infection rates of citrus or coffee plants inoculated by vectors at different times of the year. In citrus, the transmission rate by single insects was significantly higher for H. ignorata (30%) in relation to B. xanthophis (5%) and O. facialis (1.1%), but there was no difference among vector species in coffee, whose transmission rates ranged from 1.2 to 7.2%. Comparing host plants, H. ignorata was more effective in transmitting X. fastidiosa to citrus (30%) in relation to coffee (2.2%), whereas the other vectors transmitted the bacterium to both hosts with similar efficiencies. Despite these variations, vector efficiency in coffee and citrus is lower than that reported in other hosts.

Hypervariations of a Protease-Encoding Gene, PD0218 ( pspB ), in Xylella fastidiosa Strains Causing Almond Leaf Scorch and Pierce's Disease in California

Xylella fastidiosa is a gram-negative plant pathogenic bacterium that causes almond leaf scorch disease (ALSD) and Pierce's disease (PD) of grape in many regions of North America and Mexico. Of the two 16S rRNA gene genotypes described in California, A genotype strains cause ALSD only and G genotype strains cause both PD and ALSD. While G genotype strains cause two different diseases, little is known about their genetic variation. In this study, we identified a putative protease locus, PD0218 (pspB), in the genome of X. fastidiosa and evaluated the variation at this locus in X. fastidiosa populations. PD0218 contains tandem repeats of ACDCCA, translated to threonine and proline (TP), upstream of the putative protease conserved domain. Among 116 X. fastidiosa ALSD and PD strains isolated from seven locations in California, tandem repeat numbers (TRNs) varied from 9 to 47, with a total of 30 TRN genotypes, indicating that X. fastidiosa possesses an active mechanism for contracting and expanding tandem repeats at this locus. Significant TRN variation was found among PD strains (mean = 29.9), which could be further divided into two TRN groups: PD-G(small) (mean = 17.3) and PD-G(large) (mean = 44.3). Less variation was found in ALSD strains (mean = 21.7). The variation was even smaller after ALSD strains were subdivided into the A and G genotypes (mean = 13.3, for the G genotype; mean = 27.1, for the A genotype). Genetic variation at the PD0218 locus is potentially useful for sensitive discrimination of X. fastidiosa strains. However, TRN stability, variation range, and correlation to phenotypes should be evaluated in epidemiological applications such as pathotype identification and delineation of pathogen origin.

Evaluation and Molecular Characterization of Beauveria bassiana for the Control of the Glassy-winged Sharpshooter (Homoptera: Cicadellidae) in California

The glassy-winged sharpshooter, Homalodisca coagulata (Say), is an important pest on grapes, citrus, almonds and other commercial crops in California as it is a vector of Xylella fastidiosa Wells, a bacterium that causes Pierce's disease in grapes, citrus variegated chlorosis, almond leaf scorch and other plant diseases. Various entomopathogenic fungi isolated from natural infections of H. coagulata, its habitats and other insect hosts were evaluated against this insect vector. Based on these studies, 3 isolates of the hyphomycetous fungus, Beauveria bassiana (Balsamo) Vuillemin, were selected for further evaluation. Two of these were California isolates, one each from the three-cornered alfalfa hopper, Spissistilus festinus (Say), and soil from H. coagulata habitat, and the third was a Texas isolate from natural infections of H. coagulata. All three were similar in their virulence to H. coagulata under laboratory conditions. The genetic relatedness of the B. bassiana isolates also was compared using single sequence repeat (SSR) markers which showed genetic diversity of this species based on the source of the isolate. Some isolates were 4× more infectious than others demonstrating that virulence of B. bassiana is not necessarily associated with their genetic relatedness.

First report of ‘Candidatus Phytoplasma mali’ in oriental lilies and its association with leaf scorch in Poland

First report of ‘Candidatus Phytoplasma mali’ in oriental lilies and its association with leaf scorch in Poland

Separate loci underlie resistance to root infection and leaf scorch during soybean sudden death syndrome

Soybean [Glycine max (L.) Merr.] cultivars show differences in their resistance to both the leaf scorch and root rot of sudden death syndrome (SDS). The syndrome is caused by root colonization by Fusarium virguliforme (ex. F. solani f. sp. glycines). Root susceptibility combined with reduced leaf scorch resistance has been associated with resistance to Heterodera glycines HG Type 1.3.6.7 (race 14) of the soybean cyst nematode (SCN). In contrast, the rhg1 locus underlying resistance to Hg Type 0 was found clustered with three loci for resistance to SDS leaf scorch and one for root infection. The aims of this study were to compare the inheritance of resistance to leaf scorch and root infection in a population that segregated for resistance to SCN and to identify the underlying quantitative trait loci (QTL). "Hartwig", a cultivar partially resistant to SDS leaf scorch, F. virguliforme root infection and SCN HG Type 1.3.6.7 was crossed with the partially susceptible cultivar "Flyer". Ninety-two F5-derived recombinant inbred lines and 144 markers were used for map development. Four QTL found in earlier studies were confirmed. One contributed resistance to leaf scorch on linkage group (LG) C2 (Satt277; P = 0.004, R2 = 15%). Two on LG G underlay root infection at R8 (Satt038; P = 0.0001 R2 = 28.1%; Satt115; P = 0.003, R2 = 12.9%). The marker Satt038 was linked to rhg1 underlying resistance to SCN Hg Type 0. The fourth QTL was on LG D2 underlying resistance to root infection at R6 (Satt574; P = 0.001, R2 = 10%). That QTL was in an interval previously associated with resistance to both SDS leaf scorch and SCN Hg Type 1.3.6.7. The QTL showed repulsion linkage with resistance to SCN that may explain the relative susceptibility to SDS of some SCN resistant cultivars. One additional QTL was discovered on LG G underlying resistance to SDS leaf scorch measured by disease index (Satt130; P = 0.003, R2 = 13%). The loci and markers will provide tagged alleles with which to improve the breeding of cultivars combining resistances to SDS leaf scorch, root infection and SCN HG Type 1.3.6.7.

Effects of Almond Leaf Scorch Disease on Almond Yield: Implications for Management.

Almond leaf scorch (ALS) disease has been present in California's almond-growing regions for over 60 years. This disease is caused by the bacterium Xylella fastidiosa and the pathogen is vectored by xylem-feeding sharpshooters and spittlebugs. Currently, there are no effective management techniques that prevent trees from becoming infected. Within affected orchards throughout California's Central Valley, disease incidence and the risk of tree-to-tree spread appears to be low. Consequently, the decision to remove or keep infected trees depends on lost productivity. We compared yield and vitality between infected and uninfected almond for cvs. Sonora and Nonpareil. Sonora was examined at three sites over 3 years and Nonpareil was examined at one site over 2 years. Yields of ALS-affected trees were significantly lower for both cultivars, although yield losses of Sonora were proportionally greater than those of Nonpareil. Yields of infected trees did not decline incrementally over years; rather, they fluctuated similarly to those of uninfected trees. In addition, no infected trees died during the course of the study. These results are in direct contrast to previous anecdotal reports which suggest that yields of infected trees incrementally decline and infected trees eventually die. A simple economic model was developed to determine conditions under which rouging infected trees would increase returns. Based on the model, orchard age, yield loss due to infection, and the value of a maximally producing almond tree should be considered when deciding to remove ALS-affected trees.

Conserved salt tolerance quantitative trait locus (QTL) in wild and cultivated soybeans

In the present study, we investigated salt tolerance heredity in wild soybean, Glycine soja Sieb & Zucc., and compared the salt tolerance quantitative trait locus (QTL) of G. soja with that of Glycine max (L.) Merr. An F2 population (n = 225) derived from a cross between the salt sensitive soybean cultivar Jackson (PI548657) and a salt-tolerant wild soybean accession (JWS156-1) was used. Evaluation of salt tolerance in the seedling stage was carried out in hydroponic culture with half-strength Hoagland and Arnon nutrient solution containing 120 mM NaCl. Visual ratings of symptoms based on leaf scorching and chlorophyll content (SPAD value) were taken for each plant 20 days after salt treatment. Both traits showed continuous distribution; however, salt-tolerant plants (i.e. plants with a high salt tolerance rating (STR) and SPAD value) were predominant. QTL analysis revealed a major salt-tolerant QTL with a large dominant effect, which accounted for 68.7% of the total variance of the STR scale, on the soybean linkage group N. Our results indicated that the salt tolerance QTL confers a large dominant effect over salt sensitivity and that the salt tolerance QTL is conserved in both wild and cultivated soybeans.

Analysis of the bacterial community in glassy‐winged sharpshooter heads

AbstractThe glassy‐winged sharpshooter (GWSS), Homalodisca vitripennis, is an important vector of various strains of Xylella fastidiosa, which cause disease in a variety of economically important plants. These diseases include citrus variegated chlorosis, oleander leaf scorch and Pierce's Disease of grapevines. Symbiotic control (SC) is a new strategy that uses symbiotic endophytes as biological control agents to antagonize or displace the pathogenic strains of X. fastidiosa. Candidate endophytes for use in SC must occupy the xylem of host plants and attach to the pre‐cibarium and cibarium of sharpshooter insects in order to have access to the pathogen. The study of the bacterial community of GWSS heads by isolation and denaturing gradient gel electrophoresis (DGGE) revealed the presence of species that may be suitable for use in SC. In addition, the results indicated that two important factors, insect age and choice of host plant, affect the composition of the bacterial community in GWSS heads. The main bacterial genera isolated as colonizers of GWSS heads were identified, using partial 16S rRNA gene sequencing, as Bacillus, Pseudomonas, Pedobacter and Methylobacterium, as well as the species Curtobacterium flaccumfaciens. DGGE patterns revealed a diversity of endophytic species able to colonize the GWSS head. The main genera isolated in culture were also identified using this technique. Principal component analysis (PCA) from polymerase chain reaction (PCR)‐DGGE patterns indicated that the bacteria inhabiting the GWSS head are similar to those found as endophytes inside the host plants, and that insect developmental stage and preferential feeding on one host plant species over another are important factors in determining the composition of the bacterial community in the GWSS head. However, a shift in host plants for a small period of time did not cause changes in the compositions of these communities.

Leaf scorch symptoms are not correlated with bacterial populations during Pierce's disease

Xylella fastidiosa (Xf) is a xylem-limited bacterium that lives as a harmless endophyte in most plant species but is pathogenic in several agriculturally important crops such as coffee, citrus, and grapevine (Vitis vinifera L.). In susceptible cultivars of grapevine, Xf infection results in leaf scorch, premature leaf senescence, and eventually vine death; a suite of symptoms collectively referred to as Pierce's disease. A qPCR assay was developed to determine bacterial concentrations in planta and these concentrations were related to the development of leaf-scorch symptoms. The concentration of Xf in leaves of experimental grapevines grown in the greenhouse was similar to the concentration of Xf in leaves of naturally infected plants in the field. The distribution of Xf was patchy within and among leaves. Some whole leaves exhibited severe leaf-scorch symptoms in the absence of high concentrations of Xf. Despite a highly sensitive assay and a range of Xf concentrations from 10(2) to 10(9) cells g(-1) fresh weight, no clear relationship between bacterial population and symptom development during Pierce's disease was revealed. Thus, high and localized concentrations of Xf are not necessary for the formation of leaf-scorch symptoms. The results are interpreted as being consistent with an atiology that involves a systemic plant response.

Bacterial Leaf Scorch of Landscape Trees: What We Know and What We Do Not Know

Xylella fastidiosa has been recognized as a pathogen of landscape trees for over 25 years. Collectively, these diseases are referred to as bacterial leaf scorch (BLS). Arborists, property owners, and communities are now beginning to recognize BLS as a serious threat to the urban forest. Although advances in symptom awareness and diagnostic techniques have enabled arborists to diagnose BLS, there are many questions regarding host range, transmission, pathogeneses, disease management, and individual tree therapy that remain unanswered.

Xylella fastidiosagenotype differentiation by SYBR®Green-based QRT-PCR

Pathotype differentiation within Xylella fastidiosa (Wells) is necessary to accurately identify the causal agents of several leaf scorch diseases in agronomic crops and ornamentals. Three strain groups, Pierce's disease (PD), almond leaf scorch (ALS), and oleander leaf scorch (OLS), have been described in California and inhabit multiple hosts with varied severity. An SYBR Green-based quantitative real-time PCR (QRT-PCR) protocol with a single primer set was developed for X. fastidiosa genotype differentiation utilizing melting temperature (T(m)) analysis. The goal of this work was to develop a rapid diagnostic tool that could separate X. fastidiosa strains from one another similar to the placement in previous reports. Placement of eight PD, six ALS, and six OLS strains into respective strain groups was consistent with T(m) curve analysis using a single primer set. These groupings were consistent with published data on the phylogenetic placement of the strains.

Genetic characterization of Xylella fastidiosa isolated from citrus and coffee plants

The Citrus Variegated Chlorosis and the Coffee Leaf Scorch are some of the many destructive diseases caused by Xylella fastidiosa, a gram-negative bacterium limited to the xylem of affected plants. As its genetic characterization is still not well established, different isolates of X. fastidiosa from citrus and coffee were evaluated through RAPD (Random Amplified Polymorphic DNA) technique to characterize and classify these isolates based on similarity coefficients. Sixteen isolates of X. fastidiosa were used on this trial, obtained from citrus, coffee and almond. The genetic polymorphism evaluation was performed using six arbitrary 10-base primer pairs. It was possible to establish a dendogram in which the isolates were classified into five groups (A, B, C, D and E). A prevalence of citrus isolates in groups A and D was observed. In groups B and C, there was a prevalence of coffee isolates meanwhile the group D consisted of the almond isolate, solely.

Natural occurrence of Xylella fastidiosa in a commercial nursery in Maryland

Bacterial leaf scorch caused by Xylella fastidiosa is a serious problem in the landscape, but its importance in the nursery trade is undetermined. Therefore, a survey was conducted in 2003 and 2004 in a commercial wholesale and production nursery in Maryland to determine the natural occurrence of X. fastidiosa in nursery plants and surrounding vegetation using enzyme-linked immunosorbent and nested polymerase chain reaction assays specific for the bacterium. Xylella fastidiosa was associated with crape myrtle (Lagerstroemia indica), Japanese stewartia (Stewartia pseudocamellia), Schipka laurel (Prunus laurocerasus ‘Schipkaensis’), and Japanese flowering cherry (Prunus serrulata ‘Kwanzan’) in the nursery, as well as with mimosa (Albizia julibrissin) and sassafras (Sassafrass albidum) growing in the perimeter of the nursery. All were previously unknown hosts of the bacterium. Xylella fastidiosa was also detected in dogwood (Cornus florida) and willow oak (Quercus phellos) in the nursery, as well as in red oak (Quercus spp.), box-elder (Acer negundo), and wild grape (Vitis spp.) surrounding the nursery, previously reported hosts of the bacterium. The red oak and box-elder plants on the edge of the nursery showed leaf scorch symptoms and harboured high populations of the bacterium. However, the Xylella-positive nursery plants and other plants from the local environment were symptomless, and the bacterial populations were low. These data suggest that red oak and box-elder are sources of the X. fastidiosa in the nursery plants.

Effect of the Growth Regulator Paclobutrazol on Growth of the Bacterial Pathogen Xylella fastidiosa Xylella fastidiosa

Xylella fastidiosa is a fastidious, xylem-limited, insect-transmitted, bacterial plant pathogen with a wide host range that causes bacterial leaf scorch (BLS) in shade trees. BLS is a chronic disorder characterized by late season leaf scorch and dieback and is common in urban and suburban areas of the mid-Atlantic and southeast United States. BLS has been recognized since the 1980s and attempted treatments have included antibiotics and plant growth regulators. Application of paclobutrazol (PBZ), a diastereomeric triazole with both fungistatic and growth regulation properties, has been observed to alleviate symptoms of BLS, but it has not been established whether PBZ has a direct effect on the organism. In this study, we investigated the effect of PBZ on in vitro growth of two X. fastidiosa isolates. Our results showed no significant effect of PBZ on colony growth of X. fastidiosa at the manufacturers recommended rate of 20 ‘ºg/mL ’àí1. However, significant reductions in bacterial growth were observed at a rate of 200 ‘ºg/mL ’àí1, indicating that high levels of PBZ may have a direct effect on the growth of X. fastidiosa. This direct effect and growth regulator effects of PBZs suggest that PBZ may provide a promising treatment for BLS in shade trees.

Phylogenetic Relationships of Xylella fastidiosa Strains Isolated from Landscape Ornamentals in Southern California

ABSTRACT Xylella fastidiosa is an insect-borne, xylem-limited pathogenic bacterium that has been associated with a rise in incidence of diseased landscape ornamentals in southern California. The objective of this study was to genetically characterize strains isolated from ornamental hosts to understand their distribution and identity. Strains of X. fastidiosa isolated from ornamentals were characterized using a multiprimer polymerase chain reaction (PCR) system, random amplified polymorphic DNA (RAPD)-PCR, and sequence analysis of the 16S-23S rDNA intergenic spacer region (ISR). Based on RAPD-PCR and 16S-23S rDNA ISR, strains isolated from daylily, jacaranda, and magnolia clustered with members of X. fastidiosa subsp. sandyi and caused oleander leaf scorch but not Pierce's disease symptoms in glasshouse assays on oleander and grape, respectively. This demonstrated both that our groupings based on genetic characterization were valid and that strains of X. fastidiosa subsp. sandyi are present in hosts other than oleander. Strains isolated from Spanish broom, cherry, and one strain isolated from western redbud clustered with X. fastidiosa subsp. fastidiosa members. Strains isolated from purple-leafed plum, olive, peach, plum, sweetgum, maidenhair tree, crape myrtle, and another western redbud strain clustered with members of X. fastidiosa subsp. multiplex. All strains isolated from mulberry and one from heavenly bamboo formed a separate cluster that has not yet been defined as a subspecies.