Seasonal abundance of Draeculacephala minerva and other Xylella fastidiosa vectors in California almond orchards and vineyards

Almond leaf scorch (ALS) disease is caused by the bacterium Xylella fastidiosa and transmitted by xylem-feeding insects. Reports of increased incidence of ALS-diseased trees in California prompted surveys in three almond [Prunus dulcis (Mill.) D. A. Webb]-growing regions, from June 2003 to September 2005, to determine insect vector species composition and abundance. For comparison, sampling in and near vineyards in the San Joaquin Valley, California, also was completed. Sampling in or near almond orchards collected >42,000 Cicadomorpha of which 4.8% were xylem feeders, including 1912 grass sharpshooter, Draeculacephala minerva Ball; five Xyphon fulgida Nottingham; and a single spittlebug, Philaenus spumarius L. The most abundant vector was D. minerva. Season-long sampling indicated that D. minerva was a year-round resident in and/or near almonds in the Sacramento Valley, but not in the San Joaquin Valley. Similarly, D. minerca was rare in vineyards in the San Joaquin Valley, but was abundant in irrigated pastures near vineyards. D. minerva was most frequently collected along orchard margins, and peak densities were observed in summer, the period of time when bacterial titers are reported to increase in infected trees. Screening of D. minerva for presence of X.fastidiosa found that 1.1% of insects collected near almond orchards and 4.5% of insects collected from pastures tested positive. The X. fastidiosa subspecies and genotype detected in insects collected from orchards matched those collected from ALS-diseased almond trees in the same orchard. Of the few X. fulgida and P. spumarius collected, none tested positive for X. fastidiosa. Results are discussed with respect to X. fastidiosa vector control and detection methods.     

Seasonal Behavior of Xylella fastidiosa Causing Almond Leafscorch Disease under Field Conditions and Improved Detection of the Bacteria by Means of Array-PCR

Almond leaf scorch disease (ALSD) caused by Xylella fastidiosa is potentially a serious threat to the almond industry in San Joaquin Valley of California. Knowledge of X. fastidiosa behaviour in the plant host under field conditions is important for disease control and this issue is being addressed in this project. Occurrence of ALSD is strongly influenced by environmental factors. In 2006, the earliest leaf scorching symptoms were observed in June, whereas in 2007, the earliest occurrence of leaf scorching symptoms was in July, a delay of 1 month. In both years, PCR detected X. fastidiosa 1 month before of symptom expression. PCR was slightly more sensitive than cultivation method for early bacterial detection. However, uneven bacterial distribution and random sampling errors may have contributed to the differences among the assays. Correlation between cultivation and PCR detection was greater than 90%. During the processing of a large number of samples, we noticed occasional failures in PCR amplifications of some samples, interfering result interpretation. We developed an array-PCR protocol using primers from seven housekeeping genes to correct the deficiency.     

High throughput PCR detection of Xylella fastidiosa directly from almond tissues

Xylella fastidiosa, the causal agent of almond leaf scorch disease (ALSD), is currently re-emerging as a serious concern in California. Efficient pathogen detection is critical for ALSD epidemiological studies, particularly when a large sample size is involved. We here report a PCR procedure to detect X. fastidiosa directly from infected almond tissue without the laborious DNA extraction. Plant samples were prepared by freeze-drying and pulverized. Appropriate dilutions of the pulverized freeze-dried tissue (PFT) were determined to minimize the effect of enzyme inhibitors from plant tissue and retain PCR detection of X. fastdiosa cells at a single digit number level. This PFT-PCR procedure was evaluated by comparing to the in vitro cultivation method using 102 symptomatic samples and resulted in a predictive value of 90.8%. PFT-PCR was further applied to monitor the seasonal occurrence of X. fastidiosa from four selected almond trees in two orchards in 2005. The results matched with those of the cultivation method at 92.3%. Considering the simplicity and reliability, we conclude that PFT-PCR is a valuable option for high throughput rapid detection of X. fastidiosa.     

Evaluation of monitoring traps with novel bait for navel orangeworm (Lepidoptera: Pyralidae) in California almond and pistachio orchards

Experiments conducted in three almond, Prunus dulcis (Rosales: Rosaceae), orchards and three pistachio, Pistacia vera (Sapindales: Anicardiaceae), orchards in 2009 and 2010, and determined that sticky bottom wing traps baited with ground pistachio mummies, or a combination of ground pistachio plus ground almond mummies, trapped more adult female navel orangeworm, Amyelois transitella (Walker) (Lepidoptera: Pyralidae), than did traps baited with ground almond mummies alone. During both years of this study, 2.9 and 1.8 more moths were caught in traps baited with pistachio mummies compared with traps baited with almond mummies in almond orchards and pistachio orchards, respectively. Also, traps located in pistachio orchards caught 5.9 and 8.3 times more navel orangeworm than were trapped from almond orchards in 2009 and 2010, respectively. Implications for use of this novel baited trap in almond and pistachio orchard integrated pest management programs are discussed.     

A scalable and spatiotemporally resolved agricultural life cycle assessment of California almonds

Purpose

California’s Central Valley produces more than 75% of global commercial almond supply, making the life cycle performance of almond production in California of global interest. This article describes the life cycle assessment of California almond production using a Scalable, Process-based, Agronomically Responsive Cropping System Life Cycle Assessment (SPARCS-LCA) model that includes crop responses to orchard management and modeling of California’s water supply and biomass energy infrastructure.

Methods

A spatially and temporally resolved LCA model was developed to reflect the regional climate, resource, and agronomic conditions across California’s Central Valley by hydrologic subregion (San Joaquin Valley, Sacramento Valley, and Tulare Lake regions). The model couples a LCA framework with region-specific data, including water supply infrastructure and economics, crop productivity response models, and dynamic co-product markets, to characterize the environmental performance of California almonds. Previous LCAs of California almond found that irrigation and management of co-products were most influential in determining life cycle CO₂eq emissions and energy intensity of California almond production, and both have experienced extensive changes since previous studies due to drought and changing regulatory conditions, making them a focus of sensitivity and scenario analysis.

Results and discussion

Results using economic allocation show that 1 kg of hulled, brown-skin almond kernel at post-harvest facility gate causes 1.92 kg CO₂eq (GWP₁₀₀), 50.9 MJ energy use, and 4820 L freshwater use, with regional ranges of 2.0–2.69 kg CO₂eq, 42.7–59.4 MJ, and 4540–5150 L, respectively. With a substitution approach for co-product allocation, 1 kg almond kernel results in 1.23 kg CO₂eq, 18.05 MJ energy use, and 4804 L freshwater use, with regional ranges of 0.51–1.95 kg CO₂eq, 3.68–36.5 MJ, and 4521–5140 L, respectively. Almond freshwater use is comparable with other nut crops in California and globally. Results showed significant variability across subregions. While the San Joaquin Valley performed best in most impact categories, the Tulare Lake region produced the lowest eutrophication impacts.

Conclusion

While CO₂eq and energy intensity of almond production increased over previous estimates, so too did credits to the system for displacement of dairy feed. These changes result from a more comprehensive model scope and improved assumptions, as well as drought-related increases in groundwater depth and associated energy demand, and decreased utilization of biomass residues for energy recovery due to closure of bioenergy plants in California. The variation among different impact categories between subregions and over time highlight the need for spatially and temporally resolved agricultural LCA.

     

A multigene phylogenetic study of clonal diversity and divergence in North American strains of the plant pathogen Xylella fastidiosa

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 ( approximately 3%), 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 synonymous-site 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 ( approximately 0.1%) 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.     

Evaluation of Arabidopsis thaliana as a model host for Xylella fastidiosa

The bacterium Xylella fastidiosa causes a number of plant diseases of significant economic impact. To date, progress determining mechanisms of host-plant susceptibility, tolerance, or resistance has been slow, due in large part to the long generation time and limited available genetic resources for grape, almond, and other known hosts of X. fastidiosa. To overcome many of these limitations, Arabidopsis thaliana has been evaluated as a host for X. fastidiosa. A pin-prick inoculation method has been developed to infect Arabidopsis with X. fastidiosa. Following infection, X. fastidiosa multiplies and can be detected by microscopy, polymerase chain reaction, and isolation. The ecotypes Van-0, LL-0, and Tsu-1 all allow more growth of strain X. fastidiosa Temecula than the reference ecotype Col-0. Affymetrix ATH1 microarray analysis of inoculated vs. noninoculated Tsu-1 reveals gene expression changes that differ greatly from changes seen after infection with apoplast-colonizing bacteria such as Psuedomonas syringae pvs. tomato or syringae. Many genes responsive to oxidative stress are differentially regulated, while classic pathogenesis-related genes are not induced by X. fastidiosa infection.     

Detection of Xylella fastidiosa in potential insect vectors by immunomagnetic separation and nested polymerase chain reaction

A sensitive and specific assay for detecting Xylella fastidiosa in potential insect vectors was developed. This assay involves immunomagnetic separation of the bacteria from the insect, followed by a two-step, nested polymerase chain reaction (PCR) amplification using previously developed oligonucleotide primers specific to X. fastidiosa . A total of 347 leafhoppers representing 16 species were captured and sampled from American elm ( Ulmus americana L.) trees growing in a nursery where bacterial leaf scorch caused by X. fastidiosa occurs. Two of these leafhopper species, Graphocephala coccinea and G. versuta , regularly tested positive for X. fastidiosa using this technique. These insects are therefore potential vectors of X. fastidiosa . Using immunocapture and nested PCR, it was possible to detect as few as five bacteria per sample.     

Biological traits of Xylella fastidiosa strains from grapes and almonds

Xylella fastidiosa is a xylem-limited bacterium that causes various diseases, among them Pierce's disease of grapevine (PD) and almond leaf scorch (ALS). PD and ALS have long been considered to be caused by the same strain of this pathogen, but recent genetic studies have revealed differences among X. fastidiosa isolated from these host plants. We tested the hypothesis that ALS is caused by PD and ALS strains in the field and found that both groups of X. fastidiosa caused ALS and overwintered within almonds after mechanical inoculation. Under greenhouse conditions, all isolates caused ALS and all isolates from grapes caused PD. However, isolates belonging to almond genetic groupings did not cause PD in inoculated grapes but systemically infected grapes with lower frequency and populations than those belonging to grape strains. Isolates able to cause both PD and ALS developed 10-fold-higher concentrations of X. fastidiosa in grapes than in almonds. In the laboratory, isolates from grapes overwintered with higher efficiency in grapes than in almonds and isolates from almonds overwintered better in almonds than in grapes. We assigned strains from almonds into groups I and II on the basis of their genetic characteristics, growth on PD3 solid medium, and bacterial populations within inoculated grapevines. Our results show that genetically distinct strains from grapes and almonds differ in population behavior and pathogenicity in grapes and in the ability to grow on two different media.     

Spatial and temporal dynamics of overwintering Homalodisca coagulata (Hemiptera: Cicadellidae)

A 4-yr landscape-scale study was conducted to investigate spatial and temporal dynamics of overwintering Homalodisca coagulata (Say) (Hemiptera: Cicadellidae) in the lower San Joaquin Valley, California. Spatial structures of H. coagulata distributions were characterized with Moran's I index, and spatial associations between H. coagulata and the surrounding environment were investigated with a geographic information system. H. coagulata was caught consistently with sticky traps throughout the winter, and trap catches formed a distinctive peak in December or January, indicating active flight of H. coagulata during the winter. In 2000-2001, the mean +/-SE trap count was 4.8 +/- 1.21 per trap per wk, and H. coagulata trap catches were spatially autocorrelated within approximately 1.3 km. Approximately 49% of H. coagulata were caught in citrus, 23% in stone fruit, and 11% in grape. After a control program began in spring 2001, the mean trap count was considerably lower (0.041 +/- 0.0004 per trap per wk), and no spatial autocorrelations were detected in 2001-2004. H. coagulata trap catch-crop associations also changed after initiation of the control program. Between 25 and 38% of H. coagulata trap catches were from citrus, between 8 and 20% were from stone fruit, and between 11 and 25% were from grape. Potential for winter-season spread and management of Xylella fastidiosa Wells et al., a pathogen causing Pierce's disease, are discussed.     

Context-dependent transmission of a generalist plant pathogen: host species and pathogen strain mediate insect vector competence

The specificity of pathogen–vector–host interactions is an important element of disease epidemiology. For generalist pathogens, different pathogen strains, vector species, or host species may all contribute to variability in disease incidence. One such pathogen is Xylella fastidiosa Wells et al., a xylem-limited bacterium that infects dozens of crop, ornamental, and native plants in the USA. This pathogen also has a diverse vector complex and multiple biologically distinct strains. We studied the implications of diversity in this pathogen–vector–host system, by quantifying variability in transmission efficiency of different X. fastidiosa strains (isolates from almond and grape genetic groups) for different host plants (grape, almond, and alfalfa) by two of the most important vectors in California: glassy-winged sharpshooter [ Homalodisca vitripennis (Germar)] and green sharpshooter ( Draeculacephala minerva Ball) (both Hemiptera: Cicadellidae). Transmission of isolates of the almond strain by H. vitripennis did not differ significantly, whereas transmission varied significantly among isolates from the grape strain (15–90%). Host plant species did not affect H. vitripennis transmission. Conversely, D. minerva efficiency was mediated by both host plant species and pathogen strain. No acquisition of an almond isolate occurred regardless of plant type (0/122), whereas acquisition of a grape isolate from alfalfa was 10-fold higher than from grape or almond plants. These results suggest that pathogen, vector, and host diversity impose contingencies on the transmission ecology of this complex disease system. Studies aimed at the development of management strategies for X. fastidiosa diseases should consider the complexity of these interactions as they relate to disease spread.     

Xylella fastidiosa subspecies: X. fastidiosa subsp. [correction] fastidiosa [correction] subsp. nov., X. fastidiosa subsp. multiplex subsp. nov., and X. fastidiosa subsp. pauca subsp. nov

Xylella fastidiosa, a fastidious bacterium causing disease in over 100 plant species, is classified as a single species, although genetic studies support multiple taxons. To determine the taxonomic relatedness among strains of X. fastidiosa, we conducted DNA-DNA relatedness assays and sequenced the 16S-23S intergenic spacer (ITS) region using 26 strains from 10 hosts. Under stringent conditions (Tm -15 degrees C), the DNA relatedness for most X. fastidiosa strains was *70%. However, at high stringency (Tm -8 degrees C), three distinct genotypes (A, B, and C) were revealed. Taxon A included strains from cultivated grape, alfalfa, almond (two), and maple, interrelated by 85% (mean); taxon B included strains from peach, elm, plum, pigeon grape, sycamore, and almond (one), interrelated by 84%; and taxon C included only strains from citrus, interrelated by 87%. The mean reciprocal relatedness between taxons A and B, A and C, and B and C, were 58, 41, and 45%, respectively. ITS results also indicated the same grouping; taxons A and B, A and C, and B and C had identities of 98.7, 97.9, and 99.2%, respectively. Previous and present phenotypic data supports the molecular data. Taxon A strains grow faster on Pierce's disease agar medium whereas B and C strains grow more slowly. Taxon B and C strains are susceptible to penicillin and resistant to carbenicillin whereas A strains are opposite. Each taxon can be differentiated serologically as well as by structural proteins. We propose taxons A, B, and C be named X. fastidiosa subsp. fastidiosa [correction] subsp. nov, subsp. multiplex, subsp. nov., and subsp. pauca, subsp. nov., respectively. The type strains of the subspecies are subsp. fastidiosa [correction] ICPB 50025 (= ATTC 35879T and ICMP 15197), subsp. multiplex ICPB 50039 (= ATTC 35871 and ICMP 15199), and subsp. pauca ICPB 50031 (= ICMP 15198).     

Effect of bait formulation and number of traps on detection of navel orangeworm (Lepidoptera: Pyralidae) oviposition using egg traps

Egg traps are the primary tool for monitoring egg deposition of the navel orangeworm, Amyelois transitella (Walker) (Lepidoptera: Pyralidae), and for timing treatments for this pest in almonds, Prunus amygdalus Batsch, and pistachios, Pistacia vera L. We compared, in almond and pistachio orchards, the number of eggs per trap in traps baited with almond meal, pistachio meal, or the current standard commercial bait. When considering cumulative eggs captured over an extended period, traps baited with pistachio meal prepared from previous-crop nuts generally captured a similar number of eggs compared with the commercial bait, and more eggs than those baited with almond meal prepared from previous-crop nuts. However, differences in eggs per trap between bait formulations were not as evident when examining individual weeks, particularly in weeks with few eggs per trap, as is typical when treatment decisions are made. The variance in eggs per trap was generally greater than the mean and increased with the mean and, when mean eggs per trap was low, most traps did not have eggs. We discuss implications of these findings for the relative importance of bait type and trap numbers for monitoring, and for experiments comparing egg trap performance.     

Microbial Transformations of Selenium and the Bioremediation of Seleniferous Environments

Selenium (Se) exists in many natural soil and water environments around the world, but anthropological activities such as irrigated agriculture on Se-laden soils has created many ecological problems with respect to this element. Seleniferous agricultural drainage water in California's San Joaquin Valley has been linked to the death and deformity of waterfowl. In the environment, microbial reduction, oxidation, methylation, and demethylation reactions predominantly control the oxidation state of Se and its subsequent behavior. In an effort to remediate these Se-contaminated environments a number of biological technologies have been investigated. Biological transformations of toxic Se oxyanions into less toxic or biologically unavailable forms, such as elemental Se or volatile Se compounds, has received much attention over the last decade. In this literature review, a major emphasis is placed on Se reduction and methylation/volatilization reactions because these processes are currently the most promising techniques being investigated for the bioremediation of seleniferous soil and water.     

Evaluation of the genetic structure of Xylella fastidiosa populations from different Citrus sinensis varieties

Xylella fastidiosa was isolated from sweet orange plants (Citrus sinensis) grown in two orchards in the northwest region of the Brazilian state of S?o Paulo. One orchard was part of a germ plasm field plot used for studies of citrus variegated chlorosis resistance, while the other was an orchard of C. sinensis cv. Pêra clones. These two collections of strains were genotypically characterized by using random amplified polymorphic DNA (RAPD) and variable number of tandem repeat (VNTR) markers. The genetic diversity (H(T)) values of X. fastidiosa were similar for both sets of strains; however, H(T)(RAPD) values were substantially lower than H(T)(VNTR) values. The analysis of six strains per plant allowed us to identify up to three RAPD and five VNTR multilocus haplotypes colonizing one plant. Molecular analysis of variance was used to determine the extent to which population structure explained the genetic variation observed. The genetic variation observed in the X. fastidiosa strains was not related to or dependent on the different sweet orange varieties from which they had been obtained. A significant amount of the observed genetic variation could be explained by the variation between strains from different plants within the orchards and by the variation between strains within each plant. It appears, therefore, that the existence of different sweet orange varieties does not play a role in the population structure of X. fastidiosa. The consequences of these results for the management of sweet orange breeding strategies for citrus variegate chlorosis resistance are also discussed.     

Multilocus sequence type system for the plant pathogen Xylella fastidiosa and relative contributions of recombination and point mutation to clonal diversity

Multilocus sequence typing (MLST) identifies and groups bacterial strains based on DNA sequence data from (typically) seven housekeeping genes. MLST has also been employed to estimate the relative contributions of recombination and point mutation to clonal divergence. We applied MLST to the plant pathogen Xylella fastidiosa using an initial set of sequences for 10 loci (9.3 kb) of 25 strains from five different host plants, grapevine (PD strains), oleander (OLS strains), oak (OAK strains), almond (ALS strains), and peach (PP strains). An eBURST analysis identified six clonal complexes using the grouping criterion that each member must be identical to at least one other member at 7 or more of the 10 loci. These clonal complexes corresponded to previously identified phylogenetic clades; clonal complex 1 (CC1) (all PD strains plus two ALS strains) and CC2 (OLS strains) defined the X. fastidiosa subsp. fastidiosa and X. fastidiosa subsp. sandyi clades, while CC3 (ALS strains), CC4 (OAK strains), and CC5 (PP strains) were subclades of X. fastidiosa subsp. multiplex. CC6 (ALS strains) identified an X. fastidiosa subsp. multiplex-like group characterized by a high frequency of intersubspecific recombination. Compared to the recombination rate in other bacterial species, the recombination rate in X. fastidiosa is relatively low. Recombination between different alleles was estimated to give rise to 76% of the nucleotide changes and 31% of the allelic changes observed. The housekeeping loci holC, nuoL, leuA, gltT, cysG, petC, and lacF were chosen to form the basis of a public database for typing X. fastidiosa (www.mlst.net). These loci identified the same six clonal complexes using the strain grouping criterion of identity at five or more loci with at least one other member.     

Transmission efficiency of Xylella fastidiosa 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.     

Effects of grazing and invasive grasses on desert vertebrates in California

Much of California's San Joaquin Valley is a desert and, like portions of other North American deserts, is experiencing an ecological shift from being dominated by ephemeral native forbs, with widely spaced shrubs, to fire-prone non-native annual grasses. Small terrestrial vertebrates, many of which are adapted to open desert habitats, are declining. One hypothesis is that the invasive plants contribute to the decline by altering vegetative structure. Although cattle may have originally been a factor in the establishment of these non-native plants, their grazing may benefit the terrestrial vertebrates by maintaining an open structure, especially during average or wet winters when the exotic grasses grow especially dense. We experimentally tested the effect of cattle grazing on invasive plants and a community of small vertebrates at a site in the southwestern San Joaquin Desert. We established and monitored 4 treatment (grazed) and 4 control (ungrazed) plots from 1997 to 2006, and assessed the abundances of blunt-nosed leopard lizards (Gambelia sila), giant kangaroo rats (Dipodomys ingens), short-nosed kangaroo rats (Dipodomys nitratoides nitratoides), and San Joaquin antelope squirrels (Ammospermophilus nelsoni), all of which are listed as threatened or endangered by state or federal agencies. We also recorded abundances of the non-protected western whiptail lizards (Aspidoscelis tigris), side-blotched lizards (Uta stansburiana), San Joaquin pocket mice (Perognathus inornatus inornatus), and Heermann's kangaroo rats (Dipdomys heermanni). Based on repeated measures analysis of variance (ANOVA) and a 0.05 alpha level, only Heermann's kangaroo rats showed a treatment effect; they were more abundant on the control plots. However, this effect could be accounted for by the natural re-establishment of saltbush (Atriplex spp.) on part of the study site. Saltbush return also favored western whiptail lizards and San Joaquin antelope squirrels. A regression analysis indicated that populations of blunt-nosed leopard lizard and giant kangaroo rat increased significantly faster in grazed plots than the ungrazed controls, and abundances of 6 of 8 species were negatively correlated with increased residual dry matter. With relaxed alpha values to decrease Type II error, populations of blunt-nosed leopard lizards (500% greater), San Joaquin antelope squirrels (85% greater), and short-nosed kangaroo rats (73% greater) increased significantly on grazed plots over the course of the study compared to ungrazed plots. We did not find grazing to negatively affect abundance of any species we studied. When herbaceous cover is low during years of below average rainfall in deserts and other arid areas, grazing may not be necessary to maintain populations of small vertebrates. However, if cattle grazing is closely monitored in space and time to minimize adverse effects on the habitat, it could be an effective tool to control dense stands of non-native grasses and benefit native wildlife. © 2011 The Wildlife Society.     

Seasonal and Scale Size Relationships between Citricola Scale (Homoptera: Coccidae) and Its Parasitoid Complex (Hymenoptera: Chalcidoidea) on San Joaquin Valley Citrus

The phenology of citricola scale, Coccus pseudomagnoliarum (Kuwana), and its associated parasitoid complex were studied on citrus in the San Joaquin Valley of central California over the period April 1995–March 1997. A total of 10,237 parasitoid specimens of 10 species were collected. Two of these species, Marietta mexicana (Howard) and Encyrtus lecaniorum (Mayr), each recovered from individually isolated scales, represent new parasitoid records for citricola scale. A third species, Encarsia citrinus citrinus (Craw), may represent a new parasitoid record, but this requires further confirmation because a single (male) specimen was recovered from individually isolated scales. The three most dominant parasitoid species, Coccophagus lycimnia (Walker), Metaphycus helvolus (Compere), and Metaphycus luteolus (Timberlake), accounted for the majority (>97%) of the specimens recovered. In contrast to the situation on citrus in southern California, where citricola scale is under effective biological control and is very rarely seen, citricola scale on citrus in the San Joaquin Valley is reemerging as a major pest, especially in groves employing integrated pest management with minimal use of broad-spectrum insecticides. Possible reasons uncovered in this study for the lack of effective biological control of citricola scale in the San Joaquin Valley include: (i) reduced presence of Metaphycus spp. because of hyperparasitism by the heteronomous hyperparasitoid C. lycimnia; (ii) absence of alternate hosts for those species of Metaphycus present; and (iii) absence of hosts of suitable size for Metaphycus at critical times of the year. Recommendations for improving the level of biological control in the San Joaquin Valley are discussed.