Search

In the salth-marshes of the Tinto River (Huelva estuary, SW Spain), are stored in stacks around 100 Mt of PG, covering a surface of 1000 ha without any type of isolation, which produce an important impact in the surrounding environment. On the other hand, this ecosystem it is affected by acid mine drainage (AMD) from sulphide mines located upstream the Tinto River. The aim of this study is to evaluate the deep pollution of the underlain salt-marsh sediments due to leachates from the PG stacks. For that purpose, 7 cores were collected from zones 2 and 3 of the stacks, and PG and salt-marsh sediments samples from different depths were analyzed. The physicochemical parameters, mineralogy, granulometry and the concentration of the main elements of interest were determined in the samples. Most analysed salt-marsh sediments are not affected by PG stacks pollution, because sediments act as a “barrier” for the leachates from the PG, concentrating the contaminants in the first decimetres (0.5 m) under PG-sediments contact, and the deep infiltration is very limited. The obtained results suggest that the perimeter channel which is projected to build in the restauration project, should has a depth of 1 m below the level of the PG stacks for assuring the complete collection of leachates from the stacks, and avoid their liberation into the Tinto River estuary.

Along the industrial process devoted to the production of titanium dioxide pigments by using ilmenite as main raw material, small residues amounts are generated, remaining clearly enriched in natural radionuclides and chemical pollutants. Between them, we can remark the scales enriched in both radium isotopes and lead, which are formed in the internal walls of pipes and some equipment. These scales are radiological anomalies that demand its mineralogical, elemental and radiometric characterization as a basis for a detailed radiological and toxicological assessment from the occupational and public point of view. In this work, several scales collected in a TiO2 pigment production plant in South of Spain have been mineralogically characterized by XRD, while information about their elemental composition and morphology have been obtained by applying the XRF and SEM techniques. In addition, radiometric determinations have been performed by gamma-ray and alpha-particle spectrometry. The performed study indicates that the radiological doses received due to the scales by the workers performing its conventional activities are clearly lower than 1 mSv/y. Special dosimetric and chemical controls could be needed for the workers in charge of the maintenance labours (which include the removal of the scales) if these workers belong to an external company devoted to perform maintenance operations in several NORM industries., This research was partially supported by the Spanish Government Department of Science and Technology (MINECO) through the project "Fluxes of Radionuclides Emitted by the Phosphogypsum Piles Located at Huelva; Assessment of the Dispersion, Radiological Risks and Remediation Proposals" (Ref. CTM 2015-68628- R), ), and the project of the Regional Government of Andalusia called “Basic processes regulating the fractionations and enrichments of natural radionuclides under acid mine drainage conditions” (Ref.: UHU-1255876) and the project of the University of Cadiz called "New methodogy of radioisotope isolation with environmental interest for its measurement by alpha spectrometry" (Ref: PR2019-024). The authors acknowledges to the staff of CITIUS at the University of Seville their help in the application of the X-ray techniques.

The accurate determination of the levels of 210Pb in environmental samples (e.g. atmospheric aerosols, waters, soils, biota, etc.) is essential due to its multiple applications in geochronology, radiation protection, groundwaters flows, air masses movement, etc. For this purpose, it has been developed a novel and thorough study of three alternative radiometric measuring methods of 210Pb activity concentrations in several types of samples. The three selected methods were low-level gamma spectrometry (210Pb), alpha-particle spectrometry (via 210Po), and Cherenkov counting (via 210Bi). This study has been performed in terms of precision, linearity and proportionality as well as spent time, type of matrix, and showing the most appropriate method of measure in each case. Several kinds of samples were analyzed such as atmospheric filters, soils, sediments and phosphogypsum (PG) matrices. These samples contain different 210 Pb activity concentrations and were measured by the three techniques carrying out radiochemical procedures when necessary. There were not significant differences in the results obtained for the samples analyzed by the three methods. The major precision was obtained by using alpha-particle spectrometry, but this method is more expensive and time consuming.

The aim of this paper is to characterize two HPGe gamma-ray detectors used in two different laboratories for environmental radioactivity measurements, so as to perform efficiency calibrations by means of Monte Carlo Simulation. To achieve such an aim, methodologies developed in previous papers have been applied, based on the automatic optimization of the model of detector, so that the differences between computational and reference FEPEs are minimized. In this work, such reference FEPEs have been obtained experimentally from several measurements of the IAEA RGU-1 reference material for specific source-detector arrangements. The models of both detectors built through these methodologies have been validated by comparing with experimental results for several reference materials and different measurement geometries, showing deviations below 10% in most cases.

The regulation existing on NORM activities requires the accurate and quick measurement of natural radionuclides in a wide range of matrices by gamma spectrometry. The aim of this work has been to develop a comprehensive and accurate methodology to determine natural long-lived radionuclides (210Pb, 228,226Ra, 234,228Th and 40K) by gamma spectrometry with Ge detectors in NORM samples. An exhaustive calibration method to obtain the full-energy peak efficiency (FEPE), for each gamma emission energy, as a function of the thickness, apparent density and matrix composition of the cylindrical sample, has been developed. The selected certified calibration standards contain only natural radionuclides belonging to the 238U- and 232Th-series as well as 40K (codes RGU-1, RGTh-1 and RGK-1 from IAEA). Then, the obtained FEPE curves were validated using Genie 2000 simulations and Certified Reference Materials, for which the self-attenuation corrections were considered. Finally, a study on 222Rn losses was done for several samples.

This paper is the second chapter of a piece entitled “Low-level determination of Th-isotopes by alpha spectrometry”. This Part 2 presents the results of the analysis of the first and last steps for Th-isotopes determination by alpha-particle spectrometry: the sample pre-treatment, total dissolution following different methods and lixiviation, and the test sample preparation methods, electrodeposition and microprecipitation, together with a brief analysis of the financial costs of the analyzed methods.

Thorium is an unusual radionuclide in laboratory intercomparisons and when it appears, the number of participants providing results is small and the results provided are not reliable enough. These facts are particularly significant for environmental and industrial solid samples. In this work, we analyze the most suitable method for Th determination by alpha-particle spectrometry, evaluating technical points and also considering cost, time and chemical reagents used. This work is divided in two parts. This part 1 deals with the analysis of different radiochemical separation methods for Th isotopes.

The industry devoted to the production of phosphoric acid by using as raw material sedimentary phosphate rock (PR) is considered as a NORM activity (Naturally Occurring Radioactive Materials), due to the high levels of U-series radionuclides contained in this ore, which are 1–2 orders of magnitude higher than those in unperturbed soils. This fact allowed us to develop a deep characterization of the raw materials, wastes, main intermediate materials, and final products obtained at a typical phosphoric acid factory. The elemental composition (major, minor and trace elements), radionuclide concentrations, grain size distribution, mineralogy and micro-structural composition were analyzed. The aim of this characterization was to obtain information for operators and maintenance personnel involved in clean-up and waste management operations. The highest concentrations of some heavy metals and radionuclide activity concentrations were found in the “scales” (or internal incrustations)from the pipes that carry either phosphoric acid (PA) or the phosphogypsum waste (PG). The highest concentrations where found for 226Ra and 40K,with values up to 9 and 5 Bq g−1, respectively. In addition, high concentrations of many toxic heavy metals and trace elements, such as Cd, Cr, Ni, Sr, Y, V, Zn, Th, and U, were found in some sludge samples. The shielding effect of the containers/vessels/pipes has an essential role in the measured external dose in the intermediate products. The radiological implications of natural radionuclides with higher activity showed that if the maximum particulate matter concentration established in the Spanish regulation is verified, and taking into account the most conservative scenario, the annual limit of 1 mSv y−1 is not exceeded.

Next to the city of Huelva (SW of Spain), around 100 Mt of phosphogypsum (PG) are stored in stacks on the salt-marshes of the Tinto River estuary covering a surface of about 1000 ha. Due to the high content of 238U series natural radionuclides of the PG, its acidic nature (pH about 3), and the fact that PG stacks were disposed without any kind of isolation from the substrate, they could produce a potential radioactive impact into the underlying sediments. The aim of this work is to assess the pollution of the underlying sediments by natural radionuclides coming from the PG stacks. To this end, seven cores were taken, and PG and sediments samples collected at different depths were analysed. The activity concentrations of the main long half-live natural radionuclides of interest were determined by applying both gamma-ray and alpha-particle spectrometry radiometric techniques. The results of this study showed that the first decimeters of salt-marsh sediment act as a “barrier” for the radionuclides coming from the PG stacks decreasing rapidly its activity concentration in depth, affecting mainly sediments located in the first 20 cm below the contact due to mixing processes. While 230Th, 226Ra and 210Pb pollution is mainly restricted to the first 20 cm of sediments, U-isotopes can reach higher depths (up to around 50 cm) by leaching processes due to their lower reactivity and higher concentration in the polluted leachates. The obtained results have high relevance for the design of the perimeter channel which is projected to build in the restoration project, suggesting that should has around 1 m deep under the base of the PG stacks, to ensure the full collection of polluting leachates, and to prevent their release into the estuary of the Tinto River.

This work is focused on obtaining a general function for calculating the mass attenuation coefficients of samples whose compositions are known such as waste, biota or NORM (Naturally Occurring Radioactive Materials) samples. After determining the general function, it is possible to assess the self-absorption effects by the self-absorption correction factor, which is really important to consider for samples measured by gamma spectrometry. In order to obtain a general function of the mass attenuation coefficients dependent on the gamma emission energy ( E γ ) and the chemical elements ( Z i ) with their respective concentrations ( x i ), it was necessary to fit the tabulated values of the mass attenuation coefficients versus gamma energy in the range of 20 keV–2000 keV for each chemical element. This is the energy range for the majority of gamma emissions of natural radionuclides. Once those fittings for each chemical element, Z, from 1 (H) to 92 (U) were done, the parameters, that resulted from those fittings, were fitted versus the atomic number, Z. This provides a general function that allows to calculate the mass attenuation coefficient by knowing the gamma emission energy and the sample composition. Finally, both the general function for calculating the mass attenuation coefficient and the self-absorption correction factor, which is used in gamma spectrometry with coaxial Ge detectors, were validated by comparisons made with the values provided by databases for several samples with widely different known densities, compositions and activity concentrations.

This research was focused on the analyses of the hydrochemical behaviour of long-lived natural radionuclides (210Po, 210Pb, 234,238U, 230,232Th, 226,228Ra) in groundwaters from Spanish aquifers with different geological characteristics and the evaluation of the dose by ingestion of these waters. The study was performed by the gross alpha-beta, alpha spectrometry and physicochemical analysis of a set of bottled drinking waters. Radionuclide activities in the investigated aquifers were highly variable. High concentration of 210Pb is associated with granitic bedrock and reducing conditions. On the other hand, the highest 238,234U concentrations were found in carbonate aquifers. Activities of 226Ra are largely controlled by ion exchange reactions due to the presence of relatively high concentration of chloride. Several waters exceeded the parametric value for gross alpha mainly due to 234U, 226Ra and/or 210Pb concentrations. Moreover, the content of 226Ra and 210Pb in the water contributed mainly to the indicative dose.

In order to fill a gap in the open literature, occupational exposures and activity concentrations have been assessed in two NORM industrial plants, located in the south-west of Spain, devoted to the production of mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP) fertilisers. The annual effective doses received by the workers from these plants are clearly below 1 mSv yr(-1) and the contribution due to external radiation is similar to that due to inhalation. The contribution to the maximum effective doses due to inhalation of particulate matter has been estimated to be about 0.12 mSv yr(-1), while the (222)Rn concentrations inside the plants are of no concern. Consequently, no additional actions or radiological protection measures need to be taken to decrease the natural radiation received by the workers in these facilities.

The Huelva Estuary in Huelva, Spain, has been one of the most studied environmental compartments in the past years from the point of view of naturally occurring radioactive material (NORM) releases. It has been historically affected by waste releases, enriched in radionuclides from the U-decay series, from factories located in the area devoted to the production of phosphoric acid and phosphate fertilizers. Nevertheless, changes in national regulations forced a new waste management practice in 1998, prohibiting releases of phosphogypsum into the rivers. The input of natural radionuclides from phosphate factories to rivers was drastically reduced. Because of this there was a unique opportunity for the study of the response of a contaminated environmental compartment, specifically an estuary affected by tidal influences, after the cessation of the contaminant releases to, in this case, the Huelva Estuary (henceforth referred to as the Estuary). To investigate the environmental response to this new discharge regime, the specific activities of radionuclides 226Ra and 210Pb in water and sediment samples collected in four campaigns (from 1999 to 2005) were determined and compared with pre-1998 values. From this study it is possible to infer the most effective mechanisms of decontamination for the Estuary. Decontamination rates of 210Pb and 226Ra in the sediments and water have been calculated using exponential fittings and corresponding half-lives have been deduced from them. The cleaning half-life in the whole area of the Estuary is about 6 and 3.5 years for 226Ra and 210Pb respectively. The observed trend clearly shows that contamination of the Estuary by natural radionuclides is now decreasing and radioactive levels in waters and sediments are approaching the natural background references. This work attempts to evaluate whether it can be expected that the decontamination of the enhanced levels of natural radioactivity in the Estuary can be performed via natural processes.

Trabajo presentado al "EU NORM 2 Symposium" celebrado en Praga (República Checa) del 17 al 19 de junio de 2014., En programa: "Preliminary Steps in the Restoration of a Phosphogypsum Disposal Site: Radioactive Characterization of Leachates and Efflorescences Affecting the Surrounding Area"., After the recent closing of some phosphoric acid plants located in the South-West of Spain, it has been decided to restore a big extension (more than sixty hectares) of saltmarshes where historically some million tonnes of phosphogypsum (PG), the main byproduct generated by these plants, have been disposed of. This PG is characterized for containing high activity concentrations of several radionuclides from the uranium series, mainly 226Ra, 210Pb and 210Po and to a lesser extent U-isotopes. The PG disposal area can be considered as a potential source or radionuclides to their nearby environment, through the waters which percolate from them and through the efflorescences formed in their surroundings. A previous detailed radioactive characterization of the mentioned waters and efflorescences has been considered essential for a proper planification of the restoration plan to be applied in the near future in the zone.

In 2002, Compania Mexicana de Geofisica (CMG) conducted an integrated study of Vernet Field. Located in the southern part of Macuspana Basin and discovered in the 1930s, this field has produced oil and gas from 13 intervals in approximately 25 wells. Macuspana, a mature basin in southeast Mexico where hydrocarbon production dates to the early part of the 20th century, is predominantly gas-prone with production of 5.4 trillion ft3 of gas and limited oil from Mio-Pliocene clastics in approximately 35 fields of varying sizes (Figure 1). Figure 1. Map of Macuspana Basin and Vernet Field. Pemex estimates substantial undiscovered hydrocarbons remain in this 160 × 50 km basin. At the end of the study, several recommendations were made for additional exploration opportunities. The first proposed location, drilled by Pemex in December 2002, resulted in a success that will be described in this article. Pemex has subsequently planned additional drilling on this discovery to delineate the reservoir and to produce from other recommended prospects. During the integrated study and after the discovery, a 3D stratigraphic inversion and an AVO analysis were performed. Those results will also be presented. Figure 2 is a line through the field showing our interpreted horizons. It is an extremely complex area consisting of a major growth fault and numerous subsidiary synthetic and antithetic faults that create small traps at various intervals. Production comes from the crestal fault-bounded traps on the downthrown side of the major fault. Most production comes from the Z9-E15 interval on the structure. Production from AS 50 and deeper sands is very limited (40 000 bbls and 1 million ft3) and from only two wells. Initially, it was believed that the sand quality deteriorated with depth; this is no longer the case. The depositional facies varied from nonmarine, channel, near shore to offshore. …

SUMMARY Introduction: Hematopoietic stem cell transplantation, which requires accurate enumeration of stem cells, is routinely used in clinical settings. Flow cytometry provides a qualitative and quanti- tative assessment of CD34 + cells. Precision, linearity, and stability of the novel BD Stem Cell Enumeration (SCE) Kit were evaluated on two flow cytometry platforms using a modified ISHAGE gating strat- egy and including a viability dye for data acquisition and analysis. Methods: Precision and linearity were evaluated on BD FACSCanto II and BD FACSCalibur systems. Stability was evaluated on the BD FACSCanto II system. Precision was tested using both high and low controls. Linearity was evaluated using dilutions from CD34 + cell pools, while stability was evaluated using fresh leukapheresis specimens. Results: Both systems showed precision with limited variability in abso- lute counts and percentages of viable CD34 + cells. The linearity range of viable CD34 + cells in both systems was established at 0-1000 cells/ lL, showing a linear relationship (R 2 = 0.99). Stability of CD34 + cells in mobilized leukapheresis samples was confirmed up to 24 h after collection and up to 60 min after the end of stain/lyse procedures. Conclusion: The BD SCE Kit on both flow cytometry systems shows consistent and reproducible results.

Liquid scintillation counting (LSC) is an easy and straightforward technique, and combined with its low limit of detection, makes it a powerful tool for both routine and low level measurements that can be applied to 210Pb low level counting in environmental samples. 210Pb can be easily measured following a sulphate co‐precipitation method; the addition of a carrier and the weighing of the recovered amount is a widespread technique to evaluate radiochemical yield, however, this evaluation of the recovery is sometimes questioned. The samples employed in this work were recollected in 1999 and 2005 from the estuary of the Odiel and Tinto rivers (SW of Spain), which were affected by phosphogypsum (pg.) discharges until 1998. Phosphogypsum contains most of the 210Pb from the treated raw material, for that reason analysed riverbed sediments have enhanced 210Pb activity concentrations and hence, enhanced activity concentration of its daughter 210Po, both in secular equilibrium after two years.

The estuary formed by the confluence of Tinto and Odiel river‐mouths is located in the South of Spain, close to Huelva town. This estuary has been deeply studied through the years because it has a double particularity. On one hand, since the beginning of the 1960s, the estuary has been affected by direct and indirect phosphogypsum (pg.) releases from two phosphoric acid and fertilizers factories that are working in the area. On the other hand, the pyrite mining operations upstream the Odiel and Tinto rivers has caused historically the formation of H2SO4, through oxidation of the natural sulphur deposits, the acidification of the waters and the consequent mobilisation of heavy metals from the mining area to the Huelva estuary. As a consequence, enhancement contamination levels in natural emitters from the 238U series were found in the surroundings of the factories in the previous years to 1998. However, in 1998 the management policy of waste releases drastically changed in the area, and direct discharges t...

The determination of 90Sr through the Cerenkov radiation emitted by its descendant 90Y is a well-known method and firmly established in literature. Nevertheless, in order to obtain an accurate result based on a Cerenkov measurement, the experimental work must be extremely rigorous because the efficiency of Cerenkov counting is especially sensitive to the presence of colour. Any traces of colour in the sample produce a decrease in the number of photons detected in the photomultipliers and, therefore, this might cause a diminution in Cerenkov counting efficiency. It is essential not only to detect the effect of colour quenching in the sample but also to correct the decrease in counting efficiency. For this reason, colour quenching correction curves versus counting efficiency are usually done when measuring through Cerenkov counting. One of the most widely used techniques to evaluate colour quenching in these measurements is the channel ratio method, which consists of the measurement of the shift of the spectrum measuring the ratio of counts in two different windows. The selection of the windows for the application of the corrections might have an influence on the quality of the fitting parameters of the correction curves efficiency versus colour quenching degree and hence on the final 90Sr result. This work is focused on the calculation of the counting efficiency decrease using the channel ratio method and on obtaining the best fitting correction curve. For this purpose, empirical curves obtained through artificial quenchers have been studied and the results have been tested in real samples. Additionally, given that the Packard Tri-Carb 3170 TR/SL liquid scintillation counter is a novel detector for use in Cerenkov counting, the previous calibration of the Tri-Carb 3170 TR/SL detector, necessary for the measurement of 90Sr, is included.

Curly top disease is caused by Beet curly top virus (BCTV) and related curtovirus species, and is transmitted by the beet leafhopper (Circulifer tenellus). The disease occurs in several large, but geographically separate regions of western North America. BCTV re-emerged in 2001 as a serious threat to agriculture in the San Joaquin Valley of California and has continued to exert pressure on agriculture in this region. BCTV infects a broad range of crop hosts including sugar beet, pepper, and tomato, as well as numerous native weeds. Prior molecular characterization of a limited number of curtoviruses from broad areas of the western United States suggested that two distinct curtovirus species, Beet severe curly top virus (BSCTV or CFH strain) and Beet mild curly top virus (BMCTV or Worland strain) were responsible for most crop disease, but little information existed on curtovirus species distribution among weed hosts or species prevalence in the California sugarbeet crop. The aim of this study was to clarify the genetic variability among curtovirus isolates in California, and to determine if specific weed hosts might be reservoirs for exceptionally severe virus species, such as BSCTV. Data collected over 2 years focused on molecular characterization of large numbers of BCTV isolates from weed and crop hosts of the beet leafhopper in the San Joaquin Valley. Total nucleic acid was isolated from individual plants, and both universal and specific primers were used to amplify viral DNA. PCR amplification coupled with sequence analysis identified the prevalence of both BSCTV and BMCTV as the predominant curtovirus species in California, infecting both weeds and crops. The Logan strain of BCTV, historically associated with California, was not identified among over 200 isolates characterized. SUMMARY Beet curly top virus (BCTV) and related viruses (collectively known as curtoviruses) transmitted by the beet leafhopper Circulifer tenellus (Baker) have caused significant problems to irrigated agriculture in the western US since 1899 (Carsner and Stahl, 1924). BCTV is known to infect a broad range of crop and weed hosts in many plant families (Bennett, 1971). Crop hosts for which natural BCTV infection has been reported include sugarbeet, tomato, pepper, bean, spinach, and cucurbits. The leafhopper vector also feeds and breeds on an extensive range of plant hosts from different families (Cook, 1967). C. tenellus transmits curtoviruses most efficiently after a 48-hour acquisition-access feed on an infected source plant, but shorter feeding times (220 min.) also result in a low frequency of transmission. Curtovirus transmission by the vector requires a 4 hour latent period following ingestion, and leafhoppers can inoculate healthy plants by feeding for as little as a 1 min inoculation access period. Symptoms generally develop in plants within two weeks, depending on the host and age at infection. Leafhopper vectors retain the ability to transmit BCTV for days to weeks. BCTV is a monopartite geminivirus and the type member of the genus Curtovirus within the family Geminiviridae (Fauquet et al., 2003). Viruses within this group are characterized by circular ssDNA genomes of approximately 3.0 kilobases encapsidated within twin spherical particles. Many strains (up to 14) of BCTV were initially distinguished on the basis of differential symptomatology in sugarbeet (reviewed in Klein, 1992). The disease occurs in several large, but geographically separate regions of western North America. Curly top re-emerged in 2001 as a serious threat to agriculture in the San Joaquin Valley of California and has continued to exert pressure on agriculture in this region (Wintermantel and Kaffka, 2006). BCTV infects a broad range of crop hosts including sugarbeet, pepper, and tomato, as well as numerous native weeds. Prior molecular characterization of a limited number of curtoviruses from broad areas of the western United States suggested that two distinct curtovirus species, Beet severe curly top virus (BSCTV or CFH strain) and Beet mild curly top virus (BMCTV or Worland strain) were responsible for most crop disease (Stenger and McMahon, 1997), but little information existed on curtovirus species distribution among weed hosts or species prevalence in the California sugarbeet crop. The aim of this study was to clarify the genetic variability among curtovirus isolates in California, and to determine if specific weed hosts might be reservoirs for exceptionally severe virus species, such as BSCTV. Data collected over 2 years focused on molecular characterization of large numbers of curly top isolates from weed and crop hosts of the beet leafhopper in the San Joaquin Valley. Using the extensive host range information available for curly top, reported weed and crop hosts of the virus were collected from throughout California. The majority of beet leafhopper flights are reported to be shorter than 100 miles, and the spring breeding grounds of the leafhopper, the foothills of western San Joaquin Valley, are well documented in California. Weed samples for this study were collected primarily from this area, with some samples originating from the southern portion of the Salinas Valley, as well. Collection locations were made using global positioning systems (GPS) in order to map the locations where curtoviruses were detected. Crop samples, consisting of sugarbeet, tomato, and pepper were also collected from the San Joaquin Valley. Sample collection was conducted from May through September over a three year period from 2002-2004. Samples were scored as positive or negative for curtoviruses using PCRbased virus detection methods described below. Based on this information, some areas were clearly “hot-spots” for the presence of curly top virus species, although no strainspecific hot-spots were identified. Polymerase chain reaction (PCR)-based detection methods and DNA sequencing were used to confirm curtovirus infection and to identify different curtovirus species. This method involved using short strands of DNA (primers) that bind to complementary DNA sequences present in all curtovirus species (formerly known as different BCTV strains). After primer binding, an enzyme was used to extend the primers to make multiple copies of the original strand. The end result of this process is known as a PCR product. Samples that did not contain BCTV or related curtoviruses did not produce PCR products. The resulting PCR product was then directly sequenced. Sequencing results were compared with known sequences of curtovirus species to determine which species the isolate in question was most closely related to. Results indicated that the highest incidence of infection was in sugarbeet (78%) and wild mustard (73%), with somewhat lower incidence in Russian thistle (57%), tomato (55%), and London rocket (46%). Other weed and crop hosts had considerably lower incidence of curly top, as confirmed by detection of curtoviruses in plant tissue. Overall, 200 of 562 (36%) samples tested positive for BSCTV (formerly known as CFH strain) or BMCTV (formerly known as Worland strain). No traditional BCTV (formerly California/Logan strain) was found, although small pieces of DNA corresponding to the traditional BCTV (California/Logan) sequence were occasionally found interspersed among BSCTV or BMCTV sequences. Some recombinant curtoviruses were also identified. These involved sections of both BSCTV and BMCTV, suggesting recombination (exchange of viral genetic material) may readily occur between the different species within the region sequenced. The abundance of BSCTV and BMCTV, along with the lack of BCTV indicated a clear transition between curtovirus species prevalent in California during the mid 1900s and those present today, suggesting evolution and emergence of new curly top (curtovirus) species. Studies also addressed whether specific curtovirus species were associated with specific weed or crop hosts. Results demonstrated that all species were equally capable of infecting the different host species examined in this study, and that there appears to be little difference in host range between the different curtovirus species. ACKNOWLEDGEMENT Thanks to Rod Clark and Kelly Brannigan of the California Department of Food and Agriculture–Curly Top Virus Control Board for collection and GPS mapping of weed samples. This research was supported in part by funding provided by the California Beet Growers Association Industry Research Committee. LITERATURE CITED Bennett, C. W. 1971. The curly top disease of sugarbeet and other plants. The Am. Phytopathol. Soc. Monogr. No. 7. Carsner, E. and Stahl, C. F. 1924. Studies on curly-top disease of the sugarbeet. J. Agr. Res. 28:297-320. Cook, W. C. 1967. Life history, host plants, and migrations of the beet leafhopper in the western United States. U.S.D.A. Tech. Bull. 1365. 122 p. Fauquet, C.M., Bisaro, D.M., Briddon, R.W, Brown, J.K., Harrison, B.D., Rybicki, E.P., Stenger, D.C. and Stanley, J. 2003. Revision of taxonomic criteria for species demarcation in the family Geminiviridae, and an updated list of begomovirus species. Arch. Virol. 148: 405-421. Klein, M. 1992. Role of Circulifer / Neoaliturus in the transmission of plant pathogens. Pages 152-193 in: Advances in Disease Vector Research, Vol. 9. Springer-Verlag, New York, NY. Stenger, D. C. and McMahon, C. L. 1997. Genotypic diversity of beet curly top virus populations in the western United States. Phytopathology 87:737-744. Wintermantel, W.M. and Kaffka, S.R. 2006. Sugarbeet performance with curly top is related to virus accumulation and age at infection. Plant Disease 90: 000-000 (in press).