bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Beet Yellows Closterovirus-Specific RNAs in Infected Plants and Protoplasts
Authors: He X-H. , Rao A. L. N. , Creamer R.
Published: May 2007

Tetragonia expansa plants infected with a California isolate of beet yellows virus (BYV-60) contained multiple BYV-specific RNAs identified by Northern blot hybridization. These RNAs were identified by cDNA probes specific to six open reading frames (ORFs). One genomic RNA and five subgenomic (sg) RNAs representing the p65/p6.4, p64, p24, p22, and p21 ORFs were identified. A probe derived from the 3′-terminal ORF (p21) hybridized to each of the sgRNAs, indicating the RNAs are 3′ coterminal. Hybridization with 5′- and 3′-end probes indicated that preparations of BYV particles contained the genomic RNA as well as two additional RNA molecules corresponding in size to the coat protein (CP) sgRNA and an unidentified RNA. A Chenopodium quinoa protoplast system also was used to study BYV replication. The temporal accumulation of BYV-specific RNAs and CP was investigated in protoplasts transfected with purified virion RNA. Accumulation of genomic plus-strand RNA was evident as early as 15 h postinoculation. The development of this protoplast system is significant for studies of closterovirus replication.
Gene Flow and Sexual Reproduction in the Wheat Glume Blotch Pathogen Phaeosphaeria nodorum (Anamorph Stagonospora nodorum)
Authors: Keller S. M. , McDermott J. M. , Pettway R. E. , Wolfe M. S. , McDonald B. A.
Published: May 2007

Restriction fragment length polymorphisms (RFLPs) were used to characterize the genetic structures of three field populations of Phaeosphaeria nodorum from Texas, Oregon, and Switzerland. Data from seven nuclear RFLP loci were used to estimate gene diversity and genetic distances and to make indirect measures of gene flow between populations. Three of the seven RFLP loci differed significantly in allele frequencies across populations. On average, 96% of the total gene diversity was found within populations. There was little evidence for population subdivision, suggesting that gene flow was not restricted among populations. Based on an average population differentiation of 0.04, we estimated that the exchange of 11 migrants among populations per generation would be needed to account for the present level of population subdivision. Genotype diversity based on DNA fingerprints was at a maximum for the Swiss population, whereas populations in Texas and Oregon had lower genotype diversities. Many multilocus haplotypes were found in each population. Ninety-five percent of RFLP allele pairs were in gametic equilibrium. The data were consistent with random mating within each population.
Angular Leaf Spot of Phaseolus Beans: Relationships Between Disease, Healthy Leaf Area, and Yield
Authors: Filho A. Bergamin , Carneiro S. M. T. P. G. , Godoy C. V. , Amorim L. , Berger R. D. , Hau B.
Published: May 2007

Five field experiments were conducted to investigate the relationship between the severity of visible disease (X), area under the disease progress curve (AUDPC), healthy leaf area index on any given day (HLAI), radiation intercepted by healthy leaf area on any given day (HRI), healthy leaf area duration (HAD), total healthy leaf area absorption (HAA), and yield of Phaseolus beans, cultivars Rosinha and Carioca, inoculated with Phaeoisariopsis griseola at several doses. In general, yield was not related to disease severity (X) or AUDPC. In contrast, the highest yields were always related to the highest values of HAD and HAA. The relationship between yield and HAD was linear in each of five trials (29.9 < R2 < 70.2%, P < 0.001). The relationship between yield and HAA was linear in four of the trials (52.3 < R2 < 70.3%, P < 0.001) and exponential in one of them (in which the plant canopy was the largest). Singlepoint models using HRI to estimate yield at various times during the crop season were developed. The slope of the yield-HRI relationship proved to be stable (26.8 ±2.4 g MJ-1), regardless of cultivar, locale, planting date, and bean growth stage (from R5 to R8). The yield-HLAI relationship proved to be less consistent. HRI is proposed as a key explanatory variable for a transportable system of disease management; it may be useful in producing precise recommendations at the farm level.
Identification of Colletotrichum Species Responsible for Anthracnose and Root Necrosis of Strawberry in Israel
Authors: Freeman S. , Katan T.
Published: May 2007

Strawberry anthracnose was observed for the first time in Israel in 1995. The disease reached epidemic proportions in Israeli nurseries and production fields in 1995 and 1996. Using morphological and cultural characteristics, the species responsible for anthracnose was identified as Colletotrichum acutatum. A reliable semi-selective medium, amended with iprodione and lactic acid, was used to isolate the fungus from infected tissues. In addition, C. acutatum was subsequently isolated from necrotic roots of stunted, chlorotic plants that exhibited no symptoms of anthracnose. High levels of the pathogen from naturally infested field soil and perlite growth substrate were quantified from the rhizosphere of diseased plants on the iprodione-amended medium. Both foliar- and rootinfecting isolates were equally pathogenic to strawberry, causing 95 to 100% plant mortality, when inoculated on roots and foliage. In complementation (heterokaryon) tests using nitrate nonutilizing mutants, 113 out of 115 isolates from different plant parts and locations belonged to a single vegetative compatibility group. Arbitrarily primed polymerase chain reaction of genomic DNA using four repetitive-motif primers produced nearly uniform amplified DNA banding patterns for 141 of the Israeli strawberry isolates from different sites, plots, plant tissues, and cultivars. When compared to reference isolates from the US, these band patterns suggested that a single introduction of C. acutatum was responsible for strawberry anthracnose on foliage and necrosis of roots in Israel.
Comparison of Soil Receptivity to Thielaviopsis basicola, Aphanomyces euteiches, and Fusarium solani f. sp. pisi Causing Root Rot in Pea
Authors: Oyarzun P. J. , Dijst G. , Zoon F. C. , Maas P. W. Th.
Published: May 2007

Soil receptivity as a quantifiable characteristic ranging from conduciveness to suppressiveness to soilborne pea pathogens Thielaviopsis basicola and Aphanomyces euteiches was determined by analysis of differences in disease response curves obtained by artificial introduction of inoculum into natural field soil samples. Several parameters, including maximum root rot severity, the area under the health index curve, scores on the first axis of a principal component analysis (PCA) on dose responses, and Weibull model fitting were used to describe the disease responses. In all cases, the Weibull model gave satisfactory fits. PCA yielded a first axis that comprised 86% of the variance found when using Weibull predicted responses for T. basicola and 74% of the variance found for A. euteiches. This PCA axis essentially represented the average increase in disease severity due to the addition of increasing doses of inoculum to the soil. The Weibull scale parameter B, which represents the amount of inoculum necessary to increase root rot severity by 63% with respect to the level caused by pathogens naturally present in the soil, is another means of quantifying the receptivity of soils to these plant pathogens. Weibull parameter B, maximum root rot severity, the areaunder the health index curve, and the scores on the first PCA axis were strongly correlated for each of the pathogens tested individually. To compare the extent and behavior of soil receptivity responses to different pathogens, Weibull parameters B and C (slope at dose B) were chosen because of their universal definition, in contrast to PCA scores. Comparison of the average levels of Weibull parameters B and C indicated significant differences between the pathogens. Yet, no significant similarity in the ranking of the soils was found for the three pathogens, demonstrating that individual soils may interact with different pathogens in totally different ways. In general, soils were suppressive to T. basicola but conducive to A. euteiches, whereas their response to Fusarium solani f. sp. pisi ranged from conducive to suppressive. Therefore, risk assessment of soils prior to planting may require different strategies for each pathogen. Bioassays with soil samples taken before the last pea crop in 1987 and 1991 revealed a significant increase in the natural inoculum potential of soils that mainly was accounted for by A. euteiches and Pythium spp. These results strongly indicate that A. euteiches must be considered one of the most threatening pathogens to pea crops in the Netherlands.
Selection of Drug-Tolerant Strains of Pythium sylvaticum Using Sublethal Enrichment
Authors: Martin Frank N. , Semer Charles R.
Published: May 2007

Sublethal enrichment was used to generate mutants of Pythium sylvaticum tolerant to kanamycin and tetracycline. Kanamycin tolerance was readily generated, and mutants had growth rates similar to wild-type isolates at antibiotic concentrations lethal to wild-type isolates. Based on crosses between wild-type and mutant isolates, two types of inheritance of tolerance were identified. A high level of kanamycin tolerance was inherited in progeny only when the maternal parent was drug tolerant and was correlated with the inheritance of maternal mitochondrial DNA. A lower level of tolerance was observed in some progeny when the paternal parent was tolerant to the antibiotic and, based on the lack of inheritance of paternal mitochondrial DNA, was presumably nuclear-encoded. Selection of mutants tolerant to tetracycline took longer to generate than kanamycin-tolerant mutants. Based on crosses between tolerant and wild-type parents, tolerance to tetracycline was nuclear-encoded. Tolerance to both antibiotics was stable, with cultures retaining tolerance following repeated transfers on nonamended medium and after storage for 7 years.
Modeling Dose-Response Relationships in Biological Control: Partitioning Host Responses to the Pathogen and Biocontrol Agent
Authors: Smith Kevin P. , Handelsman Jo , Goodman Robert M.
Published: May 2007

Breeding plants to improve the effectiveness of biocontrol agents is a promising approach to enhance disease suppression by microorganisms. Differences in biocontrol efficacy among cultivars suggest there is genetic variation for this trait within crop germplasm. The ability to quantify host differences in support of biological control is influenced by variation in host response to the pathogen and the dose of pathogen and biocontrol agent applied to the host. To assess the contribution of each of these factors to successful biocontrol interactions, we measured disease over a range of pathogen (Pythium) and biocontrol agent (Bacillus cereus UW85) inoculum doses. We fit dose-response models to these data and used model parameter estimates to quantify host differences in response to the pathogen and biocontrol agent. We first inoculated eight plant species separately with three species of Pythium and evaluated three dose-response models for their ability to describe the disease response to pathogen inoculum level. All three models fit well to at least some of the host-pathogen combinations; the hyperbolic saturation model provided the best overall fit. To quantify the host contribution to biological control, we next evaluated these models with data from a tomato assay, using six inbred tomato lines, P. torulosum, and UW85. The lowest dose of pathogen applied revealed the greatest differences in seedling mortality among the inbred lines, ranging from 40 to 80%. The negative exponential (NE) pathogen model gave the best fit to these pathogen data, and these differences corresponded to model parameter values, which quantify pathogen efficiency, of 0.023 and 0.091. At a high pathogen dose, we detected the greatest differences in biocontrol efficacy among the inbred lines, ranging from no effect to a 68% reduction in mortality. The NE pathogen model with a NE biocontrol component, the NE/NE biocontrol model, gave the best fit to these biocontrol data, and these reductions corresponded to model parameter values, which quantify biocontrol efficiency, of 0.00 and 0.038, respectively. There was no correlation between the host response to the pathogen and biocontrol agent for these inbred lines. This work demonstrates the utility of epidemiological modeling approaches for the study of biological control and lays the groundwork to employ manipulation of host genetics to improve biocontrol efficacy.
Cauliflower Mosaic Virus Isolate NY8153 Breaks Resistance in Arabidopsis Ecotype En-2
Authors: Tang W. , Leisner S. M.
Published: May 2007

Arabidopsis thaliana ecotype En-2 was previously shown to be resistant to cauliflower mosaic caulimovirus (CaMV) isolate CM4-184. In this study, En-2 plants were screened with eight other isolates of CaMV to identify viruses capable of overcoming resistance and to determine if the mechanism of resistance was the same for each virus. En-2 resistance to most CaMV isolates was mediated by the same mechanism, i.e., preventing virus long-distance movement. One CaMV isolate, NY8153, was found that produced a severe systemic infection on En-2 plants. In addition, the CM1841 isolate was able to spread systemically through En-2 plants, to a limited extent, without producing visible symptoms. These data indicate that the resistance shown by En-2 plants is not an all-or-none phenomenon. En-2 plants were susceptible to turnip mosaic potyvirus, suggesting that resistance is specific to CaMV.
Superoxide Generation in Extracts from Isolated Plant Cell Walls Is Regulated by Fungal Signal Molecules
Authors: Kiba Akinori , Miyake Chizu , Toyoda Kazuhiro , Ichinose Yuki , Yamada Tetsuji , Shiraishi Tomonori
Published: May 2007

Fractions solubilized with NaCl from cell walls of pea and cowpea plants catalyzed the formation of blue formazan from nitroblue tetrazolium. Because superoxide dismutase decreased formazan production by over 90%, superoxide anion (O2¯) may participate in the formation of formazan in the solubilized cell wall fractions. The formazan formation in the fractions solubilized from pea and cowpea cell walls was markedly reduced by exclusion of NAD(P)H, manganese ion, or p-coumaric acid from the reaction mixture. The formazan formation was severely inhibited by salicylhydroxamic acid and catalase, but not by imidazole, pyridine, quinacrine, and diphenyleneiodonium. An elicitor preparation from the pea pathogen Mycosphaerella pinodes enhanced the activities of formazan formation nonspecifically in both pea and cowpea fractions. The suppressor preparation from M. pinodes inhibited the activity in the pea fraction in the presence or absence of the elicitor. In the cowpea fraction, however, the suppressor did not inhibit the elicitor-enhanced activity, and the suppressor alone stimulated formazan formation. These results indicated that O2¯ generation in the fractions solubilized from pea and cowpea cell walls seems to be catalyzed by cell wall-bound peroxidase(s) and that the plant cell walls alone are able to respond to the elicitor non-specifically and to the suppressor in a species-specific manner, suggesting the plant cell walls may play an important role in determination of plant-fungal pathogen specificity.
Sequence and Relationships of Sugarcane Mosaic and Sorghum Mosaic Virus Strains and Development of RT-PCR-Based RFLPs for Strain Discrimination
Authors: Yang Z. N. , Mirkov T. E.
Published: May 2007

Sugarcane mosaic is the most widespread virus disease affecting sugarcane production. We have established a collection of seven prominent sugarcane mosaic potyvirus (SCMV) strains currently causing disease in sugarcane throughout the world and originally found in sugarcane in the United States. This collection includes SCMV strains A, B, D, and E, and the sorghum mosaic virus (SrMV) strains SCH, SCI, and SCM. These viruses were propagated on Sorghum bicolor cv. Rio and purified. Cloned cDNAs representing 2.0 kb of the 3′ termini, obtained after a reversetranscriptase-polymerase chain reaction (RT-PCR) on purified virions using an oligo(dT) primer and degenerate primers with sequences located in the NIb gene, have been sequenced for each of these strains. A comparative analysis of the deduced amino acid sequences in the NIb and coat protein genes and of the nucleotide sequences in the 3′-untranslated region, among these seven viruses and among six other members of the SCMV subgroup, confirms that there are at least four, but suggests that there are five, distinct viruses in the SCMV complex. Based on these seven new sequences and on the available sequence data for six other members of the SCMV subgroup, we have developed group-specific primers for use in RT-PCR-based restriction fragment length polymorphism analysis for rapid discrimination between strains of SCMV and SrMV. This is the first assay for differentiating strains of SCMV and SrMV that does not require interpretation of symptoms on differential hosts.
Mutation in the avrBs1 Avirulence Gene of Xanthomonas campestris pv. vesicatoria Influences Survival of the Bacterium in Soil and Detached Leaf Tissue
Authors: O'Garro Leonard W. , Gibbs Harold , Newton Anthony
Published: May 2007

The role of the avrBs1 avirulence gene of Xanthomonas campestris pv. vesicatoria in survival of the bacterium was investigated by testing two strains that differ in the structure and function of the gene in 37 different soil types of Barbados and detached leaf tissue of four pepper genotypes. One strain carried a mutation in the avrBs1 gene and lacked avirulence activity, while the other expressed wild-type avrBs1 activity. In 30 to 32 soil types and all leaf tissue tested, the mutant strain persisted longer and more abundantly than the wild-type strain over a 2- to 6-week period. During this time, the mutant strain generally replaced the wild-type strain completely in soil initially infested with a mixture of equal amounts of each strain and by a factor of 6.7 in similarly infested pepper leaves. Nine selected soil factors, namely pH, clay and cation content, and percent nitrogen, carbonates, carbon, K+, Na+, and Ca2+ did not affect bacterial survival significantly.
Binding of Tomato Spotted Wilt Virus to a 94-kDa Thrips Protein
Authors: Kikkert Marjolein , Meurs Cor , van de Wetering Fennet , Dorfmüller Simone , Peters Dick , Kormelink Richard , Goldbach Rob
Published: May 2007

Using protein blot assays, a 94-kDa thrips protein was identified that exhibited specific binding to tomato spotted wilt virus (TSWV) particles. Renaturation of the 94-kDa protein, which is conserved among the two major vector species of TSWV, Frankliniella occidentalis and Thrips tabaci, was crucial for its virus-binding properties, whereas under the same conditions no specific binding was observed with aphid (Myzus persicae) proteins. The 94-kDa protein species was present in all developmental stages of both vectoring thrips, whereas it was present mainly in the adult stage of a nonvectoring thrips species, Parthenothrips dracenae. Using antibodies against the different TSWV structural proteins, the G2 envelope glycoprotein was identified as the viral determinant involved. Because the virus-binding protein is present throughout the thrips body, but not in the gut, it may represent a receptor protein involved during circulation of the virus through its vector but probably not during viral uptake in the midgut.
Primary Haustorial Development ofStriga asiaticaon Host and Nonhost Species
Authors: Hood M. E. , Condon J. M. , Timko M. P. , Riopel J. L.
Published: May 2007

Initial interactions of Striga asiatica with a susceptible host and non-host plants were examined by histological methods. Haustorial development was initiated when radicles of S. asiatica were placed in contact with host or nonhost roots. Reorganization of the S. asiatica root apical meristem was rapid and involved the formation of a distal group of cells that penetrated the host or nonhost root. Penetration of the epidermis of the host (sorghum) roots and advance into the cortex occurred within 24 to 48 h of inoculation. Penetration of the endodermis by the developing endophyte was delayed for 72 to 96 h after initial contact. However, upon penetration vascular continuity was established between parasite and host. In contrast, interactions with nonhosts provided evidence of active resistance mechanisms. Penetration of lettuce, marigold, and cowpea roots by S. asiatica was most frequently arrested in the cortex, and endophytic cells were necrotic 72 h after inoculation. Some species-specific differences were observed in the reactions of nonhosts to penetration, although in their general nature the interactions with S. asiatica were similar.
Chromosomal Location and Genetic Relationship of Leaf Rust Resistance Genes Rph9 and Rph12 in Barley
Authors: Borovkova I. G. , Jin Y. , Steffenson B. J.
Published: May 2007

Barley lines Hor 2596 and Triumph are the sources of leaf rust resistance genes Rph9 and Rph12, respectively. An allelism test was performed with F2 progeny of the cross Triumph/Hor 2596 inoculated with Puccinia hordei. No recombinants were found in a population of 3,858 progeny, indicating Rph9 and Rph12 are alleles. Molecular and morphological markers were used to identify the chromosomal location of these genes in the crosses Bowman/Hor 2596 and Triumph/I91-533-va. A linkage was detected between Rph9 and the flanking sequence-tagged site (STS) markers ABC155 and ABG3 on chromosome 7(5H) at a distance of 20.6 and 20.1 centimorgans (cM), respectively, and to the microsatellite marker dehydrin-9 (HVDHN9) at a distance of 10.2 cM in the Bowman/ Hor 2596 cross. Analysis of isozymes in bulks of the same population showed that Rph9 may be closely linked to the Est9 locus on chromosome 7(5H). The Rph12 locus was linked to the morphological trait locus va (controlling variegated leaf color) on chromosome 7(5H) at a distance of 22.6 cM in the Triumph/I91-533-va cross. Rph12 also was linked with STS marker ABC155 (24.4 cM) and RAPD marker OPA19 (1.5) (17.8 cM). These data indicate that Hor 2596 and Triumph carry a leaf rust resistance gene at the same locus on the long arm of chromosome 7(5H) of barley.
Characterization of the Beet Yellow Stunt Virus Coat Protein Gene
Authors: Karasev Alexander V. , Nikolaeva Olga V. , Lee Richard F. , Wisler Gail C. , Duffus James E. , Dawson William O.
Published: May 2007

The beet yellow stunt virus (BYSV) genome contains at least nine open reading frames (ORFs) that code for proteins ranging from 6 to 66 kDa. Based on amino acid sequence comparisons, the coat protein (CP) was previously identified as the product of ORF7. We expressed the product of ORF7 in bacteria and confirmed that ORF7 codes for the BYSV CP by immunoblotting. BYSV is a phloem-limited virus, and virus CP antigen of a quality sufficient for diagnostic antisera production has not been available. To produce BYSV antigen free of plant host contaminants, ORF7 was cloned into a pMAL bacterial expression vector. The resulting fusion protein was affinity-purified and used as an antigen to raise anti-BYSV CP antisera in rabbits and guinea pigs. Using these antisera, an indirect double-antibody sandwich (DAS) enzyme-linked immunosorbent assay (ELISA)-based diagnostic system was developed. This indirect DAS-ELISA format enabled reliable detection of BYSV in tissue extracts from virus-infected lettuce diluted up to 5,000 times. The diagnostic system developed may enable large-scale epidemiological studies of BYSV using simple serological techniques. The antisera raised had a titer exceeding 1 × 105 in immunoblots and easily detected the 23.7-kDa BYSV CP in virus-infected lettuce and sowthistle plants. In these two plant species, BYSV CP was detected as two closely migrating bands during electrophoresis, which may suggest posttranslational CP modifications. To further characterize the BYSV CP gene, the 5′-untranslated region (UTR) of the BYSV CP subgenomic RNA (sgRNA) was cloned and sequenced. The CP-encoding, approximately 1.9-kb sgRNA has an AT-rich, 66-nucleotide-long 5′-UTR colinear to the genomic sequence upstream of ORF7.
Association of Binucleate Rhizoctonia with Soybean and Mechanism of Biocontrol of Rhizoctonia solani
Authors: Poromarto Susilo H. , Nelson Berlin D. , Freeman Thomas P.
Published: May 2007

The association of binucleate Rhizoctonia (BNR) AG-K with soybean and the interaction of BNR, R. solani AG-4, and soybean seedlings were investigated to elucidate the mechanism of biocontrol of R. solani by BNR. Sixty-hour-old seedlings were inoculated and incubated in a growth chamber at 24°C; plants were examined with light microscopy and with scanning and transmission electron microscopy at various times following inoculation. BNR grew over hypocotyls, roots, and root hairs, but only colonized epidermal cells. Hyphae of BNR appeared to attach to the epidermis and, 5.5 h following inoculation, began penetrating cells by means of penetration pegs without forming distinct appressoria or infection cushions. There was evidence of cuticle degradation at the point of penetration. Infection hyphae moved to adjacent epidermal cells by direct penetration of epidermal radial walls. There were epidermal and cortical cell necrosis, beginning with the fragmentation of the tonoplast and followed by the disintegration of cytoplasm, organelles, and plasma membranes. Cell necrosis was also observed in adjacent cells where there was no evidence of BNR hyphae. Cell walls were not destroyed. After 144 h, there was noevidence of BNR hyphae in cortical cells. Attempted penetrations were observed, but papillae formed on the inside of cortical cell walls. Pre-inoculation of soybean seedlings with BNR 24 or 48 h before inoculation with R. solani (1 cm between inocula) affected the growth of R. solani on soybean tissue. There were fewer hyphae of R. solani, the hyphae branched sparingly, and infection cushions were rare when compared with hyphal growth on soybean inoculated only with R. solani. These effects were observed before the BNR hyphae began to intermingle with the hyphae of R. solani on the surface of the inoculated host. Preinoculation of soybean seedlings 24 h before inoculation with R. solani significantly (P = 0.05) reduced disease incidence and severity caused by R. solani AG-4. The lesions caused by R. solani always appeared distally, not proximally, to the BNR inoculum. The interactions of intermingling hyphae of BNR and R. solani were examined in vitro and on the surface of the host. There was no evidence of lysis, mycoparasitism, inhibition of growth, or any other form of antagonism between hyphae. The results of these studies strongly suggest that induced resistance is the mechanism of biocontrol of R. solani on soybean by BNR. The inhibition of hyphal growth of R. solani on the surface of soybean tissue preinoculated with BNR appears to be a novel characteristic of induced resistance.
Replication Specificity Elements of the Worland Strain of Beet Curly Top Virus Are Compatible with Those of the CFH Strain But Not Those of the Cal/Logan Strain
Authors: Stenger Drake C.
Published: May 2007

Cloned genomes of the CFH, Worland, and Cal/Logan strains of beet curly top virus (BCTV) served as helper viruses to trans-replicate defective (D) DNAs that are incapable of self-replication due to deletions within the C1 open reading frame encoding the replication initiator (Rep) protein. The Logan Rep protein could trans-replicate a Logan-derived D DNA in a transient replication assay conducted in Nicotiana benthamiana leaf disks. However, the Logan Rep protein was unable to trans-replicate D DNAs derived from the CFH or Worland strains. In contrast, the Rep proteins of the CFH and Worland strains could trans-replicate CFH or Worland D DNAs, but not a Logan D DNA. These results indicate that the cis- and trans-acting replication specificity elements of the CFH and Worland strains are compatible and that the three strains of BCTV may be divided into two groupings based upon replication specificity determinants. A comparison of amino acid sequences of the Rep protein for the three BCTV strains suggests that the trans-acting replication specificity element may reside in one or more of 12 amino acid residues that are identical; in two amino acid residues that are chemically similar among the CFH and Worland Rep proteins, yet are different in the Logan Rep protein; or in both. Properties including replication specificity, nucleotide sequence identity, and symptom expression were used as criteria to propose separate species designations for each of the three BCTV strains. In this proposal, the Cal/ Logan strain retains the name BCTV, CFH and the closely related Iranian isolate are designated beet severe curly top virus, and Worland is designated beet mild curly top virus.
Unusual Cytoplasmic Inclusions Induced in Tobacco by Peanut Stunt Virus Subgroup II Strains Map to RNA3
Authors: Sanger Margaret , Järlfors Ulla E. , Ghabrial Said A.
Published: May 2007

Infection of tobacco protoplasts or leaf tissues with peanut stunt virus (PSV) subgroup II strains induced the production of unusual cytoplasmic ribbon-like inclusions. The inclusion structures appeared as long, thin, densely staining sheets that were prevalent within the cytoplasm, accumulating most commonly near vacuoles. Numerous virions and ribosomes could be seen adjacent to the inclusion surfaces. The formation of these novel inclusions appeared to be subgroup specific, since infection of tobacco with PSV strains W and B (subgroup II), but not strains ER, V, and J (subgroup I), induced the inclusions. Furthermore, inclusion formation was shown to be host specific, because the inclusions were not detected in either of two leguminous host species infected with PSV subgroup II strains. Using tobacco protoplasts electroporated with various assortments of infectious RNA transcripts derived from cDNA clones of genomic RNAs of PSV-ER and PSV-W, we demonstrated that induction of the unusual ribbon-like inclusions maps to PSV-W (subgroup II) RNA3. This conclusion is consistent with the finding that PSV strain BV-15, a natural intraspecific reassortant that derives its RNA2 and RNA3 from a subgroup I strain, did not induce inclusion formation.
Synthetic Antigen from a Peptide Library Can Be an Effective Positive Control in Immunoassays for the Detection and Identification of Two Geminiviruses
Authors: Ziegler A. , Mayo M. A. , Torrance L.
Published: May 2007

Phage-displayed peptides were selected from the Cys 1 random phage display peptide library that bound strongly to the monoclonal antibody (MAb) SCR 20. The binding peptides were fused to the N-terminus of the phage protein pVIII. Preparations of the phage were shown to be effective as controls for the functionality of the SCR 20 MAb in both enzyme-linked immunosorbent assays and dot blot immunoassays. UV irradiation that eliminated phage infectivity did not greatly alter the antigenicity. Peptides displayed on phage are quick and cheap to prepare, and preparations can be standardized to ensure comparability among different assays. The peptide library approach can be readily extended for use with other MAbs to obtain inexpensive and safe standardized positive control reagents for use in immunoassays to diagnose plant disease.
Phytoplasma Identity and Disease Etiology
Authors: Davis R. E. , Sinclair W. A.
Published: May 2007

Many plant diseases believed to be caused by phytoplasmas were described before phytoplasma groups were delineated through molecular analyses. It is now possible to assess the relationships between phytoplasma identity or classification and specific plant diseases. Data were consistent with the hypothesis of a common ancestral origin of pathogenicity genes in many phytoplasmas and a limited repertoire of plant responses to certain pathogen signals. Observations also were consistent with the hypotheses that the botanical host ranges of some phytoplasmas reflect specificities in transmission by vectors and vector feeding preferences; phytoplasma-insect vector relationships are keys to understanding evolutionary divergence of phytoplasma lineages; small differences in a highly conserved phytoplasma gene may be regarded as potential indicators of separate gene pools; the reliability of a diagnosis based on symptoms must be learned empirically (i.e., through case study for each syndrome); and some discrete diseases can be ascribed to phytoplasma taxa at the 16S rRNA group level, whereas others are clearly associated with phytoplasma taxa below this level.
A Major Gene for Powdery Mildew Resistance Transferred to Common Wheat from Wild Einkorn Wheat
Authors: Shi A. N. , Leath S. , Murphy J. P.
Published: May 2007

A major gene for resistance to wheat powdery mildew (Blumeria graminis f. sp. tritici = Erysiphe graminis f. sp. tritici) has been successfully transferred into hexaploid common wheat (Triticum aestivum, 2n = 6x = 42, AABBDD) from wild einkorn wheat (Triticum monococcum subsp. aegilopoides, 2n = 2x = 14, AA). NC96BGTA5 is a germ plasm line with the pedigree Saluda × 3/PI427662. The response patterns for powdery mildew resistance in NC96BGTA5 were tested with 30 differential isolates of B. graminis f. sp. tritici, and the line was resistant to all tested isolates. The analyses of P1, P2, F1, F2, and BC1F1 populations derived from NC96BGTA5 revealed two genes for wheat powdery mildew resistance in the NC96BGTA5 line. One gene, Pm3a, was from its recurrent parent Saluda, and the second was a new gene introgressed from wild einkorn wheat. The gene was determined to be different from Pm1 to Pm21 by gene-for-gene and pedigree analyses. The new gene was identified as linked to the Pm3a gene based on the F2 and BC1F1 populations derived from a cross between NC96BGTA5 and a susceptible cultivar NK-Coker 68-15, and the data indicated that the gene was located on chromosome 1A. It is proposed that this new gene be designated Pm25 for wheat powdery mildew resistance in NC96BGTA5. Three random amplified polymorphic DNA markers, OPX061050, OPAG04950, and OPAI14600, were found to be linked to this new gene.
Genetics of Stem Rust Resistance in Wheat Cvs. Pasqua and AC Taber
Authors: Liu J. Q. , Kolmer J. A.
Published: May 2007

Canadian wheat cvs. Pasqua and AC Taber were examined genetically to determine the number and identity of stem rust resistance genes in both. The two cultivars were crossed with stem rust susceptible line RL6071, and sets of random F6 lines were developed from each cross. The F6 lines, parents, and tester lines with single stem rust resistance genes were grown in a field rust nursery, inoculated with a mixture of stem and leaf rust races, and evaluated for rust resistance. The same wheat lines were tested by inoculation with specific stem rust races in seedling tests to postulate which Sr genes were segregating in the F6 lines. Segregation of F6 lines indicated that Pasqua had three genes that conditioned field resistance to stem rust and had seedling genes Sr5, Sr6, Sr7a, Sr9b, and Sr12. Leaf rust resistance gene Lr34, which is in Pasqua, was associated with adult-plant stem rust resistance in the segregating F6 lines. Adult-plant gene Sr2 was postulated to condition field resistance in AC Taber, and seedling genes Sr9b, Sr11, and Sr12 also were postulated to be in AC Taber.
Interaction of Tomato Spotted Wilt Tospovirus (TSWV) Glycoproteins with a Thrips Midgut Protein, a Potential Cellular Receptor for TSWV
Authors: Bandla M. D. , Campbell L. R. , Ullman D. E. , Sherwood J. L.
Published: May 2007

Interactions between viral and cellular membrane fusion proteins mediate virus penetration of cells for many arthropod-borne viruses. Electron microscope observations and circumstantial evidence indicate insect acquisition of tomato spotted wilt virus (TSWV) (genus Tospovirus, family Bunyaviridae) is receptor mediated, and TSWV membrane glycoproteins (GP1 and GP2) serve as virus attachment proteins. The tospoviruses are plant-infecting members of the family Bunyaviridae and are transmitted by several thrips species, including Frankliniella occidentalis. Gel overlay assays and immunolabeling were used to investigate the putative role of TSWV GPs as viral attachment proteins and deter mine whether a corresponding cellular receptor may be present in F. occidentalis. A single band in the 50-kDa region was detected with murine monoclonal antibodies (MAbs) to the TSWV-GPs when isolated TSWV or TSWV-GPs were used to overlay separated thrips proteins. This band was not detected when blots were probed with antibody to the non-structural protein encoded by the small RNA of TSWV or the TSWV nucleocapsid protein, nor were proteins from nonvector insects labeled. Anti-idiotype antibodies prepared to murine MAbs against GP1 or GP2 specifically labeled a single band at 50 kDa in Western blots and the plasmalemma of larval thrips midguts. These results support the putative role of the TSWV GPs as viral attachment proteins and identified potential cellular receptor(s) in thrips.
Diversity of Virulence in Phytophthora parasitica on Tobacco, as Reflected by Nuclear RFLPs
Authors: Colas V. , Lacourt I. , Ricci P. , Vanlerberghe-Masutti F. , Poupet A. , Panabières F.
Published: May 2007

A worldwide collection of P. parasitica isolates was investigated for the ability to infect tobacco and tomato, as related to elicitin production. Elicitin was produced by all nontobacco isolates, and nonproducing strains all were isolated from tobacco. In addition, producing strains were isolated from tobacco and coexisted with nonproducing (TE¯) strains. Elicitin production generally was associated with low virulence on tobacco and frequent pathogenicity on tomato, whereas TE¯ isolates generally were highly virulent and specialized to tobacco. Analysis of both mitochondrial and nuclear DNA restriction fragment length polymorphisms indicated, for the first time, that black shank isolates can be distinguished from other P. parasitica isolates on the basis of genetic criteria. Our results suggest that severe black shank is caused by a limited number of TE¯ strains that have been disseminated by clonal evolution. Mutations in the TE¯ phenotype seem to have arisen independently in several genetic backgrounds and distinct geographic areas. The fortuitous absence of elicitin production has precluded population replacements in areas of intensive tobacco cultivation. Thus, monitoring the loss of elicitin production in developing tobacco areas should be considered in disease management.
Nematode-Trapping Fungi in Organic and Conventional Cropping Systems
Authors: Jaffee B. A. , Ferris H. , Scow K. M.
Published: May 2007

Nematode-trapping fungi, nematodes, and microbial biomass were quantified in conventionally and organically managed field plots in the Sustainable Agriculture Farming Systems Project at the University of California at Davis. There were four replicate plots (0.135 ha per plot) for each management system, and plots were sampled three times each year for 2 years. The hypothesis that nematode-trapping fungi would be more abundant in organically managed plots was partially supported: the number of species of nematode-trapping fungi was slightly but significantly greater in organic than in conventional plots, two species (Arthrobotrys dactyloides and Nematoctonus leiosporus) were detected more frequently in organic plots, and the population densities of A. dactyloides and N. leiosporus were greater in organic than in conventional plots. Two other species (A. haptotyla and A. thaumasia), however, tended to be more numerous in conventional than in organic plots, and the total density of nematode-trapping fungi was similar in organic and conventional plots. Bacterivorous nematodes were more abundant and microbial biomass (substrate-induced respiration) was greater in organic than in conventional plots. Suppression of the root-knot nematode Meloidogyne javanica, as measured in a bioassay, was not related to management system or population density of nematode-trapping fungi but was positively related to microbial biomass.
Dispersal of Spores of Fusarium culmorum in Aquatic Systems
Authors: Smither-Kopperl M. L. , Charudattan R. , Berger R. D.
Published: May 2007

The dispersal of spores of Fusarium culmorum, a biological control agent for the aquatic weed Hydrilla verticillata, was investigated in aquatic systems. Macroconidia and chlamydospores that were applied to the surface of the liquid settled rapidly in deionized water, tap water, 5% Hoagland's solution, natural spring water, or river water held in glass containers. The rate of fall, as measured for 50% of the spores, was determined to be 9 cm h-1. Rapid lateral dispersal of spores from a point source occurred in still water. This initial spore movement occurred at a rate of >9 m h-1 (15 cm min-1), approximately 100 times faster than the rate of settlement. The spores attained an even lateral distribution in a still, closed system. Spores dispersed rapidly in moving water and were transported with the water current. Spores were determined to carry positive electrostatic charges as they migrated towards the negative pole during electrophoresis. The physical components of dispersal of F. culmorum spores were defined in a still aquatic system to consist of rapid lateral dispersal and sinking due to gravity. In moving water, the dynamics of water movement was superimposed over the other two factors.
Polymerase Chain Reaction Detection and Phylogenetic Characterization of an Agent Associated with Yellow Vine Disease of Cucurbits
Authors: Avila Francisco J. , Bruton Benny D. , Fletcher Jacqueline , Sherwood J. L. , Pair Sam D. , Melcher Ulrich
Published: May 2007

Diagnosis of yellow vine disease (YVD) in cucurbits, an important disease in the south-central United States, relies on external symptom appearance, phloem discoloration, and the presence of bacterium-like organisms (BLOs) in phloem. Polymerase chain reaction (PCR) amplification of BLO nucleotide sequences was explored as a means to improve diagnostic techniques. PCR, using a primer pair based on sequences of the citrus-greening BLO, amplified a 0.15-kilobase (kb) fragment from the DNA of symptomatic plants, but not from that of asymptomatic plants. Its nucleotide sequence suggested that the DNA amplified was of pro-karyotic origin. A primer pair, designed to amplify nonspecific prokaryotic 16S rDNA, amplified a 1.5-kb DNA fragment in both the symptomatic and asymptomatic plants. The 1.5-kb fragment from the asymptomatic plants corresponded to chloroplast 16S rDNA, and the band from the symptomatic plants was composed of 16S rDNAs from both chloroplasts and a prokaryote. The nucleotide sequence of the prokaryotic DNA was determined and used to design three primers (YV1, YV2, and YV3). Fragments of 0.64 and 1.43 kb were amplified with primers YV1-YV2 and primers YV1-YV3, respectively, from symptomatic plants. Neither primer set yielded fragments from asymptomatic plants, unrelated bacteria, or selected soilborne fungal pathogens of cucurbits. Phylogenetic analysis indicated that the prokaryote is a gamma-3 proteobacterium. The consistent association of the 0.64- and 1.43-kb fragments with symptomatic plants suggests that the gamma-3 proteobacterium may be the causal agent of YVD of cantaloupe, squash, and watermelon.
Resveratrol Oxidation inBotrytis cinereaConidia
Authors: Adrian Marielle , Rajaei Homa , Jeandet Philippe , Veneau Jérôme , Bessis Roger
Published: May 2007

Observations using light microscopy showed that approximately 30% of Botrytis cinerea conidia treated with semi-lethal concentrations (i.e., 60 μg/ml) of the grapevine phytoalexin resveratrol possessed intracellular brown coloration. This coloration was never observed in the absence of resveratrol or in conidia treated with resveratrol together with sulfur dioxide (antioxidant compound) or sodium diethyldithiocarbamate (inhibitor of laccase action), suggesting that discoloration resulted from the laccase-mediated oxidation of resveratrol. Further studies using transmission electron microscopy enabled the observation of particular intravacuolar spherical vesicles and of granular material deposits along the tonoplast. These observations are likely to be related to the oxidation of resveratrol by an intracellular laccase-like stilbene oxidase of B. cinerea.
Establishment of Bacterial Antagonists of Erwinia amylovora on Pear and Apple Blossoms as Influenced by Inoculum Preparation
Authors: Stockwell V. O. , Johnson K. B. , Loper J. E.
Published: May 2007

The influence of inoculum preparation on the establishment of bacterial antagonists that suppress fire blight and Erwinia amylovora on blossoms was evaluated. Aqueous suspensions of Pseudomonas fluorescens A506, E. herbicola C9-1R, or E. amylovora 153N were prepared from cells harvested from the surface of an agar medium or from cells that were lyophilized after culture under similar conditions. Bacterial suspensions (1 × 108 CFU/ml) were sprayed on pear and apple trees at 50% bloom near midday. The incidence of recovery (proportion of blossoms containing detectable populations) and the population sizes of the bacteria on individual blossoms with detectable populations were followed over a period of several days. Fluorescent microspheres (1 μm in diameter) were added to sprays at a concentration of 1 × 107 microspheres per ml to mark blossoms that were open during application of bacteria. After dilution-plating, the stigmas and styles of each blossom were examined for the presence of microspheres with an epifluorescence microscope. In three of five trials, bacteria applied as suspensions of lyophilized cells were recovered from a greater proportion of blossoms than bacterial cells harvested directly from culture media. Every blossom harvested within 6 days after spraying had microspheres present on the surfaces of the styles and stigmas; thus, lack of establishment of detectable populations, rather than escape of blossoms from spray inoculation, accounted for the differences in proportion of blossoms colonized by the different preparations of bacteria. The use of lyophilized cells in field trials decreased variability in the establishment of bacteria on blossoms.
Ingress of Clavibacter michiganensis subsp. michiganensis into Tomato Leaves Through Hydathodes
Authors: Carlton W. M. , Braun E. J. , Gleason M. L.
Published: May 2007

Hydathodes of tomato leaves served as extremely efficient infection courts for the bacterial canker pathogen, Clavibacter michiganensis subsp. michiganensis. Chlorotic lesions developed at the tips of leaflet lobes about 2 weeks after inoculation of guttation droplets. Lesions expanded along the leaflet margins and became necrotic. Movement of C. michiganensis subsp. michiganensis from the inoculated leaflet into the rachis was slow and erratic. Histological observations revealed that pathogen populations first developed within large intercellular spaces lying beneath the stomata, which serve as water pores in tomato hydathodes. Bacteria were first observed within vessels of the large marginal fimbriate veins 7 days after inoculation. By 14 days after inoculation, large populations could be seen within the vessels; and by 21 days after inoculation, tissue collapse was widespread and masses of bacteria could be seen in the intercellular spaces and within necrotic cells.