bluuebunny/crossref_metadata_2025 huggingface dataset:
Deoxynivalenol and Nivalenol Production by Fusarium culmorum Isolates Differing in Aggressiveness Toward Winter Rye
Authors: Gang G. , Miedaner T. , Schuhmacher U. , Schollenberger M. , Geiger H. H.
Published: May 2007

A susceptible synthetic winter rye population was inoculated with 42 isolates of Fusarium culmorum, originating from nine European countries and Australia, at two field locations in Germany. Significant (P = 0.01) genetic variation in aggressiveness of isolates of F. culmorum was observed across both field locations. Field samples were used to determine deoxynivalenol (DON), nivalenol (NIV), and ergosterol (ERG) contents. The 42 isolates also were incubated on rye grain in vitro, and DON and NIV contents were analyzed. Thirty-four isolates produced DON, and seven isolates produced NIV at both field locations and in vitro. Mean DON contents ranged from 0.5 to 64.6 mg/kg in grain from field trials and from 0.3 to 376.3 mg/kg in grain incubated in vitro; mean NIV contents ranged from 17.6 to 30.4 mg/kg in grain from field trials and from 0.8 to 381.0 mg/kg in grain incubated in vitro. No correlation was found between the DON content of field-grown grain and grain incubated in vitro. NIV-producing isolates originated from the Netherlands, Germany, Italy, and Australia. More aggressive isolates produced higher mean DON contents in grain in field trials (r = 0.69; P = 0.01). However, DON production rate per unit of fungal biomass, estimated as the DON/ERG ratio at harvest, was not correlated with aggressiveness. Toxin production seemed to be a common feature in F. culmorum. In vitro assays reliably distinguished DON- and NIV-producing types of F. culmorum; however, these assays could not predict production of DON by these isolates in the field.
Immunofluorescent Localization of Tobacco Ringspot Nepovirus in the Vector Nematode Xiphinema americanum
Authors: Wang Shouhua , Gererich Rose C.
Published: May 2007

An indirect immunofluorescent technique was developed to localize tobacco ringspot nepovirus (TRSV) in the vector nematode Xiphinema americanum sensu stricto. A population of this nematode that efficiently transmitted TRSV was given an acquisition access period of 10 days on TRSV-infected cucumber. Treatment of fragments of viruliferous nematodes with a polyclonal antiserum against TRSV followed by fluorescein isothiocyanate-conjugated goat anti-rabbit immunoglobulin G resulted in virus-specific bright fluorescence only in the lumen of the stylet extension and esophagus. Virus-specific fluorescent signals were observed in the virus-retention region of 44% of the nematode fragments examined. The percentage of nematodes labeled with virus-specific fluorescence increased as the acquisition access period increased from 0 to 22 days; the increase paralleled the increase in the transmission efficiency of the nematode population. Visualization of the entire virus-retention region of individual nematodes within a population of vector or nonvector nematodes provides a rapid and simple means of monitoring specific attachment of plant viruses.
Natural Hybrids of Phytophthora nicotianae and Phytophthora cactorum Demonstrated by Isozyme Analysis and Random Amplified Polymorphic DNA
Authors: Man in 't Veld Willem A. , Veenbaas-Rijks Wil J. , Ilieva Elena , de Cock Arthur W. A. M. , Bonants Peter J. M. , Pieters Rob
Published: May 2007

Three similar isolates of Phytophthora (Phytophthora sp-h) were obtained from diseased Spathiphyllum and Primula plants. Cultural characteristics did not fit any known description of Phytophthora species. The Phytophthora sp-h isolates are papillate, are homothallic, possess 80 to 86% amphigynous antheridia, and have a maximum temperature for growth of 36.5°C. Isozyme analysis of the Phytophthora sp-h isolates revealed a three-banded pattern with malic enzyme and a three-banded pattern with malate dehydrogenase on the second putative locus. The fastest band at both enzyme loci comigrated with the single P. nicotianae band, the slowest band comigrated with the single P. cactorum (and also P. pseudotsugae) band, and one band in between was concluded to represent the heterodimeric isozyme. The random amplified polymorphic DNA patterns of the Phytophthora sp-h isolates almost exclusively consisted of bands that were also present in either P. nicotianae or P. cactorum. Southern hybridization showed that bands specific for P. nicotianae were present as comigrating bands in the Phytophthora sp-h isolates. The same was found for species-specific bands of P. cactorum. It is concluded that the three Phytophthora sp-h isolates represent interspecific hybrids, P. nicotianae being the one parent and P. cactorum the other. Analysis of mito-chondrial DNA with restriction enzymes revealed banding patterns in all the Phytophthora sp-h isolates identical with those of P. nicotianae, confirming that indeed P. nicotianae was one of the parents.
Elucidation of the Microbial Complex Having a Causal Role in the Development of Apple Replant Disease in Washington
Authors: Mazzola Mark
Published: May 2007

Systematic studies were conducted to elucidate the role of different soil microbial groups in the development of apple replant disease. Populations of targeted microorganisms were reduced by the application of semiselective biocides and soil pasteurization. Bacteria were not implicated in the disease, because application of the antibiotic chloramphenicol reduced soil populations of bacteria but failed to improve growth of apple transplants, while enhanced growth was achieved at pasteurization temperatures that did not alter attributes of the bacterial community recovered from apple roots. Populations of Pratylenchus penetrans were below the damage threshold level in eight of nine orchards surveyed, and nematicide applications failed to enhance apple growth in four of five replant soils tested, indicating that plant parasitic nematodes have a minor role or no role in disease development. Application of the fungicide difenconazole or metalaxyl enhanced growth of apple in all five replant soils, as did fludioxinil in the two soils tested. Soil pasteurization enhanced growth of apple and resulted in specific changes in the composition of the fungal community isolated from the roots of apple seedlings grown in these treated soils. Cylindrocarpon destructans, Phytophthora cactorum, Pythium spp., and Rhizoctonia solani were consistently isolated from symptomatic trees in the field and were pathogenic to apple. However, the composition of the Pythium and Rhizoctonia component and the relative contribution of any one component of this fungal complex to disease development varied among the study orchards. These findings clearly demonstrate that fungi are the dominant causal agents of apple replant disease in Washington state.
Serological Relationships Between the Cylindrical Inclusion Proteins of Potyviruses
Authors: Hammond John
Published: May 2007

Antisera to the cytoplasmic inclusion proteins (CIPs) of bean yellow mosaic (BYMV), clover yellow vein (ClYVV), turnip mosaic (TuMV), sweet potato feathery mottle (SPFMV), and maize dwarf mosaic (MDMV) potyviruses were used to examine the relationships between the CIPs of 18 potyviruses. The antisera to CIPs of BYMV, ClYVV, TuMV, and SPFMV cross-reacted to most or all of the purified CIPs tested in western blot assays. The MDMV CIP antiserum reacted significantly only to the MDMV and sorghum mosaic virus CIPs. Reactivity of antisera to CIPs of dicot-infecting viruses was generally higher with CIPs of other dicot-infecting than with monocot-infecting potyvirus CIPs. Analysis of amino acid sequences of the CI genes of 11 well-characterized potyviruses suggested that epitopes specific for individual potyviruses are primarily in the C-terminal domains of the CIP, whereas epitopes shared among different viruses are clustered in the N-terminal domains. The most highly conserved predicted epitope overlaps the nucleotide binding motif of the N-terminal helicase domain of the CIP. Antibodies to this domain will probably be present in antisera to any potyvirus CIP and contribute to the cross-reactivity observed. Differences in the C-terminal domains may correlate with interactions between the CIP and coat protein necessary for replication and movement.
Inheritance of Resistance to Gray Leaf Spot in Crosses Involving Selected Resistant Inbred Lines of Corn
Authors: Coates S. T. , White D. G.
Published: May 2007

Three populations derived from crosses of selected resistant inbreds (061, B37HtN, and DS:74:1071) with susceptible inbred FR1141 and a population derived from a cross of B37 × B37HtN were evaluated for gray leaf spot severity in 1992 and 1993 at Urbana, IL, and Andrews, NC. Populations included the susceptible parent (P1), the resistant parent (P2), F1 and F2 generations, backcrosses BCP1 and BCP2, and, when space and seed were available, one or more of the F3, BCP1S1, and BCP2S1 generations. Plants at Urbana were inoculated, and naturally occurring disease was relied upon at Andrews. Individual plants were rated by visually estimating the percent leaf area blighted (necrotic). Generation mean analysis of data combined over years or locations indicated that a simple additive-dominance model was able to explain the inheritance of resistance for all populations. Dominance effects were detected in all population evaluated. For the FR1141 × 061 and FR1141 × B37HtN populations, dominance was significant at early ratings, but not at late ratings. Results from generation mean analysis for individual years, locations, and rating were variable.
Characterization of Ribosomal DNA from Venturia inaequalis and Its Phylogenetic Relationship to rDNA from Other Tree-Fruit Venturia Species
Authors: Schnabel G. , Schnabel E. L. , Jones A. L.
Published: May 2007

A portion of the 18S ribosomal DNA (rDNA) gene, the internal transcribed spacers (ITS1 and ITS2), and the 5.8S rDNA gene were polymerase chain reaction-amplified from strains and field populations of Venturia inaequalis and assessed for genetic variation. A previously reported optional group I intron in the 18S rDNA gene of V. inaequalis was detected in 75.0% of 92 strains collected worldwide and in 61.1 and 71.2% of 54 and 59 strains from two Michigan orchards, respectively. Sequence and restriction analysis of rDNA revealed four intron alleles, three of which were present both in worldwide strains and in each field population. Two ITS1 alleles were detected and found to be linked to specific intron alleles. The ITS1-5.8S-ITS2 sequences from V. asperata V. carpophila, V. cerasi, V. inaequalis, V. nashicola, V. pyrina, and Cladosporium caryigenum were compared using phylogenetic analysis. Strains of the Venturia species were placed in three distinct monophyletic groups in a phylogenetic tree. The first group comprised V. inaequalis; the second, V. pyrina and V. nashicola; and the third, V. cerasi, V. carpophila, and V. asperata. The described intron and ITS1 alleles in V. inaequalis provide genetic markers for subdividing populations of V. inaequalis, and the ITS1-5.8S-ITS2 sequences are valuable in determining the relationship of the species from tree-fruit crops with other Venturia species.
Causal Role of Xylella fastidiosa in Oleander Leaf Scorch Disease
Authors: Purcell A. H. , Saunders S. R. , Hendson M. , Grebus M. E. , Henry M. J.
Published: May 2007

A lethal leaf scorch disease of oleander (Nerium oleander) appeared in southern California in 1993. A bacterium, Xylella fastidiosa, was detected by culturing, enzyme-linked immunoassay, and polymerase chain reaction in most symptomatic plants but not in symptomless plants or negative controls. Inoculating oleanders mechanically with X. fastidiosa cultures from diseased oleanders caused oleander leaf scorch (OLS) disease. The bacterium was reisolated from inoculated plants that became diseased. Three species of xylem sap-feeding leafhoppers transmitted the bacterium from oleander to oleander. The bacterium multiplied, moved systemically, and caused wilting in Madagascar periwinkle (Catharanthus rosea) and leaf scorch in periwinkle (Vinca major) in a greenhouse after inoculation with needle puncture. No bacterium was reisolated from grapevine (Vitis vinifera), peach (Prunus persica), olive (Olea europaea), California blackberry (Rubus ursinus), or valley oak (Quercus lobata) mechanically inoculated with OLS strains of X. fastidiosa. A 500-bp sequence of the 16S-23S ribosomal intergenic region of oleander strains showed 99.2% identity with Pierce's disease strains, 98.4% identity with oak leaf scorch strains, and 98.6% identity with phony peach, plum leaf scald, and almond leaf scorch strains.
A Generalized Two-Dimensional Gaussian Model of Disease Foci of Head Blight of Wheat Caused by Gibberella zeae
Authors: Paulitz T. C. , Dutilleul P. , Yamasaki S. H. , Fernando W. G. D. , Seaman W. L.
Published: May 2007

A generalized two-dimensional Gaussian model is proposed to describe disease foci of head blight of wheat in plots (100 to 2,500 m2) originating from small areas (1 to 16 m2) inoculated with Gibberella zeae-colonized corn kernels. These anisotropic, asymmetrical foci arose from ascospores produced in perithecia. The model is Z = exp[-(AX2 + BY2 + CXY + DX + EY + F)], in which Z = the incidence of seed or spikelet infection at point (X,Y) located in the plot, exp = the exponential function, X = the abscissa or spatial coordinate of the point along a given axis (approximately parallel to the average wind vector during the period of spore release in these experiments), Y = the ordinate or spatial coordinate of the point along the axis perpendicular to the X axis (approximately perpendicular to the wind direction in these experiments), A and B = the quadratic coefficients of the second-order polynomial AX2 + BY2 + CXY + DX + EY + F, C = the bilinear coefficient, D and E = the linear coefficients, and exp(-F) = the incidence of seed or spikelet infection at the focus peak in which X = 0 and Y = 0. The generalized two-dimensional Gaussian model was tested on data from a circular or isotropic focus, an elliptical or anisotropic focus with two axes of symmetry, and two anisotropic foci with one and zero axis of symmetry. Its goodness-of-fit (r2 and adjusted r2) was compared with the inverse power, modified inverse power, exponential, and classical Gaussian models. Submodels using only the linear terms, only the quadratic terms, or combinations selected from stepwise regression procedures using various probabilities to enter and to stay and a procedure maximizing the adjusted r 2 were also considered. Spatial analysis of the residuals was performed using Geary's c coefficient at the first distance class. For the circular and elliptical foci, our model provided a fit similar to the modified inverse power and exponential models. However, for anisotropic foci with one or zero axis of symmetry arising from ascospores influenced by wind direction, the generalized two-dimensional Gaussian model provided a better fit. For these anisotropic foci, the linear term X but not the quadratic term X2 was generally retained in the model, indicating an exponential gradient in the direction parallel to the wind. In all models, the quadratic term Y2 was retained, along with Y in some cases, indicating that the gradient in the direction roughly perpendicular to the wind was Gaussian or Gaussian-exponential in shape. The bilinear term XY provided an indication of the orientation of the focus in relation to the axes of the sampling grid. This model has the versatility and parameters (quadratic, bilinear, and linear) to better describe the anisotropy of foci from wind-dispersed spores.
Dynamics of Primary and Secondary Infection in Take-All Epidemics
Authors: Bailey D. J. , Gilligan C. A.
Published: May 2007

Using a combination of experimentation and mathematical modeling, the effects of initial (particulate) inoculum density on the dynamics of disease resulting from primary and secondary infection of wheat by the take-all fungus, Gaeumannomyces graminis var. tritici, were tested. A relatively high inoculum density generated a disease progress curve that rose monotonically toward an asymptote. Reducing the initial inoculum density resulted in a curve that initially was monotonic, rising to a plateau, but which increased sigmoidally to an asymptotic level of disease thereafter. Changes in the infectivity of particulate inoculum over time were examined in a separate experiment. Using a model that incorporated terms for primary and secondary infection, inoculum decay, and host growth, we showed that both disease progress curves were consistent with consecutive phases dominated, respectively, by primary and secondary infection. We examined the spread of disease from a low particulate inoculum density on seminal and adventitious root systems separately. Although seminal roots were affected by consecutive phases of primary and secondary infection, adventitious roots were affected only by secondary infection. We showed that the characteristic features of disease progress in controlled experiments were consistent with field data from crops of winter wheat. We concluded that there is an initial phase of primary infection by G. graminis var. tritici on winter wheat as seminal roots grow through the soil and encounter inoculum, but the rate of primary infection slows progressively as inoculum decays. After the initial phase, there is an acceleration in the rate of secondary infection on both seminal and adventitious roots that is stimulated by the increase in the availability of infected tissue as a source of inoculum and the availability of susceptible tissue for infection.
Oligonucleotides as Hybridization Probes to Localize Phytoplasmas in Host Plants and Insect Vectors
Authors: Webb D. R. , Bonfiglioli R. G. , Carraro L. , Osler R. , Symons R. H.
Published: May 2007

Protocols have been developed using 20- to 24-mer oligodeoxynucleotides, originally designed as polymerase chain reaction primers, as hybridization probes for the nonradioactive detection of Italian clover phyllody (ICPh) phytoplasma in plant (Chrysanthemum carinatum) and leafhopper (Euscelidius variegatus) tissue. In situ hybridization of paraffin-embedded tissue sections was carried out using oligodeoxynucleotides 5′ end-labeled with either Cy5 fluorochrome, biotin, or digoxigenin. The Cy5-labeled oligonucleotide probes that hybridized to phytoplasmas present in plant tissue were visualized by confocal microscopy. The biotin- and digoxigeninlabeled probes were detected in both plant and insect tissue using a chromogenic alkaline phosphatase-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl-phosphate reaction. An enhancement of a signal was observed in plant tissue when a tyramide signal-amplification procedure was incorporated into the biotin or digoxigenin detection systems. The results obtained using these techniques with the ICPh phytoplasma system showed that they can provide a rapid means of confirming vector status in insects. Due to the potential ability of short, labeled, oligonucleotide probes to specifically distinguish between different phytoplasmas present in multiple infections, this technique should provide a powerful new tool for epidemiological and vector ecology studies.
Distribution of Xanthomonas oryzae pv. oryzae Strains Virulent to Xa21 in Korea
Authors: Lee S. W. , Choi S. H. , Han S. S. , Lee D. G. , Lee B. Y.
Published: May 2007

Strains of Xanthomonas oryzae pv. oryzae that are virulent to rice lines carrying the Xa21 resistance gene were widely distributed in Korea. A total of 105 strains collected during 1987 to 1996 in Korea was characterized by pathogenicity tests and restriction fragment length polymorphism analysis of the XorII methyltransferase (xorIIM) and avrXa10 genes. Although the lesion lengths on rice line IRBB21, which carries Xa21, decreased as plant age increased, resistance and susceptibility of the plants to 31 strains were clearly differentiated at the seedling (14, 21, and 28 days old), maximum tillering, and flag leaf stages. The resistance or susceptibility of seedlings was correlated with bacterial populations within an inoculated leaf. There was a significant change in the population structure of X. oryzae pv. oryzae with regard to virulence to Xa21 over the last 10 years; this change in population was confirmed by genome analysis. Lineage I, which is avirulent to Xa21 and does not have a genomic xorIIM homolog, was the predominant lineage found between 1987 and 1989, while lineage II, which is virulent to Xa21 and contains the xorIIM homolog, was predominant in strains collected between 1994 and 1995. Our results demonstrate that introduction of Xa21 into commercial rice should be based on the regional structure of X. oryzae pv. oryzae populations and suggest that Xa21 will not be useful in Korea.
Characterization of Mycorrhizal Isolates of Rhizoctonia solani from an Orchid, Including AG-12, a New Anastomosis Group
Authors: Carling D. E. , Pope E. J. , Brainard K. A. , Carter D. A.
Published: May 2007

Isolates of Rhizoctonia solani collected from mycorrhizal orchid (Pterostylis acuminata) plants and adjacent leaf litter were characterized. Of 23 selected isolates, 20 were members of a new anastomosis group (AG-12) and the rest were members of AG-6. There were no bridging anastomosis reactions observed between AG-12 and other AGs of R. solani. Among the 20 isolates of AG-12 evaluated, 18 vegetatively compatible populations were detected, indicating diversity within the AG. Mature cultures were dark brown, as were mature sclerotia. Some cultures produced alternating dark- and light-colored concentric rings, with sclerotia forming in the darker rings. Most cultures were appressed to the agar surface. In tests run to characterize pathogenic potential, selected mycorrhizal isolates of AG-12 and AG-6 did little damage to potato and barley seedlings, moderate damage to head lettuce seedlings, and more extensive damage to seedlings of cauliflower and radish. Isolates of AG-12 have not been observed to fruit in nature, and all attempts to induce formation of the teleomorph (Thanatephorus cucumeris) in the laboratory by selected isolates of AG-12 failed.
Two Sibling Species of the Botrytis cinerea Complex, transposa and vacuma, Are Found in Sympatry on Numerous Host Plants
Authors: Giraud T. , Fortini D. , Levis C. , Lamarque C. , Leroux P. , LoBuglio K. , Brygoo Y.
Published: May 2007

Strains of Botrytis cinerea (the anamorph of Botryotinia fuckeliana) were collected from 21 different plant species around vineyards in the Champagne region (France). Strains were analyzed using three new polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) markers that were found by SWAPP (sequencing with arbitrary primer pairs), in addition to 15 other markers (PCR-RFLP, transposable elements, and resistance to fungicides). The markers revealed a high degree of genetic diversity and were used to investigate population structure. The two sympatric species transposa and vacuma, previously identified on grapes in these vineyards, were also detected on many of the plant species sampled. A new type of strain was also detected, having only the transposable element Boty. We did not detect any differentiation between strains from different organs or locations, but the prevalences of transposa and vacuma were significantly different on the different host plants. Fungicide resistance frequencies were significantly different in transposa and vacuma species. This study confirms that B. cinerea is a complex of sibling species and shows that the sibling species occur sympatrically on many host plants. However, the two species seemed to have different pathogenic behaviors. These findings contradict the traditional view of B. cinerea as a clonal population without specialization.
Effect of Antibiosis on Antagonist Dose-Plant Disease Response Relationships for the Biological Control of Crown Gall of Tomato and Cherry
Authors: Johnson K. B. , DiLeone J. A.
Published: May 2007

The crown gall pathosystem was used to evaluate a model that describes the dose-response relationship between biological control agents and plant pathogens. The model predicts that this relationship can become asymptotic, such that increased antagonist doses cannot compensate for deficiencies in disease suppression. Wounded roots of tomato (Lycopersicon esculentum) and cherry (Prunus mahaleb) plants were dipped into different concentrations of the biological control organism Agrobacterium radiobacter strain K84 prior to inoculation with the pathogen A. tumefaciens. Pathogen strains sensitive or resistant to the antibiotic agrocin 84 were used, and for tomato experiments, a derivative of A. radiobacter strain K84 that does not produce agrocin 84 also was included as an experimental treatment. As predicted by the dose-response model, the amount of disease suppression per unit of antagonist decreased with increasing antagonist dose and became asymptotic at high antagonist densities. Control of crown gall of tomato was nearly complete with the combination of A. radiobacter K84 and an agrocin 84-sensitive strain of A. tumefaciens. Pathogen resistance to agrocin 84 or lack of agrocin 84 production by A. radiobacter resulted in antagonist dose-crown gall incidence relationships that were apparently asymptotic at levels of control significantly less than 100%. For field-grown cherry, similar dose-response relationships were observed with higher asymptotic levels of disease suppression obtained when trees were inoculated with an agrocin 84-sensitive A. tumefaciens strain compared with an agrocin 84-resistant pathogen strain. The differences among bacterial strain combinations in the magnitude of the asymptote defined by the dose-response relationships suggest that A. radiobacter impacts a smaller proportion of the pathogen population when the activity of agrocin 84 is muted.
Fusion Proteins of Single-Chain Variable Fragments Derived from Phage Display Libraries Are Effective Reagents for Routine Diagnosis of Potato Leafroll Virus Infection in Potato
Authors: Toth R. L. , Harper K. , Mayo M. A. , Torrance L.
Published: May 2007

A panel of 11 different single-chain variable fragment antibodies (scFv) that bind to potato leafroll virus (PLRV) has been studied to assess each one's suitability as practical diagnostic tools. The scFv, previously obtained from naive phage display libraries, were expressed in Escherichia coli as fusion proteins. The fusion proteins comprised scFv joined to either the human light chain kappa constant domain (CL), an amphipathic helix (Zip), a combination of CL and Zip, or alkaline phosphatase (AP/S). The fusion proteins were tested for their ability to detect, or trap on enzymelinked immunosorbent assay (ELISA) plates, PLRV in extracts of infected potato leaves. The tests done with the different scFv fusion proteins were compared with a standard triple-antibody sandwich (TAS)-ELISA that employs a rabbit polyclonal antibody preparation to coat microtiter plates and a monoclonal antibody, SCR3, to detect PLRV. Of 11 scFvCL fusion proteins, 7 detected PLRV as readily as SCR3 when used as detecting antibodies in TAS-ELISA. The limit of detection of purified PLRV for the different scFvCL fusion proteins ranged from 250 to 5 ng/ml; that for SCR3 is 5 ng/ml. Of the 11 scFv, 4 cross-reacted with some other luteoviruses. Several scFvCL and scFvCLZip fusion proteins trapped PLRV from extracts of infected potato leaves as effectively as the polyclonal antibody preparation. Four scFv fusion proteins were used in a stem print assay to detect PLRV, and the results were similar to those obtained in tests using SCR3. The scFvCL fusion proteins retained activity for at least 6 months at 4°C, and all scFv fusion proteins were fully active on reconstitution after lyophilization. A fully recombinant ELISA was devised that detected PLRV in extracts of infected potato, with results comparable to those obtained using the standard TAS-ELISA. The advantages of using scFv fusion proteins for the routine detection of plant viruses include the ability to produce large quantities of reagents cheaply in bacterial fermenters and to incorporate them into standardized tests.
Interaction of Fusarium graminearum and F. moniliforme in Maize Ears: Disease Progress, Fungal Biomass, and Mycotoxin Accumulation
Authors: Reid L. M. , Nicol R. W. , Ouellet T. , Savard M. , Miller J. D. , Young J. C. , Stewart D. W. , Schaafsma A. W.
Published: May 2007

To investigate the interaction between two major ear-rotting pathogens, maize ears were inoculated with either Fusarium graminearum, F. moniliforme, or an equal mixture of the two. Silk and kernel tissues were periodically harvested throughout the growing season so that a time course of the experimental variables (disease severity, ergosterol content, fungal DNA content, and mycotoxin concentration) could be recorded. Over the 3 years tested (1992 to 1994), the highest levels of disease and ergosterol were found in the F. graminearum treatment, followed by the mixture treatment (F. graminearum plus F. moniliforme) and, finally, the F. moniliforme treatment. Kernel ergosterol content and disease rating were correlated for both pathogens, but the highest correlation coefficients were obtained in the F. graminearum treatment. The DNA analysis revealed that, in the mixed inoculum, F. moniliforme had a greater growth rate than did F. graminearum. In 1994, appreciable F. moniliforme from natural inoculum was found in the F. graminearum treatment. Fumonisin B1 levels did not differ between the F. moniliforme treatment and the mixed inoculum treatment. The effect of temperature on the growth rate of the two species explained some of the field results, with temperatures in the silks being more favorable to F. moniliforme. Data on the growth rate on silks obtained by the incorporation of radiolabeled precursor to ergosterol demonstrated that F. graminearum was able to grow well at 26 to 28°C, whereas F. moniliforme grew well over a broader range, including at higher temperatures.
Sampling in Seed Health Testing
Authors: Morrison R. H.
Published: May 2007

Seed health tests are usually performed on a sample of a seed lot; therefore, it is crucial that the test sample be as homogeneous as possible and representative of the lot. Seed sampling procedures appropriate for seed health testing have been developed by seed testing organizations such as the International Seed Testing Association and the Association of Official Seed Analysts. Seed lot size is generally not a constraint when the distribution of contaminated/infected seed in the lot is relatively homogeneous; if the distribution is heterogeneous, increased sampling intensity is required. Sample size is determined by the damage threshold (intolerable infection level) for the pathogen and the probability of detection desired, commonly 95 or 99%. The probability of detection at a given damage threshold is greater as sample size increases, and this probability and the appropriate sample size can be determined by statistical methods. Most seed health tests utilize qualitative data based on the presence or absence of the pathogen in the test sample, with the lot being rejected if the pathogen is detected in the sample and accepted if the sample is negative.
Epidemiology in Mixed Host Populations
Authors: Garrett K. A. , Mundt C. C.
Published: May 2007

Although plant disease epidemiology has focused on populations in which all host plants have the same genotype, mixtures of host genotypes are more typical of natural populations and offer promising options for deployment of resistance genes in agriculture. In this review, we discuss Leonard's classic model of the effects of host genotype diversity on disease and its predictions of disease level based on the proportion of susceptible host tissue. As a refinement to Leonard's model, the spatial structure of host and pathogen population can be taken into account by considering factors such as autoinfection, interaction between host size and pathogen dispersal gradients, lesion expansion, and host carrying capacity for disease. The genetic composition of the host population also can be taken into account by considering differences in race-specific resistance among host genotypes, compensation, plant competition, and competitive interactions among pathogen genotypes. The magnitude of host-diversity effects for particular host-pathogen systems can be predicted by considering how the inherent characteristics of a system causes it to differ from the assumptions of the classic model. Because of the limited number of studies comparing host-diversity effects in different systems, it is difficult at this point to make more than qualitative predictions. Environmental conditions and management decisions also influence host-diversity effects on disease through their effect on factors such as host density and epidemic length and intensity.
Loss of Avirulence and Reduced Pathogenicity of a Gamma-Irradiated Mutant of Fusarium oxysporum f. sp. lycopersici
Authors: Mes Jurriaan J. , Wit Robbert , Testerink Christa S. , de Groot Francis , Haring Michel A. , Cornelissen Ben J. C.
Published: May 2007

The tomato Fusarium resistance gene I-2 confers resistance to F. oxy-sporum f. sp. lycopersici race 2, which expresses the corresponding aviru-lence gene avrI-2. To elucidate the molecular basis of this gene-for-gene interaction, we initiated a search for the avrI-2 gene. Gamma irradiation mutagenesis, using 137Cs, was performed to generate an avrI-2 mutant of F. oxysporum f. sp. lycopersici. To this end, a race 2 isolate was first transformed with a phleomycine resistance gene and a GUS marker gene in order to distinguish mutants from contaminating isolates. A total of 21,712 mutagenized colonies was tested for loss of avirulence on I-2-containing tomato seedlings. One mutant was selected that showed the expected loss of avirulence but, surprisingly, also showed reduced pathogenicity toward susceptible tomato plants. DNA analysis was subsequently used to visualize genomic changes in the mutant. Southern analysis on contour-clamped homogeneous electrophoretic field blots demonstrated a translocation of a 3.75-Mb chromosome in the mutant. Random amplified polymorphic DNA and amplified fragment length polymorphism analysis identified at least nine polymorphisms between the wild-type and mutant isolates. Most of these polymorphisms appeared as extra fragments in the mutant and contained repetitive DNA sequences.
Epiphytic Colonization of Pear Stigmas and Hypanthia by Bacteria During Primary Bloom
Authors: Stockwell V. O. , McLaughlin R. J. , Henkels M. D. , Loper J. E. , Sugar D. , Roberts R. G.
Published: May 2007

Pear blossoms were sampled during various stages of bloom in 1991 and 1992 from orchards at Cashmere, WA, and Corvallis and Medford, OR, for epiphytic populations of culturable bacteria. On stigmatic surfaces, bacteria were isolated from 2 to 32% of blossoms prior to petal expansion and from 47 to 94% of blossoms by petal fall. In general, a lower percentage of hypanthia than stigmas supported bacterial populations. Randomly selected bacteria isolated at population levels of ≥104 CFU/tissue were identified by fatty acid methyl ester analysis. Diverse genera of gram-negative and -positive bacteria were identified from the Medford and Cashmere field sites. Pseudomonas syringae and Pseudomonas viri-diflava were isolated from all sites and were the predominant species detected at Corvallis, where they were isolated from 28% of the blossoms sampled on a given date. Because most pear blossoms do not support detectable populations (≥102 CFU/tissue) of culturable bacteria prior to petal expansion, we speculate that introduced biocontrol agents may become established with minimal competition from indigenous epiphytes at early bloom stages.
Fully Biologically Active In Vitro Transcripts of the Eriophyid Mite-Transmitted Wheat Streak Mosaic Tritimovirus
Authors: Choi Il-Ryong , French Roy , Hein Gary L. , Stenger Drake C.
Published: May 2007

Infectious RNA of wheat streak mosaic virus (WSMV) has been produced using a full-length cDNA clone as a template for in vitro transcription with SP6 RNA polymerase. Infectivity was dependent on the use of template plasmid DNA that had not undergone spontaneous rearrangement during amplification in Escherichia coli. The presence of WSMV in systemically infected wheat plants inoculated with in vitro transcripts was confirmed by reverse-transcription polymerase chain reaction of the WSMV P3 gene and by accumulation of WSMV coat protein as detected by immunoblotting. Maintenance of the full-length WSMV cDNA in the high copy number plasmid pUC18 was problematic because of spontaneous rearrangement of WSMV sequences during growth in liquid media for more than ˜8 h or if the clone was subcultured. Stability of the WSMV cDNA clone was improved by the use of the low copy number plasmid pACYC177, and it could be grown in large scale volumes (up to 1 liter) of liquid culture for ˜14 h without noticeable rearrangements. Both the original WSMV culture and the progeny virus derived from infectious in vitro transcripts were efficiently transmitted by the natural eriophyid mite vector Aceria tosichella. This is the first report of infectious in vitro transcripts for any eriophyid mite-transmitted plant virus and represents the only monopartite member of the family Potyviridae infecting monocotyledonous hosts for which infectious in vitro transcripts are available.
Further Evidence that Shallot Yellow Stripe Virus (SYSV) Is a Distinct Potyvirus and Reidentification of Welsh Onion Yellow Stripe Virus as a SYSV Strain
Authors: van der Vlugt R. A. A. , Steffens P. , Cuperus C. , Barg E. , Lesemann D.-E. , Bos L. , Vetten H. J.
Published: May 2007

An antiserum to shallot yellow stripe virus (SYSV) was raised and used in combination with a range of other antisera to potyviruses of Allium spp. in electron microscopic decoration experiments. The serological results corroborated an earlier finding that the type isolates of SYSV and Welsh onion yellow stripe virus (WoYSV) are closely related to each other and only distantly related to onion yellow dwarf (OYDV) and leek yellow stripe (LYSV) viruses, the two other major potyviruses infecting Allium spp. Moreover, the decoration results indicated that Japanese potyviruses named OYDV and Wakegi yellow dwarf virus are isolates of SYSV. Sequence analysis of the 3′-terminal regions of the SYSV and WoYSV ge-nomes revealed coat protein (CP) amino acid and 3′-nontranslated region (3′-NTR) nucleotide sequence identities of 95 and 89%, respectively. The CP amino acid and 3′-NTR nucleotide sequences of these viruses differed from those of OYDV and LYSV by >25 and >67%, respectively. The serological and molecular studies showed that SYSV and WoYSV are different strains of a potyvirus distinct from OYDV and LYSV. For priority reasons, we propose that these strains together with the Wakegi-type isolates of OYDV described in Japan be referred to as SYSV and that SYSV isolates from Allium spp. other than shallot be designated as the Welsh onion strain of SYSV (SYSV-Wo).
Variability in Geminivirus Isolates Associated with Phaseolus spp. in Brazil
Authors: Faria Josias C. , Maxwell Douglas P.
Published: May 2007

Bean golden mosaic geminivirus (BGMV) is the single most devastating virus of common beans in the tropical and subtropical Americas and the Caribbean Basin. The BGMV from Brazil, named BGMV-BZ, is considered distinct from BGMV-PR isolates from Puerto Rico, Guatemala, and the Dominican Republic because of DNA sequence data, the ability to form pseudorecombinants, and mechanical transmissibility properties. In bean-growing areas of Brazil, samples were collected from beans, lima beans, and the weed Leonurus sibiricus displaying typical symptoms of infection by geminiviruses. Viral DNA fragments comprising part of the rep gene, the common region, and part of the cp gene were amplified by polymerase chain reaction, cloned, and sequenced. The bean samples had geminivirus with sequences nearly identical to that of BGMV-BZ collected in Goiânia, state of Goiás, in 1986. The sample from lima bean contained a new species of geminivirus that induces symptoms similar to those induced by BGMV-BZ and was named lima bean golden mosaic virus (LBGMV-BR). While all sequences from bean samples clustered with BGMV-BZ, the sequence from the lima bean isolate stood alone. A mixed infection with abutilon mosaic geminivirus was also found in a single sample from the state of São Paulo. DNA sequence comparisons indicate that the virus isolate from L. sibiricus represents a new geminivirus species, designated here as leonurus mosaic virus.
Genetic Analysis of Sensitivity to a Pyrenophora tritici-repentis Necrosis-Inducing Toxin in Durum and Common Wheat
Authors: Anderson J. A. , Effertz R. J. , Faris J. D. , Francl L. J. , Meinhardt S. W. , Gill B. S.
Published: May 2007

The fungus Pyrenophora tritici-repentis produces a toxin (Ptr ToxA) that causes rapid cell necrosis in sensitive wheat genotypes. A single recessive gene (tsn1) on chromosome 5BL in common wheat confers insensitivity to this toxin. Our objectives were to analyze the allelic relationships of genotypes that have shown insensitivity to a P. tritici-repentis necrosis-inducing toxin, map the gene for insensitivity to the necrosis-inducing factor produced by P. tritici-repentis in a durum wheat population, and determine the reaction to P. tritici-repentis of aneuploid genotypes that do not contain the gene. Greenhouse-grown plants of seven populations from crosses of insensitive genotypes; an F2 population of durum wheat; and ‘Chinese Spring’ aneuploid, substitution, and deletion lines were infiltrated with Ptr ToxA. All crosses involving insensitive genotypes failed to produce sensitive progeny, indicating that the same gene is present in these genotypes. The gene for insensitivity in the durum population was mapped to the same region on 5BL as in common wheat using restriction fragment length polymorphism markers. ‘Chinese Spring’, its homoeologous group 5 nullisomic-tetrasomic stocks, and 5BL deletion lines were insensitive to the toxin. Substitution of a 5B chromosome from sensitive genotypes into ‘Chinese Spring’ resulted in sensitivity. Therefore, insensitivity is not conferred by a gene product per se, but rather conferred by absence of a gene for sensitivity.
Biological Suppression and Natural Population Decline ofHeterodera schachtiiin a California Field
Authors: Westphal A. , Becker J. O.
Published: May 2007

Soil suppressiveness to Heterodera schachtii was demonstrated in a field at the research station of the University of California, Riverside. In two field trials planted to Swiss chard (Beta vulgaris), introduced H. schachtii multiplied 2.7 and 1.7 times more in preplant metam sodium-fumigated plots than in nontreated plots in 1994 and 1995, respectively. In greenhouse experiments, preplant treatments with metam sodium, methyl bromide, methyl iodide, formaldehyde, and aerated steam reduced suppressiveness of soil against H. schachtii to undetectable levels. H. schachtii multiplied significantly less in nontreated soil than in treated soil on Swiss chard. At harvest, the number of infective second-stage juveniles in suppressive soil was close to the lowest detection level, whereas high numbers were encountered in soils initially treated. In a crop rotation trial with host crops of H. schachtii, introduced H. schachtii populations were monitored for five cropping periods over 30 months in initially fumigated versus nontreated suppressive field plots. In fumigated plots, H. schachtii population levels increased in the first and second cropping periods and then declined in the third cropping period. In the fourth and fifth cropping periods, the nematode reproduction factor in the initially fumigated plots was not significantly different from that in suppressive plots.
Increasing Complexity of Resistance in Host Populations Through Intermating to Manage Rust of Pearl Millet
Authors: Tapsoba H. , Wilson J. P.
Published: May 2007

Pearl millet inbreds Tift 23DB, Tift 85DB, PS748BC, and Tift 89D2 were used to develop three categories of host mixtures (physical mixtures, random-mated populations, and mixtures of two-way and three-way crosses) representing different levels of complexity of resistance through increased heterogeneity within populations and through stacking of resistance genes within the heterogeneous populations. The potential of these mixtures to reduce rust epidemics was evaluated in the field. Area under the disease progress curves (AUDPCs) of all physical mixtures were less than the mean of the components in 1995 and were less than the mean of the components for five of the six mixtures in 1997. In 1996, AUDPCs of the physical mixtures were consistently greater than the mean of their components. AUDPCs of the random-mated mixtures and the mixtures of crosses were consistently less than the mean of the components in 1996 and 1997, with reductions ranging from 12 to 71%. Dry matter yield (DMY) of physical mixtures relative to the mean DMY of the components was inconsistent, ranging from 18% less to 50% more than the mean of the components. The random-mated populations and the mixtures of crosses yielded 18 to 40% more DMY than the mean yield of the pure stands of their components.
Effect of Population Density of Pseudomonas fluorescens on Production of 2,4-Diacetylphloroglucinol in the Rhizosphere of Wheat
Authors: Raaijmakers Jos M. , Bonsall Robert F. , Weller David M.
Published: May 2007

The role of antibiotics in biological control of soilborne pathogens, and more generally in microbial antagonism in natural disease-suppressive soils, often has been questioned because of the indirect nature of the supporting evidence. In this study, a protocol for high pressure liquid chromatography/mass spectrometry is described that allowed specific identification and quantitation of the antibiotic 2,4-diacetylphloroglucinol (Phl) produced by naturally occurring fluorescent Pseudomonas spp. on roots of wheat grown in a soil suppressive to take-all of wheat. These results provide, for the first time, biochemical support for the conclusion of previous work that Phl-producing fluorescent Pseudomonas spp. are key components of the natural biological control that operates in take-all—suppressive soils in Washington State. This study also demonstrates that the total amount of Phl produced on roots of wheat by P. fluorescens strain Q2-87, at densities ranging from approximately 105 to 107 CFU/g of root, is proportional to its rhizosphere population density and that Phl production per population unit is a constant (0.62 ng/105 CFU). Thus, Phl production in the rhizosphere of wheat is strongly related to the ability of the introduced strain to colonize the roots.
The Mycoparasite Ampelomyces quisqualis Expresses exgA Encoding an exo-β-1,3-Glucanase in Culture and During Mycoparasitism
Authors: Rotem Yaniv , Yarden Oded , Sztejnberg Abraham
Published: May 2007

Ampelomyces quisqualis, a mycoparasite of fungi causing powdery mildews, exhibited high levels of extracellular exo-β-1,3-glucanase activity in culture compared with Neurospora crassa and Gliocladium roseum. A. quisqualis culture filtrates affected powdery mildew caused by Sphaerotheca fusca in a manner indicative of cell wall degradation, as determined by microscopic examination. A gene encoding an exo-β-1,3-glucanase in A. quisqualis, designated exgA, was isolated and sequenced. The predicted polypeptide deduced from exgA had 46, 42, and 30% identity with amino acid sequences of Trichoderma harzianum exo-β-1,3-glucanase and Cochliobolus carbonum EXG1 (both encoding exo-β-1,3-glucanase) and T. harzianum bng13.1 (encoding an endo-β-1,3-glucanase), respectively. The exgA gene had a predicted molecular mass of 84 kDa and a pI of 4.79. The gene was expressed during the late stages of growth in culture, and transcription was induced by fungal cell wall components. Transcript levels for exgA were present during late stages of hyperpar-asitism and were abundant along A. quisqualis mycelium and were slightly less abundant in A. quisqualis pycnidia.