bluuebunny/crossref_metadata_2025 huggingface dataset:
Sampling for the Incidence of Aphid-Transmitted Viruses in Snap Bean
Authors: Shah Denis A. , Dillard Helene R. , Nault Brian A.
Published: May 2007

Data collected in 2002 and 2003 on Alfalfa mosaic virus and Cucumber mosaic virus incidences of infection in commercial snap bean fields in New York State were used to develop relationships between disease incidence (plow) and sample size while accounting for the inherent spatial aggregation of infected plants observed with these two viruses. For a plan consisting of 300 sampled plants (N = 60 quadrats, n = 5 plants per quadrat), estimating plow from the incidence of positive groups (phigh; testing of N = 60 grouped samples) provides the same precision in plow as testing 200 plants individually, up to about plow = 0.2. Above that, the confidence interval width for plow obtained via group testing becomes markedly larger than the width obtained by testing individual plants. Our results suggest using group testing until phigh is in the range [0.35, 0.59], which corresponds to plow in [0.1, 0.2]. Results indicate that group testing can be more economical than the testing of individual plants without loss of precision, at lower incidences of infection. The approach presented provides a general framework for sampling and the estimation of incidence of other aphid-transmitted viruses in snap bean.
Analysis of Aggressiveness of Erwinia amylovora Using Disease-Dose and Time Relationships
Authors: Cabrefiga Jordi , Montesinos Emilio
Published: May 2007

The aggressiveness of an extensive collection of strains of Erwinia amylovora was analyzed using immature fruit and detached pear flower assays under controlled environmental conditions. The analysis was performed by means of a quantitative approach based on fitting data to mathematical models that relate infection incidence to pathogen dose and time. Probit and hyperbolic saturation models were used for disease-dose relationships and provided information on the median effective dose (ED50). Values of ED50 ranged from 103 to 106 CFU/ml (10 to 104 CFU per site of inoculation). A modified Gompertz model was used for disease-time relationships and provided information on the rate of infection incidence progression (rg) and time delayed to start of the incidence progress curve (t0). Values of rg ranged from near 0 to 1.90, and t0 varied from 1.3 to more than 10 days. The more aggressive strains showed high rg, low ED50 values, and short t0, whereas the less aggressive strains showed low rg, high ED50, and long t 0. The aggressiveness was dependent on plant material type and pear cultivars and was significantly different between strains of E. amylovora. Infectivity titration and kinetic analysis of progression of incidence of infections using the immature pear test and a standardized scale are proposed for assessment of strain aggressiveness. The implications of rg, ED50, and t0 for the epidemiology and management of fire blight are discussed, particularly the wide range of aggressiveness among strains, the degree of host specificity observed in pear isolates, the very high infective potential of this pathogen, the independent action of pathogen cells during infection, and the possible advantage of including aggressiveness parameters into fire blight risk forecasting systems.
Identification of a Maize Kernel Pathogenesis-Related Protein and Evidence for Its Involvement in Resistance to Aspergillus flavus Infection and Aflatoxin Production
Authors: Chen Z.-Y. , Brown R. L. , Rajasekaran K. , Damann K. E. , Cleveland T. E.
Published: May 2007

Aflatoxins are carcinogens produced by Aspergillus flavus and A. parasiticus during infection of susceptible crops such as maize. Several aflatoxin-resistant maize genotypes have been identified and kernel proteins have been suggested to play an important role in resistance. In the present study, one protein (#717), which was expressed fivefold higher in three resistant lines compared with three susceptible ones, was identified using proteomics. This protein was sequenced and identified as a pathogenesis-related protein (PR-10) based on its sequence homology. To assess the involvement of this PR-10 protein (ZmPR-10) in host resistance of maize against fungal infection and aflatoxin production, the corresponding cDNA (pr-10) was cloned. It encodes a protein of 160 amino acids with a predicted molecular mass of 16.9 kDa and an iso-electric point of 5.38. The expression of pr-10 during kernel development increased fivefold between 7 and 22 days after pollination, and was induced upon A. flavus infection in the resistant but not in the susceptible genotype. The ZmPR-10 overexpressed in Escherichia coli exhibited a ribonucleolytic and antifungal activities. Leaf extracts of transgenic tobacco plants expressing maize pr-10 also demonstrated RNase activity and inhibited the growth of A. flavus. This evidence suggests that ZmPR-10 plays a role in kernel resistance by inhibiting fungal growth of A. flavus.
Suppression of the Plant-Parasitic Nematode Heterodera schachtii by the Fungus Dactylella oviparasitica
Authors: Olatinwo Rabiu , Yin Bei , Becker J. Ole , Borneman James
Published: May 2007

This study examined the role of the fungi Dactylella oviparasitica and Fusarium oxysporum in the beet-cyst nematode (Heterodera schachtii) suppressiveness exhibited by a southern Californian soil. In prior research, the abundance of D. oviparasitica rRNA genes positively correlated with high levels of suppressiveness, whereas the abundance of F. oxysporum rRNA genes positively correlated with minimal to moderate levels of suppressiveness. In this report, both fungi were added to fumigation-induced nonsuppressive soil, planted with Swiss chard, and infested with H. schachtii juveniles. After two nematode generations, D. oviparasitica strain 50 reduced the population densities of H. schachtii eggs and juveniles to those in the suppressive soil and H. schachtii cysts to levels lower than in the suppressive soil. F. oxysporum did not significantly reduce H. schachtii populations. These results suggest that D. oviparasitica strain 50 plays a major role in the suppression of H. schachtii population development in this southern Californian soil.
The Genetic Architecture of Disease Resistance in Maize: A Synthesis of Published Studies
Authors: Wisser Randall J. , Balint-Kurti Peter J. , Nelson Rebecca J.
Published: May 2007

Fifty publications on the mapping of maize disease resistance loci were synthesized. These papers reported the locations of 437 quantitative trait loci (QTL) for disease (dQTL), 17 resistance genes (R-genes), and 25 R-gene analogs. A set of rules was devised to enable the placement of these loci on a single consensus map, permitting analysis of the distribution of resistance loci identified across a variety of maize germplasm for a number of different diseases. The confidence intervals of the dQTL were distributed over all 10 chromosomes and covered 89% of the genetic map to which the data were anchored. Visual inspection indicated the presence of clusters of dQTL for multiple diseases. Clustering of dQTL was supported by statistical tests that took into account genome-wide variations in gene density. Several novel clusters of resistance loci were identified. Evidence was also found for the association of dQTL with maturity-related QTL. It was evident from the distinct dQTL distributions for the different diseases that certain breeding schemes may be more suitable for certain diseases. This review provides an up-to-date synthesis of reports on the locations of resistance loci in maize.
Identification and Characterization of Novel Genetic Markers Associated with Biological Control Activities in Bacillus subtilis
Authors: Joshi Raghavendra , McSpadden Gardener Brian B.
Published: May 2007

Suppressive subtractive hybridization (SSH) was used to identify genetic markers associated with biological control of plant pathogens by Bacillus subtilis. The genomes of two commercialized strains, GB03 and QST713, were compared with that of strain 168, which has no defined biocontrol capacities, to obtain a pool of DNA fragments unique to the two biocontrol strains. The sequences of 149 subtracted fragments were determined and compared with those present in GenBank, but only 80 were found to correspond to known Bacillus genes. Of these, 65 were similar to genes with a wide range of metabolic functions, including the biosynthesis of cell wall components, sporulation, and antibiotic biosynthesis. Sixteen subtracted fragments shared a high degree of similarity to sequences found in multiple B. subtilis strains with proven biocontrol capacities. Oligonucleotide primers specific to nine of these genes were developed. The targeted genes included five genes involved in antibiotic synthesis (bmyB, fenD, ituC,srfAA, and srfAB) and four additional genes (yndJ, yngG, bioA, and a hypothetical open reading frame) not previously associated with biological control. All nine markers were amplified from the commercialized B. subtilis strains GB03, QST713, and MBI600, with the exception of ituC, which was not detected in GB03. The markers also were amplified from four other B. subtilis isolates, but they were not amplified from other related Bacillus strains, including the plant growth-promoting rhizobacteria IN937a and IN937b. Sequencing of the amplified markers revealed that all seven of the isolates that scored positive for multiple markers were genotypically distinct strains. Interestingly, strains scored positive for the amplifiable markers generally were more effective at inhibiting the growth of Rhizoctonia solani and Pythium ultimum than other Bacillus isolates that lacked the markers. The potential utility of the defined genetic markers to further define the diversity, ecology, and biocontrol activities of B. subtilis are discussed.
Systemic Resistance Induced by Trichoderma spp.: Interactions Between the Host, the Pathogen, the Biocontrol Agent, and Soil Organic Matter Quality
Authors: Hoitink H. A. J. , Madden L. V. , Dorrance A. E.
Published: May 2007

Several factors affect the ability of Trichoderma spp. to provide systemic disease control. This paper focuses on the role of the substrate in which plants are grown, resistance of the host to disease, and the ability of introduced Trichoderma inoculum to spread under commercial conditions. Several reports reveal that foliar disease control provided by Trichoderma spp. is more effective on plants grown in compost-amended media compared with in lower-in-microbial-carrying-capacity sphagnum peat media. In Rhododendron spp., host resistance affects control of Phytophthora dieback provided by Trichoderma spp. For example, T. hamatum 382 (T382) significantly (P = 0.05) suppressed the disease on susceptible cv. Roseum Elegans while plant vigor was increased. The disease was not suppressed, however, on highly susceptible cvs. Aglo and PJM Elite even though the vigor of these plants was increased. Using a strain-specific polymerase chain reaction assay under commercial conditions, it was demonstrated that introduced inoculum of T382 did not spread frequently from inoculated to control compost-amended media. Other Trichoderma isolates typically are abundant in control media within days after potting unless inoculated with a specific Trichoderma isolate. Thus, the low population of isolates that can induce systemic resistance in composting and potting mix environments may explain why most compost-amended substrates do not naturally suppress foliar diseases.
Analysis of Quantitative Trait Loci for Resistance to Southern Leaf Blight in Juvenile Maize
Authors: Balint-Kurti P. J. , Carson M. L.
Published: May 2007

A set of 192 maize recombinant inbred lines (RILs), derived from a cross between the inbred lines Mo17 and B73, were evaluated as 3-week-old seedlings in the greenhouse for resistance to southern leaf blight, caused by Cochliobolus heterostrophus race O. Six significant (LOD >3.1) quantitative trait loci (QTL) were identified for disease resistance, located on chromosomes 1, 2, 3, 6, 7, and 8. Results were compared with a previous study that had used the same RIL population and pathogen isolate, but had examined resistance in mature rather than juvenile plants. There was a very weak but significant correlation between the overall resistance phenotypes of the RILs scored as mature and juvenile plants. Two QTL were found in similar positions on chromosomes 1 and 3 at both growth stages. Other QTL were specific to one growth stage or the other. Twenty-three of these RILs, together with the parental lines, were inoculated in the greenhouse with four C. heterostrophus isolates. Results indicated that the quantitative resistance observed was largely isolate non-specific.
Vertical Distribution of the Plant-Parasitic Nematode,Pratylenchus penetrans, Under Four Field Crops
Authors: Pudasaini Mahesh P. , Schomaker Cornelia H. , Been Thomas H. , Moens Maurice
Published: May 2007

The vertical distribution of Pratylenchus penetrans was monitored in four fields cropped with maize, black salsify, carrot, or potato. Soil samples were collected at 21-day intervals from May 2002 until April 2003 from five plots (2 × 5 m2) per field. Per plot, 15 cores were taken to a depth of 70 cm and split into seven segments of 10 cm each. Within the plots, segments from corresponding depths were pooled. After mixing, 200-g subsamples were taken and nematodes were extracted by zonal centrifugation from the root fraction and the mineral soil fraction separately. In most crops, the root fraction contained more than 50% of the total number of P. penetrans. Because the ratio between the numbers of nematodes in the root fraction and mineral soil fraction changes during the growing season, numbers of P. penetrans found in the mineral soil fraction cannot be used to estimate the total number in the soil. Therefore, both fractions have to be processed to obtain a reliable estimate of the density. No nematodes were recovered below 50 cm soil depth, except in the maize field where nematodes were found at 70 cm. The optimum sampling depth for maize, black salsify, carrot, and potato was 45, 25, 25, and 35 cm, respectively. The percentage of nematodes per soil layer was independent of the sampling date, indicating that a defined optimum sampling depth will be applicable throughout all seasons. The cumulative vertical distribution, modeled with a logistic equation, can be used to estimate the sampling error when samples are collected at different depths.
Synergistic Disease in Pepper Caused by the Mixed Infection of Cucumber mosaic virus and Pepper mottle virus
Authors: Murphy John F. , Bowen Kira L.
Published: May 2007

The occurrence of more than one virus species in a single plant is not uncommon in cultivated and native plant species. A mixed virus infection may lead to greater disease severity than individual viral components and this is sometimes referred to as a synergistic disease. Although, in some cases, synergism has been demonstrated for various plant growth parameters such as plant height, weight, and yield, proof of synergy typically has not been demonstrated for symptom severity when the mixed virus infection was not lethal. We demonstrated synergy in bell pepper plants co-infected with Cucumber mosaic virus (CMV) and Pepper mottle virus (PepMoV) relative to each virus alone for stem height (two of three trials) and aboveground fresh weight (one of three trials) using factorial analysis and Abbott's equation for synergy. This approach allowed affirmation of the type of response (i.e., synergistic rather than antagonistic) and statistical proof of synergy. A detailed evaluation of symptom severity for each viral treatment revealed three phases associated with host plant developmental stages. A numerical symptom severity rating scale was developed and used in each of two equations to demonstrate statistical proof for synergy based on symptom severity for co-infected plants. Virus accumulation in noninoculated leaves was determined by enzyme-linked immunosorbent assay. In singly infected plants, CMV titers declined in mildly symptomatic leaves representing later stages of plant development, but titers increased in similar leaves of co-infected plants. In contrast, PepMoV titers did not differ in singly or co-infected plants.
Factors Affecting Pseudomonas savastanoi pv. savastanoi Plant Inoculations and Their Use for Evaluation of Olive Cultivar Susceptibility
Authors: Penyalver R. , García A. , Ferrer A. , Bertolini E. , Quesada J. M. , Salcedo C. I. , Piquer J. , Pérez-Panadés J. , Carbonell E. A. , del Río C. , Caballero J. M. , López M. M.
Published: May 2007

Pseudomonas savastanoi pv. savastanoi causes olive knot disease, which is present in most countries where olive trees are grown. Although the use of cultivars with low susceptibility may be one of the most appropriate methods of disease control, little information is available from inoculation assays, and cultivar susceptibility assessments have been limited to few cultivars. We have evaluated the effects of pathogen virulence, plant age, the dose/response relationship, and the induction of secondary tumors in olive inoculation assays. Most P. savastanoi pv. savastanoi strains evaluated were highly virulent to olive plants, but interactions between cultivars and strains were found. The severity of the disease in a given cultivar was strongly dependent of the pathogen dose applied at the wound sites. Secondary tumors developed in noninoculated wounds following inoculation at another position on the stem, suggesting the migration of the pathogen within olive plants. Proportion and weight of primary knots and the presence of secondary knots were evaluated in 29 olive cultivars inoculated with two pathogen strains at two inoculum doses, allowing us to rate most of the cultivars as having either high, medium, or low susceptibility to olive knot disease. None of the cultivars were immune to the disease.
Quantitative and Qualitative Influence of Inoculation Methods on In Planta Growth of Rice Blast Fungus
Authors: Berruyer Romain , Poussier Stéphane , Kankanala Prasanna , Mosquera Gloria , Valent Barbara
Published: May 2007

Molecular analyses of early disease events require infected plant tissue in which the pathogen is present in high quantities and interacts with the plant in a way found in the field. In this study, a quantitative polymerase chain reaction (Q-PCR) assay was developed to determine an “infection ratio” of fungal to plant cells in infected tissues. This assay was used to evaluate four inoculation methods (spray, mist, dip, and sheath) as well as use of whole plants or excised parts. Fluorescence stereomicroscopy was used to follow individual lesions developing from appressoria to macroscopic symptoms. Disease progression and outcomes were documented from 24 to 96 h postinoculation (hpi), as well as effectiveness of Pi-ta-mediated resistance. Even at 96 hpi, fungus proliferated well ahead of visible plant damage, especially in veins. Developing lesions sometimes were surrounded by greener areas in detached leaves. Spray inoculation was not sufficient for detecting fungal gene expression in planta before 96 h. Alternatively, a leaf sheath assay produced infected tissues containing 10 to 30% fungal DNA by 34 h. Used together, Q-PCR quantification and fluorescence stereomicroscopy will facilitate studies of early plant invasion because infection density and fungal growth stages are directly observed, not assumed from incubation time.
Chickpea chlorotic stunt virus: A New Polerovirus Infecting Cool-Season Food Legumes in Ethiopia
Authors: Abraham A. D. , Menzel W. , Lesemann D.-E. , Varrelmann M. , Vetten H. J.
Published: May 2007

Serological analysis of diseased chickpea and faba bean plantings with yellowing and stunting symptoms suggested the occurrence of an unknown or uncommon member of the family Luteoviridae in Ethiopia. Degenerate primers were used for reverse transcriptase-polymerase chain reaction amplification of the viral coat protein (CP) coding region from both chickpea and faba bean samples. Cloning and sequencing of the amplicons yielded nearly identical (96%) nucleotide sequences of a previously unrecognized species of the family Luteoviridae, with a CP amino acid sequence most closely related (identity of ≈78%) to that of Groundnut rosette assistor virus. The complete genome (5,900 nts) of a faba bean isolate comprised six major open reading frames characteristic of polero-viruses. Of the four aphid species tested, only Aphis craccivora transmitted the virus in a persistent manner. The host range of the virus was confined to a few species of the family Fabaceae. A rabbit antiserum raised against virion preparations cross-reacted unexpectedly with Beet western yellows virus-like viruses. This necessitated the production of murine monoclonal antibodies which, in combination with the polyclonal antiserum, permitted both sensitive and specific detection of the virus in field samples by triple-antibody sandwich, enzyme-linked immunosorbent assay. Because of the characteristic field and greenhouse symptoms in chickpea, the name Chickpea chlorotic stunt virus is proposed for this new member of the genus Polerovirus (family Luteoviridae).
Molecular and Biological Characterization of a Mitovirus in Chalara elegans (Thielaviopsis basicola)
Authors: Park Yunjung , Chen Xiaobang , Punja Zamir K.
Published: May 2007

A 2.8-kb double-stranded RNA (dsRNA) element in strain BK18 of Chalara elegans originally isolated from cotton soil in California was characterized by obtaining a full-length cDNA sequence (2,896 nucleotides in length) from a series of overlapping clones. Sequence analysis revealed the presence of one large open reading frame (ORF I) using the mitochondrial genetic code, with 20 to 34% amino acid identity to the ORF I of other previously reported fungal mitochondrial RNA viruses. The ORF I encoded a putative protein of 705 amino acids and contained the conserved motif characteristic of RNA-dependent RNA polymerases. Purification of mitochondria from strain BK18 confirmed the co-localization of this dsRNA, and northern blot hybridization with a strand-specific probe revealed the (+) single-stranded nature. This Chalara elegans mitovirus (CeMV) is designated as a new member of the genus Mitovirus of the family Narnaviridae. Using dsRNA-specific primers, the ORF I region (positions 427 to 2544) was obtained from an additional 2.8-kb dsRNA element in strain HA2 originating from carrot roots in the Netherlands. Both ORFs had 98% homology at the nucleotide and amino acid levels. CeMV was also found to be present in five additional strains of C. elegans from different geographic locations worldwide, and a 97 to 100% nucleotide sequence identity was observed within a 300-bp region of ORF I in these strains. To determine the biological effects of CeMV on C. elegans, attempts to cure strain BK18 of the dsRNA were made. Sequential transfers of mycelium at 35 to 37°C yielded a colony which lacked the 2.8-kb dsRNA when visualized on agarose gels and also in northern blot hybridization analysis. However, reverse transcription-polymerase chain reaction with specific primer sets revealed a band, indicating that dsRNA replication had been significantly repressed (latent). The wild type and latently infected strains were compared for colony morphology, growth rate, melanin production, various enzymatic assays (polyphenoloxidase, laccase, tyrosinase, and esterase), and virulence on carrot roots. Colony morphology on V8 agar was comparable between the two strains, while growth rate, melanin production, and virulence were enhanced in the latently infected strain. There were no detectable differences in enzymatic activity. Transmission electron microscopy of hyphae of the wild type and latently infected strains revealed differences in the number and size of the mitochondria, which were enhanced in the latently infected strain. Our results show that CeMV is a new member of the genus Mitovirus with some disruptive effects on its fungal host and is present in C. elegans strains from different locations worldwide.
Main Effects, Epistasis, and Environmental Interactions of Quantitative Trait Loci for Fusarium Head Blight Resistance in a Recombinant Inbred Population
Authors: Ma H.-X. , Bai G.-H. , Zhang X. , Lu W.-Z.
Published: May 2007

Chinese Spring Sumai 3 chromosome 7A disomic substitution line (CS-SM3-7ADS) is highly resistant to Fusarium head blight (FHB), and an F7 population of recombinant inbred lines derived from the cross CS-SM3-7ADS × Annong 8455 was evaluated for resistance to FHB to investigate main effects, epistasis, and environmental interactions of quantitative trait loci (QTLs) for FHB resistance. A molecular linkage map consists of 501 simple sequence repeat and amplified fragment length polymorphism markers. A total of 10 QTLs were identified with significant main effects on the FHB resistance using MapQTL and QTLMapper software. Among them, CS-SM3-7ADS carries FHB-resistance alleles at five QTLs on chromosomes 2D, 3B, 4D, and 6A. One QTL on 3BS had the largest effect and explained 30.2% of the phenotypic variance. Susceptible QTLs were detected on chromosomes 1A, 1D, 4A, and 4B. A QTL for enhanced FHB resistance was not detected on chromosome 7A of CS-SM3-7ADS; therefore, the increased FHB resistance in CS-SM3-7ADS was not due to any major FHB-resistance QTL on 7A of Sumai 3, but more likely was due to removal of susceptible alleles of QTLs on 7A of Chinese Spring. QTLMapper detected nine pairs of additive-additive interactions at 17 loci that explained 26% phenotypic variance. QTL-environment interactions explained 49% of phenotypic variation, indicating that the environments significantly affected the expression of the QTLs, especially these epistasis QTLs. Adding FHB-enhancing QTLs or removal of susceptible QTLs both may significantly enhance the degree of wheat resistance to FHB in a wheat cultivar.
Characterization of Four Viral Species Belonging to the Family Potyviridae Isolated from Ranunculus asiaticus
Authors: Turina M. , Ciuffo M. , Lenzi R. , Rostagno L. , Mela L. , Derin E. , Palmano S.
Published: May 2007

Four different viral species were isolated from diseased Ranunculus asiaticus plants growing in Imperia Province (Italian Riviera-Liguria Region). Infected plants exhibited mosaic symptoms and growth abnormalities. The viruses were mechanically inoculated to a range of herbaceous hosts and differentiated biologically. Long flexuous particles were present in leaf dip extracts observed by electron microscopy. A general protocol for the amplification of potyvirus genome fragments through reverse transcription-polymerase chain reaction generated products that were cloned and sequenced. Sequence and phylogenetic analysis suggested that three of these isolates could be considered new viral species belonging to the genus Potyvirus. The fourth isolate is a new member of the genus Macluravirus. Purified virus was used as antigen to produce a specific polyclonal antiserum in rabbit; serological features were established through double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), antigen coated plate (ACP)-ELISA, and western blot analysis. DAS-ELISA was highly specific for each virus isolate, whereas some cross-reactivity was shown in ACP-ELISA and western blot analysis. Aphid transmission by Myzus persicae was demonstrated in a controlled environment for each of the four viral isolates, whereas no transmission through seed was observed.
Phylogenetic Analyses of Phytopathogenic Isolates of Verticillium spp.
Authors: Qin Qing-Ming , Vallad Gary E. , Wu Bo Ming , Subbarao Krishna V.
Published: May 2007

To better understand the genetic relationships between Verticillium dahliae isolates from lettuce and other phytopathogenic Verticillium spp. isolates from various hosts and geographic locations, the complete intergenic spacer (IGS) region of the nuclear ribosomal RNA gene (rDNA) and the β-tubulin gene were amplified and sequenced. The sequences of the complete IGS region and the β-tubulin gene were used alone and in combination to infer genetic relationships among different isolates of Verticillium with the maximum-likelihood distance method. Phylogenetic analyses set sequences into four distinct groups comprising isolates of V. albo-atrum, V. tricorpus, and V. dahliae from cruciferous and noncruciferous hosts. Within the four Verticillium groups, isolates of V. dahliae from cruciferous hosts displayed the closest affinity to V. dahliae from noncruciferous hosts. Isolates of V. dahliae from noncruciferous hosts could be further divided into four subgroups based on sequence similarities within the IGS region. Cross-pathogenicity tests demonstrated that most Verticillium isolates were as virulent on other hosts as on their hosts of origin. A phenogram based on the cross pathogenicity of individual isolates resembled those derived from the IGS and β-tubulin sequence comparisons. On the basis of the data presented, the potential origin of some isolates of V. dahliae pathogenic on lettuce is proposed.
Heritability and Components of Resistance to Cercospora zeae-maydis Derived from Maize Inbred VO613Y
Authors: Gordon Stuart G. , Lipps Patrick E. , Pratt Richard C.
Published: May 2007

Gray leaf spot (GLS), caused by the fungus Cercospora zeae-maydis, is one of the most important foliar diseases of maize. This study was undertaken to estimate heritability of C. zeae-maydis resistance and examine the relationship between previously identified resistance loci and certain components of resistance including incubation period, lesion number, and maximum lesion length. Partially inbred progenies arising from hybridization between maize inbred lines VO613Y (high level of partial resistance) and Pa405 (susceptible) were examined in Ohio and South Africa. Heritability estimates of resistance were calculated based on severity and incubation period values. The range of heritability estimates based on severity was broad, with values ranging from approximately 0.46 to 0.81 (mean = 0.59). Estimates of mean heritability for incubation period were lowest (0.18), indicating that this component would likely be unsuitable for selection of germ plasm intended for deployment in diverse regions. Length of GLS lesions was significantly affected by host genotype, with resistant genotypes having shorter lesions from one site in Ohio during two seasons. Genotype also had a significant effect on incubation period and lesion number; the lower values for these components also were associated with resistant genotypes. The combined action of these resistance components resulted in lower overall disease severity.
Molecular Identification, Reverse Transcription-Polymerase Chain Reaction Detection, Host Reactions, and Specific Cytopathology of Artichoke yellow ringspot virus Infecting Onion Crops
Authors: Maliogka Varvara I. , Dovas Chrisostomos I. , Lesemann Dietrich E. , Winter Stephan , Katis Nikolaos I.
Published: May 2007

An isometric virus ca. 25 nm in diameter with angular contour was isolated from onion plants showing yellow leaf striping and necrotic tips. The virus was mechanically transmitted onto 28 species of indicator plants belonging to five families, viz. Amaranthaceae, Chenopodiaceae, Cucurbitaceae, Leguminosae, and Solanaceae where it causes ring spots, malformations, and/or tip necrosis. Cytopathological studies in infected Nicotiana benthamiana tissues revealed cytoplasmic inclusions resembling those caused by Artichoke yellow ringspot virus (AYRSV), a member of the family Comoviridae. Host range and symptomatology of the onion virus were also similar to AYRSV. A high seed transmission rate (20%) was found in onion. Reverse transcription-polymerase chain reaction using degenerate primers specific for the family Comoviridae allowed amplification of RNA-dependent RNA polymerase sequences, which upon sequence analysis and comparison with AYRSV isolates from Cynara scolymus (AYRSV-AtG) and Vicia faba (AYRSV-F) were highly similar, thus providing evidence that the nepovirus AYRSV is infecting onion in the field.
Stock-Type Susceptibility and Delineation of Treatment Areas for a Cryptic Pinus radiata Root Disease
Authors: Hood I. A. , Kimberley M. O. , Gardner J. F.
Published: May 2007

Planting material with superior resistance to Armillaria root disease was identified in a field trial established to investigate variation in Armillaria infection among different Pinus radiata nursery stock types. At stand age 6.4 years, total infection incidence, mortality, and degree of root collar girdling by Armillaria spp. were all significantly lower among trees derived from both rooted stool bed cuttings (physiological age 1 to 3 years) and rooted field cuttings (physiological age 3 to 6 years) than among those grown from seedlings. Cutting types did not differ significantly from one another. No significant differences were found between stock types in stem diameter, but trees from stool bed cuttings were significantly taller than seedling trees. Whether these differences remain detectable later in the rotation, initial results suggest that it may be advantageous to plant robust stock, of either cuttings or seedlings, on Armillaria-infested sites. The incidence of infection in living, green-crowned trees was unevenly distributed across the trial site, and was greater nearer to trees killed by Armillaria spp. than further away (significant within a radius of 10 m). By mapping visible Armillaria-caused mortality prior to thinning, it may be possible to delineate areas with a higher incidence of concealed chronic infection, thus defining infested sites for postharvest treatment.
Identification and Quantification of Pathogenic Pythium spp. from Soils in Eastern Washington Using Real-Time Polymerase Chain Reaction
Authors: Schroeder K. L. , Okubara P. A. , Tambong J. T. , Lévesque C. A. , Paulitz T. C.
Published: May 2007

Traditional methods of quantifying Pythium spp. rely on the use of selective media and dilution plating. However, high variability is inherent in this type of enumeration and counts may not be representative of the pathogenic population of Pythium spp. Variable regions of the internal transcribed spacer of the rDNA were used to design species-specific primers for detection and quantification of nine Pythium spp. from soils in eastern Washington. Primer pairs were designed for Pythium abappressorium, P. attrantheridium, P. heterothallicum, P. irregulare group I, P. irregulare group IV, P. paroecandrum, P. rostratifingens, P. sylvaticum, and P. ultimum and used with real-time polymerase chain reaction. Standard curves were generated for each of the species using SYBR Green I fluorescent dye for detection of amplification. Seventy-seven isolates of Pythium were screened to confirm specificity of each primer set. DNA was extracted from soil and standard curves were generated for P. irregulare group I, P. irregulare group IV, and P. ultimum to correlate populations of each species in the soil with quantities of DNA amplified from the same soil. Examination of raw field soils revealed results similar to those observed in previous studies. This new technique for the quantification of Pythium spp. is rapid and accurate, and will be a useful tool in the future study of these pathogenic Pythium spp.
A Review of Fusarium Wilt of Oil Palm Caused by Fusarium oxysporum f. sp. elaeidis
Authors: Flood Julie
Published: May 2007

Vascular wilt is the most destructive disease of oil palm in Africa and causes severe losses in some areas. Symptoms include initial wilting followed by desiccation of the fronds, which finally break and hang around the trunk. Internally, characteristic browning of the vascular elements is seen both in adult palms and in seedlings. Two disease syndromes are commonly seen in the field in adult palms—“acute wilt” where the palm dies within a few weeks and “chronic wilt” where the palm may remain alive for many months and even years but becomes progressively stunted. The pathogen (Fusarium oxysporum f. sp. elaeidis) is a soilborne fungus and the perennial nature of the crop ensured that, in the past, disease management was difficult. Over a period of 30 to 40 years, screening for resistance at the nursery stage was introduced in many plantations and research stations, and successful breeding programs in West Africa, notably in Ivory Coast, have resulted in more resistant oil palm material becoming available. The disease has not yet been detected in South East Asia (largest producer of palm oil) and rigorous quarantine measures have been imposed to prevent introduction of the pathogen into these highly productive areas.
Current Knowledge of Coffee Wilt Disease, a Major Constraint to Coffee Production in Africa
Authors: Rutherford Mike A.
Published: May 2007

Coffee is vital to the economy of East and Central Africa, providing a major source of foreign exchange earnings and, as a cash crop, supporting the livelihoods of millions involved in cultivation, processing, marketing, and export. Coffee wilt disease (CWD), attributed to Gibberella xylarioides (Fusarium xylarioides), has caused losses to coffee production in Africa since 1927 but has been largely contained through the use of host resistance and in some instances wide-scale sanitation practices. A reemergence of CWD on Coffea canephora (Robusta coffee) in Uganda, Democratic Republic of Congo, and Tanzania has already led to heavy losses and threatens future production in these countries and elsewhere in the region. The relevance of CWD is all the more pertinent given the impact of a considerable fall in world coffee prices over the last decade. Recent research has clarified the extent of the problem in the region and revealed a low level of diversity within the pathogen, suggesting that two genetically and biologically distinct forms are responsible for current problems. These findings and related research and development initiatives undertaken under the auspices of the Regional Coffee Wilt Programme are of fundamental importance in providing an urgently needed solution to this devastating disease.
Tolerance of Senecio vulgaris to Infection and Disease Caused by Native and Alien Rust Fungi
Authors: Inglese S. J. , Paul N. D.
Published: May 2007

Plant defense strategies against pathogen attack can be divided into either resistance or tolerance. Variation in tolerance is expressed as differences in the relationship between host fitness (or yield) and the degree of infection. Plant tolerance of pathogen attack remains poorly understood both in terms of its specific mechanisms and in terms of the evolutionary processes by which it has arisen. Theoretical models predict that it is the result of coevolution between host and pathogen, suggesting greater tolerance in interactions with native as opposed to introduced pathogens. Therefore, we quantified and compared the degree of tolerance expressed in the interaction of Senecio vulgaris with the rust fungus Coleosporium tussilginis, which is native to the UK, and the introduced rust fungus Puccinia lagenophorae. We used the reaction norm approach to quantify tolerance and its components. The S. vulgaris-C. tussilaginis interaction expressed a significantly greater degree of tolerance, as reductions in host growth and fitness per unit infection were significantly less than with P. lagenophorae. The key mechanism for this greater tolerance to C. tussilaginis was a significantly smaller reduction in photosynthesis per unit infection than with P. lagenophorae, at both leaf and whole plant scales. There was no significant difference in the relationship between whole plant photosynthesis and host reproduction. We discuss these responses in the context of coevolution for tolerance in host-pathogen interactions.
A Fusarium Wilt Resistance Gene in Gossypium barbadense and Its Effect on Root-Knot Nematode-Wilt Disease Complex
Authors: Wang C. , Roberts P. A.
Published: May 2007

Fusarium wilt, caused by the soilborne pathogen Fusarium oxysporum f. sp. vasinfectum race 1, is a vascular disease in cotton (Gossypium spp.), and is a component of a disease complex with root-knot nematodes (Meloidogyne incognita). Genetic analysis of two interspecific crosses (G. barbadense Pima S-7 × G. hirsutum Acala NemX and Pima S-7 × Acala SJ-2) showed that one major gene (designated Fov1) with allele dosage effect conferred resistance to F. oxysporum f. sp. vasinfectum race 1 in Pima S-7. Two amplified fragment length polymorphism (AFLP) markers were linked to Fov1 in Pima S-7, with genetic distance from the gene of 9.3 and 14.6 centimorgans. Less severe wilt symptoms in Acala NemX than Acala SJ-2 indicated that Acala NemX possesses one or more minor genes contributing to delay of wilt symptoms. Highly resistant plants in F2 and F3 (Pima S-7 × NemX) families indicated transgressive segregation effects of minor genes in Acala NemX combined with Fov1 from Pima S-7. The effects of wilt and nematode resistance on the nematode-wilt disease complex were assayed with two inoculation methods. In the presence of both pathogens, wilt damage measured as shoot and root weight reductions was greatest on wilt- and nematode-susceptible Acala SJ-2 and least in root-knot nematode-resistant and wilt-susceptible Acala NemX. Intermediate damage occurred in wilt-resistant and root-knot nematode-susceptible Pima S-7. The results indicated that nematode resistance was more effective than wilt resistance in suppressing wilt symptoms when either resistance was present alone. Nematode resistance combined with intermediate wilt resistance, as in the F1 (Pima S-7 × NemX), was highly effective in protecting plants from root-knot nematodes and race 1 of Fusarium wilt as a disease complex.
Genetic Regulation of Polerovirus and Luteovirus Transmission in the Aphid Schizaphis graminum
Authors: Burrows M. E. , Caillaud M. C. , Smith D. M. , Benson E. C. , Gildow F. E. , Gray S. M.
Published: May 2007

Sexual forms of two genotypes of the aphid Schizaphis graminum, one a vector, the other a nonvector of two viruses that cause barley yellow dwarf disease (Barley yellow dwarf virus [BYDV]-SGV, luteovirus and Cereal yellow dwarf virus-RPV, polerovirus), were mated to generate F1 and F2 populations. Segregation of the transmission phenotype for both viruses in the F1 and F2 populations indicated that the transmission phenotype is under genetic control and that the parents are heterozygous for genes involved in transmission. The ability to transmit both viruses was correlated within the F1 and F2 populations, suggesting that a major gene or linked genes regulate the transmission. However, individual hybrid genotypes differed significantly in their ability to transmit each virus, indicating that in addition to a major gene, minor genes can affect the transmission of each virus independently. Gut and salivary gland associated transmission barriers were identified in the nonvector parent and some progeny, while other progeny possessed only a gut barrier or a salivary gland barrier. Hemolymph factors do not appear to be involved in determining the transmission phenotype. These results provide direct evidence that aphid transmission of luteoviruses is genetically regulated in the insect and that the tissue-specific barriers to virus transmission are not genetically linked.
Global Hierarchical Gene Diversity Analysis Suggests the Fertile Crescent Is Not the Center of Origin of the Barley Scald PathogenRhynchosporium secalis
Authors: Zaffarano Pascal L. , McDonald Bruce A. , Zala Marcello , Linde Celeste C.
Published: May 2007

A total of 1,366 Rhynchosporium secalis isolates causing scald on barley, rye, and wild barley (Hordeum spontaneum) were assayed for restriction fragment length polymorphism loci, DNA fingerprints, and mating type, to characterize global genetic structure. The isolates originated from 31 field populations on five continents. Hierarchical analysis revealed that more than 70% of the total genetic variation within regions was distributed within a barley field. At the global level, only 58% of the total genetic variation was distributed within fields, while 11% was distributed among fields within regions, and 31% was distributed among regions. A significant correlation was found between genetic and geographic distance. These findings suggest that gene flow is common at the local level while it is low between regions on the same continent, and rare between continents. Analyses of multilocus associations, genotype diversity, and mating type frequencies indicate that sexual recombination is occurring in most of the populations. We found the highest allele richness in Scandinavia followed by Switzerland. This suggests that R. secalis may not have originated at the center of origin of barley, the Fertile Crescent, nor in a secondary center of diversity of barley, Ethiopia.
Genetic Structure of Atmospheric Populations of Gibberella zeae
Authors: Schmale III David G. , Leslie John F. , Zeller Kurt A. , Saleh Amgad A. , Shields Elson J. , Bergstrom Gary C.
Published: May 2007

Gibberella zeae, causal agent of Fusarium head blight (FHB) of wheat and barley and Gibberella ear rot (GER) of corn, may be transported over long distances in the atmosphere. Epidemics of FHB and GER may be initiated by regional atmospheric sources of inoculum of G. zeae; however, little is known about the origin of inoculum for these epidemics. We tested the hypothesis that atmospheric populations of G. zeae are genetically diverse by determining the genetic structure of New York atmospheric populations (NYAPs) of G. zeae, and comparing them with populations of G. zeae collected from seven different states in the northern United States. Viable, airborne spores of G. zeae were collected in rotational (lacking any apparent within-field inoculum sources of G. zeae) wheat and corn fields in Aurora, NY in May through August over 3 years (2002 to 2004). We evaluated 23 amplified fragment length polymorphism (AFLP) loci in 780 isolates of G. zeae. Normalized genotypic diversity was high (ranging from 0.91 to 1.0) in NYAPs of G. zeae, and nearly all of the isolates in each of the populations represented unique AFLP haplotypes. Pairwise calculations of Nei's unbiased genetic identity were uniformly high (>0.99) for all of the possible NYAP comparisons. Although the NYAPs were genotypically diverse, they were genetically similar and potentially part of a large, interbreeding population of G. zeae in North America. Estimates of the fixation index (GST) and the effective migration rate (Nm) for the NYAPs indicated significant genetic exchange among populations. Relatively low levels of linkage disequilibrium in the NYAPs suggest that outcrossing is common and that the populations are not a result of a recent bottleneck or invasion. When NYAPs were compared with those collected across the United States, the observed genetic identities between the populations ranged from 0.92 to 0.99. However, there was a significant negative correlation (R = -0.59, P < 0.001) between genetic identity and geographic distance, suggesting that some genetic isolation may occur on a continental scale. The contribution of long-distance transport of G. zeae to regional epidemics of FHB and GER remains unclear, but the diverse atmospheric populations of G. zeae suggest that inoculum may originate from multiple locations over large geographic distances. Practically, the long-distance transport of G. zeae suggests that management of inoculum sources on a local scale, unless performed over extensive production areas, will not be completely effective for the management of FHB and GER.