bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal Dynamics of Phytophthora Blight on Bell Pepper in Relation to the Mechanisms of Dispersal of Primary Inoculum of Phytophthora capsici in Soil
Authors: Sujkowski L. S. , Parra G. R. , Gumpertz M. L. , Ristaino J. B.
Published: May 2007

The effect of components of primary inoculum dispersal in soil on the temporal dynamics of Phytophthora blight epidemics in bell pepper was evaluated in field and growth-chamber experiments. Phytophthora capsici may potentially be dispersed by one of several mechanisms in the soil, including inoculum movement to roots, root growth to inoculum, and root-to-root spread. Individual components of primary inoculum dispersal were manipulated in field plots by introducing (i) sporangia and mycelia directly in soil so that all three mechanisms of dispersal were possible, (ii) a plant with sporulating lesions on the soil surface in a plastic polyvinyl chloride (PVC) tube so inoculum movement to roots was possible, (iii) a wax-encased peat pot containing sporangia and mycelia in soil so root growth to inoculum was possible, (iv) a wax-encased peat pot containing infected roots in soil so root-to-root spread was possible, (v) noninfested V8 vermiculite media into soil directly as a control, or (vi) wax-encased noninfested soil as a control. In 1995 and 1996, final incidence of disease was highest in plots where sporangia and mycelia were buried directly in soil and all mechanisms of dispersal were operative (60 and 32%) and where infected plants were placed in PVC tubes on the soil surface and inoculum movement to roots occurred with rainfall (89 and 23%). Disease onset was delayed in 1995 and 1996, and final incidence was lower in plants in plots where wax-encased sporangia (6 and 22%) or wax-encased infected roots (22%) were buried in soil and root growth to inoculum or root-to-root spread occurred. Incidence of root infections was higher over time in plots where inoculum moved to roots or all mechanisms of dispersal were possible. In growth-chamber studies, ultimately all plants became diseased regardless of the dispersal mechanism of primary inoculum, but disease onset was delayed when plant roots had to grow through a wax layer to inoculum or infected roots in tension funnels that contained small volumes of soil. Our data from both field and growth-chamber studies demonstrate that the mechanism of dispersal of the primary inoculum in soil can have large effects on the temporal dynamics of disease.
Early Brown Rot Infections in Sweet Cherry Fruit Are Detected by Monilinia-Specific DNA Primers
Authors: Förster H. , Adaskaveg J. E.
Published: May 2007

Visible and nonvisible quiescent infections of immature and mature fruit are an integral component of the disease cycle of brown rot of sweet cherry in California. Detection of these infections is critical for developing efficient and efficacious fungicide management programs. The previously published DNA amplification primers mfs3 and NS5 for the identification of Monilinia fructicola were very specific in amplifying DNA of M. fructicola only and not M. laxa. This primer set, however, only detected DNA from some of the California isolates of M. fructicola. This genetic diversity was supported by random amplified polymorphic DNA (RAPD) analysis. Using eight 10-mer primers, seven M. fructicola isolates from California were all identified as genetically distinct. Using the same primers, only one polymorphism was detected among seven isolates of M. laxa. The multiple genotypes identified within the small population sample of M. fructicola, but not of M. laxa, using RAPD analysis could be indicative of genetic recombination within M. fructicola but not within M. laxa. To detect early brown rot infections in fruit, two primer sets that were developed from DNA sequences of either ribosomal DNA (MF5/ITS4/ITS3) or a RAPD fragment (X-09intF3/X-09R) specifically amplified DNA from isolates of M. fructicola and Monilinia species, respectively. No amplification products were present when using DNA from Botrytis cinerea or from other fungi commonly found on sweet cherry fruit. Primers X-09intF3 and X-09R were more sensitive and reliable for detecting small amounts of target DNA either extracted from conidia or from laboratory-inoculated cherry fruit with early brown rot infections that showed no visual symptoms or with visible quiescent infections. Furthermore, these primers also were effective for detecting visible quiescent infections in cherry fruit that were collected in the field.
Application of Phage Display in Selecting Tomato spotted wilt virus-Specific Single-Chain Antibodies (scFvs) for Sensitive Diagnosis in ELISA
Authors: Griep Remko A. , Prins Marcel , van Twisk Charlotte , Keller Hans J. H. G. , Kerschbaumer Randolf J. , Kormelink Richard , Goldbach Rob W. , Schots Arjen
Published: May 2007

A panel of recombinant single-chain antibodies (scFvs) against structural proteins of Tomato spotted wilt virus (TSWV) was retrieved from a human combinatorial scFv antibody library using the novel phage display technique. After subcloning the encoding DNA sequences in the expression vector pSKAP/S, which allowed the scFvs to be expressed as alkaline phosphatase fusion proteins, 17 different scFv antibodies were obtained. Of these, 12 scFvs were directed against the nucleoprotein (N) and 5, putatively, against the glycoproteins (G1 and G2). Five of the N-specific antibodies cross-reacted with two other tospoviruses (Tomato chlorotic spot virus and Groundnut ringspot virus), but none recognized the more distantly related tospoviruses Impatiens necrotic spot virus, Watermelon silverleaf mottle virus, Iris yellow spot virus, or Physalis severe mottle virus. The successful use of one of the antibodies as coating and detection reagent in a double-antibody sandwich enzyme-linked immunosorbent assay showed the potential of the phage display system in obtaining antibodies for routine TSWV diagnosis.
Sequence Diversity Within the Three Agents of Groundnut Rosette Disease
Authors: Deom C. M. , Naidu R. A. , Chiyembekeza A. J. , Ntare B. R. , Subrahmanyam P.
Published: May 2007

Sequence diversity was examined in the coat protein (CP) gene of Groundnut rosette assistor virus (GRAV), the overlapping open reading frames (ORFs) 3 and 4 of Groundnut rosette virus (GRV), and the satellite RNA (sat-RNA) of GRV obtained from field isolates from Malawi and Nigeria. These three agents cause groundnut rosette disease, a major disease of groundnut in sub-Saharan Africa (SSA). Sequence analysis showed that the GRAV CP gene was highly conserved (97 to 99%) independent of its geographic source. The nucleotide sequence of the overlapping ORFs 3 and 4 of GRV was highly conserved (98 to 100%) from isolates within a geographic region but less conserved (88 to 89%) between isolates from the two distinct geographic regions. Phylogenetic analysis of the overlapping ORFs 3 and 4 show that the GRV isolates cluster according to the geographic region from which they were isolated, indicating that Malawian GRV isolates are distinct from Nigerian GRV isolates. Similarity within the sat-RNA sequences analyzed ranged from 88 to 99%. Phylogenetic analysis also showed clustering within the sat-RNA isolates according to country of origin, as well as within isolates from two distinct regions of Malawi. Because the GRAV CP sequence is highly conserved, independent of the geographic source of the GRAV isolates, the GRAV CP sequence represents the most likely candidate to use for pathogen-derived resistance in groundnut and may provide effective protection against groundnut rosette disease throughout SSA.
Heteroduplex Mobility and Sequence Analyses for Assessment of Variability of Zucchini yellow mosaic virus
Authors: Lin Shih-Shun , Hou Roger F. , Yeh Shyi-Dong
Published: May 2007

A heteroduplex mobility assay (HMA) was used to analyze the variability among five isolates of Zucchini yellow mosaic virus (ZYMV; TW-TC1, TW-CY2, TW-TN3, TW-TNML1, and TW-NT1) collected from cucurbit fields in different areas of Taiwan. A cDNA fragment of 760 bp covering the variable region of the N terminal half of the coat protein (CP) gene was amplified by reverse transcription-polymerase chain reaction (RT-PCR) and subsequently subjected to HMA analysis for sequence variation. When TW-NT1 combined with any of the other Taiwan isolates, the heteroduplexes obtained migrated much more slowly than did the heteroduplexes obtained in combinations among the other four Taiwan isolates, indicating that TW-TC1, TW-CY2, TW-TN3, and TW-TNML1 share a high degree of sequence homology, while the TW-NT1 isolate is more distinct. The complete nucleotide sequences of the CP genes and the 3′ noncoding regions of the five isolates were determined from RT-PCR-derived cDNA clones. A phylogenetic tree derived from the actual sequences of the 760-bp fragments of the five Taiwan and another six ZYMV isolates from different geographic areas revealed four genotypes. TW-TNML1, TW-TC1, TWC-Y2, and TW-TN3 were in genotype I, while TW-NT1 and U.S. isolates were in genotype II. The Singapore and Reunion Island isolates were separated into genotypes III and IV, respectively. Comparison of the CP genes of the five Taiwan isolates indicated that they share 92.8 to 98.7% nucleotide identities and 96.4 to 99.3% amino acid identities. The amino acid positions 73, 102, 109, and 149 of the CP gene, where lysine, serine, arginine, and aspartic acid reside, respectively, were uniquely conserved for genotype I Taiwan isolates. Thus, results of HMA agreed well with those of phylogenetic analysis based on the sequence data of the five Taiwan ZYMV isolates. These five ZYMV isolates of known sequence can be used as reference strains for HMA to analyze the variability of ZYMV in Taiwan.
Induction of Terpenoid Synthesis in Cotton Roots and Control of Rhizoctonia solani by Seed Treatment with Trichoderma virens
Authors: Howell C. R. , Hanson L. E. , Stipanovic R. D. , Puckhaber L. S.
Published: May 2007

Research on the mechanisms employed by the biocontrol agent Trichoderma virens to suppress cotton (Gossypium hirsutum) seedling disease incited by Rhizoctonia solani has shown that mycoparasitism and antibiotic production are not major contributors to successful biological control. In this study, we examined the possibility that seed treatment with T. virens stimulates defense responses, as indicated by the synthesis of terpenoids in cotton roots. We also examined the role of these terpenoid compounds in disease control. Analysis of extracts of cotton roots and hypocotyls grown from T. virens-treated seed showed that terpenoid synthesis and peroxidase activity were increased in the roots of treated plants, but not in the hypocotyls of these plants or in the untreated controls. Bioassay of the terpenoids for toxicity to R. solani showed that the pathway intermediates desoxyhemigossypol (dHG) and hemigossypol (HG) were strongly inhibitory to the pathogen, while the final product gossypol (G) was toxic only at a much higher concentration. Strains of T. virens and T. koningii were much more resistant to HG than was R. solani, and they thoroughly colonized the cotton roots. A comparison of biocontrol efficacy and induction of terpenoid synthesis in cotton roots by strains of T. virens, T. koningii, T. harzianum, and protoplast fusants indicated that there was a strong correlation (+0.89) between these two phenomena. It, therefore, appears that induction of defense response, particularly terpenoid synthesis, in cotton roots by T. virens may be an important mechanism in the biological control by this fungus of R. solani-incited cotton seedling disease.
Mechanism of Broccoli-Mediated Verticillium Wilt Reduction in Cauliflower
Authors: Shetty K. G. , Subbarao K. V. , Huisman O. C. , Hubbard J. C.
Published: May 2007

Broccoli is resistant to Verticillium dahliae infection and does not express wilt symptoms. Incorporation of broccoli residues reduces soil populations of V. dahliae. The effects of broccoli residue were tested on the colonization of roots by V. dahliae, plant growth response, and disease incidence of both broccoli and cauliflower in soils with different levels of V. dahliae inoculum and with or without fresh broccoli residue amendments. The three soils included a low-Verticillium soil, a high-Verticillium soil, and a broccoli-rotation soil (soil from a field after two broccoli crops) with an average of 13, 38, and below-detectable levels of microsclerotia per g of soil, respectively. Cauliflower plants in broccoli-amended high-Verticillium soil had significantly (P ≤ 0.05) lower wilt incidence and severity than did plants in unamended soil. An immunohistochemical staining assay utilizing a monoclonal antibody specific to V. dahliae was used to determine colonization of the root cortex. Despite the absence of wilt symptoms, broccoli roots were colonized by V. dahliae. In high-Verticillium soil, the broccoli residue amendment caused a marked reduction in colonization rate of V. dahliae per unit of inoculum on both cauliflower and broccoli roots. In addition to its detrimental effects on the viability of microsclerotia in soil, broccoli residue may also have an inhibitory effect on the root-colonizing potential of surviving microsclerotia.
Estimation of Rates of Recombination and Migration in Populations of Plant Pathogens
Authors: Brown J. K. M.
Published: May 2007

A method of estimating the frequency of recombination in field populations of ascomycete fungi is presented. This is a development of a technique described by J. Zhan, C. C. Mundt, and B. A. McDonald, in which field plots were inoculated with isolates of Mycosphaerella graminicola of known genotypes and then, later, the proportions of the fungal populations in the plots that originated as recombinant progeny of matings between the inoculated isolates or as immigrants from the local population were estimated. It is argued that J. Zhan, C. C. Mundt, and B. A. McDonald's method is inappropriate in terms of its approach to statistical inference and that the lack of confidence intervals for rates of recombination (r) or immigration (m) is a further weakness. An improvement to their method would involve the use of inoculated isolates that can be clearly distinguished from the local population, for instance, by the use of DNA markers. There should also be sufficient markers for there to be a negligible probability that progeny of matings between inoculated isolates will have the same genotype as either of their parents. With this experimental design, estimates of r and m can be found by a standard maximum likelihood method, while confidence intervals for the parameters can be estimated by a simple bootstrap procedure. The method appears to be about an order of magnitude more efficient than are the existing ways of estimating the frequency of recombination in ascomycete fungi.
Rhizoctonia spp. Recovered from Strawberry Roots in Central Coastal California
Authors: Martin Frank N.
Published: May 2007

Rhizoctonia spp. were commonly recovered from the roots of strawberry plants growing in nonfumigated soil in the central coastal region of California. With the exception of one multinucleate isolate of R. solani (frequency of recovery of 0.8%), all other isolates were binucleate and were in anastomosis groups (AG) A, G, or I. AGs-A and -I were recovered from all five collection sites, whereas AG-G was recovered from only two sites. AG-A was the most commonly isolated AG, followed by AGs-I and -G. Similar levels of virulence were observed among the different AGs, but differences in virulence were observed among isolates in the same AG. Evaluating anastomosis grouping by pairing isolates recovered from strawberry with known tester isolates did not always yield a positive anastomosis reaction, even though both isolates anastomosed with other members of the same AG. Subsequent investigations with multiple isolates in the same AG from the same collection location confirmed that there was a lack of anastomosis or weak anastomosis reactions for some combinations of pairings, highlighting the need for to use multiple tester isolates or molecular techniques for AG determination. Restriction fragment length polymorphism (RFLP) analysis of a polymerase chain reaction-amplified region of the rDNA was effective for differentiating AGs. Sixteen RFLP groups were observed after cluster analysis with data for the size of the amplified products and fragment sizes after digestion with four restriction enzymes. Although each AG had isolates in multiple RFLP groups, any one individual RFLP group contained isolates of only a single AG. There was no consistent correlation between RFLP group and location of isolate collection.
Role of Ice Nucleation and Antifreeze Activities in Pathogenesis and Growth of Snow Molds
Authors: Snider Christopher S. , Hsiang Tom , Zhao Guiying , Griffith Marilyn
Published: May 2007

We examined the ability of snow molds to grow at temperatures from -5 to 30°C and to influence the growth of ice through assays for ice nucleation and antifreeze activities. Isolates of Coprinus psychromorbidus (low temperature basidiomycete variant), Microdochium nivale, Typhula phacorrhiza, T. ishikariensis, T. incarnata, and T. canadensis all grew at -5°C, whereas Sclerotinia borealis and S. homoeocarpa did not grow at temperatures below 4°C. The highest threshold ice nucleation temperature was -7°C. Because snow molds are most damaging to their hosts at temperatures above this, our results imply that the pathogenesis of these fungi is not dependent on ice nucleation activity to cause freeze-wounding of host plants. All snow molds that grew at subzero temperatures also exhibited antifreeze activity in the growth medium and in the soluble and insoluble hyphal fractions, with the exception of M. nivale and one isolate of T. canadensis. The lack of high ice nucleation activity combined with the presence of antifreeze activity in all fungal fractions indicates that snow molds can moderate their environment to inhibit or modify intra- and extracellular ice formation, which helps explain their ability to grow at subzero temperatures under snow cover.
Response of Two Fungi in the Apple Sooty Blotch Complex to Temperature and Relative Humidity
Authors: Johnson Eric M. , Sutton Turner B.
Published: May 2007

Peltaster fructicola and Leptodontium elatius, two of the causal fungi of apple sooty blotch, responded differently to temperature and relative humidity in vitro. Conidia of L. elatius germinated from 12 to 32°C at relative humidities ≥97%, whereas conidia of P. fructicola germinated from 12 to 24°C at relative humidities ≥95%. Germination of conidia of L. elatius was optimum at 32°C and 99% relative humidity compared with 24°C and 97 or 99% relative humidity for P. fructicola. When L. elatius and P. fructicola were grown in Parafilm culture, sporulation was greatest at relative humidities of 97 to 99%. In agar culture, mycelia of L. elatius expanded radially from 12 to 32°C, and that of P. fructicola at 12 to 28°C. Mycelia of P. fructicola did not survive exposure for 7 days or more to temperatures ≥32°C. Mycelial growth was inhibited at relative humidities <95% for both fungi and no growth occurred at 88% relative humidity. Conidia of P. fructicola were more sensitive to air drying than were those of L. elatius. Conidial viability of P. fructicola was reduced significantly after 8 h of air drying and nearly completely inhibited after 12 h. Conidia of L. elatius required 24 h of air drying before a significant reduction in conidial viability was observed. These results support the hypothesis that environmental factors influence the temporal and geographical distributions of the fungi associated with the apple sooty blotch disease.
Genetic Differentiation and Host Specificity Among Populations of Alternaria spp. Causing Brown Spot of Grapefruit and Tangerine × Grapefruit Hybrids in Florida
Authors: Peever T. L. , Olsen L. , Ibañez A. , Timmer L. W.
Published: May 2007

Alternaria spp. were sampled from brown spot lesions in several geographically separated citrus groves and different grapefruit and tangerine × grapefruit hybrid cultivars in Florida and screened for variation at 16 putative random amplified polymorphic DNA loci. Populations of the pathogen on two hybrids, Minneola and Orlando, in five locations throughout Florida were moderately differentiated (Nei's coefficient of gene differentiation [GST] = 0.12) among locations. The hypothesis that host-specialized forms of Alternaria spp. cause brown spot on different Citrus spp. and cultivars was tested by estimating genetic differentiation among isolates sampled from different hosts and by pathogenicity assays. Isolates sampled from grapefruit and the hybrid cv. Nova were genetically distinct from isolates sampled from other hybrid cultivars including Robinson, Sunburst, Minneola, Orlando, and Murcott. No differentiation could be detected among isolates sampled from this latter group of hybrids. Quantitative pathogenicity assays on leaves using spray inoculation revealed that ‘Nova’ isolates were not significantly more pathogenic on ‘Nova’ compared with isolates from ‘Minneola’ and ‘Orlando’. Similarly, grapefruit isolates were not significantly more pathogenic on grapefruit compared with isolates from ‘Minneola’. Isolates from all hosts had similar disease rankings on each inoculated cultivar, with ‘Minneola’ the most susceptible, followed in decreasing order of susceptibility by ‘Orlando’, ‘Sunburst’, ‘Nova’, and ‘Duncan’ grapefruit. Rough lemon was generally immune to all isolates tested; however, occasional brown spot lesions were observed on leaves of this host with isolates from grapefruit. No evidence was found to support the hypothesis that unique genotypes of the pathogen, which are more virulent on ‘Sunburst’ or grapefruit, have been introduced to Florida. Populations of Alternaria spp. causing brown spot of citrus on grapefruit and ‘Nova’ in Florida are genetically distinct from isolates on other cultivars, and we speculate that these populations are in the early stages of adaptation to and possible speciation on these hosts.
Virulence and Molecular Polymorphism in International Collections of the Wheat Leaf Rust Fungus Puccinia triticina
Authors: Kolmer J. A. , Liu J. Q.
Published: May 2007

Collections of Puccinia triticina, the wheat leaf rust fungus, were obtained from Great Britain, Slovakia, Israel, Germany, Australia, Italy, Spain, Hungary, South Africa, Uruguay, New Zealand, Brazil, Pakistan, Nepal, and eastern and western Canada. All single-uredinial isolates derived from the collections were tested for virulence polymorphism on 22 Thatcher wheat lines that are near-isogenic for leaf rust resistance genes. Based on virulence phenotype, selected isolates were also tested for randomly amplified polymorphic DNA (RAPD) using 11 primers. The national collections were placed into 11 groups based on previously established epidemiological zones. Among the 131 single-uredinial isolates, 105 virulence phenotypes and 82 RAPD phenotypes were described. In a modified analysis of variance, 26% of the virulence variation was due to differences in isolates between groups, with the remainder attributable to differences within groups. Of the RAPD variation, 36% was due to differences in isolates between groups. Clustering based on the average virulence distance (simple distance coefficient) within and between groups resulted in eight groups that differed significantly. Collections from Australia-New Zealand, Spain, Italy, and Britain did not differ significantly for virulence. Clustering of RAPD marker differences (1 - Dice coefficient) distinguished nine groups that differed significantly. Collections from Spain and Italy did not differ significantly for RAPD variation, neither did collections from western Canada and South America. Groups of isolates distinguished by avirulent/virulent infection types to wheat lines with resistance genes Lr1, Lr2a, Lr2c, and Lr3 also differed significantly for RAPD distance, showing a general relationship between virulence and RAPD phenotype. The results indicated that on a worldwide level collections of P. triticina differ for virulence and molecular backgrounds.
Detection of Potato mop top virus and Tobacco rattle virus Using a Multiplex Real-Time Fluorescent Reverse-Transcription Polymerase Chain Reaction Assay
Authors: Mumford R. A. , Walsh K. , Barker I. , Boonham N.
Published: May 2007

Tobacco rattle virus (TRV) and Potato mop top virus (PMTV) are important diseases of potato that are difficult to diagnose reliably by visual symptoms. Effective control strategies rely on accurate diagnosis. This paper describes the development of a multiplex assay for the detection of TRV and PMTV directly from potato tubers and leaves by polymerase chain reaction (PCR) combined with in-tube fluorescent product detection (TaqMan). This technology obviates any post-PCR manipulations and has many advantages including reducing contamination risks, eliminating the need for ethidium bromide staining, and removing the time and cost of gel running. The new assay also allows the replacement of the two separate tests (a TRV reverse-transcription-PCR and a PMTV enzyme-linked immuno-sorbent assay) currently used with a single-tube multiplex format. In addition to greatly simplifying the detection of these two viruses, the multiplex TaqMan assay was also shown to be more sensitive than either of the tests that it replaces, allowing 100- and 10,000-fold increases in sensitivity for TRV and PMTV detection, respectively. The test reliably detected over 40 different isolates of TRV and PMTV obtained from a wide range of different cultivars and geographical locations, including some samples in which existing tests failed to detect virus. The use of an assay of this kind in routine diagnosis helps to speed up and streamline the diagnostic laboratory; in addition, more reliable diagnosis should help in the control of this damaging disease.
Characterization of a Novel Potyvirus Isolated from Maize in Israel
Authors: Seifers D. L. , Salomon R. , Marie-Jeanne V. , Alliot B. , Signoret P. , Haber S. , Loboda A. , Ens W. , She Y.-M. , Standing K. G.
Published: May 2007

A potyvirus (proposed name of Zea mosaic virus [ZeMV]) isolated from maize in Israel was analyzed by serology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of capsid proteins, symptomatology, and sequencing. Parts of the nuclear inclusion b, coat protein, and 3′ regions were sequenced; the amino acid sequence of ZeMV capsid was determined by time-of-flight mass spectrometry (TOFMS). The results of these analyses were compared with those of similar analyses of the following potyviruses: Maize dwarf mosaic virus (MDMV), Sugarcane mosaic virus strain MDB (SCMV-MDB), Johnsongrass mosaic virus(JGMV), Sorghum mosaic virus (SrMV), and an isolate of MDMV from Israel. Indirect enzyme-linked immunosorbent assay using ZeMV antiserum detected only ZeMV, and reciprocal tests using MDMV, JGMV, or SrMV antisera failed to detect ZeMV. ZeMV cross-reacted weakly when SCMV-MDB antiserum was used. The mass of ZeMV capsid was determined to be 36,810 Da by SDS-PAGE and 34,216 Da by TOFMS. The ZeMV systemically infected johnsongrass (Sorghum halepense), but did not infect oat (Avena sativa), pearl millet (Pennisetum glaucum), barley (Hordeum vulgare), or rye (Secale cereale). Necrosis was caused in 19 sorghum lines by SrMV, in 15 by ZeMV, in 14 by MDMV, and in 5 by JGMV and SCMV-MDB. The nucleic acid and amino acid sequences of ZeMV clearly showed that it is not a strain of JGMV, MDMV, SCMV, or SrMV.
Measuring the Genetic Diversity of Xanthomonas axonopodis pv. manihotis Within Different Fields in Colombia
Authors: Restrepo S. , Vélez C. M. , Verdier V.
Published: May 2007

Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis, is a widespread disease that affects cassava (Manihot esculenta). We collected 238 X. axonopodis pv. manihotis strains by intensively sampling single fields in four edaphoclimatic zones (ECZs) in Colombia. DNA polymorphism of different X. axonopodis pv. manihotis populations was assessed by restriction fragment length polymorphism (RFLP) analyses, repetitive sequence-based polymerase chain reaction (rep-PCR), and amplified fragment length polymorphism (AFLP) assays. Genetic diversity, phenetic relationships among strains, and the coefficient of genetic differentiation were determined. All strains were tested for aggressiveness on the susceptible cassava cv. MCOL 1522. Strains were also tested for virulence on cassava differentials adapted to the strains' respective ECZs. Our study showed that the Colombian X. axonopodis pv. manihotis population has a high degree of genetic diversity. The hierarchical analysis of diversity showed genotypic differentiation at all levels, among ECZs, among fields within ECZs, and among strains within fields planted to several cassava genotypes. New RFLP haplotypes were detected, leading to the characterization of a new pathotype. Dendrograms from AFLP were more robust than those from RFLP data. A close association between the strains' geographical origin and DNA polymorphism was obtained using RFLP and AFLP data. We suggest that the host played a role in causing pathogen differentiation.
Nematicidal Activity of Essential Oils and Their Components Against the Root-Knot Nematode
Authors: Oka Yuji , Nacar Sengul , Putievsky Eli , Ravid Uzi , Yaniv Zohara , Spiegel Yitzhak
Published: May 2007

Nematicidal activity of essential oils extracted from 27 spices and aromatic plants were evaluated in vitro and in pot experiments. Twelve of the twenty-seven essential oils immobilized more than 80% of juveniles of the root-knot nematode Meloidogyne javanica at a concentration of 1,000 μl/liter. At this concentration, most of these oils also inhibited nematode hatching. Essential oils of Carum carvi, Foeniculum vulgare, Mentha rotundifolia, and Mentha spicata showed the highest nematicidal activity among the in vitro tested oils. These oils and those from Origanum vulgare, O. syriacum, and Coridothymus capitatus mixed in sandy soil at concentrations of 100 and 200 mg/kg reduced the root galling of cucumber seedlings in pot experiments. The main components of these essential oils were tested for their nematicidal activity. Carvacrol, t-anethole, thymol, and (+)-carvone immobilized the juveniles and inhibited hatching at >125 μl/liter in vitro. Most of these components mixed in sandy soil at concentrations of 75 and 150 mg/kg reduced root galling of cucumber seedlings. In 3-liter pot experiments, nematicidal activity of the essential oils and their components was confirmed at 200 and 150 mg/kg, respectively. The results suggest that the essential oils and their main components may serve as nematicides.
Biological and Molecular Properties of a Begomovirus from Dicliptera sexangularis
Authors: Lotrakul Pongtharin , Valverde Rodrigo A. , Landry Angela D.
Published: May 2007

Sixangle foldwing, Dicliptera sexangularis (Acanthaceae), showing severe yellow mottle and leaf distortion symptoms was collected from the shoreline of Calusa Island (Lee County, FL). The putative virus was transmitted from infected D. sexangularis to healthy seedlings by mechanical, whitefly (Bemisia tabaci biotype B), and graft-inoculations. Different forms of geminivirus-like DNAs were detected in total DNA extracted from infected plants by Southern blot hybridization analyses using DNA-A and -B of Bean golden mosaic virus (BGMV) from Guatemala as probes. Preliminary polymerase chain reaction experiments and sequence comparisons indicated that the virus was a distinct bipartite begomovirus. The virus was designated Dicliptera yellow mottle virus (DiYMV). Replicative dsDNAs of DiYMV were extracted, digested with selected restriction enzymes, and cloned into a plasmid vector. Both DNA-A and -B were sequenced and compared with those of other begomoviruses. Phylogenetic analyses using AV1, AC1, and BV1 nucleotide sequences indicated that DiYMV has a close relationship with the New World begomoviruses, especially those distributed in the nearby geographic areas of the Florida coast and the Caribbean Basin. However, different percent nucleotide sequence identities and phylogenetic relationships were detected when different open reading frames (ORFs) of DiYMV were compared with their counterparts from begomoviruses from the Caribbean Basin. Based on phylogenetic analyses of the AC1 and BV1 ORFs, DiYMV was closely related to BGMV type II isolates, whereas sequence comparisons of the common region and the AC4-derived amino acid sequences indicated its close relationship with Potato yellow mosaic virus from Venezuela.
Suppression of Phytophthora cinnamomi in Potting Mixes Amended with Uncomposted and Composted Animal Manures
Authors: Aryantha I. P. , Cross R. , Guest D. I.
Published: May 2007

We examined the effects of fresh and composted animal manures on the development of root rot, dieback, and plant death caused by Phytophthora cinnamomi. Fresh chicken manure, or chicken manure composted for 5 weeks before incorporation into the potting mix (25%, vol/vol), significantly reduced pathogen survival and the development of symptoms on Lupinus albus seedlings. Chicken manure composted for 2 weeks was less suppressive. Cow, sheep, and horse manure, whether fresh or composted, did not consistently suppress populations of P. cinnamomi or disease symptoms at the rates used (25%, vol/vol). All composts increased organic matter content, total biological activity, and populations of actinomycetes, fluorescent pseudomonads, and fungi. Only chicken manure stimulated endospore-forming bacteria, a factor that was strongly associated with seedling survival. Fallowing the potting mix for an additional 8 weeks after the first harvest increased the survival of lupin seedlings in a second bioassay, with survival rates in chicken manure compost-amended potting mix exceeding 90%. These data suggest that the ability of composted manure to stimulate sustained biological activity, in particular the activity of endospore-forming bacteria, is the key factor in reducing disease symptoms caused by P. cinnamomi. Supporting these results, the survival of rooted cuttings of Thryptomene calycina was significantly higher in sand-peat potting mix following amendment with commercially available chicken manure (15% vol/vol). However, this protection was reduced if the potting mix was steam pasteurized before amendment, indicating that suppression was due to endogenous as well as introduced microbes. Chicken manure compost incorporated at 5% (vol/vol) or more was strongly phytotoxic to young Banksia spinulosa plants and is not suitable as an amendment for phosphorus-sensitive plants.
Coupling Disease-Progress-Curve and Time-of-Infection Functions for Predicting Yield Loss of Crops
Authors: Madden L. V. , Hughes G. , Irwin M. E.
Published: May 2007

A general approach was developed to predict the yield loss of crops in relation to infection by systemic diseases. The approach was based on two premises: (i) disease incidence in a population of plants over time can be described by a nonlinear disease progress model, such as the logistic or monomolecular; and (ii) yield of a plant is a function of time of infection (t) that can be represented by the (negative) exponential or similar model (ζ(t)). Yield loss of a population of plants on a proportional scale (L) can be written as the product of the proportion of the plant population newly infected during a very short time interval (X′(t)dt) and ζ(t), integrated over the time duration of the epidemic. L in the model can be expressed in relation to directly interpretable parameters: maximum per-plant yield loss (α, typically occurring at t = 0); the decline in per-plant loss as time of infection is delayed (γ; units of time-1); and the parameters that characterize disease progress over time, namely, initial disease incidence (X0), rate of disease increase (r; units of time-1), and maximum (or asymptotic) value of disease incidence (K). Based on the model formulation, L ranges from αX0 to αK and increases with increasing X0, r, K, α, and γ-1. The exact effects of these parameters on L were determined with numerical solutions of the model. The model was expanded to predict L when there was spatial heterogeneity in disease incidence among sites within a field and when maximum per-plant yield loss occurred at a time other than the beginning of the epidemic (t > 0). However, the latter two situations had a major impact on L only at high values of r. The modeling approach was demonstrated by analyzing data on soybean yield loss in relation to infection by Soybean mosaic virus, a member of the genus Potyvirus. Based on model solutions, strategies to reduce or minimize yield losses from a given disease can be evaluated.
Analyzing and Modeling Temporal Disease Progress of Barley yellow dwarf virus Serotypes in Barley Fields
Authors: Leclercq-Le Quillec F. , Plantegenest M. , Riault G. , Dedryver C. A.
Published: May 2007

Population dynamics of Padi avenae (PAV), Macrosiphum avenae (MAV), and Rhopalosiphum padi (RPV) virus serotypes of Barley yellow dwarf virus (BYDV) and of their main aphid vectors were studied in winter barley (Hordeum vulgare) fields for three successive years in western France. An epidemiological model of the spread of viruses in the field was developed based on vector populations as forcing variables and the population dynamics of each virus serotype. This model accurately simulated the kinetics of the epidemic for PAV serotypes, which are the most common ones. For RPV and to some extent for MAV, the results were less satisfactory. The occurrence and spread of PAV and MAV serotypes in the field was clearly and easily related to that of their main vector species. Conversely, the spread of RPV serotypes showed no consistent relationships with the dynamics of their vectors. Incidence of PAV in 1989 to 1990 and 1990 to 1991 showed a bimodal distribution, with maximums in fall (December) and spring (May) that were linked to fall infestations by R. padi and spring infestations by three (R. padi, Sitobion avenae, and Metopolophium dirhodum) or two (S. avenae and M. dirhodum) aphid species. In 1991 to 1992, the PAV infection curve was monomodal and mainly due to a primary spread of the virus by very large populations of alate R. padi. MAV incidence was low in fall and winter and reached a maximum in spring 1990 and 1991 related to the occurrence of S. avenae and M. dirhodum. RPV incidence was low every year, despite the abundance of its vector, R. padi. Mixed infections were more frequent than expected by chance and were assumed to be partly related to heterologous encapsidation. The occurrence of each serotype is discussed in relation to the time of crop infection and possible damage.
Gene Genealogies and AFLP Analyses in the Fusarium oxysporum Complex Identify Monophyletic and Nonmonophyletic Formae Speciales Causing Wilt and Rot Disease
Authors: Baayen Robert P. , O'Donnell Kerry , Bonants Peter J. M. , Cigelnik Elizabeth , Kroon Laurens P. N. M. , Roebroeck Eugène J. A. , Waalwijk Cees
Published: May 2007

The monophyletic origin of host-specific taxa in the plant-pathogenic Fusarium oxysporum complex was tested by constructing nuclear and mitochondrial gene genealogies and amplified fragment length polymorphism (AFLP)-based phylogenies for 89 strains representing the known genetic and pathogenic diversity in 8 formae speciales associated with wilt diseases and root and bulb rot. We included strains from clonal lineages of F. oxysporum f. spp. asparagi, dianthi, gladioli, lilii, lini, opuntiarum, spinaciae, and tulipae. Putatively nonpathogenic strains from carnation and lily were included and a reference strain from each of the three main clades identified previously in the F. oxysporum complex; sequences from related species were used as outgroups. DNA sequences from the nuclear translation elongation factor 1α and the mitochondrial small subunit (mtSSU) ribosomal RNA genes were combined for phylogenetic analysis. Strains in vegetative compatibility groups (VCGs) shared identical sequences and AFLP profiles, supporting the monophyly of the two single-VCG formae speciales, lilii and tulipae. Identical genotypes were also found for the three VCGs in F. oxysporum f. sp. spinaciae. In contrast, multiple evolutionary origins were apparent for F. oxysporum f. spp. asparagi, dianthi, gladioli, lini, and opuntiarum, although different VCGs within each of these formae speciales often clustered close together or shared identical EF-1α and mtSSU rDNA haplotypes. Kishino-Hasegawa analyses of constraints forcing the monophyly of these formae speciales supported the exclusive origin of F. oxysporum f. sp. opuntiarum but not the monophyly of F. oxysporum f. spp. asparagi, dianthi, gladioli, and lini. Most of the putatively nonpathogenic strains from carnation and lily, representing unique VCGs, were unrelated to F. oxysporum f. spp. dianthi and lilii, respectively. Putatively nonpathogenic or rot-inducing strains did not form exclusive groups within the molecular phylogeny. Parsimony analyses of AFLP fingerprint data supported the gene genealogy-based phylogram; however, AFLP-based phylogenies were considerably more homoplasious than the gene genealogies. The predictive value of the forma specialis naming system within the F. oxysporum complex is questioned.
Components of Partial Resistance in the Slash Pine-Fusiform Rust Pathosystem
Authors: Schmidt R. A. , Gramacho K. P. , Miller T. , Young C. H.
Published: May 2007

Progeny of rust-resistant, open-pollinated slash pine families exhibited components of partial resistance in greenhouse tests. Nine-month-old seedlings of some resistant families had (i) a greater frequency of short galls (≤ 25 mm long), and (ii) fewer sporulating galls compared with other resistant and susceptible families when exposed to Cronartium quercuum f. sp. fusiforme in greenhouse tests. These components of partial resistance were evident with (i) mixed- or single-gall inocula, (ii) varying inoculum concentration, and (iii) inoculation of wounds or intact tissues. Inocula effects were nonsignificant, but family effects were highly significant (P = 0.001 - 0.0001) for total galls (TG) and short galls (SG). Multivariate family means (TG-;SG clusters) were highly significant (P = 0.0001). Short galls decreased on all families with an increase in inoculum concentration. In 36 field trials (12 locations × 3 years) a family which exhibited components of partial resistance in greenhouse tests exhibited lower disease incidence (% trees infected), lower disease severity (galls per tree), and less variability in disease incidence and severity among trials compared with other resistant families. Partially resistant families would be useful for tree improvement and for research into the genetics and mechanisms of resistance in the slash pine-fusiform rust pathosystem.
The Influence of Silicon on Components of Resistance to Blast in Susceptible, Partially Resistant, and Resistant Cultivars of Rice
Authors: Seebold K. W. , Kucharek T. A. , Datnoff L. E. , Correa-Victoria F. J. , Marchetti M. A.
Published: May 2007

The application of silicon (Si) fertilizers reduces the severity of blast, caused by Magnaporthe grisea, in irrigated and upland rice; however, little research has been conducted to examine the epidemiological and etiological components of this reduction. Four cultivars of rice with differential susceptibilities to race IB-49 of M. grisea were fertilized with three rates of a calcium silicate fertilizer and inoculated with the pathogen to test the effects of Si on the following components of resistance to blast: incubation period, latent period, infection efficiency, lesion size, rate of lesion expansion, sporulation per lesion, and diseased leaf area. For each cultivar, the incubation period was lengthened by increased rates of Si, and the numbers of sporulating lesions, lesion size, rate of lesion expansion, diseased leaf area, and number of spores per lesion were reduced. Lesion size and sporulation per lesion were lowered by 30 to 45%, and the number of sporulating lesions per leaf and diseased leaf area were significantly reduced at the highest rate of Si. The net effect of Si on these components of resistance is an overall reduction in the production of conidia on plants infected with M. grisea, thereby slowing the epidemic rate of blast.
Survey of Prunus necrotic ringspot virus in Rose and Its Variability in Rose and Prunus spp.
Authors: Moury Benoît , Cardin Loïc , Onesto Jean-Paul , Candresse Thierry , Poupet Alain
Published: May 2007

A survey for viruses in rose propagated in Europe resulted in detection of only Prunus necrotic ringspot virus (PNRSV) among seven viruses screened. Four percent of cut-flower roses from different sources were infected with PNRSV. Progression of the disease under greenhouse conditions was very slow, which should make this virus easy to eradicate through sanitary selection. Comparison of the partial coat protein gene sequences for three representative rose isolates indicated that they do not form a distinct phylogenetic group and show close relations to Prunus spp. isolates. However, a comparison of the reactivity of monoclonal antibodies raised against these isolates showed that the most prevalent PNRSV serotype in rose was different from the most prevalent serotype in Prunus spp. All of the 27 rose isolates tested infected P. persica seedlings, whereas three of the four PNRSV isolates tested from Prunus spp. were poorly infectious in Rosa indica plants. These data suggest adaptation of PNRSV isolates from Prunus spp., but not from rose, to their host plants. The test methodologies developed here to evaluate PNRSV pathogenicity in Prunus spp. and rose could also help to screen for resistant genotypes.
Mathematical Models of Cross Protection in the Epidemiology of Plant-Virus Diseases
Authors: Zhang X.-S. , Holt J.
Published: May 2007

Mathematical models of plant-virus disease epidemics were developed where cross protection occurs between viruses or virus strains. Such cross protection can occur both naturally and through artificial intervention. Examples of diseases with continuous and discontinuous crop-host availability were considered: citrus tristeza and barley yellow dwarf, respectively. Analyses showed that, in a single host population without artificial intervention, the two categories of host plants, infected with a protecting virus alone and infected with a challenging virus, could not coexist in the long term. For disease systems with continuous host availability, the virus (strain) with the higher basic reproductive number (R0) always excluded the other eventually; whereas, for discontinuous systems, R0 is undefined and the virus (strain) with the larger natural transmission rate was the one that persisted in the model formulation. With a proportion of hosts artificially inoculated with a protecting mild virus, the disease caused by a virulent virus could be depressed or eliminated, depending on the proportion. Artificial inoculation may be constant or adjusted in response to changes in disease incidence. The importance of maintaining a constant level of managed cross protection even when the disease incidence dropped was illustrated. Investigations of both pathosystem types showed the same qualitative result: that managed cross protection need not be 100% to eliminate the virulent virus (strain). In the process of replacement of one virus (strain) by another over time, the strongest competition occurred when the incidence of both viruses or virus strains was similar. Discontinuous crop-host availability provided a greater opportunity for viruses or virus strains to replace each other than did the more stable continuous cropping system. The process by which one Barley yellow dwarf virus replaced another in New York State was illustrated.