bluuebunny/crossref_metadata_2025 huggingface dataset:
Simultaneous One-Tube Quantification of Host and Pathogen DNA with Real-Time Polymerase Chain Reaction
Authors: Winton L. M. , Stone J. K. , Watrud L. S. , Hansen E. M.
Published: May 2007

Phaeocryptopus gaeumannii is a widespread foliar parasite of Douglas-fir. Although normally innocuous, the fungus also causes the defoliating disease Swiss needle cast in heavily infected needles. The extent of P. gaeumannii colonization in Douglas-fir foliage was estimated with real-time quantitative polymerase chain reaction (PCR) using TaqMan chemistry. In order to derive a normalized expression of colonization, both pathogen and host DNA were simultaneously amplified but individually detected by species-specific primers and TaqMan probes labeled with different fluorescent dyes. Detection of host DNA additionally provided an endogenous reference that served as both an internal positive control and adjusted for variation introduced by sample-to-sample differences in DNA extraction and PCR efficiencies. The genes employed for designing the TaqMan probes and primers were β-tubulin for the pathogen and a LEAFY/FLORICAULA-like gene involved in floral development for the tree host. Both probe/primer sets exhibited high precision and reproducibility over a linear range of 4 orders of magnitude. This eliminated the need to analyze samples in multiple dilutions when comparing lightly with heavily infected needles. Quantification of the fungus within needles was successful as early as 1 month after initial infection. Real-time PCR is the only method currently available to quantify P. gaeumannii colonization early in the first year of the colonization process.
Expression of an Antifungal Peptide in Saccharomyces: A New Approach for Biological Control of the Postharvest Disease Caused by Colletotrichum coccodes
Authors: Jones Richard W. , Prusky Dov
Published: May 2007

A cecropin A-based peptide inhibited germination of Colletotrichum coccodes at 50 μM. The DNA sequence encoding the peptide was cloned in pRS413, using the Saccharomyces cerevisiae invertase leader sequence for secretion of the peptide, and expressed in yeast. Yeast transformants inhibited the growth of germinated C. coccodes spores and inhibited decay development caused by C. coccodes in tomato fruits. Expression of the antifungal peptide in yeast therefore represents a new approach for the biological control of postharvest diseases.
Benzothiadiazole Activates Resistance in Sunflower (Helianthus annuus) to the Root-Parasitic Weed Orobanche cuman
Authors: Sauerborn J. , Buschmann H. , Ghiasi K. Ghiasvand , Kogel K.-H.
Published: May 2007

The study was conducted to evaluate the potential of induced resistance to infestation of sunflower (Helianthus annuus L.) by the parasitic weed Orobanche cumana Wallr. Treatment of sunflower seeds with 40 ppm of benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) for 36 h completely prevented infection in root chambers. In pot studies using 2.86 × 10-4 g of Orobanche seeds per gram of soil as inoculum, the total number of O. cumana shoots was reduced by 84 and 95% in the 60-ppm BTH treatment in the first and second trial, respectively. Evaluation of the disease incidences revealed that attachment of O. cumana at the sunflower root and the stage of early penetration was reduced in the BTH-treated plants. Chemical analysis of root extracts revealed synthesis of the phytoalexin scopoletin and of hydrogen peroxide in the BTH-treated sunflower roots, but no increase in lignification. Western blot analysis demonstrated accumulation of the pathogenesis-related protein chitinase in roots and stems of induced resistant plants. These results show that the phenomenon of induced resistance is not restricted to viral, bacterial, and fungal disease and demonstrate the great potential of this protection strategy as an effective component of future plant production systems.
Quantification of Xylella fastidiosa from Citrus Trees by Real-Time Polymerase Chain Reaction Assay
Authors: Oliveira Antonio C. , Vallim Marcelo A. , Semighini Camile P. , Araújo Welington L. , Goldman Gustavo H. , Machado Marcos A.
Published: May 2007

Xylella fastidiosa is the causal agent of citrus variegated chlorosis (CVC), a destructive disease of sweet orange cultivars in Brazil. Polymerase chain reaction (PCR)-based assays constitute the principal diagnostic method for detection of these bacteria. In this work, we established a real-time quantitative PCR (QPCR) assay to quantify X. fastidiosa in naturally and artificially infected citrus. The X. fastidiosa cell number detected in the leaves increased according to the age of the leaf, and bacteria were not detected in the upper midrib section in young leaves, indicating temporal and spatial distribution patterns of bacteria, respectively. In addition, the X. fastidiosa cell number quantified in leaves of ‘Pera’ orange and ‘Murcott’ tangor reflected the susceptible and resistant status of these citrus cultivars. None of the 12 endophytic citrus bacteria or the four strains of X. fastidiosa nonpathogenic to citrus that were tested showed an increase in the fluorescence signal during QPCR. In contrast, all 10 CVC-causing strains exhibited an increase in fluorescence signal, thus indicating the specificity of this QPCR assay. Our QPCR provides a powerful tool for studies of different aspects of the Xylella-citrus interactions, and can be incorporated into breeding programs in order to select CVC-resistant plants more quickly.
Influence of Grape Berry Age on Susceptibility to Guignardia bidwellii and Its Incubation Period Length
Authors: Hoffman Lisa Emele , Wilcox Wayne F. , Gadoury David M. , Seem Robert C.
Published: May 2007

The period of fruit susceptibility to Guignardia bidwellii (anamorph Phyllosticta ampelicida), the causal agent of grape black rot, was determined in the field. Intact fruit were inoculated weekly from bloom until 8 weeks later with a suspension containing 2 × 105 conidia per ml. Disease progress was monitored approximately every 2 days until 3 to 5 weeks after inoculation, depending on the year and variety. Fruit of Vitis × labruscana ‘Concord’ exhibited a period of maximum susceptibility from midbloom until 2 to 4 weeks later, although some berries became symptomatic when inoculated 4 to 5 weeks postbloom. Fruit of V. vinifera ‘Chardonnay’ and ‘Riesling’ exhibited a period of maximum susceptibility from midbloom until 3 to 5 weeks later, although some berries retained their susceptibility until 6 to 7 weeks postbloom. These susceptible periods were approximately 2 to 4 weeks shorter than previously assumed. Fruit age at the time of inoculation affected the length of the incubation period (time from inoculation until symptom appearance). When the incubation period was defined in terms of degree hours (base = 0°C) accumulated after inoculation, DH50 values (the number of degree hours required to reach 50% of final disease severity) increased by at least 50% as berries neared the end of their susceptible period. Newly symptomatic berries continued to appear for over 1 month after inoculation of older fruit. Thus, age-related or ontogenic, host resistance was manifested as both a decline in susceptibility and a significant increase in incubation period length. The control of black rot is likely to be improved by tailoring the intensity of fungicidal protection to the abbreviated period of fruit susceptibility defined in this study. Furthermore, the efficacy of management programs and the results of epidemiological studies are likely to be misinterpreted unless the variable effect of fruit age on incubation period length is recognized.
Identification of Unique or Elevated Levels of Kernel Proteins in Aflatoxin-Resistant Maize Genotypes Through Proteome Analysis
Authors: Chen Z.-Y. , Brown R. L. , 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 (Zea mays L.). Resistant maize genotypes have been identified, but the incorporation of resistance into commercial lines has been slow due to the lack of selectable markers. Here we report the identification of potential markers in resistant maize lines using a proteomics approach. Kernel embryo proteins from each of two resistant genotypes have been compared with those from a composite of five susceptible genotypes using large format two-dimensional gel electrophoresis. Through these comparisons, both quantitative and qualitative differences have been identified. Protein spots have been sequenced, and based on peptide sequence homology analysis, are categorized as follows: storage proteins (globulin 1 and globulin 2), late embryogenesis abundant (LEA) proteins related to drought or desiccation (LEA3 and LEA14), water- or osmo-stress related proteins (WSI18 and aldose reductase), and heat-stress related proteins (HSP16.9). Aldose reductase activity measured in resistant and susceptible genotypes before and after infection suggests the importance of constitutive levels of this enzyme to resistance. Results of this study point to a correlation between host resistance and stress tolerance. The putative function of each identified protein is discussed.
Silicon-Induced Cell Wall Fortification of Rice Leaves: A Possible Cellular Mechanism of Enhanced Host Resistance to Blast
Authors: Kim Sang Gyu , Kim Ki Woo , Park Eun Woo , Choi Doil
Published: May 2007

Locations of silicon accumulation in rice leaves and its possible association with resistance to rice blast were investigated by electron microscopy and X-ray microanalysis. A blast-susceptible cultivar, Jinmi, and a partially resistant cultivar, Hwaseong, were grown under a hydroponic culture system with modified Yoshida's nutrient solution containing 0, 50, 100, and 200 ppm of silicon. Electron-dense silicon layers were frequently found beneath the cuticle in epidermal cell walls of silicon-treated plants. Increasing levels of silicon were detected in the outer regions of epidermal cell walls. Silicon was present mainly in epidermal cell walls, middle lamellae, and intercellular spaces within subepidermal tissues. Furthermore, silicon was prevalent throughout the leaf surface, with relatively small deposition on stomatal guard cells in silicon-treated plants. Silicon accumulation and epidermal cell wall thickness in leaves were greater in cv. Jinmi than in cv. Hwaseong. However, the thickness ratios of the silicon layers to epidermal cell walls were greater in cv. Hwaseong (53.25 to 93.28%) than in cv. Jinmi (36.58 to 66.54%). Leaf blast severity was lower in cv. Hwaseong than in cv. Jinmi and was significantly reduced in silicon-treated plants of both cultivars. These results suggest that silicon-induced cell wall fortification of rice leaves may be closely associated with enhanced host resistance to blast.
Frequency and Seasonal Distribution of Pear Psylla Infected with the Pear Decline Phytoplasma in California Pear Orchards
Authors: Blomquist C. L. , Kirkpatrick B. C.
Published: May 2007

Pear decline (PD) is an important disease of Pyrus communis fruiting cultivars in Europe, Asia, and the Americas. PD is caused by a phloem-limited phytoplasma that, in California, is transmitted from diseased to healthy trees by pear psylla, Cacopsylla pyricola. The percentage of phytoplasma-infected pear psylla has never been assessed in the United States in field-collected insects. Pear psylla were collected monthly from PD-infected trees from three orchards in northern California. Individual psylla were tested for the presence of PD phytoplasma, using both a quantitative DNA hybridization and PD phytoplasma-specific polymerase chain reaction (PCR) assays. The percentage of infected psylla ranged from 0 to 45% depending on the orchard, the month and year collected, and the method of detection. The PD phytoplasma was detected in both the winterform and summerform pear psylla. Significantly more infected psylla were detected with PCR than with DNA hybridization analysis in two of the three orchards. The number of PD phytoplasma per pear psylla was estimated to range from 1 × 106 to 8.2 × 107. The percentage of PD-infected pear psylla found in the three northern California pear orchards suggests that both winterform and summerform pear psylla could be important in the transmission of PD.
Variation Among and Within Populations of the Parasitic Weed Orobanche crenata from Spain and Israel Revealed by Inter Simple Sequence Repeat Markers
Authors: Román Belén , Satovic Zlatko , Rubiales Diego , Torres Ana M. , Cubero José Ignacio , Katzir Nurit , Joel Daniel M.
Published: May 2007

The patterns of genetic variation among Orobanche crenata populations from Spain and Israel were studied using radiolabeled inter simple sequence repeat amplification products that were separated in sequencing polyacrylamide gels. The analysis of molecular variance indicated that most of the genetic diversity was attributable to differences among individuals within a population although significant divergences were found between regions. The Jaccard's similarity matrix was analyzed by unweighted pair-group method with arithmetic average and the resultant dendrogram clearly divided six populations by region, with the Spanish populations being more similar to each other than the Israeli populations. These results are consistent with the predominantly allogamous behavior of O. crenata and the extremely efficient dispersal of its seeds.
Preinoculation Effects of Light Quantity on Infection Efficiency of Puccinia striiformis and P. triticina on Wheat Seedlings
Authors: de Vallavieille-Pope C. , Huber L. , Leconte M. , Bethenod O.
Published: May 2007

In a previous study under controlled conditions, a model was developed to predict the infection efficiency for the wheat leaf and stripe rust fungi based on temperature and dew period during the 24 h after inoculation. The two pathogens differed in their maximum infection efficiency under controlled conditions for temperature and dew period, the infection efficiency was 12 times greater for Puccinia triticina than for P. striiformis. In the present study, the model was validated by field results to predict P. triticina infection efficiency as a function of temperature and dew period only. However, this model failed to predict infection efficiency caused by P. striiformis in the field. The model was adapted to include the effects of light quantity on infection efficiency. Wheat seedlings, grown in climate-controlled rooms and exposed to various regimes of light duration and intensity for 24 h in either field or controlled conditions, were inoculated and incubated in climate-controlled rooms under optimal dew and temperature conditions. Quantity of natural or artificial light (light intensity × duration) received by the plants prior to inoculation enhanced infection efficiency of wheat seedlings inoculated by P. striiformis. Infection efficiency increased from 0.4 to 36% depending on the light quantity according to a Richards' function. For stripe rust, three environmental variables, preinoculation light quantity received by the plants, postinoculation temperature, and postinoculation dew period, were used for fitting a model for infection efficiency measured in the field.
Four Near-Isogenic Lines of Cotton with Different Genes for Bacterial Blight Resistance
Authors: Essenberg Margaret , Bayles Melanie B. , Samad Roushan A. , Hall Judy A. , Brinkerhoff L. A. , Verhalen Laval M.
Published: May 2007

The development and genetic characterization of four near-isogenic lines (NILs) of cotton (Gossypium hirsutum) is described herein. Each line contains a single, but different, gene for resistance to bacterial blight caused by Xanthomonas campestris pv. malvacearum. The lines were derived using at least six backcrosses to the susceptible recurrent parent ‘Acala 44’, followed by single plant-progeny row selection for uniformity. The NILs are homozygous for the B2, B4, BIn, or b7 genes and are designated as AcB2, AcB4, AcBIn, and Acb7, respectively. In the ‘Acala 44’ background, B2, B4, and BIn are partially dominant genes; b7 is partially recessive. Relative strengths of resistance conferred by those genes toward race 1 of the pathogen were B4˜b7>BIn˜B2. B4, BIn, and b7 each conferred resistance toward X. campestris pv. malvacearum carrying a single avirulence gene, whereas B2 was less specific.
Isolation of Nonpathogenic Mutants of Fusarium oxysporum f. sp. melonis for Biological Control of Fusarium Wilt in Cucurbits
Authors: Freeman Stanley , Zveibil Aida , Vintal Haim , Maymon Marcel
Published: May 2007

Two nonpathogenic mutant strains 4/4 and 15/15 of Fusarium oxysporum f. sp. melonis (race 1,2) were isolated by a continuous dipinoculation technique following UV mutagenesis of the virulent wild-type isolate FOM1.2. No disease symptoms or detrimental effects were observed following inoculation of muskmelon seedlings by strain 4/4. In contrast, strain 15/15 caused mortality of susceptible cultivars although to a lesser extent than the wild-type isolate. Strain 4/4 colonized a variety of muskmelon and watermelon cultivars. In muskmelon cv. Ein Dor, seedlings were dipped in a conidial suspension of strain 4/4 and planted in medium amended with the mutant to achieve 100% colonization of roots and between 30 to 70% of the lower stem tissues 7 days after planting. Similar percent colonization of watermelon seedlings by strain 4/4 was recorded. In cross-protection experiments with muskmelon cultivars, significant reduction in seedling mortality was observed between 4/4-colonized FOM1.2. challenged plants compared with that of wild-type challenged plants alone. Similarly, strain 4/4 was able to significantly reduce mortality of watermelon seedlings caused by F. oxysporum f. sp. niveum race 2. This novel approach of generating nonpathogenic mutants for biological control in Fusarium spp. and other fungal pathogens from virulent wild-type isolates may be beneficial for control, because the mutant strains, lacking only in pathogenicity, may compete more efficiently than other biocontrol organisms against the pathogen of origin.
Effect of Synthetic and Organic Soil Fertility Amendments on Southern Blight, Soil Microbial Communities, and Yield of Processing Tomatoes
Authors: Bulluck L. R. , Ristaino J. B.
Published: May 2007

Soil fertility amendments, including composted cotton-gin trash, swine manure, a rye-vetch green manure, or synthetic fertilizers, were applied to subplots and tillage on bare soil; or tillage followed by surface mulch with wheat straw were applied to main plots to determine the effect on the incidence of southern blight caused by Sclerotium rolfsii, yield of processing tomato, and soil microbial communities. The amendment-tillage interaction was significant in 1997 and disease incidence was 67% in tilled bare soil receiving synthetic fertilizers; whereas disease incidence was 3, 12, and 16% in surface-mulched plots amended with a composted cotton-gin trash, swine manure, or a rye-vetch green manure. The amendment effect was significant in 1998, and disease incidence was 61% in plots receiving synthetic fertilizer and was 23, 44, and 53% in plots receiving cotton-gin trash, swine manure, or rye-vetch green manure, respectively. In 1997, yields were highest in tilled surface-mulched plots amended with synthetic fertilizers, cotton-gin trash, or swine manure, respectively. In 1998, yields were low in all plots and there were no significant differences in yield due to treatment. Propagule densities of antagonistic soil fungi in the genus Trichoderma were highest in soils amended with composted cotton-gin trash or swine manure in both years. Propagule densities of fluorescent pseudomonads in soil were higher in plots amended with organic amendments than with synthetic fertilizers in both years. Propagules densities of enteric bacteria were elevated in soils amended with raw swine manure biosolids in both years. Our research indicates that some organic amendments, such as cotton-gin trash, reduced the incidence of southern blight in processing tomato and also enhanced populations of beneficial soil microbes.
Histological Characterization of Resistance to Uromyces viciae-fabae in Faba Bean
Authors: Sillero J. C. , Rubiales D.
Published: May 2007

Components of resistance to the faba bean rust (Uromyces viciae-fabae) were studied at the histological level in seedlings and adult plants of nine faba bean (Vicia faba) lines differing in their level of resistance. Resistance of these lines was previously shown to be characterized macroscopically by an increased latent period, a decreased colony size, and a relatively decreased infection frequency. In some lines, the resistance also was associated with macroscopically visible necrosis. Histological investigations revealed few differences in spore germination and appressorium formation. Significant levels of aborted stomatal penetration by the rust fungus were found on all resistant lines. However, differences among lines were more evident once the stomata were penetrated by the infection structures. Resistance was mainly due to a restriction of haustorium formation with varying levels of early abortion of the colonies, a reduction in the number of haustoria per colony, and smaller colony size. In addition, necrosis of the host cells associated with infection hyphae was detectable in some lines from the beginning of colony development. This microscopically visible necrosis became stronger from 4 days after inoculation, resulting in a reduced growth of the colony. Differences in resistance levels were more marked in adult plants than in seedlings.
Molecular Variability Within Diaporthe/Phomopsis helianthi from France
Authors: Says-Lesage Véronique , Roeckel-Drevet Patricia , Viguié Anne , Tourvieille Jeanne , Nicolas Paul , de Labrouhe Denis Tourvieille
Published: May 2007

Diaporthe/Phomopsis helianthi causes brown stem canker of sunflower (Helianthus annuus) and is responsible for considerable yield loss. This species shows considerable variation for morphological characters, growth, and pathogenicity. Molecular variability of two sample groups was assessed with amplified fragment length polymorphism (AFLP) markers. Isolates of the first sample were collected from infected sunflower tissues from the main regions in France where the crop is grown, whereas isolates from the second sample came from stems within a single field of sunflower. A soybean strain was taken as an outgroup for AFLP analyses. Within sample one, the greatest genetic distance among isolates was 0.97, whereas it was 0.44 within sample two isolates. For the whole of France, the average genetic distance was 0.68, whereas in the one field it was 0.12. Nei's genetic diversity indices were 0.20 and 0.06 for France and for one field, respectively. The greatest genetic distance was found between isolates from the most northern crops. The greatest genetic distance between D. helianthi isolates and the strain isolated from soybean was similar to that observed for D. helianthi isolates from different geographical areas. The problems in defining the genus Phomopsis are discussed. It is shown that internal transcribed spacer sequencing could be a useful criteria for Diaporthe/Phomopsis species determination. The considerable genetic variability of the pathogen could lead to the occurrence of new strains that could be more aggressive or more resistant to chemical control.
Response of Hard Red Winter Wheat to Soilborne wheat mosaic virus Using Novel Inoculation Methods
Authors: Driskel Barbara A. , Hunger Robert M. , Payton Mark E. , Verchot-Lubicz Jeanmarie
Published: May 2007

Soilborne wheat mosaic virus (SBWMV) is an agronomically important pathogen of wheat that is transmitted by the soilborne plasmodiophorid vector Polymyxa graminis. In the laboratory, attempts to generate SBWMV-infected plants are often hampered by poor infectivity of the virus. To analyze the mechanism for virus resistance in wheat cultivars, we developed novel inoculation techniques. A new technique for foliar inoculation of SBWMV was developed that eliminated wound-induced necrosis normally associated with rub inoculating virus to wheat leaves. This new technique is important because we can now uniformly inoculate plants in the laboratory for studies of host resistance mechanisms in the inoculated leaf. Additionally, wheat plants were grown hydroponically in seed germination pouches and their roots were inoculated with SBWMV either by placing P. graminis-infested root material in the pouch or by mechanically inoculating the roots with purified virus. The susceptibility of one SBWMV susceptible and three field resistant wheat cultivars were analyzed following inoculation of plants using these novel inoculation techniques or the conventional inoculation technique of growing plants in P. graminis-infested soil. The results presented in this study suggest that virus resistance in wheat likely functions in the roots to block virus infection.
Cytological Aspects of Compost-Mediated Induced Resistance Against Fusarium Crown and Root Rot in Tomato
Authors: Pharand Benoît , Carisse Odile , Benhamou Nicole
Published: May 2007

The potential of a pulp and paper mill residues compost for the control of crown and root rot of greenhouse-grown tomato caused by Fusarium oxysporum f. sp. radicis-lycopersici was ultrastructurally investigated. Peat moss amended with compost substantially reduced disease-associated symptoms. Addition of Pythium oligandrum to either peat moss alone or peat moss amended with compost resulted in a considerable reduction in disease incidence compared with controls grown in peat moss alone. Histological and cytological observations of root samples from Fusarium-inoculated plants revealed that the beneficial effect of compost in reducing disease symptoms is associated with increased plant resistance to fungal colonization. One of the most prominent facets of compost-mediated induced resistance concerned the formation of physical barriers at sites of attempted fungal penetration. These structures, likely laid down to prevent pathogen ingress toward the vascular elements, included callose-enriched wall appositions and osmiophilic deposits around the sites of potential pathogen ingress. Invading hyphae, coated by the osmiophilic material, showed marked cellular disorganization. The use of the wheat germ agglutinin-ovomucoid-gold complex provided evidence that the wall-bound chitin was altered in severely damaged hyphae. A substantial increase in the extent and magnitude of the cellular changes induced by compost was observed when P. oligandrum was supplied to the potting substrate. This finding corroborates the current concept that amendment of composts with specific antagonists may be a valuable option for amplifying their beneficial properties in terms of plant disease suppression.
Diversity Among Isolates ofBean pod mottle virus
Authors: Gu Hongcang , Clark Anthony J. , de Sá P. B. , Pfeiffer Todd W. , Tolin Sue , Ghabrial Said A.
Published: May 2007

Isolates of Bean pod mottle virus (BPMV), a member of the genus Comovirus, collected from soybean fields in Kentucky, Virginia, Arkansas, and Iowa were classified into two distinct subgroups, I and II, based on nucleic acid hybridization analysis using cloned cDNA probes to RNA-1 from BPMV strains K-G7 and K-Ha1. Slot blot hybridization analysis using cloned cDNA probes to RNA-2 from the same two strains (K-G7 and K-Ha1), however, revealed that some of the isolates, initially classified as belonging to subgroup I after analysis with RNA-1 probes, are in fact natural reassortants between the two strain subgroups. This was corroborated by nucleotide sequence analysis of full-length cDNA clones of both RNA-1 and RNA-2 from a putative reassortant strain (K-Ho1). These results indicate that BPMV strain diversity is more complex than initially anticipated, and that the use of cloned probes to both genomic RNAs during nucleic acid hybridization analysis is required to unravel the extent of such diversity. In a field plot experiment, BPMV isolates that belong to distinct strain subgroups induced symptoms that varied in severity and in the level of yield losses. In this regard, the reassortant strain K-Ho1 caused the most serious damage compared with four other BPMV isolates tested. Furthermore, the soybean alleles Rsv1and Rsv4, known to confer resistance against Soybean mosaic virus, a member of the genus Potyvirus, did not provide any protection against BPMV. Additionally, we developed a reverse transcription-polymerase chain reaction procedure based on the sequence of a highly conserved region in the capsid polyprotein coding sequence that provides efficient and highly sensitive detection of all BPMV isolates tested, regardless of their strain classification.
Is Change in Electrical Potential or pH a Hatching Signal for Heterodera glycines?
Authors: Pike S. M. , Heinz R. , Walk T. , Jones C. , Kraus G. A. , Gassmann W. , Novacky A. J. , Niblack T. L.
Published: May 2007

This study explored the possibilities that changes in the egg shell/lipid layer electrical potential or pH communicate external hatching conditions to the Heterodera glycines second-stage juvenile (J2) within the mature egg and that electrophysiology could measure effects of chemicals on emergence. Potentials were measured following application of the emergence inducers (ZnSO4 and ZnCl2), ions that do not affect emergence, or synthetic emergence inhibitors. Results were compared with pH measurements and emergence bioassays. Healthy appearing eggs had negative resting potentials. Application of ZnSO4 caused a smooth depolarization. However, eggs containing J2 and immature eggs depolarized to a similar degree when ZnSO4 was added. In addition, ZnSO4, synthetic emergence inhibitors, and CaCl2 caused similar depolarization, and some depolarization was measured in dye-permeable eggs and empty shells. Results suggest that change in cation surface charge contributed to depolarization and that Cl¯ penetrated the egg shell/lipid layer without causing potential changes. In bioassays, zinc consistently stimulated emergence to a greater degree than H2O, other cations, or buffers, and counteracted emergence inhibitors. Zinc-caused emergence stimulation was independent of pH. In summary, it is concluded that depolarization and pH are not emergence signals and electrophysiology is unlikely to measure effectiveness of emergence stimulators or inhibitors.
Molecular Markers Dispute the Existence of the Afro-Andean Group of the Bean Angular Leaf Spot Pathogen, Phaeoisariopsis griseola
Authors: Mahuku George S. , Henríquez María Antonia , Munõz Jaime , Buruchara Robin A.
Published: May 2007

Coevolution of the angular leaf spot pathogen, Phaeoisariopsis griseola, with its common bean host has been demonstrated, and P. griseola isolates have been divided into Andean and Mesoamerican groups that correspond to defined bean gene pools. Recent characterization of P. griseola isolates from Africa has identified a group of isolates classified as Andean using random amplified polymorphic DNA (RAPD), but which are able to infect some Mesoamerican differential varieties. These isolates, designated Afro-Andean, have been identified only in Africa. Random amplified microsatellites, RAPD, and restriction digestion of amplified ribosomal intergenic spacer region were used to elucidate the relationships among the Afro-Andean, Andean, and Mesoamerican groups of P. griseola. Cluster and multiple correspondence analysis of molecular data separated isolates into Andean and Meso-american groups, and the Afro-Andean isolates clustered with Andean isolates. Analysis of molecular variance ascribed 2.8% of the total genetic variation to differences between Afro-Andean and Andean isolates from Africa. Gene diversity analysis revealed no genetic differentiation (GST = 0.004) between Afro-Andean and Andean isolates from Africa. However, significant levels of genetic differentiation (GST = 0.39) were observed between Afro-Andean or Andean isolates from Africa and Andean isolates from Latin America, revealing significant geographical differentiation within the Andean lineage. Results from this study showed that Afro-Andean isolates do not constitute a new P. griseola group and do not represent long-term evolution of the pathogen genome, but rather are likely the consequents of point mutations in genes for virulence. This finding has significant implications in the deployment of resistant bean genotypes.
Aggressiveness of Mycosphaerella graminicola Isolates from Susceptible and Partially Resistant Wheat Cultivars
Authors: Cowger Christina , Mundt Christopher C.
Published: May 2007

The selective effect of quantitative host resistance on pathogen aggressiveness is poorly understood. Because two previous experiments with a small number of bread wheat cultivars and isolates of Mycosphaerella graminicola had indicated that more susceptible hosts selected for more aggressive isolates, we conducted a larger experiment to test that hypothesis. In each of 2 years, six cultivars differing in their levels of partial resistance were planted in field plots, and isolates were collected from each cultivar early and late in the growing season. The isolates were inoculated as populations bulked by cultivar of origin, field replicate, and collection date on seedlings of the same six cultivars in the greenhouse. The selective impact of a cultivar on aggressiveness was measured as the difference in aggressiveness between early and late isolates from that cultivar. Regression of those differences on disease severity in the field yielded significance values of 0.0531 and 0.0037 for the 2 years, with moderately resistant cultivars selecting for more aggressive isolates. In a related experiment, the protectant fungicide chlorothalonil was applied to plots of two susceptible cultivars to retard epidemic development. When tested in the greenhouse, isolates of M. graminicola from those plots were significantly more aggressive than isolates from the same cultivars unprotected by fungicide.
Incorporation of Temperature and Solar Radiation Thresholds to Modify a Lettuce Downy Mildew Warning System
Authors: Wu B. M. , van Bruggen A. H. C. , Subbarao K. V. , Scherm H.
Published: May 2007

The effect of temperature on infection of lettuce by Bremia lactucae was investigated in controlled environment studies and in the field. In controlled conditions, lettuce seedlings inoculated with B. lactucae were incubated at 15, 20, 25, or 30°C during a 4-h wet period immediately after inoculation or at the same temperatures during an 8-h dry period after the 4-h postinoculation wet period at 15°C. High temperatures during wet and dry periods reduced subsequent disease incidence. Historical data from field studies in 1991 and 1992, in which days with or without infection had been identified, were analyzed by comparing average air temperatures during 0600 to 1000 and 1000 to 1400 Pacific standard time (PST) between the two groups of days. Days without infection had significantly higher temperatures (mean 21.4°C) than days with infection (20.3°C) during 1000 to 1400 PST (P < 0.01) but not during 0600 to 1000 PST. Therefore, temperature thresholds of 20 and 22°C for the 3-h wet period after sunrise and the subsequent 4-h postpenetration period, respectively, were added to a previously developed disease warning system that predicts infection when morning leaf wetness lasts ≥4 h from 0600 PST. No infection was assumed to occur if average temperature during these periods exceeded the thresholds. Based on nonlinear regression and receiver operating characteristic curve analysis, the leaf wetness threshold of the previous warning system was also modified to ≥3-h leaf wetness (≥0900 PST). Furthermore, by comparing solar radiation on days with infection and without infection, we determined that high solar radiation during 0500 to 0600 PST in conjunction with leaf wetness ending between 0900 and 1000 PST was associated with downy mildew infection. Therefore, instead of starting at 0600 PST, the calculation of the 3-h morning leaf wetness period was modified to start after sunrise, defined as the hour when measured solar radiation exceeded 8 W m-2 (or 41 μmol m-2 s-1 for photon flux density). The modified warning system was compared with the previously developed system using historical weather and downy mildew data collected in coastal California. The modified system was more conservative when disease potential was high and recommended fewer fungicide applications when conditions were not conducive to downy mildew development.
Localization of Transmissible and Nontransmissible Viruses in the Vector Nematode Xiphinema americanum
Authors: Wang Shouhua , Gergerich Rose C. , Wickizer Sandra L. , Kim Kyung S.
Published: May 2007

The inner lining of the food canal of nematodes that transmit plantinfecting viruses is regarded as the retention region of viruses. To characterize the location of transmissible and nontransmissible viruses in the vector nematode Xiphinema americanum, three nepoviruses, Tobacco ringspot virus (TRSV), Tomato ringspot virus(TomRSV), and Cherry leaf roll virus(CLRV), and one non-nematode-transmissible virus, Squash mosaic virus (SqMV), were evaluated for transmission efficiency and localization sites in the nematode. Transmission trials showed highest transmission efficiency for TomRSV (38% with 1 and 100% with 10 nematodes, respectively), intermediate efficiency for TRSV (27% with 1 and 65% with 10 nematodes, respectively), and no transmission for CLRV and SqMV. Electron microscopy and immunofluorescent labeling revealed that TRSV was primarily localized to the lining of the lumen of the stylet extension and the anterior esophagus, but only rarely in the triradiate lumen. Within a nematode population, particles of TRSV were no longer observed in these three regions 10 weeks after acquisition, and it is assumed that there was gradual and random loss of the virus from these areas. The percentage of nematodes that were labeled by virus-specific immunofluorescent labeling in a population of viruliferous nematodes decreased gradually after TRSV acquisition when the nematodes were placed on a nonhost of the virus, and the loss of immunofluorescent labeling paralleled the decrease in the ability of the nematode population to transmit the virus. TomRSV was localized only in the triradiate lumen based on thin-section electron microscopy. No virus-like particles were observed in any part of the food canal of nematodes that had fed on CLRV-infected plants. Virus-like particles that appeared to be partially degraded were observed only in the triradiate lumen of nematodes that had fed on SqMV-infected plants. These results clarified the status of localization of two nontransmissible viruses in X. americanum and presented evidence that two nematode-transmissible viruses, TRSV and TomRSV, are localized in different regions of the food canal of X. americanum.
Dynamics of Tomato spotted wilt virus Replication in the Alimentary Canal of Two Thrips Species
Authors: de Assis Filho F. M. , Naidu R. A. , Deom C. M. , Sherwood J. L.
Published: May 2007

Transmission of Tomato spotted wilt virus (TSWV) is dependent on virus uptake in the midgut prior to virus movement to the salivary glands. Replication of TSWV in the alimentary canal of tobacco thrips (TT, Frankliniella fusca) and western flower thrips (WFT, F. occidentalis) was investigated by immunolocalization of the nonstructural protein (NSs) encoded by the small RNA of TSWV and fluorescence microscopy. Analysis of cohorts during development from larva to adults following virus acquisition by first instar larva indicated that virus replication followed a specific time-course pattern in the foregut, regions of the midgut, salivary glands, and ligaments between the midgut and salivary glands. Initial virus replication occurred only in epithelial cells of midgut-1 but, upon infection of muscle cells, the virus moved to the midgut-2, foregut, midgut-3, and salivary glands. The ligaments between the midgut and salivary glands appeared to be a route for virus to invade the salivary glands. No virus replication was observed in the hindgut, Malpighian tubules, or tubular salivary glands. The dynamics of TSWV replication, as measured by NSs accumulation, were similar in both TT and WFT.
Spatial Distribution of Venturia inaequalis Airborne Ascospores in Orchards
Authors: Charest J. , Dewdney M. , Paulitz T. , Philion V. , Carisse O.
Published: May 2007

Apple scab (Venturia inaequalis) causes important economic losses in many apple production areas of the world. The disease is controlled by numerous fungicide applications regardless of the presence of ascospores in the orchard. Airborne ascospore concentration (AAC) can be measured in real time to time fungicide applications. However, the level of heterogeneity of the AAC in commercial orchards was unknown. Consequently, the spatial distribution of V. inaequalis ascospores was studied in a commercial apple orchard of 0.43 ha. The potential ascospore dose (PAD) and AAC were measured in 40 quadrats each of 108 m2. In each quadrat, the AAC was monitored during the major rain events in spring 1999 and 2000 using spore samplers. The variance-to-mean ratio for the PAD and for most of the AAC sampling dates was >1, indicating an aggregated pattern of distribution. None of the frequency distributions of the most important ascospore ejection events followed the Poisson probability distribution, indicating that the pattern of distribution was not random. For all events, AAC had an aggregated pattern of distribution as suggested by the negative binomial distribution. The PAD followed neither the Poisson nor the negative binomial distribution. Geostatistical analyses confirmed the aggregated pattern of distribution. The cultivars had an effect on the PAD and AAC distribution pattern, but both PAD and AAC were not uniformly distributed within a block of the same cultivar. Therefore, the number, location, and height of samplers required to estimate AAC in orchards need to be investigated before using information on AAC for decision making.
High Genetic Stability of the Begomovirus Tomato yellow leaf curl Sardinia virus in Southern Spain Over an 8-Year Period
Authors: Sánchez-Campos S. , Díaz J. A. , Monci F. , Bejarano E. R. , Reina J. , Navas-Castillo J. , Aranda M. A. , Moriones E.
Published: May 2007

The evolution of the plant single-stranded DNA virus Tomato yellow leaf curl Sardinia virus (TYLCSV) (genus Begomovirus, family Geminiviridae) has been monitored for 8 years after its appearance in southern Spain. Variation within three genomic regions of 166 TYLCSV isolates collected from three locations was assessed by single-strand conformation polymorphism (SSCP) analysis. According to SSCP, the intergenic region (IR) was the most variable. Low genetic diversity was found within the population and geographical or temporal differences were not evident. Nucleotide sequences of specific genomic regions of haplotypes identified by SSCP indicated close relationships among them. Therefore, the Spanish TYLCSV population appears to represent a single, undifferentiated population. The analysis of IR sequences for a subsample of 76 randomly chosen isolates confirmed the limited genetic diversity revealed by the SSCP analysis. A tendency to a lineal increase in diversity over time was observed in Málaga and Almería subpopulations; however, no accumulation of mutations in single isolates was evident. Negative selection to variation seems to operate to conserve certain regions of the genome. Thus, the low genetic diversity found in the studied TYLCSV population might be the result of a founder effect with subsequent selection against less fit variants arising by mutation.
Characterization of AG-13, a Newly Reported Anastomosis Group of Rhizoctonia solani
Authors: Carling D. E. , Baird R. E. , Gitaitis R. D. , Brainard K. A. , Kuninaga S.
Published: May 2007

Rhizoctonia solani anastomosis group (AG)-13 was collected from diseased roots of field grown cotton plants in Georgia in the United States. Isolates of AG-13 did not anastomose with tester isolates of AG-1 through AG-12. Mycelium of all isolates of AG-13 were light brown but darkened as cultures aged. All isolates produced aerial mycelium. Concentric rings were visible after 3 to 4 days of growth but disappeared as cultures aged and darkened. Individual sclerotia were up to 1.5 mm in diameter, similar in color to the mycelium, and generally embedded in the agar. Clumps of sclerotia up to 5 mm in diameter were produced on the agar surface. All attempts to induce basidiospore production were unsuccessful. The 5.8S region of the rDNA from isolates of AG-13 was identical in length and sequence to isolates of all other AGs of R. solani. Length and sequence of the internal transcribed spacer (ITS) regions of rDNA from isolates of AG-13 were unique among AGs of R. solani. Similarity between AG-13 and other AGs of R. solani ranged from 68 to 85% for ITS region 1 and 85 to 95% for ITS region 2. Selected isolates of AG-13 caused minor or no damage to barley, cauliflower, cotton, lettuce, potato, and radish in laboratory or greenhouse studies.
Alternative Disease Control Agents Induce Resistance to Blue Mold in Harvested ‘Red Delicious’ Apple Fruit
Authors: de Capdeville Guy , Wilson Charles L. , Beer Steven V. , Aist James R.
Published: May 2007

Alternative control agents, including UV-type C (254 nm) irradiation, yeasts antagonistic to fungal growth, chitosan and harpin, were evaluated for their ability to induce resistance in cv. Red Delicious apple fruit against postharvest blue mold caused by Penicillium expansum. Freshly harvested and controlled atmosphere (CA)-stored fruit were treated with these agents at different doses and concentrations or with paired combinations of the agents. Treated fruit were inoculated with P. expansum 24, 48, or 96 h following treatment, and stored at 24°C in the dark. The fruit were evaluated for development of disease every 2 days for 14 days by measuring the diameter of lesions that formed. The area under the disease progress curve (AUDPC) was calculated and analyzed statistically. All treatments were effective in reducing the AUDPC; UV-C was most effective, followed by harpin, chitosan, and the yeasts, respectively. Regardless of treatment, fresh fruit were more responsive to treatments than CA-stored fruit. There was a clear time-dependent response of the fruit to the treatments, in which treatments applied 96 h before inoculation provided the best results. In a few situations, the combinations of agents did provide an additive effect, but no synergistic effects were detected. Moreover, disease severity in fruit treated by any combination was markedly better than that in the controls. Although the combinations of treatments was overall less effective than the single treatments, they did provide significant reductions of the progress of disease in comparison with the controls. Because the fungus did not come into contact with any of the control agents, this study showed conclusively that the agents studied were able to induce resistance in the fruit rather than merely inhibit the pathogen directly. It also showed, for the first time, that harpin is able to induce resistance in harvested apple fruit. The use of these control agents may minimize the costs of control strategies and reduce the risks associated with the excessive use of fungicides in harvested apple fruit