bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Late Season Harvesting, Fall Grazing, and Fungicide Treatment on Verticillium Wilt Incidence, Plant Density, and Forage Yield of Alfalfa
Authors: Gray F. A. , Koch D. W.
Published: May 2007

Studies were conducted in the presence of Verticillium wilt (Verticillium albo-atrum) to determine the effect of fall harvesting and grazing over time on plant stand and forage yield of alfalfa (Medicago sativa). Resistant and susceptible cultivars were tested on established and newly seeded fields. In the fall (experiments 1 and 2), cultivars were either: (i) cut (third time); (ii) grazed; (iii) cut and grazed; or (iv) left uncut and ungrazed. Although Verticillium was present, test sites for experiments 2 and 3 were sprayed with a spore suspension of V. albo-atrum immediately following the first cutting of each experiment to standardize disease pressure. In experiment 1, the moderately resistant cultivar Apollo II, harvested twice without a late third cutting or fall grazing, produced the highest forage yield the following year. Fall grazing reduced subsequent yields in both the 2- and 3-cut treatments. In experiment 2, a third cutting decreased plant density and forage yield in both resistant and susceptible cultivars, while grazing had no effect. Neither fall treatment affected incidence of Verticillium wilt. In experiment 3, application of the fungicide benomyl to plant stubble following each harvest decreased Verticillium wilt in Apollo but not in Arrow. Overall, with the resistant cultivar Arrow, harvesting twice annually and grazing after a killing frost in lieu of late fall cutting slowed disease development, prolonged stand life, and maximized forage yield and quality.
First Detection of Phytoplasmas in Rhododendron in the Czech Republic
Authors: Mertelik J. , Kloudova K. , Vanc P. , Mokra V. , Sediva J. , Navratil M. , Valova P.
Published: May 2007

A 10-year-old plant of rhododendron (‘Cunningham's White’ PS986) with leaf malformation and variegation was observed in Prague in 1997. This plant was micropropagated, and regenerants with severe, mild and no symptoms were obtained. Phytoplasma infection was detected using nested polymerase chain reaction (PCR) in the original PS986 plant and the symptomatic regenerants but not in nonsymptomatic plants. During 2002, phytoplasmas were also detected in rhododendron hybrids, PS2716 and PS2439, grown in Pruhonice that showed similar symptoms as the plant observed in Prague. In nested PCR, performed as described previously (1), primer pairs R16F1/R16R0 and R16F2/R16R2 were used. The phytoplasmas detected were classified by restriction fragment length polymorphism analysis using R16F2/R16R2 PCR fragments. Following digestion with restriction endonuclease MseI (Promega, Madison, WI), the restriction profiles obtained were identical with the pattern of the stolbur phytoplasma group (16SrXII group) as determined previously (2). To our knowledge, this is the first finding of stolbur-type phytoplasma in rhododendron worldwide.

References: (1) R. Fialova et al. Plant Prot. Sci. 39(1):7, 2003. (2) I.-M Lee et al. Int. J. Syst. Bacteriol. 48:1153,1998.
Bacterial Canker Caused by Clavibacter michiganensis subsp. michiganensis on Greenhouse-Grown Tomato in the Western Mediterranean Region of Turkey
Authors: Basim E. , Basim H. , Dickstein E. R. , Jones J. B.
Published: May 2007

During the 2003 winter growing season, a disease occurred on tomato (Lycopersicon esculentum L.) cvs. Selin, Astona, 198 RN, Malike, and Tulin that were growing in almost all greenhouses of Serik, Aksu, Kumluca, Demre, and Kinik in the Antalya Province, and Keçiborlu, Çandir, and Şeyhler in the Isparta Province in the western Mediterranean Region of Turkey. Disease incidence ranged from 26 to 65%, resulting in economically heavy losses. Symptoms were water-soaked, dark brown-to-black lesions on the leaf margins and asymmetrical wilting of the leaflets. In advanced stages of disease, vascular tissue had a light brown discoloration. A gram-positive bacterium was consistently isolated and formed light gray colonies with internal light gray flecks approximately 2 to 3 mm in diameter on the modified semiselective medium (SCM) (2). Thirty isolates were identified as Clavibacter michiganensis subsp. michiganensis using polymerase chain reaction (PCR) with primers CMM 5 and CMM 6 on the basis of consistent amplification of a 614-bp DNA fragment (1). The isolates were also identified as C. michiganensis subsp. michiganensis (similarity 0.810 to 0.888%) using fatty acid methyl ester (FAME) analysis and Sherlock Microbial Identification System software (Microbial ID, Newark, DE). A pathogenicity test was done by spraying a 6-week-old tomato seedling (cv. Selin) with the bacterial concentration of 108 CFU/ml in sterile water. Control tomato seedlings were only sprayed with sterile water. The inoculated plants were covered with polyethylene bags for 48 h and kept in a growth chamber at 25°C. Symptoms developed on the inoculated plants within approximately 10 days and were similar to those observed in the greenhouse. No symptoms developed on control plants. The bacterium was reisolated from inoculated plants, and its identity was confirmed by colony type on SCM, PCR, and FAME analysis. Although it has been known to be present previously in Aegean, eastern Mediterranean, and the eastern Anatolia regions of Turkey, to our knowledge, this is the first report of bacterial canker on greenhouse tomatoes in the western Mediterranean Region of Turkey. Occurrence of the disease in almost all greenhouses may be the result of the high relative humidity present and the lack of protective bactericide applications in many greenhouses of the region.

References: (1) J. Dreier et al. Phytopathology 85:462, 1995. (2) M. Fatmi and N. W. Schaad. Phytopathology 78:121, 1988.
First Report of Leaf Spot Caused by Curvularia affinis on Festuca arundinacea in Hubei, China
Authors: Huang J. B. , Zheng L. , Hsiang T.
Published: May 2007

Curvularia leaf spot affects many species of grasses worldwide but has not been previously reported from central China. In October 2003, symptoms were observed on Festuca arundinacea in a city park in Yichang, Hubei. Fungicides were not used at this site. Leaf spots formed on the foliage after a few days of rainfall. In the most heavily infected areas, the incidence of disease was one leaf in five with spots. Overall, there was an average of 1 leaf in 50 with spots. Infected leaves most commonly bore a single spot, but some leaves had as many as 10 small spots. The spots were round, 5 to 10 mm across, and gray brown in the center with a dark margin. The samples were brought to the lab, and isolations were made on acidified potato dextrose agar (PDA; 3% acetic acid). After 3 days at room temperature, 10-mm-diameter white cultures with appressed mycelium were observed. Two days later, the colonies acquired a greenish cast particularly at the centers, and these yielded spores were identified as Curvularia affinis according to Ellis (1). The key characteristics were a scarcely protuberant hilum, spore size of 25 × 11 μm, and four-septate, curved conidia. DNA sequencing of the internal transcribed spacer region of ribosomal DNA was used to confirm the identity, and aside from sequence length differences, the match with a C. affinis sequence in GenBank was 100%. Inoculating the fungus onto field-grown F. arundinacea that had been washed, cut, and placed in 15-cm-diameter glass petri plates tested pathogenicity. The stems were at least 10 cm long and bore multiple attached leaves. The plates were layered with tissue paper that had been wetted and autoclaved. Sterile water was added to ensure sufficient moisture, and the tissue paper was folded over the bottom 1 cm of stems. Growing the fungus on PDA produced inoculum. Of the eight stems in the plate, one-half were used as a noninoculated check, while the remaining four stems were inoculated with three 5-mm-diameter agar plugs per stem. The plates were incubated at 25°C under continuous fluorescent light, and the inoculum plugs were removed after 48 h. Within 24 h after plug removal, lesions began to appear on the leaves. Circular brown spots were initially less than 5 mm wide, but within 2 days they darkened and grew to 10 mm wide and some of the lesions had a bright yellow margin. No lesions were found on noninoculated stems, which remained turgid and green throughout the trial. C. affinis was successfully reisolated from symptomatic tissue, completing Koch's postulates. To our knowledge, this is the first report of C. affinis on a turfgrass species, although it has been previously found on rice (1).

Reference: (1) M. B. Ellis. Dematiaceous hyphomycetes VII: Curvularia, Brachysporium, etc. Mycol. Pap. 106:1, 1966.
Copper Tolerance in Australian Populations of Xanthomonas campestris pv. vesicatoria Contributes to Poor Field Control of Bacterial Spot of Pepper
Authors: Martin Heidi L. , Hamilton Vicki A. , Kopittke Rosemary A.
Published: May 2007

Many Australian pepper producers have reported poor control of bacterial spot of pepper (caused by Xanthomonas campestris pv. vesicatoria) despite using copper bactericides at recommended rates. This prompted us to investigate whether copper-tolerant strains of the pathogen are present in Australia and whether they are a contributing factor to poor control. We screened strains of X. campestris pv. vesicatoria in the laboratory, in broths containing different concentrations of CuSO4, and established the maximum copper concentration at which each strain could grow. About one in four strains (75 total) collected between 1999 and 2000 tolerated ≥1.0 mM CuSO4 and could be considered copper tolerant. Only one strain (of 12) collected before 1987 could tolerate 1.0 mM, and comparison of the two populations by fitting regression models revealed that the mortality of strains in the two populations differed significantly across all concentrations of copper. It was necessary to apply higher rates of copper to strains collected between 1999 and 2000 to achieve mortality levels equivalent to those in the older population. This is strong evidence that the prevalence of copper-tolerant strains has increased in Australian populations. We also assessed whether copper-tolerant strains may increase substantially in a field population of X. campestris pv. vesicatoria when copper is repeatedly sprayed. In the field, the proportion of copper-tolerant strains increased after 12 weekly sprays of copper, and they were very prevalent in the population after 21 sprays. This is the first report of copper tolerance in Australian populations of X. campestris pv. vesicatoria.
Detection and Partial Molecular Characterization of Two Plum pox virus Isolates from Plum and Wild Apricot in Southeast Kazakhstan
Authors: Spiegel S. , Kovalenko E. M. , Varga A. , James D.
Published: May 2007

Plum pox virus (PPV) was detected in wild apricot and cultivated plum maintained in a germ plasm collection in Kazakhstan. Both isolates were typed as D strain, with no evidence of recombination. The virus was detected by triple-antibody sandwich enzyme-linked immunosorbent assay (ELISA) utilizing the universal PPV-specific monoclonal antibody (MAb) 5B as the secondary antibody, and by reverse-transcription polymerase chain reaction (RT-PCR) assay using primers that amplified a 243-bp fragment in the C-terminus of the coat protein (CP) coding region. Immunocapture (IC) RT-PCR was used to detect PPV in nine wild apricot accessions, including eight ELISA-negative and one ELISA-positive. The plum and apricot isolates reacted positively in Western blot assay with the universal MAb 5B, and negatively with the strain-M-specific MAb-AL. Restriction fragment length polymorphism analysis applied to the amplified 243-bp fragment showed that restriction sites for AluI and RsaI were present in the were present in the plum and apricot samples. An amplified 836-bp cDNA fragment derived from the P3-6K1 coding region of both isolates had restriction profiles typical for strain D. Nucleotide identities of 99 to 100% were observed for the 243-bp fragments of the Kazakhstan isolates when compared with the corresponding regions of strain D, and 94 to 95% identity with strain M. Nucleotide sequence analysis of the entire CP coding region of the plum and apricotisolates resulted in the identification of a unique deletion of six nucleotides (two deduced proline amino acid residues) in the N-terminal region in the plum isolate. This is the first deletion of this nature observed among PPV isolates. The DAG motif was present in both isolates. Several nucleotide substitutions in the CP coding region were common to the plum and apricot isolates and appear to be unique to the Kazakstan isolates. This suggests a close relationship between the isolates.
Involvement of Jasmonic Acid/Ethylene Signaling Pathway in the Systemic Resistance Induced in Cucumber by Trichoderma asperellum T203
Authors: Shoresh Michal , Yedidia Iris , Chet Ilan
Published: May 2007

Trichoderma spp. are effective biocontrol agents for a number of soilborne plant pathogens, and some are also known for their ability to enhance plant growth. It was recently suggested that Trichoderma also affects induced systemic resistance (ISR) mechanism in plants. Analysis of signal molecules involved in defense mechanisms and application of specific inhibitors indicated the involvement of jasmonic acid and ethylene in the protective effect conferred by Trichoderma spp. against the leaf pathogen Pseudomonas syringae pv. lachrymans. Moreover, examination of local and systemic gene expression by real-time reverse transcription-polymerase chain reaction analysis revealed that T. asperellum (T203) modulates the expression of genes involved in the jasmonate/ethylene signaling pathways of ISR (Lox1, Pal1, ETR1, and CTR1) in cucumber plants. We further showed that a subsequent challenge of Trichoderma-preinoculated plants with the leaf pathogen P. syringae pv. lachrymans resulted in higher systemic expression of the pathogenesisrelated genes encoding for chitinase 1, β-1,3-glucanase, and peroxidase relative to noninoculated, challenged plants. This indicates that Trichoderma induced a potentiated state in the plant enabling it to be more resistant to subsequent pathogen infection.
Suppression of Induced Resistance in Cucumber Through Disruption of the Flavonoid Pathway
Authors: Fofana Bourlaye , Benhamou Nicole , McNally David J. , Labbé Caroline , Séguin Armand , Bélanger Richard R.
Published: May 2007

In this study, cucumber plants (Cucumis sativus) expressing induced resistance against powdery mildew (caused by Podosphaera xanthii) were infiltrated with inhibitors of cinnamate 4-hydroxylase, 4-coumarate:CoA ligase (4CL), and chalcone synthase (CHS) to evaluate the role of flavonoid phytoalexin production in induced disease resistance. Light and transmission electron microscopy demonstrated ultrastructural changes in inhibited plants, and biochemical analyses determined levels of CHS and β-glucosidase enzyme activity and 4CL protein accumulation. Our results showed that elicited plants displayed a high level of induced resistance. In contrast, down regulation of CHS, a key enzyme of the flavonoid pathway, resulted in nearly complete suppression of induced resistance, and microscopy confirmed the development of healthy fungal haustoria within these plants. Inhibition of 4CL ligase, an enzyme largely responsible for channeling phenylpropanoid metabolites into the lignin pathway, had little effect on induced disease resistance. Biochemical analyses revealed similar levels of 4CL protein accumulation for all treatments, suggesting no alterations of nontargeted functions within inhibited plants. Collectively, the results of this study support the idea that induced resistance in cucumber is largely correlated with rapid de novo biosynthesis of flavonoid phytoalexin compounds.
Protoporphyrinogen Oxidase Inhibitor Herbicide Effects on Pythium Root Rot of Sugarcane, Pythium Species, and the Soil Microbial Community
Authors: Daugrois J. H. , Hoy J. W. , Griffin J. L.
Published: May 2007

The effects of three protoporphyrinogen oxidase inhibitor herbicides, azafenidin, flumioxazin, and sulfentrazone, on Pythium root rot of sugarcane and the soil microbial community were evaluated in greenhouse experiments. Herbicides were applied as foliar and soil treatments. There were no consistent effects on plant growth or disease parameters. However, some herbicide treatments affected the relative frequency of isolation of Pythium spp. from roots and reduced colonization by the pathogenic species Pythium arrhenomanes. A comparison of sole carbon source utilization profiles indicated that soil-applied herbicides altered the functional diversity of the soil microbial community, with some variation depending on herbicide used. All three herbicides inhibited the in vitro mycelial growth of P. arrhenomanes, P. aphanidermatum, and P. ultimum. Active ingredients were less inhibitory than formulated product for azafenidin and flumioxazin but not for sulfentrazone.
An Isolate of Alternaria alternata That Is Pathogenic to Both Tangerines and Rough Lemon and Produces Two Host-Selective Toxins, ACT- and ACR-Toxins
Authors: Masunaka A. , Ohtani K. , Peever T. L. , Timmer L. W. , Tsuge T. , Yamamoto M. , Yamamoto H. , Akimitsu K.
Published: May 2007

Two different pathotypes of Alternaria alternata cause Alternaria brown spot of tangerines and Alternaria leaf spot of rough lemon. The former produces the host-selective ACT-toxin and the latter produces ACR-toxin. Both pathogens induce similar symptoms on leaves or young fruits of their respective hosts, but the host ranges of these pathogens are distinct and one pathogen can be easily distinguished from another by comparing host ranges. We isolated strain BC3-5-1-OS2A from a leaf spot on rough lemon in Florida, and this isolate is pathogenic on both cv. Iyokan tangor and rough lemon and also produces both ACT-toxin and ACR-toxin. Isolate BC3-5-1-OS2A carries both genomic regions, one of which was known only to be present in ACT-toxin producers and the other was known to exist only in ACR-toxin producers. Each of the genomic regions is present on distinct small chromosomes, one of 1.05 Mb and the other of 2.0 Mb. Alternaria species have no known sexual or parasexual cycle in nature and populations of A. alternata on citrus are clonal. Therefore, the ability to produce both toxins was not likely acquired through meiotic or mitotic recombination. We hypothesize that a dispensable chromosome carrying the gene cluster controlling biosynthesis of one of the host-selective toxins was transferred horizontally and rearranged by duplication or translocation in another isolate of the fungus carrying genes for biosynthesis of the other host-selective toxin.
A Class IV Chitinase Is Up-Regulated by Fungal Infection and Abiotic Stresses and Associated with Slow-Canker-Growth Resistance to Cronartium ribicola in Western White Pine (Pinus monticola)
Authors: Liu Jun-Jun , Ekramoddoullah Abul K. M. , Zamani Arezoo
Published: May 2007

In the present study, in a candidate gene approach, a class IV chitinase gene (PmCh4A) of pathogenesis-related family three was cloned and characterized in western white pine (Pinus monticola). PmCh4A chitinase expression in the different organs of healthy seedlings was below levels detectable by western immunoblot analysis using an antibody raised against PmCh4A protein. However, a 27-kDa isozyme of PmCh4A accu mulated in both susceptible and slow-canker-growth (SCG) resistant seedlings after infection by Cronartium ribicola. As with fungal infection, the application of a signal chemical (methyl jasmonate) and a protein phosphatase 1 and 2A inhibitor (okadaic acid) increased the PmCh4A protein accumulation. Furthermore, another 26-kDa isozyme was expressed specifically in SCG resistant seedlings, providing a potential tool for marker-assisted selection in forest breeding. Wounding treatment also induced expression of the protein. These data suggest that the class IV chitinase PmCh4A is involved in the defense response of western white pine to infection and abiotic stresses.
β-1,3-Glucanase Activity in Peanut Seed (Arachis hypogaea) is Induced by Inoculation with Aspergillus flavus and Copurifies with a Conglutin-Like Protein
Authors: Liang X. Q. , Holbrook C. C. , Lynch R. E. , Guo B. Z.
Published: May 2007

Infection of peanut (Arachis hypogaea) seed by Aspergillus flavus and A. parasiticus is a serious problem that can result in aflatoxin contamination in the seed. Breeding resistant cultivars would be an effective approach to reduce aflatoxin accumulation. The objective of this study was to investigate the expression of the pathogenesis-related (PR) protein β-1,3-glucanase and the isoform patterns in peanut seed inoculated with A. flavus. Peanut genotypes GT-YY9 and GT-YY20 (both resistant to A. flavus infection) and Georgia Green and A100 (both susceptible to A. flavus infection) were used in this study. The activities of β-1,3-glucanase were similar in the uninfected seed of all genotypes, but increased significantly in the resistant genotypes after inoculation in comparison with the susceptible genotypes. An in-gel (native polyacrylamide gel electrophoresis [PAGE]) enzymatic activity assay of β-1,3-glucanase revealed that there were more protein bands corresponding to β-1,3-glucanase isoforms in the infected seed of resistant genotypes than in the infected seed of susceptible genotypes. Both acidic and basic β-1,3-glucanase isoforms were detected in the isoelectric focusing gels. Thin-layer chromatography analysis of the hydrolytic products from the reaction mixtures of the substrate with the total protein extract or individual band of native PAGE revealed the presence of enzymatic hydrolytic oligomer products. The individual bands corresponding to the bands of β-1,3-glucanase isoforms Glu 1 to 5 were separated on the sodium dodecyl sulfate-PAGE, resulting in two bands of 10 and 13 kDa, respectively. The sequences of fragments of the 13-kDa major protein band showed a high degree of homology to conglutin, a storage protein in peanut seed. Conglutin is reported as a peanut allergen, Ara h2. Our data provide the first evidences for peanut having β-1,3-glucanase activities and the association with the resistance to A. flavus colonization in peanut seed. We have not directly demonstrated that conglutin has β-1,3-glucanase activity.
Citrus Black Rot is Caused by Phylogenetically Distinct Lineages of Alternaria alternata
Authors: Peever T. L. , Carpenter-Boggs L. , Timmer L. W. , Carris L. M. , Bhatia A.
Published: May 2007

Phylogenetic analysis revealed that isolates of Alternaria alternata causing black rot of citrus were associated with six well-supported evolutionary lineages. Isolates recovered from brown spot lesions on Minneola tangelo, leaf spot lesions on rough lemon, and healthy citrus tissue and noncitrus hosts were related closely to isolates from black-rotted fruit. Phylogenies estimated independently from DNA sequence data from an endopolygalacturonase gene (endoPG) and two anonymous regions of the genome (OPA1-3 and OPA2-1) had similar topologies, and phylogenetic analysis was performed on the combined data set. In the combined phylogeny, isolates from diverse ecological niches on citrus and noncitrus hosts were distributed in eight clades. Isolates from all lineages, regardless of ecological or host association, caused black rot in fruit inoculation assays, demonstrating that small-spored Alternaria isolates associated with different ecological niches on citrus and other plant hosts are potential black rot pathogens. These data also indicated that the fungi associated with black-rotted fruit do not form a natural evolutionary group distinct from other Alternaria pathogens and saprophytes associated with citrus. The use of the name A. citri to describe fungi associated with citrus black rot is not justified and it is proposed that citrus black rot fungi be referred to as A. alternata.
Sequence Analysis and Classification of Apparent Recombinant Begomoviruses Infecting Tomato in the Nile and Mediterranean Basins
Authors: Fauquet C. M. , Sawyer S. , Idris A. M. , Brown J. K.
Published: May 2007

Numerous whitefly-transmitted viral diseases of tomato have emerged in countries around the Nile and Mediterranean Basins the last 20 years. These diseases are caused by monopartite geminiviruses (family Gemini viridae) belonging to the genus Begomovirus that probably resulted from numerous recombination events. The molecular biodiversity of these viruses was investigated to better appreciate the role and importance of recombination and to better clarify the phylogenetic relationships and classification of these viruses. The analysis partitioned the tomato-infecting begomoviruses from this region into two major clades, Tomato yellow leaf curl virus and Tomato yellow leaf curl Sardinia virus. Phylogenetic and pairwise analyses together with an evaluation for gene conversion were performed from which taxonomic classification and virus biodiversity conclusions were drawn. Six recombination hotspots and three homogeneous zones within the genome were identified among the tomatoinfecting isolates and species examined here, suggesting that the recombination events identified were not random occurrences.
Influence of Nutrient and Environmental Factors on Conidial Germination of Potebniamyces pyri
Authors: Liu Q. , Xiao C. L.
Published: May 2007

Potebniamyces pyri is the causal agent of Phacidiopycnis rot, a postharvest disease of pears. Infection of fruit occurs in the orchard, and symptoms develop during storage. Conidial germination of P. pyri in response to nutrient, temperature, wetness duration, relative humidity (RH), and pH was determined in vitro. Conidia germinated by either budding or developing germ tubes in various concentrations of pear juice solutions. The mode of conidial germination was nutrient-dependent. Low nutrient levels favored budding, whereas high nutrient levels favored germ tube development. Conidia germinated at 0 to 30°C but not at 35°C, with optimum temperature between 20 and 25°C. Wetness durations of 4 to 5 h and 6 to 8 h at optimum temperature were required for budding and developing germ tubes, respectively, and 20 to 24 h of wetness was required to reach germination peaks. Regardless of temperature, conidia germinated primarily by budding in 10% pear juice. Secondary conidia, produced by budding of conidia, initially increased their dimensions and later germinated at 0 to 25°C in the same manner as mother conidia. No germination of secondary conidia occurred at 30°C. Germ tubes from conidia elongated at 0 to 25°C but not at 30°C. No germination occurred at ≤95% RH. At 20°C, conidia germinated only by germ tubes at 98% RH, and conidia germinated in both manners at 100% RH but dominated by germ tubes. In free water (0.1% pear juice) the majority of conidia germinated by budding. After 5 days at 0°C, conidia germinated at 100% RH but not at ≤98% RH. More than 80% of conidia germinated in 10% pear juice at pH 4 to 7 after 24 h at 20°C, whereas there was <50% germination at pH 3. Low pH favored budding, whereas high pH favored developing germ tubes. These results contribute to a better understanding of the biology and ecology of the fungus and environmental conditions required for infection of fruit.
Interaction Between Nitrogen-Fertilized Peach Trees and Expression ofsyrB, a Gene Involved in Syringomycin Production inPseudomonas syringaepv.syringae
Authors: Cao Tiesen , Duncan Roger A. , McKenry Michael V. , Shackel Kenneth A. , DeJong Theodore M. , Kirkpatrick Bruce C.
Published: May 2007

The in vitro expression of the syrB gene that controls the synthesis of syringomycin, a non-host-specific phytotoxin produced by Pseudomonas syringae pv. syringae van Hall, was studied using aqueous extracts derived from bark tissues collected from nitrogen-fertilized and nonfertilized peach trees. Expression of the syrB gene was quantified as β- galactosidase activity expressed by P. syringae pv. syringae B3AR-132 containing a syrB::lacZ fusion. Gene expression was significantly less in three of four paired comparisons using extracts derived from fertilized versus nonfertilized trees; however, canker lengths were significantly different in only one of four comparisons. Expression was negatively correlated with plant tissue nitrogen content and positively correlated with a plant carbon/nitrogen ratio. Bark tissue from ring nematodeinfested trees had significantly higher concentrations of total soluble phenolic compounds and carbon/nitrogen ratios than bark samples from trees without nematodes, and canker size was significantly greater in trees growing in ring nematode-infested soil compared with noninfested soil. Nitrogen fertilization significantly decreased the plant carbon/nitrogen ratio, which was positively correlated with the concentration of total soluble phenolic compounds. Canker size developing after bacterial inoculation was positively correlated with higher plant carbon/nitrogen ratios and total soluble phenolic compounds. These results support the hypothesis that one reason why nitrogen fertilization decreases host susceptibility to bacterial canker is by either reducing the amount of plant metabolites that can induce syrB gene expression, or producing or increasing the concentration of compounds that antagonize syrB inducing compounds.
Transmission of Phytophthora ramorum in Mixed-Evergreen Forest in California
Authors: Davidson Jennifer M. , Wickland Allison C. , Patterson Heather A. , Falk Kristen R. , Rizzo David M.
Published: May 2007

During 2001 to 2003, the transmission biology of Phytophthora ramorum, the causal agent of sudden oak death, was studied in mixedevergreen forest, a common forest type in northern, coastal California. Investigation of the sources of spore production focused on coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica), dominant hosts that comprised 39.7 and 46.2% of the individuals at the study site, respectively. All tests for inoculum production from the surface of infected coast live oak bark or exudates from cankers were negative. In contrast, sporangia and chlamydospores were produced on the surface of infected bay laurel leaves. Mean number of zoospores produced from infected bay laurel leaves under natural field conditions during rainstorms was 1,173.0 ± SE 301.48, and ranged as high as 5,200 spores/leaf. P. ramorum was recovered from rainwater, soil, litter, and streamwater during the mid- to late rainy season in all 3 years of the study. P. ramorum was not recovered from sporadic summer rains or soil and litter during the hot, dry summer months. Concentrations of inoculum in rainwater varied significantly from year to year and increased as the rainy season progressed for the two complete seasons that were studied. Potential dispersal distances were investigated for rainwater, soil, and streamwater. In rainwater, inoculum moved 5 and 10 m from the inoculum source. For soil, transmission of inoculum was demonstrated from infested soil to bay laurel green leaf litter, and from bay laurel green leaf litter to aerial leaves of bay laurel seedlings. One-third to one-half of the hikers tested at the study site during the rainy season also were carrying infested soil on their shoes. In streamwater, P. ramorum was recovered from an unforested site in pasture ≈ 1 km downstream of forest with inoculum sources. In total, these studies provide details on the production and spread of P. ramorum inoculum in mixed-evergreen forest to aid forecasting and managing disease transmission of this environmentally destructive pathogen.
Genetic Structure of Mycosphaerella populorum (Anamorph Septoria musiva) Populations in North-Central and Northeastern North America
Authors: Feau Nicolas , Hamelin Richard C. , Vandecasteele Céline , Stanosz Glen R. , Bernier Louis
Published: May 2007

In order to characterize the genetic variation of the poplar pathogen Mycosphaerella populorum (anamorph Septoria musiva), we have studied seven North American populations using the polymerase chain reaction random amplified polymorphic DNA (RAPD) technique. The fungal populations were sampled in 2001 and 2002 by obtaining 352 isolates from cankers and leaf spots in hybrid poplar plantations and adjacent eastern cottonwood (Populus deltoides). A total of 21 polymorphic RAPD markers were obtained with the six RAPD primers used. A fine-level scale analysis of the genetic structure within the populations revealed that subpopulations sampled on P. deltoides and on hybrid trees were not significantly differentiated. In contrast, analyses performed on the entire data set showed high levels of haplotypic diversity and moderate to high genetic differentiation, with 20% of the expected genetic diversity found at the interpopulation level. Moreover, a high and significant correlation between genetic and geographic distances among populations was found, suggesting isolation by distance of the sampled populations. Although the occurrence of the sexual stage of this fungus remained unclear in field populations, five of the six populations were at gametic equilibrium for RAPD loci, suggesting the occurrence of recombination episodes in Septoria musiva populations. Overall, S. musiva appears to consist of differentiated subpopulations, with both asexual and sexual recombination contributing to the local level of genetic structure.
Genetic Mapping and Chromosomal Assignment of Magnaporthe oryzae Avirulence Genes AvrPik, AvrPiz, and AvrPiz-t Controlling Cultivar Specificity on Rice
Authors: Luo Chao-Xi , Yin Liang-Fen , Koyanagi Satomi , Farman Mark L. , Kusaba Motoaki , Yaegashi Hiroshi
Published: May 2007

A genetic map including three avirulence (Avr) genes, AvrPik, AvrPiz, and AvrPiz-t, was constructed in a genetic cross of two rice field isolates, 84R-62B and Y93-245c-2. The chromosomal locations of the Avr genes were determined by using selected markers to probe Southern blots of the parental chromosomes that had been separated by contour-clamped homogenous electric fields electrophoresis. Electrophoretic karyotyping showed that both parental isolates 84R-62B and Y93-245c-2 contained seven chromosomes greater than 3.5 megabases (Mb) in size and 84R-62B possessed a small chromosome of ≈1.6 Mb. The linkage groups containing AvrPiz and AvrPiz-t were assigned to chromosomes 3 and 7, respectively. Some markers from the linkage group that contained AvrPik hybridized with chromosome 1 and the 1.6-Mb chromosome, yet all of the cloned RAPD markers that were closely linked to AvrPik hybridized exclusively to the 1.6-Mb chromosome in 84R-62B, the parent that possesses AvrPik. Thus, we conclude that AvrPik is located on the 1.6-Mb chromosome in 84R-62B.
Development of a High-Throughput Method for Quantification of Plasmopara viticola DNA in Grapevine Leaves by Means of Quantitative Real-Time Polymerase Chain Reaction
Authors: Valsesia G. , Gobbin D. , Patocchi A. , Vecchione A. , Pertot I. , Gessler C.
Published: May 2007

Plasmopara viticola is a strictly biotrophic oomycete that causes downy mildew, which is one of the most important grapevine diseases. Control of the disease is most often achieved by fungicide applications, which may have severe environmental consequences. Therefore, alternative control strategies based on biocontrol agents (BCAs) are currently in development. Thousands of potential BCAs have to be screened for their antagonist efficacy against Plasmopara viticola. Evaluation of their effect on the pathogen can be achieved by detecting the amount of P. viticola DNA in leaves treated with potential antagonists and infected with the pathogen. In this study, a rapid high-throughput method was developed for relative quantification of P. viticola DNA directly from Vitis vinifera leaves by means of multiplex real-time quantitative polymerase chain reaction (PCR) with TaqMan chemistry. This method allows simultaneous amplification, but independent detection, of pathogen and host DNA by using species-specific primers and TaqMan probes that are labeled with different fluorescent dyes. Including detection of V. vinifera DNA in the tests is fundamental because it provides an endogenous reference and allows normalization for variations caused by sample-to-sample differences in DNA extraction, PCR efficiencies, and pipetting volumes. The developed method allows highly sensitive and specific detection of P. viticola DNA (minimal detectable quantity of 0.1 pg). Moreover, high precision and reproducibility of TaqMan assays were observed over a linear range of four orders of magnitude, confirming the reliability of the developed PCR assay. Potential applications range from screening for BCA efficiency to evaluation of fungicide efficacy, or assessment of host resistance.
Distribution and Biocontrol Potential of phlD+ Pseudomonads in Corn and Soybean Fields
Authors: McSpadden Gardener Brian B. , Gutierrez Laura J. , Joshi Raghavendra , Edema Richard , Lutton Elizabeth
Published: May 2007

The abundance and diversity of phlD+ Pseudomonas spp. colonizing the rhizospheres of young, field-grown corn and soybean plants were assayed over a 3-year period. Populations of these bacteria were detected on the large majority of plants sampled in the state of Ohio, but colonization was greater on corn. Although significant variation in the incidence of rhizosphere colonization was observed from site to site and year to year on both crops, the magnitude of the variation was greatest for soybean. The D genotype was detected on plants collected from all 15 counties examined, and it represented the most abundant subpopulation on both crops. Additionally, six other genotypes (A, C, F, I, R, and S) were found to predominate in the rhizosphere of some plants. The most frequently observed of these were the A genotype and a newly discovered S genotype, both of which were found on corn and soybean roots obtained from multiple locations. Multiple isolates of the most abundant genotypes were recovered and characterized. The S genotype was found to be phylogenetically and phenotypically similar to the D genotype. In addition, the novel R genotype was found to be most similar to the A genotype. All of the isolates displayed significant capacities to inhibit the growth of an oomycete pathogen in vitro, but such phenotypes were highly dependent on media used. When tested against multiple oomycete pathogens isolated from soybean, the A genotype was significantly more inhibitory than the D genotype when incubated on 1/10× tryptic soy agar and 1/5× corn meal agar. Seed inoculation with different isolates of the A, D, and S genotypes indicated that significant root colonization, generally in excess of log 5 cells per gram of root, could be attained on both crops. Field trials of the A genotype isolate Wayne1R indicated the capacity of inoculant populations to supplement the activities of native populations so as to increase soybean stands and yields. The relevance of these findings to natural and augmentative biocontrol of root pathogens by these bacteria is discussed.
Factors Associated with Fusarium Crown and Root Rot of Asparagus Outbreaks in Quebec
Authors: Hamel Chantal , Vujanovic Vladimir , Nakano-Hylander Aiko , Jeannotte Richard , St-Arnaud Marc
Published: May 2007

The Fusarium spp. causing Fusarium crown and root rot (FCRR) are ubiquitous and abundant in soils, but in contrast, disease expression is localized and sporadic. Previous studies have related FCRR infection to phenolic acids released by asparagus, to the repression of Mn-reducers in soil, and to various soil physicochemical conditions. Fifty commercial asparagus plantations were surveyed using an exploratory approach in order to pinpoint the ecological conditions associated with FCRR development. Twenty-eight variables were used to describe the soil environments of the asparagus crops as well as the influence of crop management practices used locally. The data set was analyzed both as a whole and parsed by main cultivars (Jersey Giant and Guelph Millenium). Both field conditions and percentage of field area affected by FCRR varied widely between asparagus plantations. Planting depth was positively correlated with percentage of field area affected by FCRR and, hence, deep planting may favor FCRR infection. Plantation age was positively correlated with percentage of field area affected by FCRR, while soil available Mn was inversely correlated. Most importantly, soil Mn availability decreased with increasing plantation age, supporting the hypothesis of an asparagusmediated negative impact on Mn-reducing bacteria and of the involvement of reduced Mn availability in FCRR development. Improving the availability of Mn could provide a solution to the problem of FCRR in asparagus plantations.
Inheritance and Molecular Mapping of Barley Genes Conferring Resistance to Wheat Stripe Rust
Authors: Pahalawatta Vihanga , Chen Xianming
Published: May 2007

Most barley cultivars are resistant to stripe rust of wheat that is caused by Puccinia striiformis f. sp. tritici. The barley cv. Steptoe is susceptible to all identified races of P. striiformis f. sp. hordei (PSH), the barley stripe rust pathogen, but is resistant to most P. striiformis f. sp. tritici races. To determine inheritance of the Steptoe resistance to P. striiformis f. sp. tritici, a cross was made between Steptoe and Russell, a barley cultivar susceptible to some P. striiformis f. sp. tritici races and all tested P. striiformis f. sp. hordei races. Seedlings of parents and F1, BC1, F2, and F3progeny from the barley cross were tested with P. striiformis f. sp. tritici races PST-41 and PST-45 under controlled greenhouse conditions. Genetic analyses of infection type data showed that Steptoe had one dominant gene and one recessive gene (provisionally designated as RpstS1 and rpstS2, respectively) for resistance to races PST-41 and PST-45. Genomic DNA was extracted from the parents and 150 F2plants that were tested for rust reaction and grown for seed of F3lines. The infection type data and polymorphic markers identified using the resistance gene analog polymorphism (RGAP) technique were analyzed with the Mapmaker computer program to map the resistance genes. The dominant resistance gene in Steptoe for resistance to P. striiformis f. sp. tritici races was mapped on barley chromosome 4H using a linked microsatellite marker, HVM68. A linkage group for the dominant gene was constructed with 12 RGAP markers and the microsatellite marker. The results show that resistance in barley to the wheat stripe rust pathogen is qualitatively inherited. These genes might provide useful resistance against wheat stripe rust when introgressed into wheat from barley.
A Single Amino Acid Mutation in thePlum pox virusHelper Component-Proteinase Gene Abolishes Both Synergistic and RNA Silencing Suppression Activities
Authors: González-Jara Pablo , Atencio Felix A. , Martínez-García Belén , Barajas Daniel , Tenllado Francisco , Díaz-Ruíz José Ramón
Published: May 2007

The effects on symptom expression of single amino acid mutations in the central region of the Plum pox virus (PPV) helper component-proteinase (HC-Pro) gene were analyzed in Nicotiana benthamiana using Potato virus X (PVX) recombinant viruses. PVX recombinant virus expressing the wild-type variant of PPV HC-Pro induced the expected enhancement of PVX pathogenicity, manifested as necrosis and plant death. Recombinant virus expressing a variant of PPV HC-Pro containing a single point mutation ( HCL134H) was unable to induce this synergistic phenotype. The RNA silencing suppressor activity of PPV HC-Pro was demonstrated in a transient silencing suppression assay. In contrast, the HCL134H mutant showed no such activity. These results indicate that a unique point mutation in PPV HC-Pro impaired its ability to suppress RNA silencing and abolished its capacity to induce synergism, and clearly shows for the first time the link between these two functions in potyvirus HC-Pro. Additionally, we compared the effects on virus accumulation in N. benthamiana plants infected with either the PVX recombinant constructs or with native viruses in double infection experiments. PVX (+) and (-) strand genomic RNA accumulated at similar levels in plants infected with PVX recombinants, leading to an increase in PVX pathology, compared with plants infected with PVX alone. This finding confirms that the enhancement of pathogenicity associated with synergistic interaction is not a consequence of more efficient PVX replication due to RNA silencing suppression by PPV HC-Pro.
Pathogenic and Genetic Relatedness Among Xanthomonas axonopodis pv. allii and Other Pathovars of X. axonopodis
Authors: Gent David H. , Al-Saadi Abdulwahid , Gabriel Dean W. , Louws Frank J. , Ishimaru Carol A. , Schwartz Howard F.
Published: May 2007

Xanthomonas axonopodis pv. allii is phenotypically and genetically diverse and its relationship to other X. axonopodis pathovars within DNA homology group 9.2 is unknown. In growth chamber experiments, disease symptoms were produced on onion only by inoculation with X. axonopodis pv. allii. Citrus bacterial spot symptoms were induced by X. axonopodis pvs. alfalfae, itrumelo, and allii on Duncan grapefruit and key lime. X. axonopodis pv. allii multiplication and persistence in Duncan grapefruit were equal to those of an aggressive strain of X. axonopodis pv. citrumelo, but populations of X. axonopodis pvs. alfalfae, betlicola, citrumelo, phaseoli, and vesicatoria were 1.3 to 4.0 log units less than X. axonopodis pv. allii in onion. Genomic fingerprinting by repetitive sequence- based polymerase chain reaction demonstrated that X. axonopodis pvs. allii, alfalfae, and citrumelo are distinct from other Xanthomonas species and X. axonopodis pathovars, but these pathovars were indistinguishable from each other. Three genotype groups were apparent among DNA homology group 9.2 strains, and generally correspond to the aggressiveness and genotype groups previously described for X. axonopodis pv. citrumelo. X. axonopodis pvs. allii, alfalfae, and citrumelo appear to have recently diverged from a common ancestral strain.
Role of Ascospores in Further Spread of QoI-Resistant Cytochrome b Alleles (G143A) in Field Populations of Mycosphaerella graminicola
Authors: Fraaije B. A. , Cools H. J. , Fountaine J. , Lovell D. J. , Motteram J. , West J. S. , Lucas J. A.
Published: May 2007

Strobilurin fungicides or quinone outside inhibitors (QoIs) have been used successfully to control Septoria leaf blotch in the United Kingdom since 1997. However, QoI-resistant isolates of Mycosphaerella graminicola were reported for the first time at Rothamsted during the summer of 2002. Sequence analysis of the cytochrome b gene revealed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Extensive monitoring using real-time polymerase chain reaction (PCR) testing revealed that fungicide treatments based on QoIs rapidly selected for isolates carrying resistant A143 (R) alleles within field populations. This selection is driven mainly by polycyclic dispersal of abundantly produced asexual conidia over short distances. In order to investigate the role of sexually produced airborne ascospores in the further spread of R alleles, a method integrating spore trapping with real-time PCR assays was developed. This method enabled us to both quantify the number of M. graminicola ascospores in air samples as well as estimate the frequency of R alleles in ascospore populations. As expected, most ascospores were produced at the end of the growing season during senescence of the wheat crop. However, a rapid increase in R-allele frequency, from 35 to 80%, was measured immediately in airborne ascospore populations sampled in a wheat plot after the first QoI application at growth stage 32. After the second QoI application, most R-allele frequencies measured for M. graminicola populations present in leaves and aerosols sampled from the treated plot exceeded 90%. Spatial sampling and testing of M. graminicola flag leaf populations derived from ascospores in the surrounding crop showed that ascospores carrying R alleles can spread readily within the crop at distances of up to 85 m. After harvest, fewer ascospores were detected in air samples and the R-allele frequencies measured were influenced by ascospores originating from nearby wheat fields.