bluuebunny/crossref_metadata_2025 huggingface dataset:
Maize fine streak virus, a New Leafhopper-Transmitted Rhabdovirus
Authors: Redinbaugh M. G. , Seifers D. L. , Meulia T. , Abt J. J. , Anderson R. J. , Styer W. E. , Ackerman J. , Salomon R. , Houghton W. , Creamer R. , Gordon D. T. , Hogenhout S. A.
Published: May 2007

A previously uncharacterized virus was isolated from fall-planted sweet corn (Zea mays L., Syngenta GSS 0966) leaves showing fine chlorotic streaks. Symptomatic plants were negative in enzyme-linked immunosorbent assay against many maize viruses, but reacted weakly with antisera to Sorghum stunt mosaic virus suggesting a distant relationship between the viruses. The virus was readily transmitted by vascular puncture inoculation (VPI), but not by leaf-rub inoculation. Symptoms on maize included dwarfing and fine chlorotic streaks along intermediate and small veins that developed 12 to 17 days post-VPI. The isolated virus was bacilliform (231 ± 5 nm long and 71 ± 2 nm wide), with a knobby surface, and obvious helical structure typical of rhabdovirus morphology. Nucleorhabdovirus virions were observed by transmission electron microscopy of infected maize leaf tissue sections. Proteins unique to infected plants were observed in extracts of infected leaves, and the isolated virion contained three proteins with molecular masses 82 ± 2, 50 ± 3, and 32 ± 2 kDa. Preliminary sequence analysis indicated the virus had similarity to members of the family Rhabdoviridae. The virus was transmitted by Graminella nigrifrons under persistent conditions. The data indicate the virus, provisionally designated Maize fine streak virus, is a new species in the genus Nucleorhabdovirus.
Antibiosis Contributes to Biological Control of Fire Blight by Pantoea agglomerans Strain Eh252 in Orchards
Authors: Stockwell V. O. , Johnson K. B. , Sugar D. , Loper J. E.
Published: May 2007

Fire blight, caused by Erwinia amylovora, is the most serious bacterial disease of pear and apple trees. Biological control with strains of Pantoea agglomerans (syn. Erwinia herbicola) may provide an effective disease management strategy for fire blight. Most strains of P. agglomerans evaluated for suppression of fire blight produce compounds that inhibit the growth of E. amylovora in culture. The role of these inhibitory compounds in fire blight suppression in orchard environments has not been studied. In seven field trials in Oregon, we compared the population dynamics and disease suppression with P. agglomerans Eh252, a strain that produces a single antibiotic, with its near-isogenic antibiotic-deficient derivative, strain 10:12. Water or suspensions of Eh252 or 10:12 (1 × 108 CFU/ml) were applied at 30 and 70% bloom to pear or apple trees. Aqueous suspensions of freeze-dried cells of E. amylovora (3 × 105 CFU/ml) were applied at full bloom. Additional trees were treated with streptomycin or oxytetracycline at 30 and 70% bloom and in some experiments, 1 day after application of the pathogen. Population sizes of Eh252 or 10:12 on pear blossoms were estimated by spreading dilutions of blossom washes on culture media. Average population sizes of Eh252 and 10:12 on blossoms ranged from 105 to 107 CFU, and in five of six trials, the relative area under the population curve of Eh252 was not significantly different than that of its derivative 10:12. Both Eh252 and 10:12 reduced the growth of the pathogen on blossoms compared with inoculated water-treated controls. Eh252 significantly decreased the incidence of fire blight in six of seven field trials compared with the incidence on water-treated trees, and 10:12 similarly reduced the incidence of fire blight in four of seven trials. In three of seven field trials, trees treated with Eh252 had a significantly lower incidence of fire blight compared with trees treated 3 with 10:12. Overall,3 Eh252 reduced the incidence of fire blight by 55 ± 8%, 10:12 by 30 ± 6%, streptomycin by 75 ± 4%, and oxytetracycline by 16 ± 14%. The effectiveness of strain 10:12 compared with water treatment indicates that other mechanisms (e.g., competitive exclusion or habitat modification) also contribute to disease suppression by P. agglomerans. The increased suppression of fire blight by the parental strain Eh252 compared with the antibiotic-deficient mutant 10:12 indicates that antibiosis is an important mechanism of biological control of fire blight.
Application of Alternative Fumigants Through Drip Irrigation Systems
Authors: Ajwa H. A. , Trout T. , Mueller J. , Wilhelm S. , Nelson S. D. , Soppe R. , Shatley D.
Published: May 2007

Strawberry fields in California (9,500 ha annually) are pre-plant fumigated with methyl bromide and chloropicrin to prevent serious soil pest and disease problems. Although soil fumigation with methyl bromide has ensured stability of strawberry production, its use is being discontinued because of its effect on stratospheric ozone. The likely short-term alternatives such as 1,3-dichloropropene, chloropicrin, and metham sodium, although not ozone depleters, are potentially hazardous to the environment and humans if applied improperly. Water-soluble formulations of alternative fumigants can be applied through drip irrigation systems established to irrigate crops. In comparison to conventional shank methods of injection, application of soluble formulations through drip irrigation systems would be economical and environmentally friendly, reduce worker exposure, and allow for simultaneous or sequential application of a combination of fumigants. This paper discusses techniques developed to apply alternative fumigants through drip irrigation systems, and reviews ongoing studies to determine optimum application rates, soil conditions, plastic mulches, and amount of irrigation water used to apply these alternative fumigants.
Comparison of Three Methods for Monitoring Populations of Different Genotypes of 2,4-Diacetylphloroglucinol-Producing Pseudomonas fluorescens in the Rhizosphere
Authors: Landa Blanca B. , de Werd Henricus A. E. , McSpadden Gardener Brian B. , Weller David M.
Published: May 2007

Pseudomonas fluorescens strains producing the antibiotic 2,4-diacetylphloroglucinol (DAPG) have biocontrol activity against a broad spectrum of root and seedling diseases. In this study, we determined the effect of genotype on the ability to isolate and quantify introduced 2,4-DAPG producers from the rhizosphere of wheat using three different methods: traditional dilution plating on selective media, colony hybridization followed by polymerase chain reaction (PCR), and phlD-specific PCR-based dilution endpoint assay. Regression analysis of the population densities of 10 2,4-DAPG-producing P. fluorescens, representing five genotypes, determined by the three different methods demonstrated that the relationship was linear (P < 0.001) and the techniques were very similar (i.e., slopes equal to 1.0). The phlD-specific PCR-based assay had a slightly lower limit of detection than the other two methods (log 3.3 versus log 4.0 CFU/g of fresh root weight). With the colony hybridization procedure, we observed that the phlD probe, derived from strain P. fluorescens Q8r1-96, hybridized more strongly to colonies of BOX-PCR genotypes D (strains W2-6, L5.1-96, Q8r1-96, and Q8r2-96) and K (strain F113) compared with strains of genotypes A (Pf-5 and CHA0), B (Q2-87), and L (1M1-96 and W4-4). Colony hybridization alone overestimated the actual densities of some strains, thus requiring an additional PCR step to obtain accurate estimates. In contrast, population densities estimated for three of the bacterial treatments (strains CHA0, W2-6, and Q8r2-96) with the PCR-based assay were significantly (P < 0.041) smaller by 7.6 to 9.2% and 6.4 to 9.4% than population densities detected by the dilution plating and colony hybridization techniques, respectively. In this paper, we discuss the relative advantages of the different methods for detecting 2,4-DAPG producers.
Genome Structure and Production of Biologically Active In Vitro Transcripts of Cucurbit-Infecting Zucchini green mottle mosaic virus
Authors: Yoon Ju Yeon , Min Byoung Eun , Choi Jang Kyung , Ryu Ki Hyun
Published: May 2007

The complete nucleotide sequence of the Zucchini green mottle mosaic virus (ZGMMV), a new member of the genus Tobamovirus, has been determined. The genome of ZGMMV is 6,513 nucleotides long and contains four open reading frames coding for proteins of 131, 189, 28, and 17 kDa from the 5′ to 3′ end, respectively. The 5′- and 3′-non-translated regions consist of 59 and 163 residues, respectively. The sequences of the viral proteins exhibit high identity to the proteins of the members of the genus Tobamovirus and are distinct from other viruses within the subgroup of cucurbit-infecting tobamoviruses. Results from phylogenetic trees of the coding regions demonstrated that ZGMMV is a very close relative of Kyuri green mottle mosaic virus and Cucumber fruit mottle mosaic virus and is less similar to Cucumber green mottle mosaic virus. Full-length cDNA of ZGMMV was directly amplified by reverse-transcription polymerase chain reaction (RT-PCR) using the 5′-end primer containing a T7 RNA promoter sequence and 3′-end primer. Capped in vitro transcript from the RT-PCR products was infectious on zucchini squash, cucumber, and Nicotiana benthamiana plants. This cell-free system to produce infectious transcripts from uncloned cDNA copies is useful for quick assessment of infectivity of transcripts from plant RNA viruses prior to cloning. Synthesized capped transcript from a full-length cDNA clone of the virus was highly infectious. Progeny virus derived from infectious transcripts had the same biological and biochemical properties as wild-type virus. To our knowledge, this is the first report of a biologically active transcript from a cucurbit-infecting tobamovirus.
The 5′ Third of Cauliflower mosaic virus Gene VI Conditions Resistance Breakage in Arabidopsis Ecotype Tsu-0
Authors: Agama K. , Beach S. , Schoelz J. , Leisner S. M.
Published: May 2007

Arabidopsis thaliana ecotypes vary in their responses to viruses. In this study, we analyzed the variation in response of A. thaliana ecotype Tsu-0 to Cauliflower mosaic virus (CaMV). This ecotype was previously reported to be resistant to two CaMV isolates (CM1841 and CM4-184), but susceptible to W260. In this study, we show that Tsu-0 is resistant to four additional CaMV isolates. CaMV propagated within the rosette leaves of Tsu-0 plants, but did not appear to spread systemically into the inflorescence. However, virus viability in rosette leaves of Tsu-0 plants apparently was not compromised because infectious CaMV could be recovered from these organs. W260 overcomes Tsu-0 resistance by a passive mechanism (i.e., this virus avoids activating plant defenses). The portion of the viral genome responsible for W260 resistance breakage was mapped to the 5′ third of gene VI, which we have termed RBR-1. This region is also responsible for controlling the ability of CaMV to infect different types of solanaceous plants. Hence, the pathways by which plants of different families interact with CaMV may be conserved through evolution.
Enhanced Resistance to Sclerotium rolfsii in Populations of Alfalfa Selected for Quantitative Resistance to Sclerotinia trifoliorum
Authors: Pratt R. G. , Rowe D. E.
Published: May 2007

Sclerotinia trifoliorum and Sclerotium rolfsii are pathogens for which similar mechanisms of parasitism have been proposed. This suggested that resistance to these pathogens may be related in a common host plant. This study was undertaken to determine whether selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also increases resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Resistance in excised leaf tissues was evaluated according to the rate of necrosis induced by Sclerotium rolfsii following inoculation with mycelium. Resistance to Sclerotium rolfsii in whole plants was evaluated according to their survival following crown inoculations. Three alfalfa populations previously selected from cv. Delta for quantitative resistance to Sclerotinia trifoliorum exhibited enhanced resistance to Sclerotium rolfsii, in comparison with Delta or with susceptible populations, in excised leaf tissues. When whole plants of Delta and two of these populations, Sclerotinia trifoliorum resistant (STR) and Mississippi Sclerotinia resistant (MSR), were inoculated with Sclerotium rolfsii at 3 to 8 weeks of age, significant (P = 0.01) differences in survival were attributed to plant age at inoculation and alfalfa populations. Survival of both MSR and STR was significantly (P = 0.05) greater than for Delta; the best differential results were obtained by inoculating plants 5 to 7 weeks old. To evaluate relationships of resistance to Sclerotinia trifoliorum and Sclerotium rolfsii over a broader genetic background, additional populations were selected for resistance to Sclerotinia trifoliorum from four other alfalfa cultivars by leaf-inoculation techniques, and this resistance was confirmed by whole-plant inoculations. In excised leaf tissues, all four of these populations also expressed enhanced resistance to Sclerotium rolfsii in comparison with either parent cultivars or populations of comparable size selected at random. These results establish that selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also confers enhanced resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Results suggest, therefore, that genes for quantitative resistance to Sclerotinia trifoliorum and Sclerotium rolfsii in alfalfa are likely to be synonymous, overlapping, or closely linked.
Genetic Specificity in the White Pine-Blister Rust Pathosystem
Authors: Kinloch Bohun B. , Dupper Gayle E.
Published: May 2007

Four of eight white pine species native to western North America surveyed for resistance to white pine blister rust by artificial inoculation showed classical hypersensitive reactions (HR) at frequencies ranging from very low to moderate. Mendelian segregation, indicating a single dominant allele for resistance (Cr3), was observed in southwestern white pine (Pinus strobiformis), as it was previously in sugar pine (P. lambertiana, Cr1) and western white pine (P. monticola, Cr2). HR was present at a relatively high frequency (19%) in one of five bulk seed lot sources of limber pine (P. flexilis), and was also presumed to be conditioned by a single gene locus, by analogy with the other three species. HR was not found in whitebark pine (P. albcaulis), Mexican white pine (P. ayacahuite), foxtail pine (P. balfouriana), or Great Basin bristlecone pine (P. longaeva), but population and sample sizes in these species may have been below the level of detection of alleles in low frequency. When challenged by (haploid) inocula from specific locations known to harbor virulence to Cr1 or Cr2, genotypes carrying these alleles and Cr3 reacted differentially, such that inoculum virulent to Cr1 was avirulent to Cr2, and inoculum virulent to Cr2 was avirulent to Cr1. Neither of these two inocula was capable of neutralizing Cr3. Although blister rust traditionally is considered an exotic disease in North America, these results, typical of classic gene-for-gene interactions, suggest that genetic memory of similar encounters in past epochs has been retained in this pathosystem.
Interactions of Peronospora tabacina with Roots of Nicotiana spp. in Gnotobiotic Associations
Authors: Heist E. P. , Nesmith W. C. , Schardl C. L.
Published: May 2007

Peronospora tabacina is an obligate plant pathogen that causes downy mildew disease on several species of Nicotiana, including N. tabacum (tobacco). The primary objective of this study was to use gnotobiotic associations to describe interactions between the pathogen and roots of either N. tabacum (cv. KY14) or N. repanda. We found that the pathogen was capable of moving systemically from shoots to roots of both host species and emerged from the root tissues as hyphae. We also demonstrated that root-associated hyphae were infectious on roots of nearby plants and resulted in new systemic infections. Following overnight darkness, sporulation of the pathogen was observed on infected roots exposed to air on both host species. We also found that within 2 months in culture, structures resembling resting stages of Peronospora tabacina were produced on hyphae emerging from roots of N. repanda but not N. tabacum. These findings appear relevant to both the epidemiology of the disease and to future studies of this and other downy mildew pathosystems.
Temporal and Spatial Spread of Soybean mosaic virus (SMV) in Soybeans Transformed with the Coat Protein Gene of SMV
Authors: Steinlage T. A. , Hill J. H. , Nutter F. W.
Published: May 2007

Soybean lines transformed with the coat protein (CP) gene of Soybean mosaic virus (SMV) were evaluated for SMV resistance by quantifying the temporal and spatial spread of SMV strain AL-5 released from a point source in the field. The temporal spread of SMV within field plots during 1999 and 2000 was quantified by enzyme-linked immunosorbent assay. The Gompertz model most appropriately described temporal spread. Two SMV CP transformed lines (genotypes) had significantly lower infection rates and significantly lower final SMV incidence values (P ≤ 0.05) compared with controls that did not contain the CP gene. Ordinary runs analysis revealed that the spatial pattern of SMV-infected quadrats was more clustered in plots with higher SMV infection rates. Soybean lines with the lowest infection rates had significantly higher yields in 2000 and significantly less seed coat mottling compared with the controls. To our knowledge, this is the first field study demonstrating the effectiveness of pathogen-derived resistance on the temporal and spatial dynamics of pathogen spread in soybean.
Real-Time Polymerase Chain Reaction for One-Hour On-Site Diagnosis of Pierce's Disease of Grape in Early Season Asymptomatic Vines
Authors: Schaad N. W. , Opgenorth D. , Gaush P.
Published: May 2007

Molecular-based techniques, such as polymerase chain reaction (PCR), can reduce the time needed for diagnosis of plant diseases when compared with classical isolation and pathogenicity tests. However, molecular techniques still require 2 to 3 days to complete. To the best of our knowledge, we describe for the first time a real-time PCR technique using a portable Smart Cycler for one-hour on-site diagnosis of an asymptomatic plant disease. Pierce's disease (PD) of grape, caused by the fastidious bacterium Xylella fastidiosa, causes serious losses in grapes in California and the southeastern United States. The disease has been difficult to diagnose because typical leaf scorching symptoms do not appear until late (June and after) in the season and the organism is very difficult to isolate early in the season. Sap and samples of macerated chips of secondary xylem from trunks of vines were used in a direct real-time PCR without extraction of DNA. Using two different sets of primers and probe, we diagnosed PD in 7 of 27 vines (26%) from four of six vineyards sampled 10 to 12 days after bud break in Kern, Tulare, and Napa counties of California. The diagnosis was confirmed by isolation of Xylella fastidiosa from two of the original PCR positive samples and later from symptomatic leaf petioles of four out of four vines from one vineyard that were originally PCR positive.
Independent and Synergistic Activity of Synthetic Peptides Against Thiabendazole-Resistant Fusarium sambucinum
Authors: Gonzalez C. F. , Provin E. M. , Zhu L. , Ebbole D. J.
Published: May 2007

Two heptapeptides with broad antifungal activity were identified and assessed for their ability to act synergistically with thiabendazole. The hexapeptide 66–10 was the progenitor of the heptapeptides and exhibited minimal inhibitory concentrations (MICs) of 9.3 to 9.8 μg/ml for thiabendazole (TBZ) resistant Fusarium sambucinum strains (MIC of 186 to 312 μg/ml). Heptapeptide derivatives 77–3 and 77–12 exhibited MICs between 3.8 and 7.5 μg/ml against the same strains. Incubation of conidia or mycelia with the peptide 77–3 showed that treated fungal structures were stained by the membrane impermeant dye SYTOX Green indicating disruption of membranes. Conidia incubated with peptide 77–3 at 10 μg/ml showed a 91 ± 3.6% reduction in viability in 15 min. A checkerboard method was used to test the peptides and TBZ individually and in combination to determine potential synergistic activity. The results indicate that small peptides can act synergistically with TBZ against TBZ-resistant F. sambucinum.
Comparative Dynamics of Adherent and Nonadherent Bacterial Populations on Maize Leaves
Authors: Beattie Gwyn A. , Marcell Lise M.
Published: May 2007

The dynamics of the adherent and nonadherent populations of three bacterial species on maize leaves were examined to identify the extent to which bacteria adhere to leaves and the importance of this adhesion to leaf colonization. Pantoea agglomerans strain BRT98, Clavibacter michiganensis subsp. nebraskensis strain GH2390, and Pseudomonas syringae pv. syringae strain HS191R all rapidly adhered to maize leaves following inoculation, but differed in the percentage of cells that adhered to the leaves. Immediately following inoculation, the percentage of adherent cells was highest for the saprophyte P. agglomerans (8 to 10%) and was much lower for the pathogens C. michiganensis subsp. nebras-kensis and P. syringae pv. syringae (2 to 3 and <1%, respectively), although the results for P. syringae pv. syringae HS191R were based on only one experiment. In the 4 days following inoculation, the percentage of the P. agglomerans populations that adhered to the leaves increased to approximately 70%. Similarly, the percentage of C. michiganensis subsp. nebraskensis and P. syringae pv. syringae cells that resisted removal steadily increased in the days following inoculation, although these increases probably reflected both adherence and localization to endophytic sites. Based on differences in the percentage of cells adhering to several cuticular wax mutants of maize, the rapid adherence of C. michiganensis subsp. nebraskensis cells to maize leaves was influenced by the cuticular wax properties, while the rapid adherence of P. agglomerans was not. Finally, bacterial adherence to leaves was advantageous to P. agglomerans survival and growth on leaves based on the finding that the nonadherent populations of the P. agglomerans strain decreased significantly more than did the adherent populations in the 24 h following inoculation, and increased much less than did the adherent populations over the next 3 days. Similar results with the C. michiganensis subsp. nebraskensis and P. syringae pv. syringae strains indicate that bacterial adherence to leaves, bacterial movement to endophytic sites, or both were advantageous to the survival and growth of these strains on leaves.
Population Structure ofMycosphaerella graminicola: From Lesions to Continents
Authors: Linde C. C. , Zhan J. , McDonald B. A.
Published: May 2007

The genetic structure of field populations of Mycosphaerella graminicola was determined across a hierarchy of spatial scales using restriction fragment length polymorphism markers. The hierarchical gene diversity analysis included 1,098 isolates from seven field populations. Spatial scales ranged from millimeters to thousands of kilometers, including comparisons within and among lesions, within and among fields, and within and among regions and continents. At the smallest spatial scale, microtransect sampling was used to determine the spatial distribution of 15 genotypes found among 158 isolates sampled from five individual lesions. Each lesion had two to six different genotypes including both mating types in four of the five lesions, but in most cases a lesion was composed of one or two genotypes that occupied the majority of the lesion, with other rare genotypes interspersed among the common genotypes. The majority (77%) of gene diversity was distributed within plots ranging from ≈1 to 9 m2in size. Genotype diversity (Ĝ / N) within fields for the Swiss, Texas, and Israeli fields was high, ranging from 79 to 100% of maximum possible values. Low population differentiation was indicated by the low GSTvalues among populations, suggesting a corresponding high degree of gene flow among these populations. At the largest spatial scale, populations from Switzerland, Israel, Oregon, and Texas were compared. Population differentiation among these populations was low (GST= 0.05), and genetic identity between populations was high. A low but significant correlation between genetic and geographic distance among populations was found (r = -0.47, P = 0.012), suggesting that these populations probably have not reached an equilibrium between gene flow and genetic drift. Gene flow on a regional level can be reduced by implementing strategies, such as improved stubble management that minimize the production of ascospores. The possibility of high levels of gene flow on a regional level indicates a significant potential risk for the regional spread of mutant alleles that enable fungicide resistance or the breakdown of resistance genes.
Internal Transcribed Spacer Regions 1 and 2 and Random Amplified Polymorphic DNA Analysis ofDidymella bryoniaeand RelatedPhomaSpecies Isolated from Cucurbits
Authors: Somai Benesh M. , Dean Ralph A. , Farnham Mark W. , Zitter Thomas A. , Keinath Anthony P.
Published: May 2007

Didymella bryoniae (anamorph Phoma cucurbitacearum) is the causal agent of gummy stem blight, although other Phoma species are often isolated from cucurbit plants exhibiting symptoms of the disease. The molecular and phylogenetic relationships between D. bryoniae and these Phoma species are unknown. Isolates of D. bryoniae and Phoma obtained from cucurbits grown at various geographical locations in the United States were subjected to random amplified polymorphic DNA (RAPD) analysis and internal transcribed spacer (ITS) sequence analysis (ITS-1 and ITS-2) to determine the molecular and phylogenetic relationships within and between these fungi. Using RAPD fingerprinting, 59 isolates were placed into four phylogenetic groups, designated RAPD group (RG) I, RG II, RG III, and RG IV. D. bryoniae isolates clustered in either RG I (33 isolates), RG II (12 isolates), or RG IV (one isolate), whereas all 13 Phoma isolates clustered to RG III. There was greater than 99% sequence identity in the ITS-1 and ITS-2 regions between isolates in RG I and RG II, whereas isolates in RG III, P. medicaginis ATCC 64481, and P. exigua ATCC 14728 clustered separately. On muskmelon seedlings, a subset of RG I isolates were highly virulent (mean disease severity was 71%), RG II and RG IV isolates were slightly virulent (mean disease severity was 4%), and RG III isolates were nonpathogenic (disease severity was 0% for all isolates). The ITS sequences indicate that RG I and RG II are both D. bryoniae, but RAPD fingerprints and pathogenicity indicate that they represent two different molecular and virulence subgroups.
Comparison of Biochemical, Molecular, and Visual Methods to Quantify Phaeocryptopus gaeumannii in Douglas-Fir Foliage
Authors: Winton L. M. , Manter D. K. , Stone J. K. , Hansen E. M.
Published: May 2007

A recent epidemic of Swiss needle cast along the Oregon coast has prompted efforts to quantify foliar infection and colonization of the causal agent Phaeocryptopus gaeumannii. In this paper, we compare four methods to quantify colonization of Douglas-fir foliage by P. gaeumannii: fruiting body abundance, ergosterol content, dot blot analysis, and TaqMan based real-time quantitative polymerase chain reaction (PCR). Results from the four techniques were all significantly correlated. Fruiting body density and quantitative PCR are two methods least affected by the presence of other needle fungi and had the highest correlation. The methods also were used to compare foliage colonization in nine field sites exhibiting a range of disease severity. All four methods provided evidence that sites differed in the degree of fungal colonization, but only quantitative PCR consistently separated sites with moderate to severe levels of disease from sites with low disease estimated by foliage color, canopy density, and growth measurements.
Expression of the Arabidopsis MCM Gene PROLIFERA During Root-Knot and Cyst Nematode Infection
Authors: Huang X. , Springer P. S. , Kaloshian I.
Published: May 2007

Expression of the Arabidopsis thaliana gene PROLIFERA (PRL) was examined during development of root-knot and cyst nematode feeding sites. These obligate plant parasites establish specialized feeding structures in roots that allow them to withdraw nutrients from the host. In the process of establishing feeding sites, nematodes alter cell cycle regulation. PRL is normally expressed specifically in dividing cells at all stages of plant development and was used here as a marker for cell division. PRL expression, reported from a PRL∷GUS fusion protein, was detected in nematode feeding sites of both root-knot and cyst nematodes from the earliest stages of infection in both giant cells and syncytia. However, unlike other cell cycle genes, expression of PRL was detected only occasionally in cells surrounding the feeding sites. PRL∷GUS activity persisted until late in the infection cycle, past the time when other cell cycle genes are expressed. These data indicate that some aspects of the PRL expression pattern during nematode infection differ from that of other cell cycle genes.
Correlations of Morphological, Anatomical, and Chemical Features of Grape Berries with Resistance toBotrytis cinerea
Authors: Gabler Franka Mlikota , Smilanick Joseph L. , Mansour Monir , Ramming David W. , Mackey Bruce E.
Published: May 2007

Resistance of mature berries of grapevine cultivars and selections to postharvest infection by Botrytis cinerea was assessed. Little or no resistance existed in most popular table grape Vitis vinifera cultivars, except in moderately resistant ‘Emperor’ and ‘Autumn Black’. Highly resistant grapes were V. rotundifolia, V. labrusca, or other complex hybrids. Morphological, anatomical, and chemical characteristics of 42 genetically diverse cultivars and selections with various levels of resistance to B. cinerea were examined to determine which features were associated with resistance. We quantified the (i) density of berries within a cluster; (ii) number of pores and lenticels on the berry surface; (iii) thickness and number of cell layers in the epidermis and external hypodermis; (iv) amount of cuticle and wax; (v) berry skin protein content; (vi) total phenolic content of the skin before and after B. cinerea inoculation; and (vii) catechin and trans- and cis-resveratrol contents of the skin before and after inoculation. The number of pores was negatively correlated with resistance. Highly resistant cultivars had few or no pores in the berry surface. The number and thickness of epidermal and hypodermal cell layers and cuticle and wax contents were positively correlated with resistance. Other characteristics evaluated were not associated with resistance. trans-Resveratrol and cis-resveratrol were induced by B. cinerea inoculation only in sensitive and moderately resistant cultivars and selections.
Strawberry Plant Extracts Stimulate Secondary Conidiation by Colletotrichum acutatum on Symptomless Leaves
Authors: Leandro L. F. S. , Gleason M. L. , Nutter F. W. , Wegulo S. N. , Dixon P. M.
Published: May 2007

Conidial suspensions of Colletotrichum acutatum were prepared in 1:27, 1:45, and 1:81 (wt/vol) dilutions of an extract of strawberry (cv. Tristar) flowers or leaves in water. Strawberry leaves and plastic coverslips were sprayed with the conidial suspensions, incubated at 25°C and continuous wetness for 48 h, and the number of conidia and appressoria were counted. In another experiment, leaves and coverslips were sprayed with a conidial suspension in water, incubated for 72 h to establish C. acutatum populations, and placed in a growth chamber under dry conditions for up to 6 weeks. At each sampling time, leaves and coverslips were sprayed with flower extracts, leaf extracts, or water, incubated for 48 h at 25°C and continuous wetness, and the number of conidia and appressoria were counted. Flower extracts significantly (P ≤ 0.05) increased the number of conidia on leaves and coverslips compared with water, both when conidia were applied with the extracts and when the extracts were applied to C. acutatum populations exposed to dryness for up to 2 weeks. Application of flower extracts resulted in up to 10- and 16-fold increases in conidia on leaves and coverslips, respectively. Number of conidia increased more when exposed to flower extracts than to leaf extracts. Production of appressoria was not significantly affected by flower or leaf extracts. Our results suggest that inoculum levels of C. acutatum on foliage may increase during flowering of strawberry plants.
Activity of the Antifungal Protein from Aspergillus giganteus Against Botrytis cinerea
Authors: Moreno Ana Beatriz , del Pozo Álvaro Martínez , Borja Marisé , Segundo Blanca San
Published: May 2007

Botrytis blight (gray mold), caused by Botrytis cinerea, is one of the most widely distributed diseases of ornamental plants. In geranium plants, gray mold is responsible for important losses in production. The mold Aspergillus giganteus is known to produce and secrete a basic low-molecular-weight protein, the antifungal protein (AFP). Here, the antifungal properties of the Aspergillus AFP against various B. cinerea isolates obtained from naturally infected geranium plants were investigated. AFP strongly inhibited mycelial growth as well as conidial germination of B. cinerea. Microscopic observations of fungal cultures treated with AFP revealed reduced hyphal elongation and swollen hyphal tips. Washout experiments in which B. cinerea was incubated with AFP for different periods of time and then washed away revealed a fungicidal activity of AFP. Application of AFP on geranium plants protected leaves against Botrytis infection. Cecropin A also was active against this pathogen. An additive effect against the fungus was observed when AFP was combined with cecropin A. These results are discussed in relation to the potential of the afp gene to enhance crop protection against B. cinerea diseases.
Diversity of Pathotypes and DNA Fingerprint Haplotypes in Populations of Magnaporthe grisea in Korea over Two Decades
Authors: Park Sook-Young , Milgroom Michael G. , Han Seong-Sook , Kang Seogchan , Lee Yong-Hwan
Published: May 2007

Using isolates collected over 2 decades, we determined the population structure and dynamics of the rice blast fungus, Magnaporthe grisea, in Korea at both the genotypic and phenotypic levels. Pathotype analysis on 6,315 isolates collected from 328 rice cultivars from 1981 to 2000 revealed the presence of a total of 91 pathotypes. Among these 91 patho-types, nine dominated, comprising 76.5% of the isolates. The expected number of pathotypes (corrected for sample size) increased significantly during the course of this study. On average, six (ranging from 0 to 20) new commercial cultivars were introduced annually between 1981 and 1998. However, the overall cultivar diversity, estimated using the Shannon index, was low. Most of the new cultivars were not planted to a large area because the seven most common cultivars each year occupied over 70% of the rice-cultivated area. The frequencies of the nine dominant patho-types from these seven cultivars were highly correlated with those from the entire set of cultivars. To understand genetic diversity within and between pathotypes, 176 isolates collected from 1984 to 1999 were randomly sampled and analyzed by DNA fingerprinting. High similarities were observed among isolates; overall similarities were greater than 63% in combined MGR586 and MAGGY DNA fingerprints. Unlike most other populations of M. grisea, DNA fingerprints showed no clear lineage structure. No groups were supported by bootstrap values greater than 10%. Furthermore, there was no significant correlation between DNA fingerprint similarities and pathotypes. Genetic similarity was significantly greater (P < 0.001) within years than between years, although the difference was small. Our data suggest that M. grisea populations in Korea have been mostly dominated by a single clonal lineage. We cannot conclude from these data that selection by the host population has been a major force in the evolution of M. grisea in Korea.
Tolerance of Mycelium of Different Genotypes of Phytophthora infestans to Freezing Temperatures for Extended Periods
Authors: Kirk William W.
Published: May 2007

Mycelium of Phytophthora infestans, the causal agent of potato late blight, can initiate crop infections over successive years by overwintering in infected potato tubers that survive as seed in fields or within cull piles. This study used four different genotypes of P. infestans to evaluate the influence of freezing temperatures on survival of mycelium in vitro. Sporangium-free mycelium of P. infestans US1, US8, US11, and US14 growing on rye agar plates was exposed to temperatures ranging from -20 and 0°C (experiment A) for different periods up to 24 h and from -5 and 0°C (experiment B) for periods up to 5 days. Cultures were incubated at 12°C after exposure, and survival of the cultures was estimated after 28 days by a digital image analysis technique that measured the average reflectance intensity (ARI) of images of the mycelium of temperature-treated cultures. The ARI values of treated cultures were compared with the growth of mycelium in negative controls (mycelium not present) and positive controls (mycelium exposed to 12°C for an equivalent period), and determination of recovery was based on statistical differences from the controls. There were significant differences in ARI values among genotypes, temperature treatment, and exposure periods in all experiments. An index of recovery was calculated for each genotype at all treatment temperatures and exposure periods for both experiments. In experiment A, exposure of mycelium of P. infestans (all genotypes) to -20 and -10°C proved lethal for exposure periods of more than 1 h. All genotypes showed some degree of recovery up to 24-h exposure at -5 and -3°C. In both experiments, exposure of mycelium of P. infestans to 0°C was not lethal to any genotype tested for any exposure period. In experiment B, all of the genotypes survived exposure up to 3 days at -3°C to some degree, but at -5°C, exposure of 1 day was lethal to all genotypes. Tolerance of freezing temperatures by mycelium of P. infestans may be an ecologically important survival mechanism and the increased tolerance of US8 and US14 may explain their predominance in cooler climates such as north-central United States.
Hypovirulence-Associated Double-Stranded RNA from Sclerotinia homoeocarpa Is Conspecific with Ophiostoma novo-ulmi Mitovirus 3a-Ld
Authors: Deng F. , Xu R. , Boland G. J.
Published: May 2007

The nucleotide sequence of the hypovirulence-associated double-stranded RNA (dsRNA) in hypovirulent isolate Sh12B of Sclerotinia homoeocarpa, the causal agent of dollar spot of turf grass, was determined. This large dsRNA (L-dsRNA) is 2,632 bp long and is A and U rich (61.0% A+U residues). One strand of this dsRNA contains an open reading frame (ORF) with the potential to encode a protein of 720 amino acids. This ORF contains 12 UGA codons, predicted to encode tryptophan in ascomycete mitochondria, and has a codon bias typical of mitochondrial genes, which is consistent with a mitochondrial localization of this dsRNA. The amino acid sequence contains conserved motifs typical of RNA-dependent RNA polymerases (RdRps). Sequence analyses of the nucleotide and RdRp-like protein revealed that the L-dsRNA is homologous with previously characterized mitochondrial viruses and dsRNAs from other phytopathogenic fungi, and shares 92.4% nucleotide and 95.1% amino acid sequence identities with the Ophiostoma novo-ulmi mitovirus 3a-Ld from Ophiostoma novo-ulmi, the causal agent of Dutch elm disease. The results indicate that these two dsRNAs are conspecific. This is the first report that a hypovirulence-associated dsRNA virus naturally occurs in two taxonomically distinct fungi, and indicates that horizontal transmission of this dsRNA virus may have occurred between these fungi.
A Quantitative Method to Screen Common Bean Plants for Resistance to Bean common mosaic necrosis virus
Authors: Strausbaugh C. A. , Myers J. R. , Forster R. L. , McClean P. E.
Published: May 2007

A quantitative method to screen common bean (Phaseolus vulgaris) plants for resistance to Bean common mosaic necrosis virus (BCMNV) is described. Four parameters were assessed in developing the quantitative method: symptoms associated with systemic virus movement, plant vigor, virus titer, and plant dry weight. Based on these parameters, two rating systems (V and VV rating) were established. Plants from 21 recombinant inbred lines (RILs) from a Sierra (susceptible) × Olathe (partially resistant) cross inoculated with the BCMNV-NL-3 K strain were used to evaluate this quantitative approach. In all, 11 RILs exhibited very susceptible reactions and 10 RILs expressed partially resistant reactions, thus fitting a 1:1 susceptible/partially resistant ratio (χ2 = 0.048, P = 0.827) and suggesting that the response is mediated by a single gene. Using the classical qualitative approach based only on symptom expression, the RILs were difficult to separate into phenotypic groups because of a continuum of responses. By plotting mean percent reduction in either V (based on visual symptoms) or VV (based on visual symptoms and vigor) rating versus enzyme-linked immunosorbent assay (ELISA) absorbance values, RILs could be separated clearly into different phenotypic groups. The utility of this quantitative approach also was evaluated on plants from 12 cultivars or pure lines inoculated with one of three strains of BCMNV. Using the mean VV rating and ELISA absorbance values, significant differences were established not only in cultivar and pure line comparisons but also in virus strain comparisons. This quantitative system should be particularly useful for the evaluation of the independent action of bc genes, the discovery of new genes associated with partial resistance, and assessing virulence of virus strains.
Comparison of Sandy Soils Suppressive or Conducive to Ectoparasitic Nematode Damage on Sugarcane
Authors: Rimé Delphine , Nazaret Sylvie , Gourbière François , Cadet Patrice , Moënne-Loccoz Yvan
Published: May 2007

Two South African sandy soils, one suppressive and the other conducive to ectoparasitic nematode damage on monoculture sugarcane, were compared. Analysis of field transects indicated that the suppressive soil displayed a comparatively higher population of the weak ectoparasite Helicotylenchus dihystera, whose predominance among ectoparasitic nematodes is known to limit yield loss caused by more virulent phytonematodes. Soil type was identical at both sites (entisols), but the suppressive soil had a higher organic matter content and a lower pH, which correlated with H. dihystera population data. In contrast, microclimatic differences between the two field sites were unlikely to be responsible for the suppressive or conducive status of the soils, as shown in a greenhouse experiment. The two soils exhibited a bacterial community of the same size but with different genetic structures, as indicated by automated ribosomal intergenic spacer analysis (RISA). The number of culturable fluorescent pseudomonads was higher for the conducive soil, probably because extensive root damage caused by ectoparasitic nematodes favored proliferation of these bacteria. This study shows that apparently small differences in soil composition between fields located in the same climatic area and managed similarly can translate into contrasted nematode communities, ectoparasitic nematode damage levels, and sugarcane yields.
The N-Terminal Region of the Readthrough Domain Is Closely Related to Aphid Vector Specificity of Soybean dwarf virus
Authors: Terauchi Hidetaka , Honda Ken-ichiro , Yamagishi Noriko , Kanematsu Seiji , Ishiguro Kiyoshi , Hidaka Soh
Published: May 2007

It has been speculated that the N-terminal half of the readthrough domain (RTD) encoded by open reading frame 5 of Soybean dwarf virus (SbDV) is related to the vector specificity. To further investigate this hypothesis, transmissibility via aphids was tested on 17 SbDV isolates and comparisons of the deduced amino acid sequences of the coat protein (CP) and other proteins encoded by the RTD were made between these isolates. Isolates were distinguished into four strains: YS, causing yellowing in soybean and transmittable by Aulacorthum solani; DS, causing dwarfing and transmittable by A. solani; YP, causing yellowing and transmittable by Acyrthosiphon pisum; and DP, causing dwarfing and transmittable by A. pisum. Phylogenetic analysis showed that the trees for the CP and the C-terminal half of the RTD sequences contained clusters of isolates of the same symptom type, whereas the tree for the N-terminal half of the RTD contained clusters of isolates of the same aphid vector type. These results agreed with our previous data of the complete nucleotide sequences of four SbDV isolates, and strongly indicated a close relationship between the N-terminal half of the RTD amino acid sequences and aphid transmission specificity of SbDV.
Effects of Irrigation and Tillage on Temporal and Spatial Dynamics of Sclerotinia minor Sclerotia and Lettuce Drop Incidence
Authors: Wu B. M. , Subbarao K. V.
Published: May 2007

The temporal and spatial dynamics of Sclerotinia minor sclerotia and the resulting incidence of lettuce drop were studied under furrow irrigation with conventional tillage and subsurface-drip irrigation with minimum tillage during 1993–95. Lettuce crops were grown each year during the spring and fall seasons. All plants were inoculated immediately after thinning in the spring of 1993. Grids of 24 contiguous quadrats (1 by 1 m2) were demarcated in the centers of each 150-m2 plot. Lettuce drop incidence in each quadrat was evaluated each season prior to harvest. One soil sample (100 cm3) was collected from each quadrat at harvest and after tillage prior to planting of the next crop for both spring and fall crops and assayed for S. minor sclerotia using wet sieving. Lloyd's index of patchiness, the β-binomial distribution, and variance of moving window averages were used to evaluate the spatial patterns of sclerotia and lettuce drop incidence under the two irrigation systems and associated tillage treatments. Disease incidence remained significantly higher under furrow irrigation than under subsurface-drip irrigation throughout the study period, and was significantly higher on fall crops than on spring crops. Under furrow irrigation, the number of sclerotia at the end of a crop season increased significantly over that at the beginning of the season, but no significant changes were detected over years. In contrast, the number of sclerotia within a single season did not increase significantly under subsurface drip irrigation, nor was year-to-year accumulation of sclerotia statistically significant. The degree of aggregation of sclerotia increased significantly during a cropping season under furrow irrigation, but not under subsurface drip irrigation. The conventional tillage after harvest under furrow irrigation decreased the degree of aggregation of sclerotia after each season, but the distribution pattern of sclerotia under subsurface-drip irrigation changed little by the associated minimum tillage. Spatial pattern analyses suggested that the aggregation of S. minor sclerotia occurred at a scale of no more than 1 m, and distribution of diseased lettuce plants was random at a scale larger than 1 m. The combination of fewer sclerotia produced by each crop and its unaltered distribution under subsurface drip irrigation and associated minimum tillage makes it a valuable cultural practice for lettuce drop management.