bluuebunny/crossref_metadata_2025 huggingface dataset:
Aggressiveness of Isolates of Phytophthora infestans from the Columbia Basin of Washington and Oregon
Authors: Miller Jeffrey S. , Johnson Dennis A. , Hamm Philip B.
Published: May 2007

The aggressiveness of 22 isolates of Phytophthora infestans collected from naturally infected potato plants in the Columbia Basin of Washington and Oregon was determined on detached potato leaflets at 18°C in an incubator. Selected isolates were evaluated on whole plants in a greenhouse. Aggressiveness was measured by using the area under the lesion expansion curve (AULEC), incubation period, latent period, sporulation capacity, and lesion size on detached leaflets and the area under the disease progress curve and sporulation capacity on whole plants. The detached-leaflet assay was useful in that a large number of isolates were tested, several components of aggressiveness were studied, and significant differences were found among isolates. Significant variation for components of aggressiveness was found within and among isolates classified according to genotype. Significant interactions among isolates and cultivars were found for some components of aggressiveness, so results were pooled according to cultivar. On average, US-8 and US-11 isolates had higher AULEC scores, indicating aggressiveness higher than that of US-7, US-6, and US-1 genotypes. One US-8 genotype isolate had a higher standardized sporulation capacity than isolates of the other genotypes. US-6 genotype isolates were the least aggressive group, as indicated by low AULEC, sporulation capacity, and lesion size values. The replacement of the US-1 genotype by the US-8 genotype in the Columbia Basin may be partially explained by the increased aggressiveness of US-8 isolates. Additionally, potato growers may need to shorten intervals between fungicide applications and begin applications earlier.
Identification of Some Oomycetes by Reverse Dot Blot Hybridization
Authors: Lévesque C. André , Harlton Colleen E. , de Cock Arthur W. A. M.
Published: May 2007

An assay was developed that can identify unknown isolates of Pythium or Phytophthora species in a single hybridization. This reverse dot blot system is based on arrays of species-specific amplified fragments or oligonucleotides derived from the internal transcribed spacer (ITS) region, which are blotted as dots on a nylon membrane. By using total DNA from a sample as the template, universal primers, and digoxigenin-dUTP, the ITS was amplified and labeled simultaneously by the polymerase chain reaction (PCR). A small aliquot of the resultant labeled and amplified product was used as a probe for hybridization to a dot blot membrane that contained the immobilized species-specific oligonucleotides or amplified PCR fragments. The reverse dot blot system based on arrays of oligonucleotides showed far fewer cross-hybridizations than one based on entire amplified ITS I fragments. Unknown species can be identified simply by visualizing the positive hybridization reaction between the DNA labeled directly from the sample and the immobilized specific oligonucleotide. Currently, the assay can be used to identify Pythium aphanidermatum, P. ultimum, P. acanthicum, and Phytophthora cinnamomi. An oligonucleotide that was originally designed to identify Phytophthora hybridized to 10 of the 14 Phytophthora species tested. Another oligonucleotide designed to identify oomycetes hybridized to the 68 species tested, which represented two of the four orders of this phylum.
Selection of Single-Chain Variable Fragment Antibodies to Black Currant Reversion Associated Virus from a Synthetic Phage Display Library
Authors: Susi Petri , Ziegler Angelika , Torrance Lesley
Published: May 2007

Single-chain variable fragment (scFv) antibodies that bind to black currant reversion associated virus (BRAV) were obtained from a synthetic phage display antibody gene library without recourse to animal immunizations. Several different BRAV-specific phage scFv were obtained quickly, after only three rounds of selection against immobilized virus antigen. The phage scFv gave enzyme-linked immunosorbent assay (ELISA) absorbance values that were greater than seven times the control healthy plant extracts. In contrast, comparative tests using a rabbit antiserum failed, because unacceptably high background values were obtained with healthy plant extracts. Two of the scFv were subcloned into the pDAP2 vector for the rapid and efficient production of scFv-alkaline phosphatase fusion proteins. Functional fusion proteins were obtained after expression in Escherichia coli, and preparations from periplasmic extracts detected BRAV in ELISA. The results demonstrate that antibody fragments obtained from a synthetic phage display library are useful research tools, and they proved to be a viable practical alternative when traditional antisera failed to detect BRAV, a weak immunogen. Furthermore, the genetic fusion of antibody fragments to alkaline phosphatase obviates the need for further chemical coupling procedures, and the fusion proteins can be obtained cheaply.
Monoterpene and Phenolic Compound Concentrations in Water-Stressed Red Pine Inoculated with Sphaeropsis sapinea
Authors: Blodgett J. T. , Stanosz G. R.
Published: May 2007

Changes in monoterpene and phenolic compounds resulting from water stress and colonization by Sphaeropsis sapinea were examined for 9- and 11-year-old red pine trees in a plantation and 3-year-old seedlings in a growth chamber. Four treatments were assigned at random to individual trees in the field: no treatment, herbicide to kill surrounding weeds, supplemental water, and both herbicide and supplemental water. In the growth chamber, seedlings were either not watered (water stressed) or watered daily (nonstressed). Shoots were inoculated with agar plugs colonized with either S. sapinea isolates of morphotype A and B (field) or only isolates of morphotype A (growth chamber). Nine monoterpenes were detected in tissue extracts; the most common were α-pinene (59 to 74% of the total), β-pinene (13 to 33% of the total), and δ-3-carene (1 to 5% of the total). Shoots inoculated with isolates of morphotype A had more severe symptoms and produced higher concentrations of monoterpenes in both experiments compared with the controls. In the growth chamber, inoculations with isolates of morphotype A caused higher concentrations of phenolics compared with the controls. In the field experiment, monoterpenes increased in quantity only in shoots of stressed trees inoculated with isolates of morphotype A. Isolates of morphotype B caused few symptoms and did not alter monoterpene concentrations. Increases in monoterpenes do not appear to be involved in the response to infection by morphotype A in nonstressed trees, and the role of phenolics is unclear. However, these results are consistent with previous observations that monoterpenes may be involved in the differences in aggressiveness between morphotypes on red pine.
Mechanisms of Subsurface Drip Irrigation-Mediated Suppression of Lettuce Drop Caused by Sclerotinia minor
Authors: Bell A. A. , Liu L. , Reidy B. , Davis R. M. , Subbarao K. V.
Published: May 2007

Subsurface drip irrigation and associated mandatory minimum tillage practices significantly reduced the incidence of lettuce drop (Sclerotinia minor) and the severity of corky root on lettuce compared with furrow irrigation and conventional tillage. Three possible mechanisms for the drip irrigation-mediated disease suppression were examined in this study: qualitative and quantitative differences in the soil microflora under furrow and subsurface drip irrigation; their antagonism and potential bio-control effects on S. minor; and the physical distribution of soil moisture and temperature relative to the two irrigation methods. To determine if the suppressive effects under subsurface drip irrigation were related to changes in soil microflora, soils were assayed for actinomycetes, bacteria, and fungi during the spring and fall seasons. The effects of the irrigation methods on microbial populations were nearly identical during both seasons. In the spring season, the total number of fungal colonies recovered on potato dextrose agar amended with rose Bengal generally was greater in soils under drip irrigation than under furrow irrigation, but no such differences were observed during the fall. Numbers of actinomycetes and bacteria were not significantly different between irrigation methods during either season. No interaction between sampling time and irrigation methods was observed for any of the microbial populations during both seasons. Thus, the significant effect of sampling time observed for actinomycete and bacterial populations during the spring was most likely not caused by the irrigation treatments. There were also no qualitative differences in the three groups of soil microflora between the irrigation treatments. Even though some fungal, actinomycete, and bacterial isolates suppressed mycelial growth of S. minor in in vitro assays, the isolates came from both subsurface drip- and furrow-irrigated soils. In in planta assays, selected isolates failed to reduce the incidence of drop in lettuce plants. The soil moisture under subsurface drip irrigation was significantly lower at all depths and distances from the bed center after an irrigation event than under furrow irrigation. The soil temperature, in contrast, was significantly higher at both 5 and 15 cm depths under drip irrigation than under furrow irrigation. The suppression of lettuce drop under subsurface drip irrigation compared with furrow irrigation is attributed to differential moisture and temperature effects rather than to changes in the soil microflora or their inhibitory effects on S. minor.
Host Specificity of Phytophthora infestans on Tomato and Potato in Ecuador
Authors: Oyarzun P. J. , Pozo A. , Ordoñez M. E. , Doucett K. , Forbes G. A.
Published: May 2007

Sixty Ecuadorian isolates of Phytophthora infestans from potato and 60 isolates from tomato were compared for dilocus allozyme genotype, mitochondrial DNA haplotype, mating type, and specific virulence on 11 potato R-gene differential plants and four tomato cultivars, two of which contained different Ph genes. Restriction fragment length polymorphism (RFLP) fingerprints of subsamples of isolates from each host were compared by using RG57 as the probe. All potato isolates had the allozyme genotype, haplotype, and mating type of the clonal lineage EC-1, which had been previously described in Ecuador. With the same markers, only one isolate from tomato was classified as EC-1; all others belonged to the globally distributed US-1 clonal lineage. RFLP fingerprints of isolate subsets corroborated this clonal lineage classification. Specific virulence on potato differentials was broadest among potato isolates, while specific virulence on tomato cultivars was broadest among tomato isolates. Some tomato isolates infected all tomato differentials but no potato differentials, indicating that specific virulence for the two hosts is probably controlled by different avirulence genes in P. infestans. In two separate experiments, the diameters of lesions caused by nine isolates from potato and 10 from tomato were compared on three tomato and three potato cultivars. All isolates produced larger lesions on the host from which they were isolated. No isolates were found that were highly aggressive on both tomato and potato. We conclude that there are two different populations of P. infestans in Ecuador and that they are separated by host.
Resistance to Aspergillus flavus in Corn Kernels Is Associated with a 14-kDa Protein
Authors: Chen Z.-Y. , Brown R. L. , Lax A. R. , Guo B. Z. , Cleveland T. E. , Russin J. S.
Published: May 2007

Corn genotypes resistant or susceptible to Aspergillus flavus were extracted for protein analysis using a pH 2.8 buffer. The profile of protein extracts revealed that a 14-kDa protein is present in relatively high concentration in kernels of seven resistant corn genotypes, but is absent or present only in low concentration in kernels of six susceptible ones. The N-terminal sequence of this 14-kDa protein showed 100% homology to a corn trypsin inhibitor. The 14-kDa protein purified from resistant varieties also demonstrated in vitro inhibition of both trypsin activity and the growth of A. flavus. This is the first demonstration of antifungal activity of a corn 14-kDa trypsin inhibitor protein. The expression of this protein among tested genotypes may be related to their difference in resistance to A. flavus infection and subsequent aflatoxin contamination.
Ultrastructural and Cytochemical Aspects of the Biological Control of Botrytis cinerea by Candida saitoana in Apple Fruit
Authors: El-Ghaouth Ahmed , Wilson Charles L. , Wisniewski Michael
Published: May 2007

Biocontrol activity of Candida saitoana and its interaction with Botrytis cinerea in apple wounds were investigated. When cultured together, yeast attached to Botrytis sp. hyphal walls. In wounded apple tissue, C. saitoana restricted the proliferation of B. cinerea, multiplied, and suppressed disease caused by either B. cinerea or Penicillium expansum. In inoculated apple tissue without the yeast, fungal colonization caused an extensive degradation of host walls and altered cellulose labeling patterns. Hyphae in close proximity to the antagonistic yeast exhibited severe cytological injury, such as cell wall swelling and protoplasm degeneration. Colonization of the wound site by C. saitoana did not cause degradation of host cell walls. Host cell walls in close contact with C. saitoana cells and B. cinerea hyphae were well preserved and displayed an intense and regular cellulose labeling pattern. In addition to restricting fungal colonization, C. saitoana induced the formation of structural defense responses in apple tissue. The ability of C. saitoana to prevent the necrotrophic growth of the pathogen and stimulate structural defense responses may be the basis of its biocontrol activity.
Characterization of an Exo-β-1,3-Glucanase Produced by Pichia anomala Strain K, Antagonist of Botrytis cinerea on Apples
Authors: Jijakli M. Haïssam , Lepoivre Philippe
Published: May 2007

The exo-β-1,3-glucanase (EC 3.2.1.58) activity of Pichia anomala strain K, an antagonistic yeast of Botrytis cinerea on postharvest apples, was studied in a synthetic medium supplemented with laminarin, a cell wall preparation (CWP) of B. cinerea, or glucose. The highest enzyme activity was detected in culture media containing a CWP of B. cinerea as the sole carbon source, whereas the lowest activity was observed in culture media supplemented with glucose. Exoglc1, an exo-β-1,3-glucanase, was purified to homogeneity from culture filtrates of strain K containing a CWP. The molecular mass of exoglc1 was estimated to be under 15 kDa. Optimum activity of exoglc1 was recorded at 50°C and pH 5.5. The exoglc1 Km value was estimated at 22.4 mg/ml. Exoglc1 showed in vitro a stronger inhibitory effect on germ tube growth of B. cinerea than on conidia germination and caused morphological changes such as leakage of cytoplasm and cell swelling. Exo-β-1,3-glucanase activity was detected on apples treated with strain K and was similar to exoglc1 on the basis of activity on native gel. Moreover, the addition of a CWP to a suspension of P. anomala stimulated both in situ exo-β-1,3-glucanase activity and protective activity against the pathogen, strengthening the hypothesis that exo-β-1,3-glucanase activity is one of the mechanisms of action involved in the suppression of B. cinerea by P. anomala strain K.
Histological Comparison of Fibrous Root Infection of Disease-Tolerant and Susceptible Citrus Hosts by Phytophthora nicotianae and P. palmivora
Authors: Widmer T. L. , Graham J. H. , Mitchell D. J.
Published: May 2007

Phytophthora nicotianae and P. palmivora infect and cause rot of fibrous roots of susceptible and tolerant citrus rootstocks in Florida orchards. The infection and colonization by the two Phytophthora spp. of a susceptible citrus host, sour orange (Citrus aurantium), and a tolerant host, trifoliate orange (Poncirus trifoliata), were compared using light and electron microscopy. Penetration by both Phytophthora spp. occurred within 1 h after inoculation, regardless of the host species. No differences were observed in mode of penetration of the hypodermis or the hosts' response to infection. After 24 h, P. palmivora had a significantly higher colonization of cortical cells in susceptible sour orange than in tolerant trifoliate orange. Intracellular hyphae of both Phytophthora spp. were observed in the cortex of sour orange, and cortical cells adjacent to intercellular hyphae of P. palmivora were disrupted. In contrast, the cortical cells of sour orange and trifoliate orange adjacent to P. nicotianae hyphae and the cortical cells of trifoliate orange adjacent to P. palmivora were still intact. After 48 h, the cortical cells of both hosts adjacent to either Phytophthora spp. were disrupted. After 48 and 72 h, P. palmivora hyphae colonized the cortex of sour orange more extensively than the cortex of trifoliate orange; P. palmivora also colonized both hosts more extensively than P. nicotianae. A higher rate of electrolyte leakage among host-pathogen combinations reflected the combined effects of greater cell disruption by P. palmivora than by P. nicotianae, and the higher concentration of electrolytes in healthy roots of trifoliate orange than of sour orange. Although cellular responses unique to the tolerant host were not observed, reduced hyphal colonization by both pathogens in the cortex of trifoliate orange compared with sour orange is evidence for a putative resistance factor(s) in the trifoliate orange roots that inhibits the growth of Phytophthora spp.
Tomato Chlorosis Virus: A New Whitefly-Transmitted, Phloem-Limited, Bipartite Closterovirus of Tomato
Authors: Wisler G. C. , Li R. H. , Liu H.-Y. , Lowry D. S. , Duffus J. E.
Published: May 2007

Tomato chlorosis virus (ToCV) is the second whitefly-transmitted, phloem-limited, bipartite closterovirus described infecting tomato. ToCV is distinct from tomato infectious chlorosis virus (TICV), based on lack of serological and nucleic acid cross-reactions and differences in vector specificity. TICV is transmitted only by the greenhouse whitefly (Trialeurodes vaporariorum), whereas ToCV is transmitted by the greenhouse whitefly, the banded-wing whitefly (T. abutilonea), and Bemisia tabaci biotypes A and B (B. argentifolii). Double-stranded (ds) RNA analyses of ToCV show two prominent dsRNAs of approximately 7,800 and 8,200 bp, with several small dsRNAs. Digoxigenin-11-UTP-labeled riboprobes derived from cDNA clones representing portions of RNAs 1 and 2 were used in Northern blot hybridizations to detect two large nonhomologous dsRNAs and a subset of smaller dsRNAs. These probes were used in dot blot hybridizations to detect ToCV in infected tomato. Inclusion bodies and cytoplasmic vesicles were consistently observed in phloem tissues of ToCV-infected Nicotiana clevelandii. Computer-assisted sequence analysis showed significant homology between ToCV clones that hybridize specifically with RNAs 1 and 2 and the lettuce infectious yellows virus methyltransferase of RNA 1 and the HSP70 heat shock protein homolog of RNA 2, respectively. Thus, ToCV is another member of the growing subgroup of bipartite closteroviruses transmitted by whiteflies.
Colonization of Host Plants by the Fire Blight Pathogen Erwinia amylovora Marked with Genes for Bioluminescence and Fluorescence
Authors: Bogs Jochen , Bruchmüller Iris , Erbar Claudia , Geider Klaus
Published: May 2007

To follow the movement of Erwinia amylovora in plant tissue without dissection, this bacterium was marked with either the lux operon from Vibrio fischeri or the gfp gene from the jellyfish Aequorea victoria, both carried on multicopy plasmids and expressed under the control of the lac promoter from Escherichia coli. Movement of the pathogen was visualized in leaves, stems, and roots of apple seedlings, and migration of E. amylovora was traced from inoculation sites in the stem to as far as the roots. Green fluorescent E. amylovora cells were observed in the xylem and later appeared to break out of the vessels into the intercellular spaces of the adjacent parenchyma. Inoculation in the intercostal region of leaves caused a zone of slow necrosis that finally resulted in bacterial invasion of the xylem vessels. Labeled bacteria could also be seen in association with the anchor sites of leaf hairs. Distortion of the epidermis adjacent to leaf hairs created openings that were observed by scanning electron microscopy. As the intercostal region, the bases of leaf hairs provided E. amylovora access to intact xylem vessels, which allowed further distribution of the pathogen in the host plant.
Microcomputer-Based Quantification of Maize Streak Virus Symptoms in Zea mays
Authors: Martin Darrin P. , Rybicki Edward P.
Published: May 2007

We investigated the use of computer-assisted image analysis techniques for the objective quantification of maize streak virus (MSV) symptoms in Zea mays. We compared independent duplicate evaluations of chlorotic lesion areas occurring on MSV-infected leaves using visual assessment, a commercial image analysis system, and a custom image analysis system employing software developed in our laboratory. Relative to visual assessments of disease severity, computer-assisted image analysis employing both the commercial and custom systems provided significant enhancements in the accuracy and precision of chlorotic area estimations. The commercial image analysis system afforded no significant improvement in precision or accuracy over the custom system. An important advantage of examining images using the custom-written software was that the software permitted a high degree of analysis automation. Digitized images of maize leaves could be automatically analyzed by the custom software five times faster than, and with the same precision and accuracy as, when the same images were analyzed with the commercial software. Because of the flexibility of the image analysis techniques described, they should be applicable to the measurement of symptom severity in other plant host-pathogen combinations.
Use of Multispectral Radiometry for Assessment of Rhizoctonia Blight in Creeping Bentgrass
Authors: Raikes C. , Burpee L. L.
Published: May 2007

The ability to identify diseases early and quantify severity accurately is crucial in plant disease assessment and management. This study was conducted to assess changes in the spectral reflectance of sunlight from plots of creeping bentgrass during infection by Rhizoctonia solani, the cause of Rhizoctonia blight, and to evaluate multispectral radiometry as a tool to quantify Rhizoctonia blight severity. After inoculation of 6-year-old creeping bentgrass turf with R. solani anastomosis group 2-2, reflectance of sunlight from the foliar canopy was measured at light wavelengths of 460 nm (blue) to 810 nm (near infrared [NIR]), at 50-nm intervals. Visual estimates of disease severity and percentage of canopy reflectance were made daily throughout each of three epidemics of Rhizoctonia blight from the onset of visible symptoms until maximum disease severity was reached. In each experiment, linear regression analysis revealed a significant reduction in the percentage of NIR (760 and 810 nm) reflectance as disease severity increased. However, in the majority of analyses, regression models explained <50% of the variability between components. Multispectrum radiometry appears to function best when used to assess differences in disease severity at discrete points in time rather than over an entire epidemic.
Naturally Occurring Variants of Satellite Tobacco Mosaic Virus
Authors: Mathews D. M. , Dodds J. A.
Published: May 2007

Four natural variants of satellite tobacco mosaic virus (STMV) were compared with each other and with the type strain. Differences were detected in double-stranded RNA, single-stranded RNA, and virion electrophoretic mobility patterns, while the size and antigenicity of the coat protein were similar for all. RNase protection assays detected differences in the genomes of each of the four new variants, which differed not only from each other, but also from that of type STMV. Infectious RNA transcripts were made from complementary DNA clones of one variant (STMV 10) with a genome apparently smaller than that of type STMV. A 71-base deletion in the region that contains the 6.8-kDa protein in type STMV was detected by sequence analysis of the STMV 10 clones, a result that is confirmed by the lack of a 6.8-kDa in vitro translation product for STMV 10. Only minor sequence differences exist elsewhere in the genome compared with that of type STMV. Type STMV and STMV 10 each successfully cross-protected against the other when tobacco plants were inoculated 10 days apart.
Herbicide Effects on Sugarcane Growth, Pythium Root Rot, and Pythium arrhenomanes
Authors: Dissanayake N. , Hoy J. W. , Griffin J. L.
Published: May 2007

Six herbicides were evaluated for their effects on Pythium root rot and growth of sugarcane in greenhouse experiments and on in vitro mycelial growth rate of Pythium arrhenomanes. Pendimethalin and atrazine were most inhibitory to mycelial growth, but neither reduced root rot severity. Asulam, atrazine, and metribuzin were not phytotoxic to sugarcane and did not affect root rot symptom severity in clay loam or silt loam field soils. Atrazine and metribuzin increased shoot number, and atrazine increased total shoot weight for treated plants in silt loam soil. Glyphosate, pendimethalin, and terbacil were phytotoxic to sugarcane. These herbicides increased root rot severity, but the extent to which growth reductions resulted from increased disease severity or from direct herbicide injury was not clear. Adverse effects on plant growth and root rot severity were greater in clay loam than in silt loam soil. The results suggest that sugarcane injury from some herbicides is compounded by increased severity of root rot.
Survival of Fusarium moniliforme, F. proliferatum, and F. subglutinans in Maize Stalk Residue
Authors: Cotten T. K. , Munkvold G. P.
Published: May 2007

The roles of residue size and burial depth were assessed in the survival of Fusarium moniliforme, F. proliferatum, and F. subglutinans in maize stalk residue. Stalk pieces (small or large sizes) were soaked in a spore suspension of F. moniliforme, F. proliferatum, or F. subglutinans and placed in a field on the soil surface or buried at 15- or 30-cm depths. Residue pieces were recovered periodically, cultured on a selective medium, and microscopically examined for the presence of the inoculated Fusarium species. After 630 days, the inoculated Fusarium species were recovered from 0 to 50% of the inoculated stalk pieces in a long-term, continuous maize field, from 0 to 28% of the inoculated stalk pieces placed in a maize/soybean/oat rotation field, and from 0 to 25% of the noninoculated stalk pieces at both locations. Residue size and residue depth had significant effects on survival, but there were significant interactions among strain, depth, residue size, and time. Up to 343 days after placement in the field, survival of the three Fusarium species was not consistently different between buried residues and surface residues, but after 630 days, survival was greater from surface residues. Overall, fungus survival decreased more slowly in the surface residues than in the buried residues. Linear coefficients of determination ranged from 0.35 to 0.82 for the surface residues and from 0.81 to 0.98 for the buried residues. Decline in survival over time followed a more linear pattern in buried residues than in surface residues. Vegetative compatibility tests confirmed that F. moniliforme, F. proliferatum, and F. subglutinans strains can survive at least 630 days in surface or buried maize residue. These results demonstrate that maize residue can act as a long-term source of inoculum for infection of maize plants by these three Fusarium species.
Role of Light and Malate in the Decreased Sensitivity of cms-T Cytoplasm Maize Leaves to Bipolaris maydis Race T Toxin
Authors: Garraway M. O. , Park D.-S. , Beltran J. D.
Published: May 2007

Leaf segments from Texas male sterile (cms-T) cytoplasm maize isolines exposed to light (50 μM s-1 m-2) for 8 h or more before or after being infiltrated with the Bipolaris maydis race T toxin (T-toxin) leaked significantly less electrolytes when immersed in distilled water (DW) for 24 to 48 h than did dark-treated leaf segments. No comparable effect of light on toxin-induced electrolyte leakage was observed with normal (N) cytoplasm isolines. Toxin-treated cms-T leaf segments incubated in DW for three consecutive 12-h periods of alternating light and dark showed significantly greater electrolyte leakage than leaf segments incubated in continuous light for 36 h and significantly less leakage than segments incubated in continuous dark for 36 h.Exposure of cms-T, but not N, cytoplasm leaves to 25 or 50 μM malic acid decreased their sensitivity to T-toxin in the dark to a level similar to that observed when leaves were incubated in the light without malic acid. The potency of T-toxin appeared to be unaffected by its exposure to light. The loss of electrolytes from T-toxin-treated cms-T cytoplasm leaf segments was at approximately the level seen with light or malate when 25 μM H2O2 was added to the DW bathing solution. Evaluation of the data points to the possibility that H2O2 might be involved with the altered sensitivity of cms-T cytoplasm leaves to T-toxin caused by either light or malate.
Molecular Evidence of Distinct Introductions of the European Race of Gremmeniella abietina into North America
Authors: Hamelin Richard C. , Lecours Nicole , Laflamme Gaston
Published: May 2007

The presence of the European (EU) race of Gremmeniella abietina var. abietina, the causal agent of Scleroderris canker of conifers, was first reported in North America in 1975 in the northeastern United States and subsequently in southern Quebec and Newfoundland during the late 1970s, where it quickly became established. We analyzed DNA profiles in samples from a historic collection of G. abietina var. abietina that included some of the first isolates of the EU race reported in the United States to test hypotheses concerning the G. abietina var. abietina epidemic in North America. Genetic diversity was partitioned by an analysis of molecular variance with haplotype frequencies and distances. Genetic differentiation was high between populations in continental North America and Newfoundland (between region differentiation, Φct = 0.665, P < 0.001). This result was not consistent with the hypothesis of a single introduction of the pathogen into the northeastern United States followed by secondary spread into northeastern Canada. In contrast, small levels of genetic differentiation were observed among continental North American populations (Φct = 0.047, P = 0.079), suggesting gene flow among these populations. A single haplotype of G. abietina var. abietina dominated the continental populations (80% of the isolates) but was absent from Newfoundland and Europe. Five haplotypes were found in the New-foundland population, all of which were either absent or very rare on the continent. Populations from continental North America clustered together and were distinct from a second cluster composed of European and Newfoundland populations. A phylogenetic analysis of the haplotypes indicated that some of the rare haplotypes may have derived from somatic mutations, whereas others probably occurred as the result of new introductions. The results are consistent with a scenario of distinct primary introductions of this pathogen into Newfoundland and continental eastern North America followed by secondary asexual propagation.
Cytochemical Investigation of the Antagonistic Interaction Between a Microsphaeropsis sp. (Isolate P130A) and Venturia inaequalis
Authors: Benyagoub Mohammed , Benhamou Nicole , Carisse Odile
Published: May 2007

In an attempt to better understand the mode of action of the antagonistic fungus Microsphaeropsis sp., the interaction between this fungus and Venturia inaequalis was studied, using both light and electron microscopy. Cytological observations indicated that the antagonistic interaction between the two fungi likely involves a sequence of events, including (i) attachment and local penetration of Microsphaeropsis sp. into V. inaequalis hyphae; (ii) induction of host structural response at sites of potential antagonist entry; (iii) alteration of host cytoplasm; and (iv) active multiplication of antagonistic cells in pathogen hyphae, leading to host cell breakdown and release of the antagonist. The interaction was investigated further by gold cytochemistry. The use of gold-complexed β-1,4-exoglucanase and wheat germ agglutinin/ovomucoid-gold complex to localize cellulosic β-1,4-glucans and chitin monomers, respectively, resulted in regular labeling of V. inaequalis cell walls. This finding supports other studies refuting the classification of ascomycetes as only a glucan-chitin group. At an advanced state of parasitism, the labeling pattern of cellulose and chitin, which clearly showed that the level of integrity of these compounds was affected, suggested the production of cellulolytic and chitinolytic enzymes by Microsphaeropsis sp. Wall appositions formed in V. inaequalis in response to the antagonist's attack contained both cellulose and chitin. However, penetration of this newly formed material frequently succeeded. This study provides the first detailed picture of the cytological events associated with mycoparasitism in V. inaequalis.
Increase and Patterns of Spread of Citrus Tristeza Virus Infections in Costa Rica and the Dominican Republic in the Presence of the Brown Citrus Aphid, Toxoptera citricida
Authors: Gottwald T. R. , Garnsey S. M. , Borbón J.
Published: May 2007

Citrus tristeza virus (CTV) was monitored for 4 years by monoclonal antibody probes via enzyme-linked immunosorbent assay in four citrus orchards in northern Costa Rica and four orchards in the Dominican Republic following the introduction of the brown citrus aphid, Toxoptera citricida. The Gompertz nonlinear model was selected as the most appropriate in most cases to describe temporal increase of CTV. Ordinary runs analysis for association of CTV-positive trees failed to show a spatial relationship of virus status among immediately adjacent trees within or across rows. The beta-binomial index of dispersion for various quadrat sizes suggested aggregations of CTV-positive trees for all plots within the quadrat sizes tested. Spatial autocorrelation analysis of proximity patterns suggested that aggregation often existed among quadrats of various sizes up to four lag distances; however, significant lag positions discontinuous from the main proximity pattern were rare. Some asymmetry was also detected for some spatial autocorrelation proximity patterns. These results were interpreted to mean that, although CTV-positive trees did not often influence immediately adjacent trees, virus transmission was common within a local area of influence that extended two to eight trees in all directions. Where asymmetry was indicated, this area of influence was somewhat elliptical. The spatial and temporal analyses gave some insight into possible underlying processes of CTV spread in the presence of T. citricida and suggested CTV spread was predominantly to trees within a local area. Patterns of longer-distance spread were not detected within the confines of the plot sizes tested. Longer-distance spread probably exists, but may well be of a complexity beyond the detection ability of the spatial analysis methods employed, or perhaps is on a scale larger than the dimensions of the plots studied.
Spread of Endosepsis in Calimyrna Fig Orchards
Authors: Michailides Themis J. , Morgan David P.
Published: May 2007

Pollination of the edible fig (Ficus carica cv. Calimyrna) is mediated by a small symbiotic wasp, Blastophaga psenes, that inhabits the syconium cavity of the spring crop of fig pollinator trees (caprifigs). These fig wasps also carry propagules, mainly of Fusarium verticillioides (formerly F. moniliforme) and other Fusarium spp., which cause endosepsis, from pollinator figs to the edible Calimyrna figs in California. Spread of endosepsis was studied in one experimental and up to four commercial Calimyrna fig orchards from 1989 through 1995. The incidence of endosepsis in fruit collected from the tree canopy at either <2.0 m (low) or >2.0 m (high) height, from the north and south of the tree canopy, and from the outer (direct sunlight) and inner (shaded) canopy were similar. More wasps were captured in fig trees located 3.5 to 10 m east or west of the source than in trees 48 to 63 m from the source. In addition, significantly more wasps entered the syconia of trees closest (9 to 12.7 m) to the source than the syconia of the second or third trees (18 to 38.2 m) from the source. Endosepsis decreased with distance from the source, decreasing faster to the south than in other directions from the source. In addition, the disease-vectoring wasps decreased with increased distance from the source, which also described the disease spread from the contamination source for most directions, with a sharper decline south of the source. A 3-year study in three commercial Calimyrna orchards showed there is no secondary spread of fig endosepsis in the field. Although endosepsis can complete as many cycles (three to four) as its vector in fig pollinator trees, in Calimyrna figs it is considered a monocyclic disease. Because fig wasp pollinators prefer to stay close to the contamination source when receptive Calimyrna figs are available in close proximity, only disease sources (caprifigs trees) found among Calimyrna trees or at a distance less than 50 m from the borders of Calimyrna orchards affect endosepsis incidence in commercial orchards.
Variation in Virulence Within Meloidogyne chitwoodi, M. fallax, and M. hapla on Solanum spp.
Authors: van der Beek J. G. , Poleij L. M. , Zijlstra C. , Janssen R. , Janssen G. J. W.
Published: May 2007

The virulence of Meloidogyne hapla, M. chitwoodi, and M. fallax was studied on genotypes of Solanum spp. in a greenhouse. Juveniles of 11 M. hapla race A isolates, 3 M. hapla race B isolates, and 5 mono-female lines of a M. hapla race A isolate were inoculated on S. chacoense, S. hougasii, and S. sparsipilum. Juveniles of eight M. chitwoodi isolates, five M. fallax isolates, and six mono-female lines of a M. chitwoodi isolate were inoculated on S. bulbocastanum, S. chacoense, S. hougasii, S. stoloniferum, and S. tuberosum. Virulence was expressed as nematode reproduction 8 weeks after inoculation. Nematode reproduction was estimated by the number of egg masses and, in one experiment, by the number of hatched second-stage juveniles per inoculated juvenile. Considerable variation in virulence and resistance was observed among M. hapla isolates and plant genotypes, respectively. The M. hapla isolate-plant species interaction was highly significant. The response to M. chitwoodi ranged from susceptible (S. tuberosum and S. chacoense) to highly resistant (S. bulbocastanum and S. hougasii). S. tuberosum was susceptible to M. fallax, whereas all four wild species were resistant. In contrast to M. hapla, no significant isolate-plant genotype interaction was obtained for M. chitwoodi or M. fallax, indicating no or little intraspecific variation in virulence. M. chitwoodi juveniles in species mixtures with M. fallax isolates appeared to be able to break the resistance of S. bulbocastanum and S. hougasii. Significant differences among mono-female lines of M. hapla and M. chitwoodi were observed, indicating heterogeneity of pathogenicity within meiotic parthenogenic Meloidogyne populations.
Salicylic Acid Biosynthetic Genes Expressed in Pseudomonas fluorescens Strain P3 Improve the Induction of Systemic Resistance in Tobacco Against Tobacco Necrosis Virus
Authors: Maurhofer M. , Reimmann C. , Schmidli-Sacherer P. , Heeb S. , Haas D. , Défago G.
Published: May 2007

Application of salicylic acid induces systemic acquired resistance in tobacco. pchA and pchB, which encode for the biosynthesis of salicylic acid in Pseudomonas aeruginosa, were cloned into two expression vectors, and these constructs were introduced into two root-colonizing strains of P. fluorescens. Introduction of pchBA into strain P3, which does not produce salicylic acid, rendered this strain capable of salicylic acid production in vitro and significantly improved its ability to induce systemic resistance in tobacco against tobacco necrosis virus. Strain CHA0 is a well-described biocontrol agent that naturally produces salicylic acid under conditions of iron limitation. Introduction of pchBA into CHA0 increased the production of salicylic acid in vitro and in the rhizosphere of tobacco, but did not improve the ability of CHA0 to induce systemic resistance in tobacco. In addition, these genes did not improve significantly the capacity of strains P3 and CHA0 to suppress black root rot of tobacco in a gnotobiotic system.
Expression of Human Lactoferrin cDNA Confers Resistance to Ralstonia solanacearum in Transgenic Tobacco Plants
Authors: Zhang Zhanyuan , Coyne Dermot P. , Vidaver Anne K. , Mitra Amitava
Published: May 2007

A construct containing a human lactoferrin cDNA was used to transform tobacco (Nicotiana tabacum) using an Agrobacterium-mediated DNA-transfer system to express this human protein in transgenic plants. Transformants were analyzed by Southern, Northern, and Western blots to determine integration of the cDNA into the plant genome and lactoferrin gene expression levels. Most transgenic plants demonstrated significant delays of bacterial wilt symptoms when inoculated with the bacterial pathogen Ralstonia solanacearum. Quantification of the expressed lactoferrin protein by enzyme-linked immunosorbent assay in transgenic plants indicated a significant positive relationship between lactoferrin gene expression levels and levels of disease resistance. Incorporation of the lactoferrin gene into crop plants may enhance resistance to other phytopathogenic bacteria as well.
Incidence and Diversity of Double-Stranded RNAs Occurring in the Chestnut Blight Fungus, Cryphonectria parasitica, in China and Japan
Authors: Peever Tobin L. , Liu Yir-Chung , Wang Kerong , Hillman Bradley I. , Foglia Robert , Milgroom Michael G.
Published: May 2007

Isolates of the chestnut blight fungus, Cryphonectria parasitica, were randomly sampled from 10 subpopulations in China and 8 subpopulations in Japan and screened for the presence of double-stranded (ds) RNA using an immunoblot procedure with a monoclonal antibody specific for dsRNA. The overall incidence of dsRNA in C. parasitica was 2 and 6% in China and Japan, respectively, much lower than the 28% found previously in North American populations. Genetic relatedness of dsRNAs within and among populations in China and Japan was examined using RNA-RNA hybridizations with labeled-dsRNA probes. The majority of Chinese and Japanese dsRNAs were members of a single hybridization group, related to Cryphonectria hypovirus 1 (CHV1) from Europe, and are referred to as CHV1-type dsRNAs. No evidence was obtained for genetic differentiation between CHV1-type dsRNAs sampled in China and Japan. Five Japanese isolates contained two genetically distinct dsRNAs. The larger segments (approximately 12 kilobases [kb]) were members of the CHV1 hybridization group, while the smaller segments (approximately 3 kb) did not hybridize with any known dsRNA from C. parasitica including the 2.7-kb dsRNA from isolate NB631 from New Jersey or dsRNA from isolate RC1 from Michigan. Two small dsRNA segments (approximately 1.8 and 2 kb) from one isolate sampled from Liaoning Province in northeastern China did not hybridize with any of the dsRNA probes tested including several described dsRNAs of similar size from C. parasitica in North America. Three dsRNAs from Anhui Province, China, hybridized to Cryphonectria hypovirus 2 (CHV2)-specific probes and are thus referred to as CHV2-type dsRNAs. Sequence analysis of 1,627 base pairs of these three CHV2-type dsRNAs from Anhui revealed that they were identical to each other in the region sequenced and very closely related to CHV2-NB58, isolated from New Jersey. We speculate that CHV2-NB58 may have been introduced into North America from this part of China. This is the first record of a North American C. parasitica dsRNA that is genetically related to a dsRNA from Asia.
Characterization of Wheatgrass-Derived Barley Yellow Dwarf Virus Resistance in a Wheat Alien Chromosome Substitution Line
Authors: Anderson Joseph M. , Bucholtz Dennis L. , Greene Ann E. , Francki Michael G. , Gray Stewart M. , Sharma Hari , Ohm Herbert W. , Perry Keith L.
Published: May 2007

Wheatgrass (Thinopyrum intermedium) possesses a high level of resistance to barley yellow dwarf virus (BYDV) subgroup I and subgroup II strains. A wheat line (P29), in which the 7D chromosome has been substituted with a group 7 chromosome from T. intermedium, was examined for the level of resistance to two subgroup I and two subgroup II BYDV strains. In P29 plants inoculated with the subgroup I PAV strains, the titer of virus in leaf and stem tissue was typically reduced 42 to 52% when compared with the BYDV-susceptible cv. Abe. P29 and ‘Abe’ had the same content of PAV in roots. These results and the absence of detectable virus in inoculated T. intermedium plants indicate that the complete resistance to subgroup I possessed by the wheatgrass has not been introgressed into P29. In contrast, P29 was completely resistant throughout the plant to the subgroup II strains, NY-RPV and NY-RMV, demonstrating that the complete resistance to subgroup II in T. intermedium was incorporated into P29. Further analysis of this resistance to NY-RPV showed that NY-RPV can replicate in mesophyll protoplasts of P29 and ‘Abe’, suggesting that this resistance is not operating at the single-cell level. Molecular marker analysis confirmed that the T. intermedium chromosome present in P29 is a different group 7 wheatgrass chromosome than that present in L1, a wheat line with BYDV resistance properties similar to those of P29.
rep-PCR-Mediated Genomic Fingerprinting: A Rapid and Effective Method to IdentifyClavibacter michiganensis
Authors: Louws F. J. , Bell J. , Medina-Mora C. M. , Smart C. D. , Opgenorth D. , Ishimaru C. A. , Hausbeck M. K. , de Bruijn F. J. , Fulbright D. W.
Published: May 2007

The genomic DNA fingerprinting technique known as repetitive-sequence-based polymerase chain reaction (rep-PCR) was evaluated as a tool to differentiate subspecies of Clavibacter michiganensis, with special emphasis on C. michiganensis subsp. michiganensis, the pathogen responsible for bacterial canker of tomato. DNA primers (REP, ERIC, and BOX), corresponding to conserved repetitive element motifs in the genomes of diverse bacterial species, were used to generate genomic fingerprints of C. michiganensis subsp. michiganensis, C. michiganensis subsp. sepedonicus, C. michiganensis subsp. nebraskensis, C. michiganensis subsp. tessellarius, and C. michiganensis subsp. insidiosum. The rep-PCR-generated patterns of DNA fragments observed after agarose gel electrophoresis support the current division of C. michiganensis into five subspecies. In addition, the rep-PCR fingerprints identified at least four types (A, B, C, and D) within C. michiganensis subsp. michiganensis based on limited DNA polymorphisms; the ability to differentiate individual strains may be of potential use in studies on the epidemiology and host-pathogen interactions of this organism. In addition, we have recovered from diseased tomato plants a relatively large number of naturally occurring avirulent C. michiganensis subsp. michiganensis strains with rep-PCR fingerprints identical to those of virulent C. michiganensis subsp. michiganensis strains.