bluuebunny/crossref_metadata_2025 huggingface dataset:
Hypovirulence and Double-Stranded RNA in Sclerotinia homoeocarpa
Authors: Zhou Ting , Boland Greg J.
Published: May 2007

One hundred and thirty-two isolates of Sclerotinia homoeocarpa, the causal agent of dollar spot of turfgrass, were evaluated for virulence on swards and detached leaves of creeping bentgrass and for the presence of double-stranded RNA (dsRNA). In at least four isolates, the hypovirulent phenotype was associated with the presence of specific segments of dsRNA. In addition, these hypovirulent isolates often grew slowly on potato dextrose agar (PDA), formed thin colonies with atypical colony margins, and failed to produce typical black stroma. The hypovirulent phenotype and dsRNA were transmitted from hypovirulent isolate Sh12B to virulent isolate Sh48B, and the converted isolate was hypovirulent and contained dsRNA. The hypovirulent phenotype and dsRNA also were transmitted to at least four other isolates of the pathogen, including the fungicide-resistant, dsRNA¯ isolate KY-7. Converted isolates of KY-7 developed the hypovirulent phenotype, grew on fungicide-amended medium, and contained dsRNA. Subcultures of hypovirulent isolate Sh12B that did not contain dsRNA were obtained through curative treatment using cycloheximide-containing medium and heat. Cured subcultures grew faster on PDA, had more typical colony morphologies, were more virulent on bentgrass leaves, and did not contain dsRNA. No cured subcultures were obtained from hypovirulent isolate Sh09B. Isolates regenerated from protoplasts of hypovirulent isolate Sh12B were not cured, remained hypovirulent, and contained dsRNA. Transmission of hypovirulence and dsRNA in S. homoeocarpa has potential as a novel approach to the management of dollar spot of turfgrass.
Effects of Antagonist Cell Concentration and Two-Strain Mixtures on Biological Control of Fusarium Dry Rot of Potatoes
Authors: Schisler D. A. , Slininger P. J. , Bothast R. J.
Published: May 2007

Eighteen bacterial strains were individually assayed against Gibberella pulicaris (5 × 105 conidia per ml) by coinoculating antagonist and pathogen in wounds in cv. Russet Burbank potatoes. All antagonist concentrations (106, 107, and 108 CFU/ml) decreased disease (38 to 76% versus control, P < 0.05). When four strains were assayed at 11 concentrations (range 105 to 108 CFU/ml) against G. pulicaris, linear regression of the log-dose, log-response data was significant for all four strains (P < 0.001 to 0.01, R2 = 0.50 to 0.74). Challenging G. pulicaris with all possible antagonist pairings within 2 sets of 10 antagonist strains (5 × 105 CFU of each strain per ml) resulted in 16 of 90 pairs controlling disease better than predicted based on averaging the performance of the individual strains making up the pair (P < 0.10). Successful pairs reduced disease by ~70% versus controls, a level of control comparable to that obtained with 100 times the inoculum dose of a single antagonist strain. Neither strain genus nor soil of origin were useful in predicting successful antagonist pairs. Factors potentially influencing dose-response relationships and the effectiveness of antagonist pairs in controlling disease are discussed.
Inoculum Densities of Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita in Relation to the Development of Fusarium Wilt and the Phenology of Cotton Plants (Gossypium hirsutum)
Authors: DeVay J. E. , Gutierrez A. P. , Pullman G. S. , Wakeman R. J. , Garber R. H. , Jeffers D. P. , Smith S. N. , Goodell P. B. , Roberts P. A.
Published: May 2007

Development of Fusarium wilt in upland cotton (Gossypium hirsutum) usually requires infections of plants by both Meloidogyne incognita and Fusarium oxysporum f. sp. vasinfectum. In this study, the soil densities of M. incognita and F. oxysporum f. sp. vasinfectum and the incidence of Fusarium wilt in three field sites were determined in 1982-1984. Multiple regression analysis of percent incidence of Fusarium wilt symptoms on population densities of M. incognita and F. oxysporum f. sp. vasinfectum yielded a significant fit (R 2 = 0.64) only on F. oxysporum f. sp. vasinfectum. Significant t-values for slope were also obtained for the interaction of M. incognita and F. oxysporum f. sp. vasinfectum, but densities of M. incognita and F. oxysporum f. sp. vasinfectum were also related on a log10 scale. The physiological time of appearance of first foliar symptoms of Fusarium wilt, based on a degree-days threshold of 11.9°C (53.5°F), was used as a basis for determining disease progress curves and the phenology of cotton plant growth and development. Effects of Fusarium wilt on plant height and boll set were determined in three successive years. Increases in both of these plant characteristics decreased or stopped before foliar symptoms were apparent. Seed cotton yields of plant cohorts that developed foliar wilt symptoms early in the season (before 2,000 F degree-days) were variable but not much different in these years. This contrasted with cohorts of plants that first showed foliar symptoms late in the season (after 2,400 F degree-days) and cohorts of plants that showed no foliar symptoms of wilt. Regression analyses for 1982-1984 indicated moderate to weak correlations (r = 0.16–0.74) of the time of appearance of the first foliar symptoms and seed cotton yields.
Ultrastructural and Cytochemical Investigations of the Antagonistic Effect of Verticillium lecanii on Cucumber Powdery Mildew
Authors: Askary Hassan , Benhamou Nicole , Brodeur Jacques
Published: May 2007

Chronological events of the intercellular interaction between Verticillium lecanii and cucumber powdery mildew, caused by Sphaerotheca fuliginea, were investigated at different times after inoculation by transmission electron microscopy. V. lecanii hyphae colonized host structures by tight binding, apparently mediated by a thin mucilaginous matrix. As early as 24 h after application of the antagonist, increased vacuolation and disorganization of the cytoplasm of the pathogen hyphae were easily detected. By 36 h after treatment, plasmalemma retraction and local cytoplasm aggregation were typical features of damage. Labeling chitin with the wheat germ agglutinin (WGA)/ovomucoid-gold complex showed that intracellular invasion of S. fuliginea by V. lecanii did not cause extensive host cell wall alterations, except in the area of hyphal penetration. By 48 h after inoculation, further cytoplasm disorganization was observed, as evidenced by the loss of cell turgor and contortion of the cell wall. Such deformation suggests that penetration of the antagonist results from mechanical pressure or localized enzymatic hydrolysis through the action of chitinases, as confirmed by the pattern of labeling obtained with the WGA/ovomucoid-gold complex. By 72 h after contact between the fungi, S. fuliginea cells were markedly collapsed, depleted of their protoplasm due to extensive multiplication of the antagonist, and totally encircled by the antagonist. Based on the current observations, the antagonism of S. fuliginea by V. lecanii appears to involve the following events: (i) attachment of the antagonist to the powdery mildew fungus; (ii) mechanical pressure and production of cell-wall degrading enzymes such as chitinases; (iii) penetration and active growth of the antagonist inside the pathogen hyphae; and (iv) digestion of host tissues and release of the antagonist from dead cells of S. fuliginea. The interaction between V. lecanii and S. fuliginea also affected the morphological and structural features of the haustorial bodies, as shown by increased vacuolation, distortion, and necrotization of the haustorial lobes. These observations provide the first experimental evidence that V. lecanii, primarily known as an entomopathogenic fungus, also has the potential to colonize mycelial structures of S. fuliginea. V. lecanii, therefore, may become a valuable alternative to current management of cucumber powdery mildew in greenhouses.
Molecular Variability of the Genomes of Capilloviruses from Apple, Japanese Pear, European Pear, and Citrus Trees
Authors: Magome H. , Yoshikawa N. , Takahashi T. , Ito T. , Miyakawa T.
Published: May 2007

The 3′-terminal regions of the genomes of apple stem grooving virus (ASGV), isolated from apple, Japanese pear, and European pear plants, and citrus tatter leaf virus (CTLV), isolated from citrus plants, were amplified by reverse transcription-polymerase chain reaction. The DNA products were cloned and sequenced. The results indicated that the ASGV isolates from apple, Japanese pear, and European pear comprise at least two to four “sequence variants” that differ considerably from each other in nucleotide sequence. Comparison of the amino acid (aa) sequences of the open reading frame (ORF) 1- and 2-encoded proteins among isolates or sequence variants, including isolates of ASGV and CTLV reported previously, showed that the aa sequences of the ORF2-encoded protein (320 aa) and the putative coat protein (CP) (237 aa), which was equivalent to aa positions 1869 to 2105 of the ORF1-encoded protein of ASGV (P-209), were highly conserved among isolates and sequence variants. Identities ranged from 92.8 to 100% for the ORF2-encoded protein among 15 isolates and sequence variants and from 92.4 to 100% for the CP among 21 isolates and sequence variants. On the other hand, the aa sequence identities of a variable 284-aa region (designated the V-region), equivalent to aa positions 1585 to 1868 of the ORF1-encoded protein (which also encoded the ORF2-encoded protein in another frame), were highly variable (53.2 to 99.3%) among 15 isolates and sequence variants. The percentage of conserved V-region aa in all 15 isolates was only 20.4%. Phylogenetic trees constructed from the aa sequences of the V-region, coat, and ORF2-encoded proteins showed that isolates and sequence variants were separated into several clusters, regardless of the source host plant species (apple, Japanese pear, European pear, citrus, and lily). In a tree constructed for the V-region proteins, two CTLV isolates were grouped into two clusters, each of which contained isolates and sequence variants from apple and Japanese pear. These results suggest that both CTLV from citrus and ASGV from Rosaceous plants should be considered isolates of ASGV.
Effects of Moderate Water Stress on Disease Development by Sphaeropsis sapinea on Red Pine
Authors: Blodgett J. T. , Kruger E. L. , Stanosz G. R.
Published: May 2007

The aggressiveness of Sphaeropsis sapinea isolates was compared on water-stressed and nonstressed 3-year-old red pines (Pinus resinosa) in greenhouse and growth chamber experiments. Water was withheld from stressed seedlings to achieve mean predawn needle water potentials (ψPD) above −1.9 MPa. The lowest mean ψPD of well-watered seedlings was maintained at or above −0.8 MPa. Young shoots were inoculated by placing colonized agar plugs on wounds made by removing a needle fascicle. Two isolates of each recognized morphotype (A and B) were used in the greenhouse experiment and two isolates of morphotype A were used in the growth chamber experiment. After 4 weeks, isolates of morphotype A caused more severe symptoms and could be recovered farther from the inoculation site on water-stressed than on nonstressed trees in both experiments. In the greenhouse experiment, isolates of mor-photype A also caused more severe symptoms and could be recovered farther from the inoculation site than isolates of morphotype B, regardless of watering regime. These results indicate that water stress at levels observed typically in the field can result in increased disease development by isolates of S. sapinea morphotype A on red pine. The reduction of water stress of red pines in the field may reduce losses due to Sphaeropsis shoot blight.
A Study on the Phylogeny of the Dyer's Woad Rust Fungus and Other Species of Puccinia from Crucifers
Authors: Kropp Bradley R. , Hansen Dane R. , Wolf Paul G. , Flint Karen M. , Thomson Sherman V.
Published: May 2007

The identity of a Puccinia species occurring on the introduced weed dyer's woad (Isatis tinctoria) was studied using sequences from the internal transcribed spacer of the nuclear ribosomal DNA. The relationship of this fungus to other Puccinia species occurring on the family Brassicaceae in Europe and North America was examined, and we tested the hypothesis that P. thlaspeos and P. monoica are correlated species. The data suggest that the Puccinia species from dyer's woad is closely related to the North American species P. consimilis and may be derived from an indigenous strain of P. consimilis that switched hosts. Thus, the Puccinia species from dyer's woad is probably native to North America and is unlikely to cause disease epidemics on indigenous plants if used as a biological control agent against dyer's woad. P. thlaspeos appears to be polyphyletic and, therefore, P. thlaspeos and P. monoica do not appear to be correlated species. Additional DNA sequence data will be needed to clarify further the phylogeny of Puccinia species on the family Brassicaceae.
Sporulation of Fusarium oxysporum f. sp. lycopersici on Stem Surfaces of Tomato Plants and Aerial Dissemination of Inoculum
Authors: Katan Talma , Shlevin E. , Katan J.
Published: May 2007

Plants exhibiting symptoms of wilt and xylem discoloration typical of Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici were observed in greenhouses of cherry tomatoes at various sites in Israel. However, the lower stems of some of these plants were covered with a pink layer of macroconidia of F. oxysporum. This sign resembles the sporulating layer on stems of tomato plants infected with F. oxysporum f. sp. radicis-lycopersici, which causes the crown and root rot disease. Monoconidial isolates of F. oxysporum from diseased plants were assigned to vegetative compatibility group 0030 of F. oxysporum f. sp. lycopersici and identified as belonging to race 1 of F. oxysporum f. sp. lycopersici. The possibility of coinfection with F. oxysporum f. sp. lycopersici and F. oxysporum f. sp. radicis-lycopersici was excluded by testing several macroconidia from each plant. Airborne propagules of F. oxysporum f. sp. lycopersici were trapped on selective medium in greenhouses in which plants with a sporulating layer had been growing. Sporulation on stems was reproduced by inoculating tomato plants with races 1 and 2 of F. oxysporum f. sp. lycopersici. This phenomenon has not been reported previously with F. oxysporum f. sp. lycopersici and might be connected to specific environmental conditions, e.g., high humidity. The sporulation of F. oxysporum f. sp. lycopersici on plant stems and the resultant aerial dissemination of macroconidia may have serious epidemiological consequences. Sanitation of the greenhouse structure, as part of a holistic disease management approach, is necessary to ensure effective disease control.
Genotypic Diversity of Beet Curly Top Virus Populations in the Western United States
Authors: Stenger Drake C. , McMahon Carrie L.
Published: May 2007

The genotypic diversity of beet curly top virus (BCTV) present in the western United States has been examined by the analysis of 58 field isolates and eight laboratory or nursery isolates of the virus. Full-length clones for each isolate have been characterized for genotype by restriction endonuclease mapping. The results indicate that most of the genotypes examined may be classified as variants of the CFH, Worland, or Cal/Logan strains of BCTV. Two genotypes were recovered that appear to share certain genotypic markers of both Worland and CFH strains. Genotypic variants of the CFH and Worland strains and the two genotypes sharing markers of both strains were recovered from field isolates collected during 1994 and 1995. In contrast, the Cal/Logan strain was recovered only from isolates maintained in laboratories or nurseries. Comparisons of restriction endonuclease maps of cloned BCTV genomes revealed considerable variability both within and between strains. Although a total of 43 distinct genotypes of BCTV were identified, only 36 (84%) were recovered from field isolates. Of 37 field isolates for which more than a single clone was recovered, 16 (43%) contained more than a single genotype of one strain, whereas 4 (11%) harbored mixed infections of the CFH and Worland strains. A phylogenetic analysis using 43 characters derived from restriction endonuclease mapping data supported the grouping of 41 genotypes into three taxa consistent with the three currently recognized strains of BCTV. The relationships of the two genotypes sharing genotypic markers of both the Worland and CFH strains to other BCTV genotypes was unresolved in the phylogenetic analysis. Based on the mild symptom phenotype of the isolates from which these two genotypes were recovered and the presence of Worland genotypic markers in portions of the genome containing both cis- and trans-acting elements determining replication specificity, these two genotypes were tentatively considered as variants of the Worland strain.
Antifungal Activity of 2-Deoxy-D-Glucose on Botrytis cinerea, Penicillium expansum, and Rhizopus stolonifer: Ultrastructural and Cytochemical Aspects
Authors: El-Ghaouth Ahmed , Wilson Charles L. , Wisniewski Michael
Published: May 2007

The effect of 2-deoxy-D-glucose on major postharvest pathogens was investigated at the ultrastructural and cytochemical level. Hyphae of Botrytis cinerea, Penicillium expansum,, and Rhizopus stolonifer grown in the absence of 2-deoxy-D-glucose were normal and showed no apparent cytological alterations. In the presence of 2-deoxy-D-glucose, however, these fungi exhibited severe cellular injuries ranging from cell wall disruption to cytoplasm disintegration. Although 2-deoxy-D-glucose caused cytoplasmic degeneration in the three fungi tested, cell wall alterations were exhibited only by B. cinerea and R. stolonifer. In the latter, the retraction of degenerated cytoplasm was often accompanied by the deposition of amorphous material in paramural spaces. Cytochemical study of fungal cell wall components showed that 2-deoxy-D-glucose caused a marked increase of chitin- and β-1,3-glucan—labeling in R. stolonifer and B. cinerea, indicating an interference of 2-deoxy-D-glucose with fungal wall biosynthesis. The observed cellular alterations indicate that 2-deoxy-D-glucose may also have affected other metabolic processes.
Distributions of Sensitivities to Three Sterol Demethylation Inhibitor Fungicides Among Populations ofUncinula necatorSensitive and Resistant to Triadimefon
Authors: Erickson Eugene O. , Wilcox Wayne F.
Published: May 2007

Single-conidial isolates of Uncinula necator from (i) a population representing two vineyards with no previous exposure to sterol demethylation inhibitor (DMI) fungicides (“unexposed,” n = 77) and (ii) a population representing two vineyards in which powdery mildew was poorly controlled by triadimefon after prolonged DMI use (“selected,” n = 82) were assayed to determine distributions of sensitivities to the DMI fungicides triadimenol (the active form of triadimefon), myclobutanil, and fenarimol. Median 50% effective dose (ED50) values (micrograms per milliliter) in the selected versus unexposed populations were 0.06 versus 1.9 for triadimenol, 0.03 versus 0.23 for myclobutanil, and 0.03 versus 0.07 for fenarimol, respectively. Isolates were grouped into sensitivity classes according to their ED50values, and those from the selected population were categorized as resistant if the frequency of their sensitivity class had increased significantly relative to levels found in the unexposed population (ED50values exceeding 0.56, 0.18, and 0.18 μg/ml for triadimenol, myclobutanil, and fenarimol, respectively). Of the 76 isolates defined as resistant to triadimenol, 64% were classified as cross-resistant to myclobutanil, 18% were classified as cross-resistant to fenarimol, and 17% were classified as resistant to all three fungicides; 25% of the isolates classified as resistant to myclobutanil also were classified as resistant to fenarimol. Similar cross-resistance relationships were revealed when all isolates were examined by regressing log ED50values for each fungicide against those for the remaining two fungicides to determine the correlation coefficients (e.g., r = 0.85 for triadimenol versus myclobutanil and 0.56 for triadimenol versus fenarimol). The restricted levels of cross-resistance indicated by these data, particularly between fenarimol and the other two fungicides, is in sharp contrast to the high levels of cross-resistance among DMIs reported for some other pathogens and has significant implications with respect to programs for managing grapevine powdery mildew and DMI resistance.
Durability of Resistance to Globodera pallida I. Changes in Pathogenicity, Virulence, and Aggressiveness During Reproduction on Partially Resistant Potato Cultivars
Authors: Schouten Henk J. , Beniers Johanna E.
Published: May 2007

We studied changes in pathogenicity, virulence, and aggressiveness of Globodera pallida populations over time. As a measure for pathogenicity, the reproduction factor on a partially resistant host was used; for aggressiveness, the reproduction factor on a susceptible reference host was used; and, for virulence, the ratio pathogenicity/aggressiveness was used. The G. pallida populations were reared in a glasshouse for four generations on potato cultivars with different levels of resistance. The cultivar Elkana did not increase pathogenicity significantly, but the more resistant cultivars Karakter and Darwina did. This increase in pathogenicity was caused by an increase in virulence, whereas aggressiveness generally was not altered significantly. The increase in virulence appeared to be caused by an enhanced ability of eggs to develop into cysts, and not by an increase in egg production per new cyst. The observed changes in virulence could be predicted reasonably well by a simple numerical model. The rate of selection depended strongly on the nematode population. Rearing a mix of two different populations on a susceptible host decreased the virulence strongly, as predicted by the Hardy-Weinberg equilibrium, but increased the aggressiveness because of heterosis.
Comparison of Lettuce Diseases and Yield Under Subsurface Drip and Furrow Irrigation
Authors: Subbarao K. V. , Hubbard J. C. , Schulbach K. F.
Published: May 2007

Subsurface drip and furrow irrigation were compared on lettuce (Lactuca sativa) cvs. Salinas and Misty Day for yield and incidence and severity of three important diseases of lettuce in the Salinas Valley, CA. Experiments were conducted between 1993 and 1995 during the spring and fall seasons. The diseases examined included lettuce drop (Sclerotinia minor), downy mildew (Bremia lactucae), and corky root (Rhizomonas suberifaciens). Replicated plots of subsurface drip and furrow irrigation were arranged in a randomized complete-block design. All plants were inoculated with S. minor at the initiation of the experiment during the 1993 spring season. Plots were not inoculated for downy mildew and corky root during any season nor were the plots reinoculated with S. minor. During each season, all plots were sprinkler irrigated until thinning, and subsequently, the irrigation treatments were begun. The furrow plots were irrigated once per week, and the drip plots received water twice per week. The distribution of soil moisture at two soil depths (0 to 5 and 6 to 15 cm) at 5, 10, and 15 cm distance on either side of the bed center in two diagonal directions was significantly lower in drip-irrigated compared with furrow-irrigated plots. Plots were evaluated for lettuce drop incidence and downy mildew incidence and severity at weekly intervals until harvest. Corky root severity and yield components were determined at maturity. Lettuce drop incidence and corky root severity were significantly lower and yields were higher in plots under subsurface drip irrigation compared with furrow irrigation, regardless of the cultivar, except during the 1994 fall season. Incidence and severity of downy mildew were not significantly different between the two irrigation methods throughout the study. The differential microclimates created by the two irrigation treatments did not affect downy mildew infection, presumably because the mesoclimate is usually favorable in the Salinas Valley. Subsurface drip irrigation is a viable, long-term strategy for soilborne disease management in lettuce in the Salinas Valley.
Where Are All the Undescribed Fungi?
Authors: Hawksworth David L. , Rossman Amy Y.
Published: May 2007

The hypothesis that there are 1.5 million fungal species on Earth, of which only about 70,000 are described, implies that 1.43 million remain undescribed. The recognition that many new species have yet to be found is of fundamental importance to plant pathologists, agronomists, and plant regulatory officials, among others, who continue to encounter diseases caused by previously unknown or understudied fungi. Unexplored habitats with their arsenal of unknown fungi are also of interest to those searching for novel organisms for use in biological control or for their pharmaceutical attributes. This paper presents data on the expected numbers of fungi in some relatively unexplored habitats, such as tropical forests, and those obligately associated with plants, lichens, and insects. In addition to undiscovered species, many have been collected but remain lost or hidden as named species and ignored for lack of modern characterization; others have been collected and recognized as new species but remain undescribed. Some fungal species are unrecognized within erroneously circumscribed species, often based on presumed host specificity, while others exist as biological species but remain buried within those broadly defined species for lack of gross morphological characterization. From these data, one must conclude that enormous numbers of unrecognized fungi can be found almost everywhere, including one's own backyard.
Effects of Leaf Wetness Duration and Temperature on Infection Efficiency, Latent Period, and Rate of Pustule Appearance of Rust in Alfalfa
Authors: Webb D. H. , Nutter F. W.
Published: May 2007

Dew and growth chamber tests were conducted on the alfalfa cultivar Ranger to determine the effect of duration of leaf wetness and temperature on several components of the alfalfa rust (Uromyces striatus) monocycle. Duration of leaf wetness and temperature both had significant effects on pustule development. Infection efficiency (number of alfalfa rust pustules per leaf) increased linearly as duration of leaf wetness was increased from 4 to 24 h after inoculation. There was an inverse linear relationship between temperature and infection efficiency as indicated by the slope (-5.73) of the regression line relating the number of pustules per leaf to increasing temperatures between 17.5 and 28°C. Infection efficiency was approximately 20 times greater at 17.5°C than at 28°C. Inoculated alfalfa plants exposed to constant temperatures of 15, 18, 21, 24, 27, or 30°C after an initial 24-h leaf wetness period (19°C) did not significantly affect infection efficiency (P ≤ 0.05), but did affect the time (from inoculation to the time when 50% of the pustules (T50) were visible (i.e., latent period). Using this state variable definition, latent period was found to decrease with increasing temperature. When latent period was measured as a rate variable, the rate of pustule appearance (as affected by temperature) was best described by the Gompertz model. Thus, temperature and length of the initial leaf wetness period had a greater impact on infection efficiency than did postinfection (post-leaf wetness) temperatures, whereas increasing temperatures from 15 to 30°C had a significant effect on shortening the latent period (T50) and increasing the rate of pustule appearance.
Rhizosphere Competence of Pythium oligandrum
Authors: Al-Rawahi A. K. , Hancock J. G.
Published: May 2007

The associations of Pythium oligandrum with the root cortex, rhizoplane, and rhizosphere were measured with 11 crop species. This work was expedited by the use of a semiselective technique for isolation of P. oligandrum from soil and plant material. Cortical colonization of roots by P. oligandrum was not detected, and the rhizoplanes of the roots of most crops were free of the fungus. However, P. oligandrum was detected in large quantities with every crop tested when roots with adhering soil (rhizosphere soil) were assayed. Different crop species and cultivars of cantaloupe, cauliflower, and tomato varied in rhizosphere densities of P. oligandrum, but rhizosphere population densities of the fungus were consistently higher than in nonrhizosphere soils with plants grown in P. oligandrum-infested sterilized potting mix or an unsterilized mineral soil. After transplanting tomatoes into potting mix infested with P. oligandrum, increases in CFU occurred over time in the rhizosphere but not in the nonrhizosphere soil. In trials on delivery methods of inoculum of P. oligandrum, the rhizosphere populations of tomato plants grown in potting mix were about sixfold higher compared to seed-coat treatments when ground, alginate pelleted oospores were applied to seedlings growing in plug containers prior to transplanting or to pots containing potting mix before direct seeding.
Extreme Reduction of Disease in Oats Transformed with the 5′ Half of the Barley Yellow Dwarf Virus-PAV Genome
Authors: Koev Gennadiy , Mohan B. R. , Dinesh-Kumar S. P. , Torbert Kimberly A. , Somers David A. , Miller W. Allen
Published: May 2007

Barley yellow dwarf viruses (BYDVs) are the most serious and widespread viruses of oats, barley, and wheat worldwide. Natural resistance is inadequate. Toward overcoming this limitation, we engineered virus-derived transgenic resistance in oat. Oat plants were transformed with the 5′ half of the BYDV strain PAV genome, which includes the RNA-dependent RNA polymerase gene. In experiments on T2- and T3-generation plants descended from the same transformation event, all BYDV-inoculated plants containing the transgene showed disease symptoms initially, but recovered, flowered, and produced seed. In contrast, all but one of the BYDV-PAV-inoculated nontransgenic segregants died before reaching 25 cm in height. Although all of the recovered transgenic plants looked similar, the amount of virus and viral RNA ranged from substantial to undetectable levels. Thus, the transgene may act either by restricting virus accumulation or by a novel transgenic tolerance phenomenon. This work demonstrates a strategy for genetically stable transgenic resistance to BYDVs that should apply to all hosts of the virus.
The Xylanolytic System of Claviceps purpurea: Cytological Evidence for Secretion of Xylanases in Infected Rye Tissue and Molecular Characterization of Two Xylanase Genes
Authors: Giesbert Sabine , Lepping Heinz-Bernd , Tenberge Klaus B. , Tudzynski Paul
Published: May 2007

Claviceps purpurea is a common phytopathogenic fungus that colonizes ovarian tissue of grasses. A concerted approach involving cytological and molecular techniques was initiated to investigate the role of the fungus' xylanolytic system in the interaction. Using enzyme-gold and immuno-gold electron-microscopic techniques, the presence of arabinoxylans in cell walls of rye ovarian tissues (i.e., along the usual path of infection of C. purpurea) was confirmed; tissue-print and immunostaining analyses indicated the presence of xylanase(s) exclusively in ovaries infected with C. purpurea. This strongly suggests that C. purpurea secretes xylanase while colonizing its host. Two xylanase genes (cpxyl1 and cpxyl2) were isolated from a genomic library of C. purpurea using genes from Cochliobolus carbonum (xyl1) and Magnaporthe grisea (xyn33) as heterologous probes. cpxyl1 of C. purpurea had an open reading frame (ORF) of 832 bp interrupted by a 181-bp intron. The derived gene product (CPXYL1) had a molecular mass of 21.5 kDa and an pI of 8.88; it showed significant homology to family G endo-β-1,4-xylanases. The cpxyl2 ORF (1,144 bp) contained two introns (76 and 90 bp) and coded for a polypeptide of 33.8 kDa with an pI of 7.01; CPXYL2 belonged to family F xylanases. Southern analyses with genomic DNA demonstrated that both genes were single-copy genes. Using reverse transcription polymerase chain reaction, it could be shown that both genes were expressed in vitro and in planta (during all infection stages). Inactivation of cpxyl1 was achieved by a gene-replacement approach. The mutant strain (Δcpxyl1) had significantly reduced xylanase activity; Western analyses confirmed that it lacked a polypeptide of approximately 23 kDa.
The Importance of Archival and Herbarium Materials in Understanding the Role of Oospores in Late Blight Epidemics of the Past
Authors: Ristaino Jean B.
Published: May 2007

Nineteenth and early twentieth century botanists and mycologists collected healthy and infected plant materials from many regions of the world. Some of these plant collections preserved in herbaria around the world contain samples that are of considerable significance to epidemiologists, population biologists, and botanists. The advent of the polymerase chain reaction (PCR) and the development of molecular marker technology has made DNA amplification from herbarium material a reality. In this mini-review, archival letters and herbarium samples are used to track the historical role of oospores in the biology of the potato late blight pathogen. DNA was successfully amplified by PCR with the Phytophthora infestans-specific PCR primer, PINF, and the universal primer, ITS 5, from oospores observed in a field sample of potato collected by G. P. Clinton in 1902. This experiment demonstrates the potential to utilize molecular methods to amplify DNA from historical samples of the late blight pathogen and represents the earliest definitive record of oospores of the pathogen in field samples in the United States. Studies based upon such materials and techniques, although high risk and laborious, have the potential to open a new window to epidemics of the past.
Fine-Level Genetic Structure of White Pine Blister Rust Populations
Authors: Hamelin Richard C. , Dusabenyagasani Mathieu , Et-touil Khalid
Published: May 2007

The fine-level genetic structure of the white pine blister rust agent, Cronartium ribicola, was investigated by sampling multiple monokaryotic spermogonia directly on cankers in four eastern Canadian white pine (Pinus strobus) plantations and assessing genetic variability, using random amplified polymorphic DNA (RAPD) markers. Ninety-eight percent of the cankers surveyed contained a single DNA haplotype, suggesting spermogonia within cankers are the result of clonal reproduction. A single canker contained two haplotypes that were divided between the upper and lower parts of the canker, suggesting it represented two confluent cankers. In contrast, genotypic diversity was high among cankers. Thirty-seven haplotypes were found among forty-three cankers sampled, and an analysis of molecular variance indicated that 93% (P < 0.001) of the total genetic diversity was attributable to sampling of different cankers, strongly suggesting that multiple infections do not take place in the white pine blister rust pathosystem, i.e., a canker is the result of infection by a single genotype. This result is in contrast with the high level of genetic diversity previously reported among dikaryotic aecidia within cankers and is consistent with the hypothesis that variability in the aecidial stage is the result of outcrossing between resident spermogonia and alien spermatia. The genetic structure of the spermogonial stage, which is the vegetative extension of infection by basidiospores and, therefore, the indirect result of meiosis, was consistent with random mating; the observed genotypic diversity was not significantly different (P > 0.05) from the genotypic diversity expected under the assumption of panmixis. The results indicate that monokaryotic cankers can be genotyped by sampling a single unopened spermogonia per canker.
Variability Among Turnip Mosaic Potyvirus Isolates
Authors: Stavolone Livia , Alioto Daniela , Ragozzino Antonio , Laliberté Jean-François
Published: May 2007

Eight turnip mosaic potyvirus (TuMV) isolates from the Campania region of Italy were characterized. Experiments based on host range and symptomatology indicated that the isolates were biologically different. In addition, the isolates, with the exception of ITA1 and ITA3, were distinguished from each other by using a combination of monoclonal antibodies recognizing the coat protein. Single-strand conformation polymorphism (SSCP) analysis of the coat protein gene revealed that each isolate produced a specific SSCP profile, except for isolates ITA1 and ITA3. This study indicates that (i) even in a small geographical region, there is a great deal of variation in TuMV isolates; (ii) the use of a set of four differential hosts does not always specify the same pathotype in different environments; (iii) the TuMV isolates with the same pathotype on Brassica napus test lines can still differ in host range, symptoms, serology, and SSCP; and (iv) there was perfect correlation between the panel of antibodies and SSCP in differentiating among the isolates; ITA1 and ITA3 were indistinguishable by either assay.
Conidia as a Substrate for Internal Transcribed Spacer-Based PCR Identification of Members of the Leptosphaeria maculans Species Complex
Authors: Balesdent M. H. , Jedryczka M. , Jain L. , Mendes-Pereira E. , Bertrandy J. , Rouxel T.
Published: May 2007

The blackleg disease of oilseed rape is caused by an ascomycete species complex termed Leptosphaeria maculans (anamorph Phoma lingam). L. maculans isolates collected worldwide were gathered in the International Blackleg of Crucifers Network (IBCN) collection. Representative IBCN isolates, along with one P. nigrificans isolate, were further analyzed using polymerase chain reaction (PCR) amplification of the internal transcribed spacer (ITS) region. ITS size polymorphism discriminated three groups: (i) P. nigrificans, (ii) Tox+ and ‘Lepidium’ isolates, and (iii) NA1, NA2, NA3, ‘Thlaspi’, and ‘Erysimum’ isolates. Digestion of the ITS region with 19 selected endonucleases showed restriction site polymorphism between the different subgroups: digestion with RsaI could discriminate Tox+ from ‘Lepidium’ isolates, whereas digestion with four enzymes, i.e., HaeIII, EcoRII, RsaI, and AluI, was needed to discriminate between NA1, NA2, NA3, ‘Thlaspi’, and ‘Erysimum’ isolates. No restriction site polymorphism was observed between isolates within the ‘Thlaspi’, Tox+, NA1, and NA2 subgroups. Direct amplification of the ITS region could be achieved using intact conidia, collected either in axenic cultures or on leaf lesions, with only a 4-min 95°C denaturation step prior to PCR reaction. A routine identification protocol requiring no DNA extraction and a sequential use of a few restriction enzymes following PCR has been used successfully for large-scale identification of French field isolates.
Insect-Mediated Dispersal of the Rhizobacterium Pseudomonas chlororaphis
Authors: Snyder William E. , Tonkyn David W. , Kluepfel Daniel A.
Published: May 2007

The southern corn rootworm, Diabrotica undecimpunctata subsp. howardi, a common and mobile insect pest, was shown to transmit the rhizobacte-rium Pseudomonas chlororaphis strain L11 between corn plants. Strain L11 has been genetically modified to contain the lacZY genes from Escherichia coli. It can reach high densities on roots and invade the roots and move into the foliage. D. undecimpunctata subsp. howardi became infested with L11 as larvae while feeding on roots of seed-inoculated corn and retained the bacteria through pupation, molting to the adult stage, and emergence from the soil. Bacterial densities on or in the insects increased 100-fold after they fed again as adults on L11-infested foliage. Adults retained the bacteria for at least 2 weeks after last exposure and could transmit L11 to new plants. The likelihood of transmission decreased with time since last exposure to L11, but increased with time spent on the new plants. This research demonstrates that rhizobacteria can escape the rhizosphere by moving in or onto foliage, where they can then be acquired and transmitted by insects. This transmission route may be common among naturally occurring rhizobacteria and facilitate the dispersal of both beneficial and harmful soilborne microorganisms.
Sites of Infection by Pythium Species in Rice Seedlings and Effects of Plant Age and Water Depth on Disease Development
Authors: Chun S.-C. , Schneider R. W.
Published: May 2007

Seedling disease, caused primarily by several species of Pythium, is one of the major constraints to water-seeded rice production in Louisiana. The disease, also known as water-mold disease, seed rot, and seedling damping-off, causes stand reductions and growth abnormalities. In severe cases, fields must be replanted, which may result in delayed harvests and reduced yields. To develop more effective disease management tactics including biological control, this study was conducted primarily to determine sites of infection in seeds and seedlings; effect of plant age on susceptibility to P. arrhenomanes, P. myriotylum, and P. dissotocum; and minimum exposure times required for infection and seedling death. In addition, the effect of water depth on seedling disease was investigated. Infection rates of seed embryos were significantly higher than those of endosperms for all three Pythium spp. The development of roots from dry-seeded seedlings was significantly reduced by P. arrhenomanes and P. myriotylum at 5 days after planting compared with that of roots from noninoculated controls. Susceptibility of rice to all three species was sharply reduced within 2 to 6 days after planting, and seedlings were completely resistant at 8 days after planting. There was a steep reduction in emergence through the flood water, relative to the noninoculated control, following 2 to 3 days of exposure to inoculum of P. arrhenomanes and P. myriotylum. In contrast, P. dissotocum was much less virulent and required longer exposure times to cause irreversible seedling damage. Disease incidence was higher when seeds were planted into deeper water, implying that seedlings become resistant after they emerge through the flood water. These results suggest that disease control tactics including flood water management need to be employed for a very short period of time after planting. Also, given that the embryo is the primary site of infection and it is susceptible for only a few days, the disease should be amenable to biological control.
Distribution and Diversity of Geminiviruses in Trinidad and Tobago
Authors: Umaharan Pathmanathan , Padidam Malla , Phelps Ralph H. , Beachy Roger N. , Fauquet Claude M.
Published: May 2007

Seven crop and eight weed species from 12 agricultural locations in Trinidad and Tobago were assayed for the presence of whitefly-transmitted geminiviruses (WTGs) by using dot blot hybridization and polymerase chain reaction (PCR) amplification of the N-terminal coat protein sequence with degenerate primers. The amplified fragments were cloned and analyzed by restriction enzyme digestion to determine fragment length polymorphism among the cloned fragments. Representative clones were then sequenced and subjected to phylogenetic analysis to determine the sequence similarity to known WTGs. WTGs were found in every location sampled and in 10 of the 15 species investigated: Lycopersicon esculentum(tomato), Capsicum annuum (pepper), Capsicum frutescens (sweet pepper), Abelmoschus esculentus (okra), Phaseolus vulgaris (beans), Alternanthera tenella, Desmodium frutescens, Euphorbia heterophylla, Malva alceifolia, and Sida acuta. The geminiviruses infecting these plants were closely related to potato yellow mosaic virus from Venezuela (PYMV-VE) and tomato leaf curl virus from Panama (ToLCV-PA). However, in pepper, sweet pepper, okra, Alternanthera tenella, Euphorbia heterophylla, Des-modium frutescens, and in one sample of tomato, a PYMV-VE-related virus was found in mixed infections with a virus related to pepper huasteco virus. Full-length infectious DNA-A and DNA-B of a tomato-infecting geminivirus from Trinidad and Tobago were cloned and sequenced. DNA-A appears to be a recombinant derived from PYMV-VE or ToLCV-PA, and Sida golden mosaic from Honduras. The implications of these findings in the control of WTGs are discussed.
Molecular and Serological Characterization of Iris Yellow Spot Virus, a New and Distinct Tospovirus Species
Authors: Cortês I. , Livieratos I. C. , Derks A. , Peters D. , Kormelink R.
Published: May 2007

A new tospovirus was identified in iris cultivations in the Netherlands. Both serological comparisons and sequence determination of the S RNA demonstrate that this virus represents a new and distinct species, belonging to a separate serogroup, and for which the name iris yellow spot virus (IYSV) is proposed. The disease symptoms on iris are characterized by yellow spots on the leaves. Its experimental host range is very narrow and, in addition to iris, only includes Nicotiana benthamiana and Datura stramonium. The nucleoprotein of IYSV shows only 30 to 44% sequence identity with those of other tospoviruses identified so far; the highest homology being found with the tospovirus species of serogroup IV.
Loss of Production of the Elicitor Protein INF1 in the Clonal Lineage US-1 of Phytophthora infestans
Authors: Kamoun Sophien , van der Lee Theo , van den Berg-Velthuis Grardy , de Groot Koen E. , Govers Francine
Published: May 2007

The extracellular protein INF1 of Phytophthora infestans is a member of the elicitin family of protein elicitors known to induce a hypersensitive response on some solanaceous and cruciferous plants. The presence of INF1 elicitin in culture filtrates of 102 P. infestans isolates from 15 countries was examined. All tested isolates produced INF1 except five isolates collected in 1976 and 1977 from infected potatoes in East Germany (the former German Democratic Republic). Based on hybridization to the multi-locus DNA fingerprint probe RG57, all the INF1-nonproducing isolates were shown to belong to the clonal lineage US-1 that dominated world populations until the 1980s. Phylogenetic analysis of a set of European US-1 isolates using amplified fragment length polymorphism fingerprint data indicated that loss of INF1 production evolved independently in separate lineages within US-1. DNA and RNA blot hybridizations showed that INF1-nonproducing isolates still retain a copy of the inf1 gene, whereas little inf1 mRNA could be detected. Hypothetical interpretations of the evolution in a restricted geographic area of P. infestans lineages deficient in the production of a specific elicitor protein are discussed.
Fungal Sensitivity to and Enzymatic Degradation of the Phytoanticipin α-Tomatine
Authors: Sandrock Robert W. , VanEtten Hans D.
Published: May 2007

α-Tomatine, synthesized by Lycopersicon and some Solanum species, is toxic to a broad range of fungi, presumably because it binds to 3β-hydroxy sterols in fungal membranes. Several fungal pathogens of tomato have previously been shown to be tolerant of this glycoalkaloid and to possess enzymes thought to be involved in its detoxification. In the current study, 23 fungal strains were examined for their ability to degrade α-tomatine and for their sensitivity to this compound and two breakdown products, β2-tomatine and tomatidine. Both saprophytes and all five non-pathogens of tomato tested were sensitive, while all but two tomato pathogens (Stemphylium solani and Verticillium dahliae) were tolerant of α-to-matine (50% effective dose > 300 μM). Except for an isolate of Botrytis cinerea isolated from grape, no degradation products were detected when saprophytes and nonpathogens were grown in the presence of α-tomatine. All tomato pathogens except Phytophthora infestans and Pythium aphani-dermatum degraded α-tomatine. There was a strong correlation between tolerance to α-tomatine, the ability to degrade this compound, and pathogenicity on tomato. However, while β2-tomatine and tomatidine were less toxic to most tomato pathogens, these breakdown products were inhibitory to some of the saprophytes and nonpathogens of tomato, suggesting that tomato pathogens may have multiple tolerance mechanisms to α-tomatine.