bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Downy Mildew Development on Transpiration of Cucumber Leaves Visualized by Digital Infrared Thermography
Authors: Lindenthal Miriam , Steiner Ulrike , Dehne H.-W. , Oerke E.-C.
Published: May 2007

Disease progress of downy mildew on cucumber leaves, caused by the obligate biotrophic pathogen Pseudoperonospora cubensis, was shown to be associated with various changes in transpiration depending on the stage of pathogenesis. Spatial and temporal changes in the transpiration rate of infected and noninfected cucumber leaves were visualized by digital infrared thermography in combination with measurements of gas exchange as well as microscopic observations of pathogen growth within plant tissue and stomatal aperture during pathogenesis. Transpiration of cucumber leaf tissue was correlated to leaf temperature in a negative linear manner (r = -0.762, P < 0.001, n = 18). Leaf areas colonized by Pseudoperonospora cubensis exhibited a presymptomatic decrease in leaf tem perature up to 0.8°C lower than noninfected tissue due to abnormal stomata opening. The appearance of chlorosis was associated with a cooling effect caused by the loss of integrity of cell membranes leading to a larger amount of apoplastic water in infected tissue. Increased water loss from damaged cells and the inability of infected plant tissue to regulate stomatal opening promoted cell death and desiccation of dying tissue. Ultimately, the lack of natural cooling from necrotic tissue was associated with an increase in leaf temperature. These changes in leaf temperature during downy mildew development resulted in a considerable heterogeneity in temperature distribution of infected leaves. The maximum temperature difference within a thermogram of cucumber leaves allowed the discrimination between healthy and infected leaves before visible symptoms appeared.
Epidemiology of Foliar Blights (Spot Blotch and Tan Spot) of Wheat in the Plains Bordering the Himalayas
Authors: Duveiller E. , Kandel Y. R. , Sharma R. C. , Shrestha S. M.
Published: May 2007

Helminthosporium leaf blight (HLB), a complex of spot blotch caused by Cochliobolus sativus and of tan spot caused by Pyrenophora tritici-repentis, is a major wheat disease in South Asia. This 2-year study elucidated HLB development and its impact on yield. Symptoms caused by C. sativus and P. tritici-repentis were first observed at the seedling and tillering stages, respectively. The number of airborne conidia and leaves infected by the two pathogens remained low for several weeks under lower temperatures, followed by a sharp rise as temperatures increased. The number of airborne conidia of C. sativus and incidence of infection by C. sativus were higher compared with P. tritici-repentis. The disease complex caused an average 30% reduction in yield, with greater losses under delayed seeding. Delayed seeding increased disease severity even in resistant genotypes and caused higher yield losses. ‘Milan/Shanghai-7’ was the most resistant among six genotypes evaluated. Despite higher disease severity, ‘BL 1473’ showed relatively lower yield losses, indicating its tolerance to foliar blight. The findings of this study bear implications for integrated foliar blight management in the warmer areas of South Asia by combining optimum seeding date, seed treatment and foliar spray of fungicides, and resistant wheat genotypes.
Vegetative Compatibility Groups in Cercospora kikuchii, the Causal Agent of Cercospora Leaf Blight and Purple Seed Stain in Soybean
Authors: Cai G. , Schneider R. W.
Published: May 2007

Nitrogen nonutilizing (Nit) mutants were used to assess vegetative compatibility of 58 isolates of Cercospora kikuchii, 55 of which were isolated from soybean plants in Louisiana. Two isolates were vegetatively self-incompatible. Of 56 self-compatible isolates, 16 were assigned to six multimember vegetative compatibility groups (VCGs), 01 to 06, with 2 or 3 isolates in each VCG. The other 40 isolates each belonged to a distinct VCG. All six multimember VCGs contained isolates from different soy bean cultivars, and three included isolates from different locations. Only one of six multimember VCGs included isolates both from soybean leaves and seed, while the other five included isolates from only leaves or seed. The likelihood of tissue specificity or preference was discussed. All isolates and tested Nit mutants produced cercosporin on potato dextrose agar under light. Significantly different amounts of cercosporin were produced among wild-type isolates, and two Nit mutants produced significantly more cercosporin than their wild-type counterparts. All isolates produced typical Cercospora leaf blight symptoms on soybean plants in greenhouse pathogenicity tests.
Pathogenicity, Internal Transcribed Spacer-rDNA Variation, and Human Dispersal of Ceratocystis fimbriata on the Family Araceae
Authors: Thorpe Daniel J. , Harrington Thomas C. , Uchida Janice Y.
Published: May 2007

Ceratocystis fimbriata is a complex of many cryptic, host-specialized species that causes wilt and canker of woody species and rot diseases of storage roots and corms of many economically important plants worldwide. With the exception of the family Araceae, all confirmed hosts of C. fimbriata are dicotyledonous plants. We hypothesized that the isolates from members of the family Araceae would form a monophyletic lineage specialized to infect these unique hosts. Analyses of sequences of the internal transcribed spacer (ITS) region of nuclear rDNA indicate that isolates and herbarium specimens of C. fimbriata from the family Araceae represent three different groups: an Xanthosoma/Syngonium group on corms of Xanthosoma spp. from the Caribbean region and on ornamental S. podophyllum from greenhouses in Florida, Hawaii, Australia, and Brazil; an inhame group on corms of Colocasia esculenta in Brazil; and a distantly related taro group on Colocasia esculenta in Hawaii and China and on X. sagittifolium in Fiji. Inoculations of three species of Araceae (Caladium bicolor, S. podophyllum, and Colocasia esculenta) showed that isolates from all three groups are pathogenic to these three hosts. Brazilian isolates from Mangifera indica and Ficus carica were only weakly pathogenic to Caladium and Syngonium sp. and were not pathogenic to Colocasia sp. Syngonium plants appeared to be most susceptible to isolates of the Xanthosoma/Syngonium group, and Colocasia plants were least susceptible to isolates from Syngonium spp. Thus, it appears that adaptations to the family Araceae have evolved more than once in the C. fimbriata complex. It is hypothesized that the three groups of C. fimbriata on the family Araceae are native to the Caribbean, Brazil, and Asia, respectively, but they have been spread elsewhere by humans.
Net Carbon Gain and Growth of Bell Peppers, Capsicum annuum ‘Cubico’, Following Root Infection by Pythium aphanidermatum
Authors: Johnstone M. , Chatterton S. , Sutton J. C. , Grodzinski B.
Published: May 2007

The first characterization of alterations in whole-plant photosynthetic rate and carbon assimilation of bell peppers associated with infection by Pythium aphanidermatum is described. Relationships of root disease caused by P. aphanidermatum to whole-plant net carbon exchange rate (NCER), total carbon accumulation, dark respiration rates, water loss, and destructive growth parameters were quantified in vegetative, hydroponically grown pepper plants (Capsicum annuum ‘Cubico’). Inoculated plants displayed lower whole-plant NCER. This translated into a loss of 28% in cumulative C gain during 7 days after inoculation and occurred before visible shoot symptoms developed. Leaf area and dry weight of shoots and roots were significantly decreased and the shoot/root ratio was higher in inoculated plants than in noninoculated plants. We propose that reduced NCER in inoculated plants was mainly due to restricted development of leaf area, because no differences in NCER and evapotranspiration were observed between control and inoculated plants when expressed based on leaf area and root dry mass, respectively. These findings indicate that Pythium infection did not affect the photosynthetic apparatus directly and that the reductions in photosynthesis and growth were not caused by inefficient water transport by diseased roots. These results enlarge on the understanding of physiological responses of host plants to early stages of root disease.
Panicle Blast and Canopy Moisture in Rice Cultivar Mixtures
Authors: Zhu You-Yong , Fang Hui , Wang Yun-Yue , Fan Jin Xiang , Yang Shi-Sheng , Mew Twng Wah , Mundt Christopher C.
Published: May 2007

Glutinous rice cultivars were sown after every fourth row of a nonglutinous, hybrid cultivar in an additive design. The glutinous cultivars were 35 to 40 cm taller and substantially more susceptible to blast than was the nonglutinous cultivar. Interplanting of glutinous and nonglutinous rice reduced the incidence and severity of panicle blast on the glutinous cultivars by >90%, and on the nonglutinous cultivar by 30 to 40%. Mixing increased the per unit area yield of glutinous rice by 80 to 90% relative to pure stand, whereas yield of the nonglutinous cultivar was essentially unaffected by mixing. To determine whether the different plant heights and canopy structures may contribute to a microclimate that is less favorable to blast infection, we monitored the moisture status of the glutinous cultivars in pure stand and mixture at 0800 h by measuring relative humidity at the height of the glutinous panicles using a swing psychrometer and by visually estimating the percentage of leaf area covered by dew. Averaged over the two seasons, the number of days of 100% humidity at 0800 h was 20.0 and 2.2 for pure stands and mixtures, respectively. The mean percentage of glutinous leaf area covered by dewwas 84 and 36% for the pure stands and mixtures, respectively. Although other mechanisms also were operative, reduced leaf wetness was likely a substantial contributor to panicle blast control in the mixtures.
Genotyping Common Bean for the Potyvirus Resistance Alleles I and bc-12 with a Multiplex Real-Time Polymerase Chain Reaction Assay
Authors: Vandemark George J. , Miklas Phillip N.
Published: May 2007

A multiplex real-time polymerase chain reaction (PCR) assay was developed to simultaneously genotype plants for the I and bc-12 alleles, which condition resistance in beans to Bean common mosaic virus and Bean common mosaic necrosis virus. A segregating F2 population was derived from the cross between pinto bean breeding line P94207-189A (bc-1 bc-1 I I) and Olathe (bc-12 bc-12 i i). Real-time PCR assays were developed that were specific for each allele, and a multiplex PCR reaction could unambiguously assign F2 plants to one of nine genotypes. Remnant F1 plants were used as a comparative reference sample. PCR results among this sample fit a normal distribution for both real-time PCR assays, and 99% probability distributions were determined for heterozygotes. F2 plants were genotyped based on results relative to the probability distributions for heterozygotes. F2 plants also were genotyped for the I and bc-12 alleles by performing F3 family progeny tests for virus resistance. Agreement between the two methods was 100% (198/198) for the bc-12 allele, and 92.4% (183/198) for the I allele. Erroneous genotyping was due to recombination between the amplicon and the I allele. Realtime PCR assays provide a robust method for genotyping seedlings and, in some cases, may eliminate the need for progeny testing.
Spiroplasma citri Spiralin Acts In Vitro as a Lectin Binding to Glycoproteins from Its Insect Vector Circulifer haematoceps
Authors: Killiny Nabil , Castroviejo Michel , Saillard Colette
Published: May 2007

In order to understand the molecular mechanisms underlying transmission of Spiroplasma citri by the leafhopper Circulifer haematoceps, we screened leafhopper proteins as putative S. citri-binding molecules using a spiroplasma overlay assay of protein blots (Far-western assay). Insect proteins were separated by one- or two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis, blotted, and probed with S. citri proteins. In this in vitro assay, we found that spiroplasma proteins exhibited affinity for seven leafhopper proteins. The interactions between S. citri proteins and insect proteins with molecular masses of 50 and 60 kDa were found to be sugar sensitive. These insect proteins were identified as high mannose N-glycoproteins, which support an interaction of glycoprotein-lectin type with S. citri proteins. Lectin detection in S. citri has revealed only one protein of 24 kDa. Using a leafhopper protein overlay assay on an S. citri protein blot, one spiroplasma protein with a similar molecular mass of 24 kDa was shown to display an insect protein-binding capacity. This protein was identified as the spiralin, which is the most abundant membrane protein of S. citri. Far-western experiments performed with purified spiralin and insect glycoproteins confirmed the binding of spiralin to the insect glycoproteins of 50 and 60 kDa. Thus, the spiralin could play a key role in the transmission of S. citri by mediating spiroplasma adherence to epithelial cells of insect vector gut or salivary gland.
Detection of the Pinewood Nematode, Bursaphelenchus xylophilus, Using a Real-Time Polymerase Chain Reaction Assay
Authors: Cao A. X. , Liu X. Z. , Zhu S. F. , Lu B. S.
Published: May 2007

The pinewood nematode, Bursaphelenchus xylophilus, has caused significant damage to pine plantations both in East Asia and North America and is an important quarantine organism. A real-time polymerase chain reaction (PCR) assay was developed to detect B. xylophilus. A set of primers and probe specific for B. xylophilus was designed to target the ribosomal DNA internal transcribed spacer region. Optimal primer concentration, Mg2+ concentration, and extension temperature were 400 nM, 3.0 mM, and 60°C, respectively. The assay was highly specific and sensitive, detecting as little as 0.01 ng of B. xylophilus DNA. The real-time PCR assay also successfully detected B. xylophilus in field samples, and it should be very useful for quarantine purposes.
Spatiotemporal Description of Epidemics Caused by Phoma ligulicola in Tasmanian Pyrethrum Fields
Authors: Pethybridge Sarah J. , Esker Paul , Hay Frank , Wilson Calum , Nutter Forrest W.
Published: May 2007

Spatial and temporal patterns of foliar disease caused by Phoma ligulicola were quantified in naturally occurring epidemics in Tasmanian pyrethrum fields. Disease assessments (defoliation incidence, defoliation severity, incidence of stems with ray blight, and incidence of flowers with ray blight) were performed four times each year in 2002 and 2003. Spatial analyses based on distribution fitting, runs analysis, and spatial analysis by distance indices (SADIE) demonstrated aggregation in fields approaching their first harvest for all assessment times between September and December. In second-year harvest fields, however, the incidence of stems with ray blight was random for the first and last samplings, but aggregated between these times. Spatiotemporal analyses were conducted between the same disease intensity measures at subsequent assessment times with the association function of SADIE. In first-year harvest fields, the presence of steep spatial gradients was suggested, most likely from dispersal of conidia from foci within the field. The importance of exogenous inoculum sources, such as wind-dispersed ascospores, was suggested by the absence of significant association between defoliation intensity (incidence and severity) and incidence of stems with ray blight in second-year harvest fields. The logistic model provided the best temporal fit to the increase in defoliation severity in each of six first-year harvest fields in 2003. The logistic model also provided the best fit for the incidence of stems with ray blight and the incidence of flowers with ray blight in four of six and three of six fields, respectively, whereas the Gompertz model provided the best fit in the remaining fields. Fungicides applied prior to mid-October (early spring) significantly reduced the area under disease progress curve (P < 0.001) for defoliation severity, the incidence of stems with ray blight, and the incidence of flowers with ray blight for epidemics at all field locations. This study provides information concerning the epidemiology of foliar disease and ray blight epidemics in pyrethrum and offers insight on how to best manage these diseases.
Ceratothripoides claratris, a New Vector of a Capsicum chlorosis virus Isolate Infecting Tomato in Thailand
Authors: Premachandra W. T. S. D. , Borgemeister C. , Maiss E. , Knierim D. , Poehling H.-M.
Published: May 2007

Ceratothripoides claratris, the predominant thrips species on tomato in Thailand, was tested for vector competence and efficiency to transmit Capsicum chlorosis virus (CaCV) (isolate AIT) to tomato. The efficiency of adult-stage transmission was influenced by the larval stage at which virus was acquired. Adult C. claratris showed 69% transmission efficiency after acquiring the virus as freshly emerged (<1 h) first-instar larvae. However, when just molted (<1 h) second-instar larvae acquired the virus, the percentage of adult transmitters significantly decreased (48%). Transmission efficiency of up to 47% was detected with second-instar larvae of C. claratris which had acquired the virus as freshly emerged first-instar larvae. Transmission efficiency did not significantly differ between adult males and females, irrespective of the larval stage at which the virus was acquired. Highest transmission efficiency for CaCV was recorded in adult C. claratris derived from second-instar larvae collected from infected tomato plants in a greenhouse. Lowest transmission efficiency was observed in adults directly collected from infected tomato plants in the greenhouse. The spread of CaCV on tomato plants in greenhouses showed a close association with thrips infestations.
Resistance of Wheat Line Kavkaz-K4500 L.6.A.4 to Septoria Tritici Blotch Controlled by Isolate-Specific Resistance Genes
Authors: Chartrain L. , Berry S. T. , Brown J. K. M.
Published: May 2007

The International Maize and Wheat Improvement Center (CIMMYT), Mexico, germplasm-derived wheat (Triticum aestivum) Kavkaz-K4500 L.6.A.4 (KK) is one of the major sources of resistance to Septoria tritici blotch (STB). KK is resistant to STB in field conditions in the UK even though a large majority of Mycosphaerella graminicola isolates are virulent to it. The genetics of the resistance of KK to four isolates of M. graminicola were investigated. KK has at least five isolate-specific resistance genes including Stb6 on chromosome 3A plus a second gene for resistance to isolate IPO323, two genes on chromosome 4A, both in the region where Stb7 is located with one designated as Stb12, and a gene designated Stb10 on chromosome 1D. Taken together, the widespread use of KK as a source of resistance to STB, its high resistance in field conditions, and its high susceptibility to M. graminicola isolates, which are virulent to all its resistance genes, suggest that high levels of field resistance to STB might be achieved by pyramiding several isolate-specific resistance genes.
Seasonal Patterns of Spore Deposition of Heterobasidion Species in Four Forests of the Western Alps
Authors: Gonthier Paolo , Garbelotto Matteo M. , Nicolotti Giovanni
Published: May 2007

Patterns of spore deposition by Heterobasidion species were studied between the spring of 1998 and December 2000 in four forests in the western Alps using woody traps. The maximum spore deposition rate (DR) ranged from 169 to 1,550 spores m-2 h-1. Although spores were captured from February to October at most sites, inoculum concentration consistently peaked in the late summer or early fall. In one of the four study sites, similar patterns of DR were recorded in 2 years of sampling. A significant correlation (r = 0.654, P = 0.001) was found between DR and the average minimum air temperature in the 4 weeks before sampling. Approximately 1,200 spores were isolated and identified at the species level by polymerase chain reaction-based methods. Single-spore isolates were consistently clampless, indicating the sampled airspora was almost exclusively composed of haploid basidiospores. No significant variations of basidiospore frequencies were detected for either H. abietinum or H. annosum among sampling periods. However, the frequency of H. parviporum spores was always significantly higher in the summer. These findings suggest different patterns of sporulation among Heterobasidion species.
Molecular Epidemiology of White Pine Blister Rust: Recombination and Spatial Distribution
Authors: Hamelin R. C. , Allaire M. , Bergeron M.-J. , Nicole M.-C. , Lecours N.
Published: May 2007

Multilocus haplotypes (MLHs) were derived for the spermogonial (monokaryotic haploid) stage of Cronartium ribicola, the causal agent of white pine blister rust. Six random amplified polymorphic DNA loci and three single-strand conformational polymorphism markers were analyzed for 246 rust samples collected from two heavily infected white pine plantations. All cankers sampled were spatially located within the plantations. The hypothesis that spores are not locally disseminated was supported by the absence of any spatial clustering in the distribution of the MLHs. A large number of MLHs was found at both sites and the haplotypic diversity was close to the maximum (one) in both populations. All measures of recombination were not different from expectations under a scenario of sexual recombination. Genetic differentiation between the two sites was very low (θ = 0.023), yet it was significantly different from zero (P < 0.01). This analysis is in agreement with a scenario of extensive sexual recombination followed by some long-distance dispersal.
Field Evaluation of Tomato Hybrids Engineered with Tomato spotted wilt virus Sequences for Virus Resistance, Agronomic Performance, and Pollen-Mediated Transgene Flow
Authors: Accotto Gian Paolo , Nervo Giuseppe , Acciarri Nazzareno , Tavella Luciana , Vecchiati Manuela , Schiavi Massimo , Mason Giovanna , Vaira Anna Maria
Published: May 2007

Tomato hybrids obtained from homozygous progeny of line 30-4, engineered for Tomato spotted wilt virus (TSWV) resistance, were tested under field conditions in two locations with their corresponding nontransgenic hybrids. No transgenic hybrid became infected, but 33 to 50% of plants of each nontransgenic hybrid became infected with a severe reduction of marketable fruit production. The transgenic hybrids conformed to the standard agronomic characteristics of the corresponding nontransgenic ones. Fruit were collected from the nontransgenic plots included in the experimental field and from border rows, and seed were used to estimate the flow of the transgene via pollen. No transgene flow was detected in the protected crops; however, in the open field experiment, 0.32% of tomato seedlings were found to contain the genetic modification. Immunity to TSWV infection in 30-4 hybrids was confirmed in laboratory conditions using mechanical inoculation and grafting. Thrips inoculation in leaf discs of line 30-4 demonstrated that TSWV replication was inhibited at the primary infection site but not in leaf discs of a commercial hybrid containing the naturally occurring resistance gene Sw-5. Due to the high economic value of tomato crops worldwide and the importance of TSWV, the engineered resistance described here is of practical value for breeding into cultivars of commercial interest, because it could be combined with naturally occurring resistance, thus greatly reducing the ability of the virus to develop resistance-breaking strains.
Resistance to Monopartite Begomoviruses Associated with the Bean Leaf Crumple Disease in Phaseolus vulgaris Controlled by a Single Dominant Gene
Authors: Monci Francisco , García-Andrés Susana , Maldonado José Antonio , Moriones Enrique
Published: May 2007

Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl Málaga virus are monopartite begomoviruses (genus Begomovirus, family Geminiviridae) that infect common bean (Phaseolus vulgaris), causing bean leaf crumple disease (BLCD). This disease was found to be widespread in southern Spain and causes stunted growth, flower abortion, and leaf and pod deformation in common bean plants. Commercial yield losses of up to 100% occur. In the present study, we have identified and characterized a resistance trait to BLCD-associated viruses in the common bean breeding line GG12. This resistance resulted in a complete absence of BLCD symptoms under field conditions or after experimental inoculation. Our analysis showed that virus replication was not inhibited. However, a severe restriction to systemic virus accumulation occurred in resistant plants, suggesting that cell-to-cell or long-distance movement were impaired. In addition, recovery from virus infection was observed in resistant plants. The reaction of P. vulgaris lines GG12 (resistant) and GG14 (susceptible), and of F1, F2, and backcross populations derived from them, to TYLCV inoculation suggested that a single dominant gene conferred the BLCD resistance described here.
Quantitative Trait Loci Analysis of Resistance to Sclerotinia sclerotiorum in Sunflower
Authors: Rönicke S. , Hahn V. , Vogler A. , Friedt W.
Published: May 2007

A quantitative trait loci (QTL) analysis of resistance to Sclerotinia sclerotiorum was carried out with 283 sunflower (Helianthus annuus) F2:3 families derived from a cross between a resistant (SWS-B-04) and a highly susceptible sunflower inbred line. For that purpose, a genetic map based on 195 amplified fragment length polymorphism and 20 simple sequence repeat markers was constructed. The map has a size of 2,273.5 centimorgans and comprises 17 linkage groups, 12 of which could be associated to already defined linkage groups. The heads of sunflower F3 families were artificially inoculated by using sclerotinia mycelium in three field environments. The lesion length was measured in centimeters 1 week postinoculation and head rot was scored according to a 1-to-8 head rot scale 2 weeks postinoculation. Using the composite interval mapping procedure, three QTL for lesion length and two QTL for head rot could be identified. These QTL explain 10.6 to 17.1% of the total phenotypic variance.
Isolation, Selection, and Efficacy of Pochonia chlamydosporia for Control of Rotylenchulus reniformis on Cotton
Authors: Wang Kening , Riggs Robert D. , Crippen Devany
Published: May 2007

The reniform nematode, Rotylenchulus reniformis, is a serious threat to cotton (Gossypium hirsutum) production in the United States, causing an annual loss of about $80 million. The objective of this study was to isolate fungi from eggs of R. reniformis and select potential biocontrol agents for R. reniformis on cotton. We focused on the fungus Pochonia chlamydosporia because it suppresses root-knot and cyst nematodes and because preliminary data indicated that it was present in Arkansas cotton fields. Soil samples were collected from six cotton fields in Jefferson County, Arkansas. A total of 117 isolates of the nematophagous fungus P. chlamydosporia were obtained. In an in vitro test, 105 of the 117 isolates parasitized fewer than 15% of R. reniformis eggs, but 12 isolates parasitized between 16 and 35% of the eggs. These 12 isolates produced from 6.8 × 104 to 6.9 × 105 chlamydospores per gram of medium in vitro, and chlamydospore production was similar on rice grain and corn grain media. In two greenhouse experiments, a single application of isolate 37 (5,000 chlamydospores per gram of soil) significantly reduced the numbers of R. reniformis on cotton roots and in soil. The three isolates (37, 26, and 14) that parasitized the most eggs in vitro were also the most effective in suppressing numbers of R. reniformis and in increasing cotton growth in the greenhouse.
Genetic Marker Analysis of a Global Collection of Isolates of Citrus tristeza virus: Characterization and Distribution of CTV Genotypes and Association with Symptoms
Authors: Hilf Mark E. , Mavrodieva Vessela A. , Garnsey Stephen M.
Published: May 2007

Genetic markers amplified from three noncontiguous regions by sequence specific primers designed from the partial or complete genome sequences of Citrus tristeza virus (CTV) isolates T3, T30, T36, and VT were used to assess genetic relatedness of 372 isolates in an international collection. Eighty-five isolates were judged similar to the T3 isolate, 81 to T30, 11 to T36, and 89 to VT. Fifty-one isolates were mixed infections by two or more identifiable viral genotypes, and 55 isolates could not be assigned unequivocally to a group defined by marker patterns. Maximum parsimony analysis of aligned marker sequences supported the grouping of isolates on the basis of marker patterns only. Specific disease symptoms induced in select citrus host plants were shared across molecular groups, although symptoms were least severe among isolates grouped by markers with the T30 isolate and were most severe among isolates grouped by markers with the T3 isolate. Isolates assigned the same genotype showed variable symptoms and symptom severity. A classification strategy for CTV isolates is proposed that combines genetic marker patterns and nucleotide sequence data.
Velocity of Spread of Wheat Stripe Rust Epidemics
Authors: Cowger Christina , Wallace LaRae D. , Mundt Christopher C.
Published: May 2007

Controversy has long existed over whether plant disease epidemics spread with constant or with increasing velocity. We conducted largescale field experiments with wheat stripe rust at Madras and Hermiston, Oregon, where natural stripe rust epidemics were rare, to test these competing models. Data from three location-years were available for analysis. A susceptible winter wheat cultivar was planted in pure stand and also in a 1:4 or 1:1 mixture with a cultivar immune to the stripe rust race utilized in the experiments. Plots were 6.1 m wide and varied from 73 to 171 m in length. A 1.5 by 1.5-m focus was inoculated in either the center (2001) or upwind of the center (2002 and 2003) of each plot. Disease severity was evaluated weekly throughout the epidemics in each plot at the same points along a transect running upwind and downwind from the focus. Velocity of spread was calculated from the severity data and regressed separately on time and on distance from the focus. In all location-years and treatments, and at all levels of disease severity, velocity consistently increased linearly with distance, at an average rate of 0.59 m/week per m, and exponentially with time. Further, across epidemics there was a significant positive relationship between the apparent infection rate, r, and the rate of velocity increase in both space and time. These findings have important implications for plant diseases with a focal or partially focal character, and in particular for the effectiveness of ratereducing disease management strategies at different spatial scales.
Nonpathogenic Binucleate Rhizoctonia spp. and Benzothiadiazole Protect Cotton Seedlings Against Rhizoctonia Damping-Off and Alternaria Leaf Spot in Cotton
Authors: Jabaji-Hare Suha , Neate Stephen M.
Published: May 2007

Recent reports have shown induction of resistance to Rhizoctonia root rot using nonpathogenic strains of binucleate Rhizoctonia spp. (np-BNR). This study evaluates the biocontrol ability of several np-BNR isolates against root and foliar diseases of cotton in greenhouse trials, provides evidence for induced systemic resistance (ISR) as a mechanism in this biocontrol, and compares the disease control provided by np-BNR with that provided by the chemical inducer benzothiadiazole (BTH). Pretreatment of cotton seedlings with np-BNR isolates provided good protection against pre- and post-emergence damping-off caused by a virulent strain of Rhizoctonia solani (AG-4). Seedling stand of protected cotton was significantly higher (P < 0.05) than that of nonprotected seedlings. Several np-BNR isolates significantly reduced disease severity. The combination of BTH and np-BNR provided significant protection against seedling rot and leaf spot in cotton; however, the degree of disease reduction was comparable to that obtained with np-BNR treatment alone. Significant reduction in leaf spot symptoms caused by Alternaria macrospora occurred on cotyledons pretreated with np-BNR or sprayed with BTH, and the np- BNR-treated seedlings had significantly less leaf spot than BTH-treated seedlings. The results demonstrate that np-BNR isolates can protect cotton from infections caused by both root and leaf pathogens and that disease control was superior to that observed with a chemical inducer.
NL-3 K Strain Is a Stable and Naturally Occurring Interspecific Recombinant Derived from Bean common mosaic necrosis virus and Bean common mosaic virus
Authors: Larsen Richard C. , Miklas Phillip N. , Druffel Keri L. , Wyatt Stephen D.
Published: May 2007

A strain of Bean common mosaic necrosis virus (BCMNV) from Idaho was identified by enzyme-linked immunosorbent assay using monoclonal antibodies and determined to be similar to the NL-3 D strain (of Drifjhout) by reaction of differential bean cultivars. However, this BCMNV strain (designated NL-3 K) caused earlier and more severe symptoms on bean plants representing host groups 0, 4, and 5. The nucleotide sequence encoding the predicted polyprotein of NL-3 K was 9,893 nucleotides (nt) in length, yielding a peptide with a molecular size of 362.1 kDa compared with a 9,626-nt, 350.9-kDa polyprotein for NL-3 D. Sequence analysis of the putative P1 protein suggests that the NL-3 K strain is a recombinant between NL-3 D and the Russian strain (RU1) of Bean common mosaic virus. The P1 protein of NL-3 K consisted of 415 amino acids compared with 317 for NL-3 D. The first 114 predicted amino acids of the NL-3 K P1 region were 98% identical with RU1. The remaining 301 amino acids of the protein shared only 34% identity with RU1 but were 98% identical with NL-3 D. Primers were designed that flanked the recombination point in the P1 coding sequence of NL-3 K. An amplicon of the expected size was produced by reverse-transcriptase polymerase chain reaction of total nucleic acid extracts of bean plants inoculated with NL-3 K, but not from those with NL-3 D or RU1. The increased symptom severity on selected common bean lines induced by NL-3 K suggests that the P1 gene may play a significant role in pathogenicity and virulence.
A Comprehensive Species to Strain Taxonomic Framework forXanthomonas
Authors: Rademaker J. L. W. , Louws F. J. , Schultz M. H. , Rossbach U. , Vauterin L. , Swings J. , de Bruijn F. J.
Published: May 2007

A comprehensive classification framework was developed that refines the current Xanthomonas classification scheme and provides a detailed assessment of Xanthomonas diversity at the species, subspecies, pathovar, and subpathovar levels. Polymerase chain reaction (PCR) using primers targeting the conserved repetitive sequences BOX, enterobacterial repetitive intergenic consensus (ERIC), and repetitive extragenic palindromic (REP) (rep-PCR) was used to generate genomic fingerprints of 339 Xanthomonas strains comprising 80 pathovars, 20 DNA homology groups, and a Stenotrophomonas maltophilia reference strain. Computer-assisted pattern analysis of the rep-PCR profiles permitted the clustering of strains into distinct groups, which correspond directly to the 20 DNA-DNA homology groups(genospecies) previously identified. Group 9 strains (X. axonopodis) were an exception and did not cluster together into a coherent group but comprised six subgroups. Over 160 strains not previously characterized by DNA-DNA hybridization analysis, or not previously classified, were assigned to specific genospecies based on the classification framework developed. The rep-PCR delineated subspecific groups within X. hortorum, X. arboricola, X. axonopodis, X. oryzae, X. campestris, and X. translucens. Numerous taxonomic issues with regard to the diversity, similarity, redundancy, or misnaming were resolved. This classification framework will enable the rapid identification and classification of new, novel, or unknown Xanthomonas strains that are pathogenic or are otherwise associated with plants.
Inoculum Dynamics, Fruit Infection, and Development of Brown Rot in Prune Orchards in California
Authors: Luo Yong , Michailides Themis J. , Morgan David P. , Krueger William H. , Buchner Richard P.
Published: May 2007

Brown rot, caused by Monilinia fructicola, is a destructive disease of stone fruit in California. Disease management requires information on inoculum dynamics and development of latent and visible fruit infections during the season to help make decisions on timing of fungicide treatments and choice of cultural practices. In this study, the daily spore concentration (ascospores and conidia) of M. fructicola in the air was monitored with spore traps in two prune orchards during the growing seasons in 2001 and 2002. The spore concentrations were low to moderate at early bloom, increased at full bloom, and decreased to the lowest level at the end of bloom. Improper timing of fruit thinning and irrigation in midseason increased spore concentration in the air and fruit infections late in the season. Artificial fruit inoculations were conducted periodically in 10 prune orchards in 2002 and 2004, and incidence of fruit rot at different inoculation dates was assessed. Fruit rot development rate increased linearly with inoculation date during the growing season. Natural blossom and fruit infections were monitored periodically in 10 prune orchards, and incidence of latent fruit infection was determined by using the overnight freezing-incubation technique. Incidence of fruit rot also was assessed 2 weeks before harvest in these orchards. The incidence of latent fruit infection at the pit hardening stage significantly correlated with that at the late stages and with the incidence of fruit rot at harvest.
Identification of Individual Powdery Mildew Fungi Infecting Leaves and Direct Detection of Gene Expression by Single Conidium Polymerase Chain Reaction
Authors: Matsuda Yoshinori , Sameshima Takeshi , Moriura Nobuyuki , Inoue Kanako , Nonomura Teruo , Kakutani Koji , Nishimura Hiroaki , Kusakari Shin-ichi , Takamatsu Susumu , Toyoda Hideyoshi
Published: May 2007

Greenhouse-grown tomato seedlings were inoculated naturally with two genera of powdery mildew conidia forming appressorial germ tubes that could not be differentiated by length alone. For direct identification, single germinated conidia were removed from leaves by means of a glass pipette linked to the manipulator of a high-fidelity digital microscope. This microscope enabled in vivo observation of the fungi without leaf decoloration or fungal staining. The isolated conidia were subjected to PCR amplification of the 5.8S rDNA and its adjacent internal transcribed spacer sequences followed by nested PCR to attain sensitivity high enough to amplify target nucleotide sequences (PCR/nested PCR). Target sequences from the conidia were completely coincident with those of the pathogen Oidium neolycopersici or Erysiphe trifolii (syn. Microsphaera trifolii), which is nonpathogenic on tomato. Using RT-PCR/nested PCR or multiplex RT-PCR/nested PCR, it was possible to amplify transcripts expressed in single conidia. Conidia at pre- and postgermination stages were removed individually from tomato leaves, and two powdery mildew genes were monitored. The results indicated that the β-tubulin homolog TUB2-ol was expressed at pre- and postgermination stages and the cutinase homolog CUT1-ol was only expressed postgermination. Combining digital microscopic micromanipulation and two-step PCR amplification is thus useful for investigation of individual propagules on the surface of plants.
Population Genetic Analysis of a Global Collection of Pyrenophora tritici-repentis, Causal Agent of Tan Spot of Wheat
Authors: Friesen T. L. , Ali S. , Klein K. K. , Rasmussen J. B.
Published: May 2007

The work presented here is the first major study to analyze the genetic diversity within the worldwide population of the economically important wheat pathogen Pyrenophora tritici-repentis. The genetic structure of field populations of P. tritici-repentis was determined using amplified fragment length polymorphism markers along with sequence data from the internal transcribed spacer region of the ribosomal DNA. Ninetyseven fungal isolates were collected from naturally infected wheat and wild grass species. The collection of 97 P. tritici-repentis isolates included races 1, 2, 3, 4, 5, ND7, and ND8 and was collected from North America, South America, and Europe. Results show no genetic grouping of fungal races nor do results indicate grouping based on geographic location indicating that the population is preferentially outcrossing in nature and that the introduction and spread of this population is either relatively recent or that there has been a constant worldwide flow of this fungus possibly by seed movement between continents.
Simulation of Potato Late Blight in the Andes. I: Modification and Parameterization of the LATEBLIGHT Model
Authors: Andrade-Piedra Jorge L. , Hijmans Robert J. , Forbes Gregory A. , Fry William E. , Nelson Rebecca J.
Published: May 2007

LATEBLIGHT, a mathematical model that simulates the effects of weather, host growth and resistance, and fungicide use on asexual development and growth of Phytophthora infestans on potato foliage, was modified so that it can be used in the Andes and, eventually, worldwide. The modifications included (i) the incorporation of improved equations for the effect of temperature on lesion growth rate (LGR) and sporulation rate (SR); (ii) the incorporation of temperature-dependent latent period (LP); and (iii) the use of experimentally measured parameters of LGR, SR, and LP for specific potato cultivars and pathogen lineages. The model was parameterized for three Peruvian potato cultivars (Tomasa, Yungay, and Amarilis) infected with isolates of a new clonal lineage of P. infestans that is currently predominant in Ecuador and Peru (EC-1).
Quantitative Real-Time Polymerase Chain Reaction for Bacterial Enumeration and Allelic Discrimination to Differentiate Xanthomonas Strains on Citrus
Authors: Cubero J. , Graham J. H.
Published: May 2007

Quantitative real-time polymerase chain reaction (QRT-PCR) was developed for identification and enumeration of bacteria in citrus plant samples infected with Xanthomonas axonopodis pvs. citri and citrumelo, the cause of citrus bacterial canker (CBC) and citrus bacterial spot (CBS), respectively. Three sets of primers based on the pathogenicity gene (pth) in X. axonopodis pv. citri, a ribosomal gene in X. axonopodis pv. citrumelo, and the leucine-responsive regulatory protein (lrp) in both pathovars were combined with TaqMan probes and applied for specific strain detection and quantification. Calibration curves for bacterial abundance in plant samples obtained with the three primer-probe combinations were congruent with colony counts on plates of semiselective medium in most of the cases. However, apparent overestimation of bacterial cells by QRT-PCR indicated the presence of nonculturable or nonviable cells in some samples. In addition to quantification, the lrp primers and probes permitted differentiation by allelic discrimination of Xanthomonas strains infecting citrus tissues. This technique is based on the utilization of two probes that detect a single nucleotide difference in the target sequence between different strains and was validated with a collection of cultured Xanthomonas strains as well as tissue with CBC and CBS lesions. Allelic discrimination is demonstrated to be a more specific and sensitive protocol than previously developed PCR-based methods for strain identification and quantification.
Metabolism of the Flavonoid Epicatechin by Laccase of Colletotrichum gloeosporioides and Its Effect on Pathogenicity on Avocado Fruits
Authors: Guetsky R. , Kobiler I. , Wang X. , Perlman N. , Gollop N. , Avila-Quezada G. , Hadar I. , Prusky D.
Published: May 2007

During avocado fruit ripening, decreasing levels of the flavonoid epicatechin have been reported to modulate the metabolism of preformed antifungal compounds and the activation of quiescent Colletotrichum gloeosporioides infections. Epicatechin levels decreased as well when C. gloeosporioides was grown in the presence of epicatechin in culture. Extracts of laccase enzyme obtained from decayed tissue and culture media fully metabolized the epicatechin substrate within 4 and 20 h, respectively. Purified laccase protein from C. gloeosporioides showed an apparent MW of 60,000, an isoelectric point at pH 3.9, and maximal epicatechin degradation at pH 5.6. Inhibitors of fungal laccase such as EDTA and thioglycolic acid reduced C. gloeosporioides symptom development when applied to ripening susceptible fruits. Isolates of C. gloeosporioides with reduced laccase activity and no capability to metabolize epicatechin showed reduced pathogenicity on ripening fruits. On the contrary, Mexican isolates with increasing capabilities to metabolize epicatechin showed early symptoms of disease in unripe fruits. Transcript levels of cglac1, encoding C. gloeosporioides laccase, were enhanced during fungal development in the presence of epicatechin at pH 6.0, where avocado fruits are susceptible to fungal attack. But transcript increase was not detected at pH 5.0, where the fruit is resistant to fungal attack. The present results suggest that biotransformation of epicatechin by C. gloeosporioides in ripening fruits is followed by the decline of the preformed antifungal diene compound, resulting in the activation of quiescent infections.
Pathogenicity and In Planta Mycotoxin Accumulation Among Members of the Fusarium graminearum Species Complex on Wheat and Rice
Authors: Goswami Rubella S. , Kistler H. Corby
Published: May 2007

Fusarium head blight (FHB), or scab, is a destructive disease of small grains caused by members of the Fusarium graminearum species complex, comprised of at least nine distinct, cryptic species. Members of this complex are known to produce mycotoxins including the trichothecenes deoxynivalenol (DON) along with its acetylated derivatives and nivalenol (NIV). In this study, 31 strains, belonging to eight species of this complex and originating from diverse hosts or substrates, were tested for differences in aggressiveness and mycotoxin production. Large variation among strains, both in terms of their aggressiveness and the ability to produce trichothecenes on a susceptible cultivar of wheat was found; variation appears to be a strain-specific rather than species-specific characteristic. While pathogenicity was not influenced by the type of mycotoxin produced, a significant correlation was observed between the amount of the dominant trichothecene (DON and its acetylated forms or NIV) produced by each strain and its level of aggressiveness on wheat. Some isolates also were tested for their ability to infect rice cv. M201, commonly grown in the United States. While tested strains were capable of infecting rice under greenhouse conditions and causing significant amount of disease, no trichothecenes could be detected from the infected rice florets.