bluuebunny/crossref_metadata_2025 huggingface dataset:
Primary Disease Gradients of Wheat Stripe Rust in Large Field Plots
Authors: Sackett Kathryn E. , Mundt Christopher C.
Published: May 2007

Field data on disease gradients are essential for understanding the spread of plant diseases. In particular, dispersal far from an inoculum source can drive the behavior of an expanding focal epidemic. In this study, primary disease gradients of wheat stripe rust, caused by the aerially dispersed fungal pathogen Puccinia striiformis, were measured in Madras and Hermiston, OR, in the spring of 2002 and 2003. Plots were 6.1 m wide by 128 to 171 m long, and inoculated with urediniospores in an area of 1.52 by 1.52 m. Gradients were measured as far as 79.2 m downwind and 12.2 m upwind of the focus. Four gradient models—the power law, the modified power law, the exponential model, and the Lambert's general model—were fit to the data. Five of eight gradients were better fit by the power law, modified power law, and Lambert model than by the exponential, revealing the non-exponentially bound nature of the gradient tails. The other three data sets, which comprised fewer data points, were fit equally well by all the models. By truncating the largest data sets (maximum distances 79.2, 48.8, and 30.5 m) to within 30.5, 18.3, and 6.1 m of the focus, it was shown how the relative suitability of dispersal models can be obscured when data are available only at a short distance from the focus. The truncated data sets were also used to examine the danger associated with extrapolating gradients to distances beyond available data. The power law and modified power law predicted dispersal at large distances well relative to the Lambert and exponential models, which consistently and sometimes severely underestimated dispersal at large distances.
The Effects of Dispersal Gradient and Pathogen Life Cycle Components on Epidemic Velocity in Computer Simulations
Authors: Sackett Kathryn E. , Mundt Christopher C.
Published: May 2007

The velocity of expansion of focal epidemics was studied using an updated version of the simulation model EPIMUL, with model parameters relevant to wheat stripe rust. The modified power law, the exponential model, and Lambert's general model were fit to primary disease gradient data from an artificially initiated field epidemic of stripe rust and employed to describe dispersal in simulations. The exponential model, which fit the field data poorly (R2= 0.728 to 0.776), yielded an epidemic that expanded as a traveling wave (i.e., at a constant velocity), after an initial buildup period. Both the modified power law and the Lambert model fit the field data well (R2= 0.962 to 0.988) and resulted in dispersive epidemic waves (velocities increased over time for the entire course of the epidemic). The field epidemic also expanded as a dispersive wave. Using parameters based on the field epidemic and modified power law dispersal as a baseline, life cycle components of the pathogen (lesion growth rate, latent period, infectious period, and multiplication rate) and dispersal gradient steepness were varied within biologically reasonable ranges for this disease to test their effect on dispersive wave epidemics. All components but the infectious period had a strong influence on epidemic velocity, but none changed the general pattern of velocity increasing over time.
Characterization of a Novel Member of the Family Closteroviridae from Mentha spp.
Authors: Tzanetakis Ioannis E. , Postman Joseph D. , Martin Robert R.
Published: May 2007

While characterizing the agents involved in symptomatology of a variegated mint, Mentha × gracilis ‘Variegata’, a nursery plant with atypical symptoms was examined. This plant, unlike ‘Variegata’, did not exhibit yellow vein banding symptoms but instead had distorted and crinkled leaves. Molecular tests for the three viruses found in ‘Variegata’ clones failed to detect any of these viruses in the plant. Double-stranded RNA was extracted and cloned, disclosing the presence of two unknown viruses. One of the viruses was a novel member of the family Closteroviridae. The complete nucleotide sequence of the virus, designated as Mint virus 1, has been obtained. A detection test was developed, and revealed the presence of the virus in several other mint clones and species. Genomic regions from three additional isolates were examined to investigate the genetic diversity of the virus. Genome and phylogenetic analysis placed Mint virus 1 in the genus Closterovirus and transmission studies have identified the mint aphid, Ovatus crataegarius, as a vector for this new member of the genus Closterovirus.
Biological Control Efficiency of Fusarium Wilt of Tomato by Nonpathogenic Fusarium oxysporum Fo-B2 in Different Environments
Authors: Shishido Masahiro , Miwa Chika , Usami Toshiyuki , Amemiya Yoshimiki , Johnson Kenneth B.
Published: May 2007

Efficiency of nonpathogenic Fusarium oxysporum Fo-B2 for the biological control of Fusarium wilt of tomato, caused by F. oxysporum f. sp. lycopersici CU1, was examined in different environments: a growth chamber with sterile soil-less medium, a greenhouse with fumigated or nonfumigated soil, and nonfumigated field plots. Inoculation of Fo-B2 onto tomato roots significantly reduced the severity of disease, but the efficiency of disease suppression decreased as the experimental environment became less controlled. Relationships between the recovery of Fo-B2 from hypocotyls and the disease severity indicated that the biocontrol agent was most effective when it colonized vascular tissues intensively. Moreover, the degree of Fo-B2 colonization was greatly reduced when the seedlings were grown in nonfumigated soil. Dose-response models (negative exponential, hyperbolic saturation, and logistic) were fit to observed data collected over a range of inoculum densities of the pathogen and the antagonist; the logistic model provided the best fit in all environments. The ratios of an 50% effective dose parameter for Fo-B2 to that of CU1 increased as the environment became less controlled, suggesting that environmentally related efficiency reduction impacted the antagonist more than the pathogen. The results suggest that indigenous soil microbes were a primary factor negatively influencing the efficiency of Fo-B2. Therefore, early establishment of the antagonist in a noncompetitive environment prior to outplanting could improve the efficacy of biological control.
Biological Control of Late Leaf Spot of Peanut (Arachis hypogaea) with Chitinolytic Bacteria
Authors: Kishore G. Krishna , Pande Suresh , Podile A. R.
Published: May 2007

Late leaf spot (LLS), caused by Phaeoisariopsis personata, is a foliar disease of groundnut or peanut (Arachis hypogaea) with high economic and global importance. Antifungal and chitinolytic Bacillus circulans GRS 243 and Serratia marcescens GPS 5, selected among a collection of 393 peanut-associated bacteria, were applied as a prophylactic foliar spray and tested for control of LLS. Chitin-supplemented application of B. circulans GRS 243 and S. marcescens GPS 5 resulted in improved biological control of LLS disease. Supplementation of bacterial cells with 1% (wt/vol) colloidal chitin reduced lesion frequency by 60% compared with application of bacterial cells alone, in the greenhouse. Chitinsupplemented application of GRS 243 and GPS 5 also resulted in improved and stable control of LLS in a repeated field experiment and increased the pod yields by 62 and 75%, respectively, compared with the control. Chitin-supplemented application of GPS 5 was tested in six onfarm trials, and the increase in pod yields was up to 48% in kharif (rainy season). A 55-kDa chitinase was purified from the cell-free culture filtrate of GPS 5 by affinity chromatography and gel filtration. Purified chitinase of S. marcescens GPS 5 (specific activity 120 units) inhibited the in vitro germination of P. personata conidia, lysed the conidia, and effectively controlled LLS in greenhouse tests, indicating the importance of chitinolysis in biological control of LLS disease by GPS 5.
Genetic Structure and Analysis of Host and Nonhost Interactions of Striga gesnerioides (Witchweed) from Central Florida
Authors: Botanga Christopher J. , Timko Michael P.
Published: May 2007

Striga gesnerioides is a root hemiparasite of wild and cultivated legumes, among which cowpea (Vigna unguiculata) and Indigofera hirsuta are suitable hosts. In this study, we examined the genetic structure and host-parasite interaction of a strain of S. gesnerioides parasitizing I. hirsuta (SGFL) from central Florida (United States). Amplified fragment length polymorphism analysis was used to compare genetic profiles from 71 individual S. gesnerioides plants (SGFL) representing four different populations in central Florida. Our results showed that these individuals are genetically similar, with pairwise genetic distances ranging from 0.00 to 0.037. A cluster analysis grouped all four S. gesnerioides populations from Florida, separating them from S. gesnerioides isolates parasitic on I. hirsuta and cowpea collected from West Africa. One EcoRI and MseI selective primer combination generated a 510-bp fragment present in individuals from the SGFL and the West African isolate parasitic on I. hirsuta, but absent in isolates parasitic on cowpea. Germination of seed from individuals of all four populations of S. gesnerioides parasitic on I. hirsuta from Florida was stimulated by root exudates from cowpea genotypes Blackeye and TVX-3236, known to be highly susceptible to all races of S. gesnerioides parasitic on cowpea in West Africa. SGFL seedlings failed to parasitize cowpea, with the development of attached SGFL seedlings arrested at the tubercle stage. The very high level of genetic uniformity observed within and among the central Florida populations suggests that there was likely a single introduction of the parasite or strong host-driven selection to genetic uniformity.These findings are important in assessing the potential of the parasite as an agronomically significant pest in the United States.
Rice Defense Mechanisms AgainstCochliobolus miyabeanusandMagnaporthe griseaAre Distinct
Authors: Ahn Il-Pyung , Kim Soonok , Kang Seogchan , Suh Seok-Cheol , Lee Yong-Hwan
Published: May 2007

Responses of rice to Magnaporthe grisea and Cochliobolus miyabeanus were compared. In Tetep, a rice cultivar resistant to both fungi, pathogen inoculation rapidly triggered the hypersensitive reaction (HR), resulting in microscopic cell death. In rice cv. Nakdong, susceptible to both pathogens, M. grisea did not cause HR, whereas C. miyabeanus caused rapid cell death similar to that associated with HR, which appeared similar to that observed in cv. Tetep, yet failed to block fungal ramification. Treatment with conidial germination fluid (CGF) from C. miyabeanus induced rapid cell death in both cultivars, suggesting the presence of phytotoxins in CGF. Pretreatment of cv. Nakdong with CGF significantly increased resistance to M. grisea, while the same treatment was ineffective against C. miyabeanus. Similarly, in cv. Nakdong, benzothiadiazole (BTH) significantly increased resistance to M. grisea, but was ineffective against C. miyabeanus. Methyl jasmonate (MeJA) treatment appeared to be ineffective against either fungus. Increased resistance of cv. Nakdong to M. grisea by BTH or CCF treatment was correlated with more rapid induction of three monitored PR genes. Application of MeJA resulted in the expression of JAmyb in cv. Nakdong being induced faster than in untreated plants in response to M. grisea infection. In contrast, the expression pattern of the PR and JAmyb genes in response to C. miyabeanus was nearly identical between cvs. Nakdong and Tetep, and neither BTH nor MeJA treatment significantly modified their expression patterns in response to C. miyabeanus infection. Our results suggest that rice employs distinct mechanisms for its defense against M. grisea versus C. miyabeanus.
Production of Didymella rabiei Pseudothecia and Dispersal of Ascospores in a Mediterranean Climate
Authors: Gamliel-Atinsky E. , Shtienberg D. , Vintal H. , Nitzni Y. , Dinoor A.
Published: May 2007

Temperature and wetness conditions required for development and maturation of Didymella rabiei pseudothecia were determined in a series of experiments conducted in controlled-environmental conditions. Initial stages of pseudothecium formation occurred at temperatures ranging from 5 to 15°C. Incubation at low temperatures was essential for subsequent pseudothecium maturation. This requirement was satisfied for chickpea stem segments incubated at 5 or 10°C for three consecutive weeks or during periods of 3 or 5 days, separated by periods at higher temperatures. Following the low-temperature requirement, subsequent pseudothecium development was independent of temperature in the range tested (5 to 20°C). Wetness was essential for pseudothecium production: pseudothecia formed and matured on stem segments maintained continuously wet but also on those exposed to periods of three or five wet days, separated by dry periods. The dispersal of D. rabiei ascospores was studied using chickpea plants as living traps in the field. Trap plants were infected mainly when exposed during rain but also in rainless periods. Results of this study enabled us to describe the developmental events leading to the production of the teleomorph stage and the dispersal of ascospores by D. rabiei in the Mediterranean climate of Israel.
Relationships Between Black Pod and Witches'-Broom Diseases in Theobroma cacao
Authors: Thevenin J.-M. , Umaharan R. , Surujdeo-Maharaj S. , Latchman B. , Cilas C. , Butler D. R.
Published: May 2007

Field observations were conducted from 1998 to 2001 at the International Cocoa Genebank, Trinidad, to evaluate 57 cacao clones for resistance to black pod (BP) and witches'-broom (WB) diseases (caused by Phytophthora sp. and Crinipellis perniciosa, respectively). Each month ripe pods were harvested and the number of healthy and diseased was recorded. The number of brooms on vegetative shoots was recorded three times a year on selected branches. Twenty-three clones showed less than 10% of infection for both BP and WB on pods. Among those, eight clones showed an absence of brooms on the observed branches: IMC 6, MAN 15/60 [BRA], PA 67 [PER], PA 195 [PER], PA 218 [PER], PA 296 [PER], PA 303 [PER], and POUND 32/A [POU]. Broad-sense heritability was estimated at 0.38 and 0.57 for WB disease on pods and shoots, respectively, and at 0.51 for BP disease. Genetic correlation between WB disease on pods and on shoots was low and estimated at 0.39, whereas the correlation between WB and BP diseases on pods was 0.48. To choose putative parents for breeding schemes, it is suggested that clones are first assessed for their level of resistance to WB on shoots, and the most promising individuals are screened for BP with a detached pods test. Further studies are needed to confirm whether the level of resistance to WB on pods can be predicted using an early test on seedlings.
Investigating Disease Spread between Two Assessment Dates with Permutation Tests on a Lattice
Authors: Thébaud Gaël , Peyrard Nathalie , Dallot Sylvie , Calonnec Agnès , Labonne Gérard
Published: May 2007

Mapping and analyzing the disease status of individual plants within a study area at successive dates can give insight into the processes involved in the spread of a disease. We propose a permutation method to analyze such spatiotemporal maps of binary data (healthy or diseased plants) in regularly spaced plantings. It requires little prior information on the causes of disease spread and handles missing plants and censored data. A Monte Carlo test is used to assess whether the location of newly diseased plants is independent of the location of previously diseased plants. The test takes account of the significant spatial structures at each date in order to separate nonrandomness caused by the structure at one date from nonrandomness caused by the dependence between newly diseased plants and previously diseased plants. If there is a nonrandom structure at both dates, independent patterns are simulated by randomly shifting the entire pattern observed at the second date. Otherwise, independent patterns are simulated by randomly reallocating the positions of one group of diseased plants. Simulated and observed patterns of disease are then compared through distance-based statistics. The performance of the method and its robustness are evaluated by its ability to accurately identify simulated independent and dependent bivariate point patterns. Additionally, two realworld spatiotemporal maps with contrasting disease progress illustrate how the tests can provide valuable clues about the processes of disease spread. This method can supplement biological investigations and be used as an exploratory step before developing a specific mechanistic model.
Use of Quantitative Molecular Diagnostic Assays to Investigate Fusarium Dry Rot in Potato Stocks and Soil
Authors: Cullen D. W. , Toth I. K. , Pitkin Y. , Boonham N. , Walsh K. , Barker I. , Lees A. K.
Published: May 2007

Specific and sensitive quantitative diagnostics, based on real-time (TaqMan) polymerase chain reaction (PCR) and PCR enzyme-linked immunosorbent assay, were developed to detect dry-rot-causing Fusarium spp. (F. avenaceum, F. coeruleum, F. culmorum, and F. sulphureum). Each assay detected Fusarium spp. on potato seed stocks with equal efficiency. Four potato stocks, sampled over two seed generations from Scottish stores, were contaminated with F. avenaceum, F. sulphureum, F. culmorum, F. coeruleum or a combination of species, and there was a general trend towards increased Fusarium spp. contamination in the second generation of seed sampled. F. sulphureum and F. coeruleum caused significantly (P < 0.05) more disease in storage than the other species when disease-free tubers of potato cvs. Spunta and Morene were inoculated at a range of inoculum concentrations (0, 104, 105, and 106 conidia/ml). Increased DNA levels were correlated with increased disease severity between 8 and 12 weeks of storage. The threshold inoculum levels resulting in significant disease development on both cultivars were estimated to be 104 conidia/ml for F. sulphureum and 105 conidia/ml for F. coeruleum. To study the effect of soil infestation and harvest date on disease incidence, seed tubers of cvs. Morene and Spunta were planted in a field plot artificially infested with the four Fusarium spp. F. culmorum and F. sulphureum were detected in soil taken from these plots at harvest, and F. sulphureum DNA levels increased significantly (P < 0.05) at the final harvest. All four Fusarium spp. were detected in progeny tubers. There was a trend toward higher levels of F. culmorum detected in progeny tubers at the earliest harvest date, and higher levels of F. sulphureum at the final harvest. The use of diagnostic assays to detect fungal storage rot pathogens and implications for disease control strategies are discussed.
Genetic and Biological Diversity of Trichoderma stromaticum, a Mycoparasite of the Cacao Witches'-Broom Pathogen
Authors: de Souza Jorge T. , Pomella Alan W. V. , Bowers John H. , Pirovani Carlos P. , Loguercio Leandro L. , Hebbar K. Prakash
Published: May 2007

The witches'-broom disease, caused by the basidiomycete Crinipellis perniciosa, is the most limiting factor for cacao cultivation in Brazil. Trichoderma stromaticum is a mycoparasite of the witches'-broom pathogen of cacao that is currently being applied in the field to manage the disease in Bahia State, Brazil. In this work, molecular and traditional methods were used to study the genetic and biological diversity of this mycoparasite. Ninety-one isolates, mostly collected from farms not sprayed with the fungus, were analyzed by amplified fragment length polymorphisms (AFLP), which showed that two genetic groups (I and II) of T. stromaticum occur in Bahia State. This classification of T. stromaticum into two distinct AFLP groups was also in agreement with several other characteristics, including growth on agar media at different temperatures and sporulation on infected stem segments (broom pieces) and rice grains. Group II favors higher temperatures compared with group I. The genetic and biological differences of the isolates, however, were not evident in field experiments, where sporulation was evaluated on the surface of brooms under natural conditions. Our results show that there is considerable genetic and biological diversity within T. stromaticum in Bahia and other cacao-growing regions of South America that are affected by the witches'-broom disease. This diversity could be explored in the development of efficient biological control agents against the disease. Factors that may affect the application and performance of this biocontrol agent in the field, such as sporulation on rice substrate and on the brooms and growth at various temperatures, are discussed.
VirE1-Mediated Resistance to Crown Gall in Transgenic Arabidopsis thaliana
Authors: Humann Jodi , Andrews Sarah , Ream Walt
Published: May 2007

Crown gall disease, caused by Agrobacterium tumefaciens, remains a serious agricultural problem despite current biocontrol methods. Agrobacterium tumefaciens transfers single-stranded DNA (T-strands) into plant cells along with several virulence proteins, including a single-stranded DNA-binding protein (VirE2). In plant cells, T-strands are protected from nucleases and targeted to the nucleus by VirE2, which is essential for efficient transmission (transfer and integration) of T-strands. VirE1 is the secretory chaperone for VirE2; it prevents VirE2 from forming aggregates and from binding the T-strands in bacterial cells. Therefore, we hypothesized that sufficient quantities of VirE1 expressed in plant cells might block T-DNA transmission by preventing VirE2 from binding T-strands. Here we show that root explants from Arabidopsis thaliana plants that expressed virE1 formed 3.5-fold fewer tumors than roots from plants without virE1. Also, this resistance was specific for VirE2-mediated Agrobacterium transformation. Plants that have been genetically altered to resist crown gall may prove more effective than biological control.
Cloning and Characterization of a Putative Antifungal Peptide Gene (Pm-AMP1) in Pinus monticola
Authors: Ekramoddoullah A. K. M. , Liu J.-J. , Zamani A.
Published: May 2007

We have been working on proteins that are involved in the defense response of western white pine (WWP) (Pinus monitcola) to the blister rust fungus Cronartium ribicola. Our objective was to identify candidate genes that could be used for improving resistance of WWP to this rust pathogen. During proteomic analysis of bark proteins extracted from WWP trees exhibiting slow-canker-growth (SCG) resistance, a 10.6-kDa peptide, termed Pm-AMP1, was found to be enriched at the receding canker margin. The cDNA encoding this peptide was cloned and characterized. A BLASTX search revealed that the Pm-AMP1 encoded by its cDNA has a 50% homology with MiAMP1, a broad-spectrum antifungal protein isolated from Macadamia integrifolia. Based on the deduced amino acid sequence, an antibody was produced against the Pm-AMP1. Immunochemical quantification of the Pm-AMP1 in bark samples of susceptible WWP trees revealed this protein to be barely detectable in the cankered tissues, but occurring in higher concentrations in healthy tissues away from canker margins. Foliage of SCG-resistant trees contained higher concentrations of the Pm-AMP1 than foliage from susceptible cankered trees. Both wounding and methyl jasmonate treatment of WWP needles induced the expression of this protein, further supporting its putative role as a defense response protein.
Overview of Mechanisms and Uses of Trichoderma spp.
Authors: Harman Gary E.
Published: May 2007

Fungi in the genus Trichoderma have been known since at least the 1920s for their ability to act as biocontrol agents against plant pathogens. Until recently, the principal mechanisms for control have been assumed to be those primarily acting upon the pathogens and included mycoparasitism, antibiosis, and competition for resources and space. Recent advances demonstrate that the effects of Trichoderma on plants, including induced systemic or localized resistance, are also very important. These fungi colonize the root epidermis and outer cortical layers and release bioactive molecules that cause walling off of the Trichoderma thallus. At the same time, the transcriptome and the proteome of plants are substantially altered. As a consequence, in addition to induction of pathways for resistance in plants, increased plant growth and nutrient uptake occur. However, at least in maize, the increased growth response is genotype specific, and some maize inbreds respond negatively to some strains. Trichoderma spp. are beginning to be used in reasonably large quantities in plant agriculture, both for disease control and yield increases. The studies of mycoparasitism also have demonstrated that these fungi produce a rich mixture of antifungal enzymes, including chitinases and β-1,3 glucanases. These enzymes are synergistic with each other, with other antifungal enzymes, and with other materials. The genes encoding the enzymes appear useful for producing transgenic plants resistant to diseases and the enzymes themselves are beneficial for biological control and other processes.
Dead Spot Severity, Pseudothecia Development, and Overwintering ofOphiosphaerella agrostisin Creeping Bentgrass
Authors: Kaminski John E. , Dernoeden Peter H.
Published: May 2007

Dead spot (Ophiosphaerella agrostis) is a damaging disease of young ≤6 years old) creeping bentgrass (Agrostis stolonifera) golf greens. The objectives of this 3-year field study were to determine the peak periods of dead spot activity based on increasing patch diameter and pseudothecia development, and to determine where O. agrostis overwinters. Patch diameter generally increased at a linear rate between mid-June and early August. Increases in pseudothecia production closely followed increasing patch diameter. Pseudothecia could be found within necrotic tissue as early as the first day of symptom expression and as many as 478 pseudothecia were found in a single patch. Periods of rapid dead spot development coincided with air and soil temperatures ranging from 22 to 26°C. Increases in patch diameter and pseudothecia development were negligible after late August. Dead spot severity was highest in the year following green construction or fumigation, but then rapidly declined in severity in subsequent years. O. agrostis was capable of overwintering as pseudothecia or as hyphae within or on bentgrass leaf sheaths, crowns, roots, and especially the nodes of stolons.
Genetic Structure of the Population of Pepino mosaic virus Infecting Tomato Crops in Spain
Authors: Pagán Israel , del Carmen Córdoba-Sellés María , Martínez-Priego Llucia , Fraile Aurora , Malpica José M. , Jordá Concepción , García-Arenal Fernando
Published: May 2007

The population structure of Pepino mosaic virus (PepMV), which has caused severe epidemics in tomato in Spain since 2000, was analyzed. Isolates were characterized by the nucleotide sequence of the triple gene block and coat protein gene and, for a subset of isolates, a part of the RNA-dependent RNA polymerase gene. The full-length sequence of the genomic RNA of a Solanum muricatum isolate from Peru also was determined. In spite of high symptom diversity, the Spanish population of PepMV mostly comprised highly similar isolates belonging to the strain reported in Europe (European tomato strain), which has been the most prevalent genotype in Spain. The Spanish PepMV population was not structured spatially or temporally. Also, isolates highly similar to those from nontomato hosts from Peru (Peruvian strain) or to isolate US2 from the United States (US2 strain) were detected at lower frequency relative to the European strain. These two strains were detected in peninsular Spain only in 2004, but the Peruvian strain has been detected in the Canary Islands since 2000. These results suggest that PepMV was introduced into Spain more than once. Isolates from the Peruvian and US2 strains always were found in mixed infections with the European tomato strain, and interstrain recombinants were detected. The presence of different strains of the virus, and of recombinant isolates, should be considered for the development of control strategies based on genetic resistance.
Importance of the Life Cycle in Sympatric Host Race Formation and Speciation of Pathogens
Authors: Giraud Tatiana , Villaréal Lorys M. M. A. , Austerlitz Frédéric , Le Gac Mickaël , Lavigne Claire
Published: May 2007

Numerous morphological species of pathogenic fungi have been shown to actually encompass several genetically isolated lineages, often specialized on different hosts and, thus, constituting host races or sibling species. In this article, we explore theoretically the importance of some aspects of the life cycle on the conditions of sympatric divergence of host races, particularly in fungal plant pathogens. Because the life cycles classically modeled by theoreticians of sympatric speciation correspond to those of free-living animals, sympatric divergence of host races requires the evolution of active assortative mating or of active host preference if mating takes place on the hosts. With some particular life cycles with restricted dispersal between selection on the host and mating, we show that divergence can occur in sympatry and lead to host race formation, or even speciation, by a mere process of specialization, with strong divergent adaptive selection. Neither active assortative mating nor active habitat choice is required in these cases, and this may explain why the phylo-genetic species concept seems more appropriate than the biological species concept in these organisms.
Genetic and Virulence Diversity in Verticillium dahliae Populations Infecting Artichoke in Eastern-Central Spain
Authors: Jiménez-Díaz Rafael M. , Mercado-Blanco Jesús , Olivares-García Concepción , Collado-Romero Melania , Bejarano-Alcázar José , Rodríguez-Jurado Dolores , Giménez-Jaime Ana , García-Jiménez José , Armengol Josep
Published: May 2007

Severe Verticillium dahliae attacks have occurred in artichoke crops in the Comunidad Valenciana region of eastern-central Spain since the late 1990s. Knowledge of genetic and virulence diversity in the pathogen population is a key factor for the management of the disease through disease risk assessment as well as development and use of resistant cultivars. V. dahliae isolates from artichoke (109 isolates) and cotton (three isolates) in that region were characterized by vegetative compatibility grouping (VCG), and specific polymerase chain reaction assays using three sets of primer pairs that differentiate the cotton-defoliating (D) and -nondefoliating (ND) V. dahliae pathotypes. In all, 35 and 39 V. dahliae isolates representative of the identified VCGs and geographic origins were tested for virulence to artichoke cvs. Nun 6374 and Nun 9444, and cotton cv. Acala SJ-2, respectively. Four VCGs were identified among 107 artichoke isolates, and 2 isolates were heterokaryon self-incompatible: VCG1A (one isolate), VCG2A (31 isolates), VCG2B (72 isolates), and VCG4B (three isolates). The three cotton isolates were VCG1A. Isolates in VCG2B were distributed across the region and were the most prevalent isolates in the northern part. Conversely, 83.9% of isolates in VCG2A were recovered from the southern part of the region. Two subgroups of isolates were identified in VCG2B based on heterokaryon compatibility with either international or local tester isolates, which further showed diversity in the amplification of 334- and 824-bp DNA fragments which are markers of the D and ND pathotypes, respectively. Virulence of isolates to artichoke and cotton correlated with VCG but the pattern of correlation varied with the host. VCG1A isolates from artichoke and cotton induced defoliation in cotton but not in artichoke. Collectively, isolates of VCG2B and VCG4B were the most virulent and isolates of VCG1A or HSI were the least virulent to artichoke; but isolates of VCG1A were more virulent to cotton than those of any other VCG. Also, molecular subgrouping in VCG2B determined by amplification of the 334- and 824-bp markers correlated with virulence of isolates to the two hosts tested.
Genetic Analysis of Host Species Specificity of Magnaporthe oryzae Isolates from Rice and Wheat
Authors: Tosa Y. , Tamba H. , Tanaka K. , Mayama S.
Published: May 2007

A Triticum isolate (pathogenic on wheat) of Magnaporthe oryzae was crossed with an Oryza isolate (pathogenic on rice) to elucidate mechanisms of their parasitic specificity on wheat. When the pathogenicity of their F 1 cultures (hybrids between a Triticum isolate and an Oryza isolate) was tested on wheat, avirulent and virulent cultures segregated in a 7:1 ratio. This result suggests that three loci are involved in avirulence of the Oryza isolate on wheat. One of the three loci conditioned papilla formation, whereas the others conditioned the hypersensitive reaction. Allelism tests revealed that the locus conditioning papilla formation is Pwt2 while one of the two loci conditioning the hypersensitive reaction is Pwt1. The other locus conditioning the hypersensitive reaction was different from any other known loci and, therefore, was designated as Pwt5.
Molecular Variability Within and Among Verticillium dahliae Vegetative Compatibility Groups Determined by Fluorescent Amplified Fragment Length Polymorphism and Polymerase Chain Reaction Markers
Authors: Collado-Romero Melania , Mercado-Blanco Jesús , Olivares-García Concepción , Valverde-Corredor Antonio , Jiménez-Díaz Rafael M.
Published: May 2007

A degree of genetic diversity may exist among Verticillium dahliae isolates within vegetative compatibility groups (VCGs) that bears phytopathological significance and is worth investigating using molecular tools of a higher resolution than VCG characterization. The molecular variability within and among V. dahliae VCGs was studied using 53 artichoke isolates from eastern-central Spain, 96 isolates from cotton, 7 from cotton soil, and 45 from olive trees in countries of the Mediterranean Basin. Isolates were selected to represent the widest available diversity in cotton- and olive-defoliating (D) and -nondefoliating (ND) pathotypes, as well as for VCG. The VCG of 96 cotton and olive isolates was determined in this present study. Molecular variability among V. dahliae isolates was assessed by fluorescent amplified fragment length polymorphism (AFLP) analysis and by polymerase chain reaction (PCR) assays for DNA fragments associated with the D (462 bp) and ND (824 bp) pathotypes, as well as a 334-bp amplicon associated with D pathotype isolates but also present in some VCG2B isolates. Isolates from cotton were in VCG1A, VCG1B, VCG2A, VCG2B, and VCG4B and those from olive trees were in VCG1A, VCG2A, and VCG4B. Artichoke isolates included representatives of VCG1A, VCG2A, VCG2B (including a newly identified VCG2Ba), and VCG4B. AFLP data were used to generate matrixes of genetic distance among isolates for cluster analysis using the neighbor-joining method and for analysis of molecular variance. Results demonstrated that V. dahliae isolates within a VCG subgroup are molecularly similar, to the extent that clustering of isolates correlated with VCG subgroups regardless of the host source and geographic origin. VCGs differed in molecular variability, with the variability being highest in VCG2B and VCG2A. For some AFLP/VCG subgroup clusterings, V. dahliae isolates from artichoke grouped in subclusters clearly distinct from those comprising isolates from cotton and olive trees. In addition, VCG2B isolates from artichoke formed two distinct clusters that correlated with PCR markers of 334 bp (VCG2B334) or 824 bp (VCG2B824). Artichoke isolates in the VCG2B334/2β334 cluster were molecularly similar to isolates of VCG1A. The molecular difference found among artichoke isolates in VCG2B correlates with virulence of isolates to artichoke and cotton cultivars demonstrated in a previous study.
Particular Structure of Plasmopara viticola Populations Evolved Under Greek Island Conditions
Authors: Rumbou Artemis , Gessler Cesare
Published: May 2007

Plasmopara viticola populations collected from three islands in the Ionian Sea—an arm of the Mediterranean Sea to the west of Greece—were analyzed with microsatellite molecular markers in order to investigate the pathogen population structure. Downy mildew populations from mainland regions previously studied were found to have high genotypic diversity and limited clonality; however, populations under Mediterranean island conditions mostly showed limited variation and the epidemics basically were driven by the multiple clonal infections of one or a few genotypes. Populations from different islands were differentiated from each other, whereas genetic divergence also was found among subpopulations of the same plot. Polyploid individuals and individuals that overwintered in asexual form were observed in some cases. The findings obtained by this population genetics study improve our understanding of the biology of the pathogen and lead to potential alternative control measures for the disease.
An Epidemiological Analysis of the Role of Disease-Induced Root Growth in the Differential Response of Two Cultivars of Winter Wheat to Infection byGaeumannomyces graminisvar.tritici
Authors: Bailey D. J. , Kleczkowski A. , Gilligan C. A.
Published: May 2007

Epidemiological modeling combined with parameter estimation of experimental data was used to examine differences in the contribution of disease-induced root production to the spread of take-all on plants of two representative yet contrasting cultivars of winter wheat, Ghengis and Savannah. A mechanistic model, including terms for primary infection, secondary infection, inoculum decay, and intrinsic and disease-induced root growth, was fitted to data describing changes in the numbers of infected and susceptible roots over time at a low or high density of inoculum. Disease progress curves were characterized by consecutive phases of primary and secondary infection. No differences in root growth were detected between cultivars in the absence of disease and root production continued for the duration of the experiment. However, significant differences in disease-induced root production were detected between Savannah and Genghis. In the presence of disease, root production for both cultivars was characterized by stimulation when few roots were infected and inhibition when many roots were infected. At low inoculum density, the transition from stimulation to inhibition occurred when an average of 5.0 and 9.0 roots were infected for Genghis and Savannah, respectively. At high inoculum density, the transition from stimulation to inhibition occurred when an average of 4.5 and 6.7 roots were infected for Genghis and Savannah, respectively. Differences in the rates of primary and secondary infection between Savannah and Genghis also were detected. At a low inoculum density, Genghis was marginally more resistant to secondary infection whereas, at a high density of inoculum, Savannah was marginally more resistant to primary infection. The combined effects of differences in disease-induced root growth and differences in the rates of primary and secondary infection meant that the period of stimulated root production was extended by 7 and 15 days for Savannah at a low and high inoculum density, respectively. The contribution of this form of epidemiological modeling to the better management of take-all is discussed.
Bioprotection of Spruce Logs Against Sapstain Using an Albino Strain of Ceratocystis resinifera
Authors: Morin Chantal , Tanguay Philippe , Breuil Colette , Yang Dian-Qing , Bernier Louis
Published: May 2007

We recovered a spontaneous albino strain from ascospores of Ceratocystis resinifera, a sapstain fungus that grows deeply and rapidly in freshly felled conifer trees. This albino strain, named Kasper, was tested for its ability to prevent discoloration of spruce sapwood caused by wild-type sapstain fungi and compared with Cartapip 97, a commercially available biological control agent of sapstain in lodgepole pine and red pine logs. In a laboratory trial, Kasper reduced sapstain of white spruce logs as much as 94.4% and was more efficient than Cartapip 97. In field trials conducted in an area north of Québec City, Kasper reduced sapstain of black spruce as much as 80%. In three of four field trials, Kasper was significantly more efficient than Cartapip 97 in reducing sapstain development. The exception was encountered in a 2003 trial conducted in a sawmill yard where Kasper did not reduce sapstain. In a field trial conducted in western Canada, at Aleza Lake forest near Prince George, Kasper almost totally prevented the development of sapstain, even after 24 weeks. These results suggest albino strains derived from C. resinifera might be an additional source of potential biocontrol agents against sapstain.
Fusarium Species from the Cassava Root Rot Complex in West Africa
Authors: Bandyopadhyay Ranajit , Mwangi Maina , Aigbe Sylvester O. , Leslie John F.
Published: May 2007

Fusarium species are a significant component of the set of fungi associated with cassava root rot. Yield losses due to root rot average 0.5 to 1 ton/ha but losses >3 ton/ha, an equivalent of 15 to 20% yield, often occur. This paper reviews previous work on cassava root rot and summarizes a few recent studies on Fusarium species associated with the disease. Our studies in Cameroon showed that 30% of rotted tubers were infected by Fusarium spp. 12 months after planting and represented 25% of all the fungal isolates recovered. Other commonly recovered fungi were Botryodiplodia theobromae and Armillaria spp. Numerous and diverse species of Fusarium were associated with rotted cassava roots in Nigeria and Cameroon. At least 13 distinct amplified fragment length polymorphism (AFLP) groups of Fusarium were distinguishable, each group probably a distinct species, and many of them might represent previously undescribed Fusarium species. The two largest of the AFLP groups correspond to F. oxysporum and F. solani species complex. The distribution of Fusarium spp. varied among countries and among locations within a country, suggesting that germ plasm resistant at one location may not be resistant at another. Fusarium spp. also cause seedling blight of cassava and can be recovered from the stems of infected plants up to 1 m above the ground. Therefore, the pathogen can spread with stems cut as planting material. Fusarium spp. also can colonize Chromolaena odorata, the dominant weed in short fallows, which could further complicate management efforts by serving as an alternative host for strains that colonize cassava.
Major Genetic Changes in Wheat with Potential to Affect Disease Tolerance
Authors: Foulkes M. J. , Paveley N. D. , Worland A. , Welham S. J. , Thomas J. , Snape J. W.
Published: May 2007

Selection through plant breeding has resulted in most elite winter wheat germplasm in the United Kingdom containing the Rht-D1b semi-dwarfing allele, the 1BL.1RS chromosome arm translocation with rye, and an allele conferring suppression of awns. Near-isogenic lines (NILs) were used to test whether these major genetic changes have had any effect on disease tolerance. The ability of the NILs to tolerate epidemics of Septoria leaf blotch or stripe rust was measured in four field experiments over two seasons. Tolerance was quantified as yield loss per unit of green canopy area lost to disease. There was a trend for the presence of the 1BL.1RS translocation to decrease tolerance; however, this was not consistent across experiments and there was no effect of semi-dwarfing. The awned NIL exhibited decreased tolerance compared with the unawned NIL. There were significant differences in tolerance between the cultivar backgrounds in which the NILs were developed. Tolerance was lower in the modern genetic background of Weston, released in 1996, than in the genetic background of Maris Hunstman, released in 1972. The data suggest that certain physiological traits were associated with the tolerance differences among the backgrounds in these experiments. Potential yield, accumulation of stem soluble carbohydrate reserves, and grain sink capacity were negatively correlated with tolerance, whereas flag leaf area was positively correlated.
Interaction of Fusarium solani f. sp. glycines and Heterodera glycines in Sudden Death Syndrome of Soybean
Authors: Xing Lijuan , Westphal Andreas
Published: May 2007

Sudden death syndrome (SDS) of soybean is caused by the soilborne Fusarium solani f. sp. glycines (synonym F. virguliforme). In a sequential approach, two multifactor factorial-design microplot experiments were conducted to investigate the effects of fungal infestation levels and soil moisture on both root necrosis and foliar SDS severity, and the interaction between F. solani f. sp. glycines and Heterodera glycines in fumigated versus nonfumigated soil. In 2003, soybean cv. Spencer was grown in nonfumigated or methyl bromide-fumigated soil and infested with increasing levels of F. solani f. sp. glycines, either under rainfall or irrigated after growth stage V6/R1. In 2004, interactions between F. solani f. sp. glycines and H. glycines were explored in a factorial inoculation design in fumigated or nonfumigated soil, planted to Williams 82 or Cyst-X20-18. In both years, higher levels of foliar SDS severity and root necrosis were found in F. solani f. sp. glycines-infested soils with H. glycines than in soils without the nematode on the soybean cultivars susceptible to both pathogens. Both natural infestations of H. glycines in 2003 and artificially amended populations of H. glycines in 2004 contributed to higher foliar SDS severity. More severe foliar SDS symptoms always were associated with more root necrosis, but elevated levels of root necrosis did not predict severe leaf symptoms. In contrast to the critical role of H. glycines, increasing fungal infestation levels had no significant effects on increasing either foliar SDS symptoms or root necrosis. Effects of moisture regime and fungal infestation levels also were examined in factorial greenhouse and growth chamber experiments. High soil moisture resulted in higher levels of SDS root necrosis. In the greenhouse, root necrosis increased at a higher rate in low soil moisture than the rate in high soil moisture. The two pathogens acted as a complex and the disease development was strongly dependent on high soil moisture.