bluuebunny/crossref_metadata_2025 huggingface dataset:
Population Structure of Dogwood Anthracnose Fungus
Authors: Zhang Ning , Blackwell Meredith
Published: May 2007

Dogwood anthracnose, caused by Discula destructiva, affects several native dogwood species in North America, especially flowering dogwood in the east and Pacific dogwood in the west. The fungus behaves as a recently introduced plant pathogen under episodic selection. Two distinct disjunct groups of fungal isolates corresponding to eastern and western groups were detected by amplified fragment length polymorphisms and sequences of the intergenic spacer (IGS) of the nuclear ribosomal DNA, translation elongation factor-1α, and β-tubulin genes. Of 20 genotypes identified among 72 isolates, 17 genotypes were from the eastern United States (n = 50), but only three were present among the western isolates (n = 22), indicating that the eastern population may be more diverse. Most eastern and western isolates belonged to a few widespread clones, and the genetic variability of this apparently asexual fungus was remarkably low compared with that of many other asexual fungi. We conclude that D. destructiva is still under intense selection pressure and that episodic selection may still be in effect. The New York City area, a possible epidemic center in the east, had relatively higher genetic variability than samples from other areas.
A Laboratory Simulation for Vectoring of Trichosporon pullulans by Conidia of Botrytis cinerea
Authors: Cook Darryl W. M.
Published: May 2007

A mechanism that could contribute to the suppression of Botrytis cinerea during pathogen sporulation was examined in this study. Yeasts capable of binding to B. cinerea were formulated with a cellulose carrier and applied to sporulating colonies of the pathogen. The particles from this yeast/cellulose product attached to B. cinerea conidia in the sporulating colony. Inoculum from treated colonies was harvested and applied to tomato stem tissue to test for subsequent pathogenicity. Disease development from inoculum obtained from cultures that had been treated with Trichosporon pullulans was significantly retarded (P = 0.0001) compared with cellulose-only controls. However, between 5 and 11% of conidia applied were attached to yeast cells. The removal of conidia not attached to yeast resulted in inoculum composed of >90% of conidia attached to yeast, and from this inoculum, disease development was significantly retarded (P < 0.05). When inoculum from treated B. cinerea colonies was applied to nutrient limiting agar and then incubated, the B. cinerea conidia germinated, and yeast cells infested the new hyphal growth. Constraints of the formulation of the yeast used in this study, and the implications of this vectoring approach for the suppression of B. cinerea during pathogen sporulation are discussed.
Wheat Genotype-Specific Induction of Soil Microbial Communities Suppressive to Disease Incited by Rhizoctonia solani Anastomosis Group (AG)-5 and AG-8
Authors: Mazzola Mark , Gu Yu-Huan
Published: May 2007

The induction of disease-suppressive soils in response to specific cropping sequences has been demonstrated for numerous plant-pathogen systems. The role of host genotype in elicitation of the essential transformations in soil microbial community structure that lead to disease suppression has not been fully recognized. Apple orchard soils were planted with three successive 28-day cycles of specific wheat cultivars in the greenhouse prior to infestation with Rhizoctonia solani anastomosis group (AG)-5 or AG-8. Suppressiveness to Rhizoctonia root rot of apple caused by the introduced isolate of R. solani AG-5 was induced in a wheat cultivar-specific manner. Pasteurization of soils after wheat cultivation and prior to pathogen introduction eliminated the disease suppressive potential of the soil. Wheat cultivars that induced disease suppression enhanced populations of specific fluorescent pseudomonad genotypes with antagonistic activity toward R. solani AG-5 and AG-8, but cultivars that did not elicit a disease suppressive soil did not modify the antagonistic capacity of this bacterial community. When soils were infested prior to the initial wheat planting, all cultivars were uniformly susceptible to R. solani AG-8. However, when pathogen inoculum was added after three growth-cycles, wheat root infection during the fourth growth-cycle varied in a cultivar specific manner. The same wheat cultivar-specific response in terms of transformation of the fluorescent pseudomonad community and subsequent suppression of Rhizoctonia root rot of apple was observed in three different orchard soils. These results demonstrate the importance of host genotype in modification of indigenous saprophytic microbial communities and suggest an important role for host genotype in the success of biological control.
Induced Systemic Protection Against Tomato Late Blight Elicited by Plant Growth-Promoting Rhizobacteria
Authors: Yan Zhinong , Reddy M. S. , Ryu Choong-Min , McInroy John A. , Wilson Mark , Kloepper Joseph W.
Published: May 2007

Two strains of plant growth-promoting rhizobacteria (PGPR), Bacillus pumilus SE34 and Pseudomonas fluorescens 89B61, elicited systemic protection against late blight on tomato and reduced disease severity by a level equivalent to systemic acquired resistance induced by Phytophthora infestans or induced local resistance by chemical inducer β-amino butyric acid (BABA) in greenhouse assays. Germination of sporangia and zoospores of P. infestans on leaf surfaces of tomato plants treated with the two PGPR strains, pathogen, and chemical BABA was significantly reduced compared with the noninduced control. Induced protection elicited by PGPR, pathogen, and BABA were examined to determine the signal transduction pathways in three tomato lines: salicylic acid (SA)-hydroxylase transgenic tomato (nahG), ethylene insensitive mutants (Nr/Nr), and jasmonic acid insensitive mutants (def1). Results suggest that induced protection elicited by both bacilli and pseudomonad PGPR strains was SA-independent but ethylene- and jasmonic acid-dependent, whereas systemic acquired resistance elicited by the pathogen and induced local resistance by BABA were SA-dependent. The lack of colonization of tomato leaves by strain 89B61 suggests that the observed induced systemic resistance (ISR) was due to systemic protection by strain 89B61 and not attributable to a direct interaction between pathogen and biological control agent. Although strain SE34 was detected on tomato leaves, ISR mainly accounted for the systemic protection with this strain.
Reducing Fumigant Emissions After Soil Application
Authors: Yates S. R. , Gan J. , Papiernik S. K. , Dungan R. , Wang D.
Published: May 2007

Volatilization and soil transformation are major pathways by which pesticides dissipate from treated agricultural soil. Volatilization is a primary source of unwanted agricultural chemicals in the atmosphere and can significantly affect fumigant efficacy. Volatile pesticides may cause other unique problems; for example, the soil fumigant methyl bromide has been shown to damage stratospheric ozone and will soon be phased out. There is also great concern about the health consequences of inhalation of fumigants by people living in proximity to treated fields. Because replacement fumigants will likely face increased scrutiny in years ahead, there is a great need to understand the mechanisms that control their emission into the atmosphere so these losses can be minimized without loss of efficacy. Recent research has shown that combinations of vapor barriers and soil amendments can be effective in reducing emissions. In this paper, some potential approaches for reducing fumigant emissions to the atmosphere are described.
The Effect of Endophytic Fusarium verticillioides on Infestation of Two Maize Varieties by Lepidopterous Stemborers and Coleopteran Grain Feeders
Authors: Schulthess F. , Cardwell K. F. , Gounou S.
Published: May 2007

A series of experiments were conducted to test the effect of the presence of Fusarium verticillioides in the maize plant on subsequent infestation by coleopteran and lepidopteran pests. The effect of percent internodes 1 to 5 infected with F. verticillioides, time after planting, and maize variety on attacks of stem and ears by lepidopterous and coleopteran pests was assessed in field experiments in early and late season 1998 and early season 1999 in Benin Republic. Artificial inoculation of the first internode with fungal-treated toothpicks was compared with a hot-water-fungicide seed treatment and a control. In 1998, two varieties that differed in husk tightness, the improved DMRLSR-W and the local Gbogbe, were used. Percentage of node 1 to 5 and plants infected was highest with the inoculation treatment but tended to be similar in the seed treatment and the control. The infection rate tended to increase with time and, within sampling date, decreased with node level. Ear infection was strongly correlated with percent infected nodes, indicating that F. verticillioides in the stem predisposed kernel infection. F. verticillioides incidence was higher in Gbogbe than in DMRLSR-W. Stem and ear infestations by the pyralid Eldana saccharina, the major pest in the area, tended to be highest in inoculation and lowest in the protection treatment. The same trends were found for the pyralid Chilo spp., the tortricid Cryptophlebia leucotreta, and beetles pooled across species. Significant positive correlations were found between ear/stem F. verticillioides infection and E. saccharina, Cryptophlebia leucotreta, Mussidia nigrivenella, and the noctuid Sesamia calamistis, but the latter three pest species were only significantly correlated with fungal infection of the upper nodes of the plant. Similar to disease incidence, E. saccharina numbers in stem and ear were higher in Gbogbe than DMRLSR-W in late 1998, whereas for the pyralid ear feeder M. nigrivenella, it was reversed. It was suggested that some lepidopterous and coleopteran pests are attracted by and survive longer (or have lower mortality) on plants infected with F. verticillioides.
Inheritance of Race-Specific Resistance to Mycosphaerella graminicola in Wheat
Authors: McCartney C. A. , Brûlé-Babel A. L. , Lamari L.
Published: May 2007

Mycosphaerella graminicola causes Septoria tritici blotch of hexaploid and tetraploid wheat. The inheritance of high-level resistance to Septoria tritici blotch was studied in controlled environment experiments. Intraspecific reciprocal crosses were made between hexaploid wheat lines Salamouni, ST6, Katepwa, and Erik, and the tetraploid wheat lines Coulter and 4B1149. Parental, F1, F2, F3, BC1F1, and BC1F2 populations were evaluated for reaction to isolates MG2 and MG96-36 of M. graminicola. Resistance was controlled by incompletely dominant nuclear genes in all cases. Salamouni had three independent resistance genes to isolate MG2, two of which also controlled resistance to isolate MG96-36. ST6 had a single resistance gene to isolate MG2 and none to isolate MG96-36. The resistance genes in Salamouni and ST6 were not allelic. Two independent genes control resistance to isolate MG2 in Coulter, one of which also controlled resistance to isolate MG96-36. These data are consistent with a gene-for-gene interaction in the wheat-M. graminicola pathosystem.
Temperature Sensitivity of the Hypersensitive Response of Bell Pepper to Xanthomonas axonopodis pv. vesicatoria
Authors: Romero A. M. , Kousik C. S. , Ritchie D. F.
Published: May 2007

When bacterial spot-resistant pepper plants carrying resistance gene Bs2 and infiltrated with incompatible strains of Xanthomonas axonopodis pv. vesicatoria carrying a functional avrBs2 gene (races P1 and P3) were incubated at 32°C, they exhibited an electrolyte leakage and bacterial multiplication pattern in planta similar to that obtained with a compatible strain (race P4) carrying a nonfunctional avrBs2 gene. They also developed disease-like symptoms. Pretreatment of incompatible bacteria at 32°C before infiltration caused a delay in electrolyte leakage less pronounced than that caused by exposing plants to 32°C. Also, plants had to be exposed to 32°C for an hour prior to inoculation to increase symptom expression. These data suggest that the Bs2 gene is temperature sensitive. In other experiments, the avrBs1-Bs1 interaction appeared to be the most heat tolerant and thus the least likely to revert to compatible, whereas the avrBs3-Bs3 interaction had an intermediate sensitivity to elevated temperatures.
Pheromone-Related Inhibitors of Ustilago hordei Mating and Tilletia tritici Teliospore Germination
Authors: Kosted Paula J. , Gerhardt Shirley A. , Sherwood John E.
Published: May 2007

Ustilago hordei, the causal agent of barley covered smut, produces mating pheromones that break down to smaller peptide compounds that act as potent inhibitors of mating and germination in several fungi. The pheromones are members of the farnesylated family of proteins. Synthetic peptide analogs of the pheromone derivatives, ranging in size from 4 mers to full length pheromones, were farnesylated, methyl esterified, or both and tested for mating or teliospore germination inhibition with U. hordei or Tilletia tritici, respectively. N-Acetyl-S-farnesylcysteine, which inhibits processing of Ras, and other sulfur-containing compounds such as homocysteine or methionine, were likewise modified and tested. The most potent inhibitors were methionine methyl ester and modified 4-mer peptides from both pheromones. Alanine scanning of the inhibitory 4 mers determined that the native amino acid sequence was specific for a high level of activity. The sulfur amino acids appear to be required for inhibition. Glasshouse studies using selected antagonists of mating and teliospore germination as seed treatments inhibited covered smut of barley and common bunt of wheat, although the level of control was inconsistent. The use of pheromone-related antagonists to mating or teliospore germination is a promising, novel strategy for control of smut and bunt diseases.
The Internet-Based Fungal Pathogen Database: A Proposed Model
Authors: Kang Seogchan , Ayers John E. , DeWolf Erick D. , Geiser David M. , Kuldau Gretchen , Moorman Gary W. , Mullins Ewen , Uddin Wakar , Correll James C. , Deckert Gerald , Lee Yong-Hwan , Lee Yin-Won , Martin Frank N. , Subbarao Krishna
Published: May 2007

A better understanding of the phenotypic and genetic diversity of significant agricultural pathogens and how their populations change in the field is critical for designing successful, long-term disease management strategies. Although efforts to determine the genetic diversity of plant pathogens have substantially increased in recent years, mainly due to the availability of various molecular tools, complementary efforts to archive and integrate the resulting data have been very limited. As a consequence, it is often difficult to compare the available data from various laboratories because the data have been generated by diverse tools, often preventing any direct comparisons, and are saved in a format that is unsuitable for comparative studies. The establishment of an internet-based database that cross-links the digitized genotypic and phenotypic information of individual pathogens at both the species and population levels may allow us to effectively address these problems by coordinating the generation of data and its subsequent archiving. We discuss the needs, benefits, and potential structure of such a database.
Pyricularia grisea Isolates Causing Gray Leaf Spot on Perennial Ryegrass (Lolium perenne) in the United States: Relationship to P. grisea Isolates from Other Host Plants
Authors: Farman Mark L.
Published: May 2007

Gray leaf spot of perennial ryegrass (prg) (Lolium perenne), caused by the fungus Pyricularia grisea (teleomorph = Magnaporthe grisea), has rapidly become the most destructive of all turf grass diseases in the United States. Fungal isolates from infected prg were analyzed with several molecular markers to investigate their relationship to P. grisea strains found on other hosts. All of the molecular markers used in this study revealed that isolates from prg are very distantly related to those found on crabgrass. Fingerprinting with MGR586 (Pot3) revealed zero to three copies of this transposon in the prg pathogens, distinguishing them from isolates pathogenic to rice, which typically have more than 50 copies of this element. RETRO5, a newly identified retroelement in P. grisea, was present at a copy number of >50 in isolates from rice and Setaria spp. but only six to eight copies were found in the isolates from prg. The MAGGY retrotransposon was unevenly distributed in the prg pathogens, with some isolates lacking this element, some possessing six to eight copies, and others having 10 to 30 copies. These results indicated that the P. grisea isolates causing gray leaf spot are distinct from those found on crabgrass, rice, or Setaria spp. This conclusion was supported by an unweighted pair-group method with arithmetic average cluster analysis of single-copy restriction fragment length polymorphism haplo-types. Fingerprints obtained with probes from the Pot2 and MGR583 transposons revealed that the prg pathogens are very closely related to isolates from tall fescue, and that they share similarity with isolates from wheat. However, the wheat pathogens had fewer copies of these elements than those found on prg. Therefore, I conclude that P. grisea isolates commonly found on other host plant species did not cause gray leaf spot epidemics on prg. Instead, the disease appears to be caused by a P. grisea population that is specific to prg and tall fescue.
Populations of Phytophthora infestans in Israel Underwent Three Major Genetic Changes During 1983 to 2000
Authors: Cohen Yigal
Published: May 2007

In this survey, 799 isolates of Phytophthora infestans collected from potato crops in Israel during 1983 to 2000 were analyzed for mating type and sensitivity to metalaxyl, and 324 were analyzed for race structure. The A2 mating type, first recorded in 1983, fully dominated the pathogen population from 1983 until 1991 (9 years). It was thereafter replaced by the A1 mating type, which dominated the population during 1993 to 2000. Metalaxyl-resistant isolates were first recorded in 1982. During 1983 to 1991, the majority of the isolates were resistant. Isolates with intermediate sensitivity (I) to this fungicide were first observed in 1993, when both A1 and A2 mating types occurred in the population. The proportion of I isolates gradually increased, reaching 39 to 41% in 1997 to 1998, and then declined to ≈15% in 1999 to 2000. Pathogenicity to nine potato differential cultivars was determined for 80 potato isolates collected in 1983 to 1991, to 11 potato differentials in 173 isolates collected in 1993 to 1998, and in 71 potato isolates collected in 1999 to 2000. The first population was composed of 5 races with race 1,3,4,7,8,10 predominating (76%), the second population was composed of 19 races with race 1,3,4,7,8,10,11 predominating (63%), and the third population exhibited 42 (34 new) races with no single predominating race. RG-57 DNA fingerprinting and allozymes loci assays of 23 isolates revealed that isolates collected during 1984 to 1986 belonged to the PO-57 lineage, whereas those collected during 1997 to 1999 belonged to the RFO-39 lineage. Among isolates collected during 1993 to 1995, two unreported DNA fingerprinting patterns were found. Severe late blight epidemics occurred in tomato crops during 1998 to 2000. Of 35 tomato isolates, 28 were A1 and only 7 were A2. Of these tomato isolates, 94% were sensitive to metalaxyl. Almost every isolate had a different race structure on the 11 potato differentials. When inoculated onto three tomato differential cultivars, tomato isolates showed a virulence much more enhanced than potato isolates. The data suggest the Israeli population of P. infestans has passed through three major genetic changes during the past 18 years: in 1983, 1993, and 1999. The recent change included host specialization to tomato.
Evidence of Two Formae Speciales in Venturia inaequalis, Responsible for Apple and Pyracantha Scab
Authors: Le Cam Bruno , Parisi Luciana , Arene Laurence
Published: May 2007

Genetic relationships, mating crosses, and host specificity of Venturia inaequalis isolates from Malus spp. and of Spilocaea pyracanthae isolates from Pyracantha spp. were evaluated. Sequence analysis of the complete internal transcribed spacer (ITS) region (ITS1-5.8S to ITS2) revealed a total similarity between these two putative species. ITS restriction fragment length polymorphism carried out with five restriction enzymes on a collection of 28 isolates confirmed a lack of diversity in this region between and within these two populations. Additional isolates from three related species (V. pirina, V. nashicola, and S. eriobotryae) were divided into two distinct monophyletic groups in a phylogenetic tree using ITS sequence comparison. These groups were related to their anamorph (i.e., Spilocaea or Fusicladium). When inoculated on their host of origin, fields isolates caused typical symptoms of scab disease, and a host specificity was demonstrated by cross pathogenicity of isolates from Malus × domestica and Pyracantha spp. Mating on dried leaves in vitro between one isolate of each putative species led to production of numerous perithecia. Ninety-six sporulating monoascosporic progenies were isolated from this cross. Based on these genetic and pathogenic data, we proposed that pathogens responsible for scab on Malus spp. and Pyracantha spp. are considered as two formae speciales belonging to V. inaequalis.
Modeling the Risk of Entry, Establishment, Spread, Containment, and Economic Impact of Tilletia indica, the Cause of Karnal Bunt of Wheat, Using an Australian Context
Authors: Stansbury Chris D. , McKirdy Simon J. , Diggle Art J. , Riley Ian T.
Published: May 2007

Modeling techniques were developed to quantify the probability of Tilletia indica entering and establishing in Western Australia (WA), and to simulate spread, containment, and the economic impact of the pathogen. Entry of T. indica is most likely to occur through imports of bulk grain or fertilizer (0.023 ± 0.017 entries per year and approximately 0.009 ± 0.009 establishments per year). Entry may also occur through straw goods, new or second-hand agricultural machinery, and on personal effects of travelers who have visited regions with infected plants. The combined probability of entry and establishment of T. indica, for all pathways of entry, is about one entry every 25 years and one establishment every 67 years. Alternatively, sensitivity analysis does show that increases in quarantine funding can reduce the probability of entry to about one entry every 50 years and less than one establishment every 100 years. T. indica is spread efficiently through contaminated farm machinery, seed and soil, rain, air currents, and animals. Depending on the rate of spread of the pathogen and the amount of resources allocated for detection, the time until first detection could range from 4 to 11 years and the economic impact could range from 8 to 24% of the total value of wheat production in WA.
Geo-Referenced Spatiotemporal Analysis of the Urban Citrus Canker Epidemic in Florida
Authors: Gottwald Tim R. , Sun Xiaoan , Riley Tim , Graham James H. , Ferrandino Frank , Taylor Earl L.
Published: May 2007

Five areas in urban Miami were identified to study the spread of Xanthomonas axonopodis pv. citri to determine if the practice of removing exposed citrus trees within 38.1 m of trees affected by citrus canker was adequate to curtail further bacterial spread. To accomplish this, 18,769 trees in dooryards were surveyed, geo-referenced by differential global positioning systems (GPS), and assayed for disease severity, age of infection, citrus cultivar, location of infection in tree, and canopy size. For each tree, the date the tree became infected was estimated and used to separate trees into contiguous 30-day categories. For each area studied, distance measurements between focal trees and newly infected trees were calculated for various temporal periods of 30, 60, 90, and 120 days in duration, corresponding to intervals of inspection survey. A visual basic application was used to calculate the distances between each newly diseased tree and all prior focal trees. The nearest distance was used because it was considered the most conservative estimate possible. It is therefore likely to be an underestimate of spread but is a good estimate of the minimum possible distances of spread. For the first four 30-day periods among the five study areas, calculated maximum distances of spread ranged from 12 to 3,474 m, indicating a broad continuum of distance for bacterial spread was possible. Disease increased during the first two-thirds of the time studied and reached an asymptote due to dry conditions in the final one-third of the duration of the study. Cross correlation analysis indicated that disease was best visualized 107 days following rainstorms with wind. Analysis of regional spatial point patterns was performed temporally for each 30-day period via a modified Ripley's K-function. Spatiotemporal analyses between periods over areas larger than previously examined were accomplished via spatiotemporal semivariogram analysis. These methods in combination demonstrated rapid increases in range of spatial dependency and range of spatiotemporal dependency for all study sites. This corresponded to rapid spread of disease across the regions studied in response to rainstorms with wind followed by a “filling in” of disease on remaining noninfected susceptible trees through time by less intense rain events. A stochastic quadratization technique demonstrated that disease incidence and disease severity were not greatly affected by urban host density but were positively correlated to host susceptibility within local 0.25-km2quadrats.
A Gene-for-Gene Relationship Between Wheat and Mycosphaerella graminicola, the Septoria Tritici Blotch Pathogen
Authors: Brading Penny A. , Verstappen Els C. P. , Kema Gert H. J. , Brown James K. M.
Published: May 2007

Specific resistances to isolates of the ascomycete fungus Mycosphaerella graminicola, which causes Septoria tritici blotch of wheat, have been detected in many cultivars. Cvs. Flame and Hereward, which have specific resistance to the isolate IPO323, were crossed with the susceptible cv. Longbow. The results of tests on F1 and F2 progeny indicated that a single semidominant gene controls resistance to IPO323 in each of the resistant cultivars. This was confirmed in F3 families of Flame × Longbow, which were either homozygous resistant, homozygous susceptible, or segregating in tests with IPO323 but were uniformly susceptible to another isolate, IPO94269. None of 100 F2 progeny of Flame × Hereward were susceptible to IPO323, indicating that the resistance genes in these two cultivars are the same, closely linked, or allelic. The resistance gene in cv. Flame was mapped to the short arm of chromosome 3A using microsatellite markers and was named Stb6. Fifty-nine progeny of a cross between IPO323 and IPO94269 were used in complementary genetic analysis of the pathogen to test a gene-for-gene relationship between Stb6 and the avirulence gene in IPO323. Avirulence to cvs. Flame, Hereward, Shafir, Bezostaya 1, and Vivant and the breeding line NSL92-5719 cosegregated, and the ratio of virulent to avirulent was close to 1:1, suggesting that these wheat lines may all recognize the same avirulence gene and may all have Stb6. Together, these data provide the first demonstration that isolate-specific resistance of wheat to Septoria tritici blotch follows a gene-for-gene relationship.
Two Loci fromLycopersicon hirsutumLA407 Confer Resistance to Strains ofClavibacter michiganensissubsp.michiganensis
Authors: Kabelka E. , Franchino B. , Francis D. M.
Published: May 2007

We used molecular markers to identify quantitative trait loci (QTL) that contribute to resistance to bacterial canker of tomato caused by Clavibacter michiganensis subsp. michiganensis. Resistance was first identified as a marker-trait association in an inbred backcross (IBC) population derived from crossing Lycopersicon hirsutum accession (LA407) with L. esculentum. Single-marker QTL analysis suggested that at least two loci originating from L. hirsutum LA407, Rcm 2.0 on chromosome 2 and Rcm 5.1 on chromosome 5, contribute to resistance in replicated trials. Two segregating F2populations were developed by crossing resistant inbred backcross lines (IBLs) to elite L. esculentum lines and used to confirm QTL associations detected in the IBC population. In these populations, realized heritability estimates were higher for selection based on maximal disease than for selection based on disease progression. Realized heritability in the population carrying Rcm 2.0 was 0.63 and 0.14, respectively, for each selection criteria. Realized heritability estimates were 0.85 for selection based on maximal disease and 0.37 for selection based on disease progression in a population carrying Rcm 5.1. The disease response of F3families selected for resistance suggested that both Rcm 2.0 and Rcm 5.1 confer resistance to bacterial strains in the repetitive sequence-based polymerase chain reaction DNA fingerprint classes A and C. Markers linked to Rcm 2.0 explained up to 56% of the total phenotypic variation for resistance in one population, and markers linked to Rcm 5.1 explained up to 73% of the total phenotypic variation for resistance in a separate population.
A Rainfall-Based Model for Predicting the Regional Incidence of Wheat Seed Infection by Stagonospora nodorum in New York
Authors: Shah Denis A. , Bergstrom Gary C.
Published: May 2007

Our goal was to develop a simple model for predicting the incidence of wheat seed infection by Stagonospora nodorum across western and central New York in any given year. The distribution of the incidence of seed infection by S. nodorum across the region was well described by the beta-binomial probability distribution (parameters p and θ). Mean monthly rainfalls in May and in June across western and central New York were used to predict p. The binary power law was used to predict θ. The model was validated with independent data collected from New York. The predicted distribution of seed infection incidence was not statistically different from the actual distribution of the incidence of seed infection.
Identification of a Chlorosis-Inducing Toxin from Pyrenophora tritici-repentis and the Chromosomal Location of an Insensitivity Locus in Wheat
Authors: Effertz R. J. , Meinhardt S. W. , Anderson J. A. , Jordahl J. G. , Francl L. J.
Published: May 2007

Culture filtrate from Pyrenophora tritici-repentis race 1 isolate 78-62 contained a genotype-specific toxin which elicited extensive chlorosis on sensitive wheat genotypes. This toxin was partially purified using gel filtration, ion exchange, and reversed-phase chromatography. The chlorosis toxin was found to be a polar, nonionic, low-molecular-weight molecule. Wheat genotypes infiltrated with crude culture filtrate and partially purified chlorosis toxin exhibited the same chlorotic symptoms seen with conidial inoculations of isolate 78-62. All tested wheat genotypes that exhibited extensive chlorosis to the toxin also exhibited extensive chlorosis to conidial inoculations, and all wheat genotypes insensitive to the toxin did not exhibit extensive chlorosis to conidial inoculations. The recombinant inbred population derived from the cross W-7984 × Opata 85 segregated for chlorosis induction from infiltration with partially purified chlorosis toxin from isolate 78-62. The locus identified by the marker XGli1, associated with resistance to conidial inoculations from race 1 isolates Pti2 and 78-62 and race 3 isolate D308, also was associated with insensitivity to infiltration of crude culture filtrate and partially purified chlorosis toxin. The marker XGli1, located on the short arm of chromosome 1A, is linked to the insensitivity locus within 5.7 cM. We propose that this chlorosis toxin be designated Ptr ToxC.
Effects of Wheat Cultivar Mixtures on Epidemic Progression of Septoria Tritici Blotch and Pathogenicity of Mycosphaerella graminicola
Authors: Cowger Christina , Mundt Christopher C.
Published: May 2007

The effects of host genotype mixtures on disease progression and pathogen evolution are not well understood in pathosystems that vary quantitatively for resistance and pathogenicity. We used four mixtures of moderately resistant and susceptible winter wheat cultivars naturally inoculated with Mycosphaerella graminicola to investigate impacts on disease progression in the field, and effects on pathogenicity as assayed by testing isolate populations sampled from the field on greenhousegrown seedlings. Over 3 years, there was a correspondence between the mixtures' disease response and the pathogenicity of isolates sampled from them. In 1998, with a severe epidemic, mixtures were 9.4% less diseased than were their component pure stands (P = 0.0045), and pathogen populations from mixtures caused 27% less disease (P = 0.085) in greenhouse assays than did populations from component pure stands. In 1999, the epidemic was mild, mixtures did not reduce disease severity (P = 0.39), and pathogen populations from mixtures and pure stands did not differ in pathogenicity (P = 0.42). In 2000, epidemic intensity was intermediate, mixture plots were 15.2% more diseased than the mean of component pure stands (P = 0.053), and populations from two of four mixtures were 152 and 156% more pathogenic than the mean of populations from component pure stands (P = 0.043 and 0.059, respectively). Mixture yields were on average 2.4 and 6.2% higher than mean component pure-stand yields in 1999 and 2000, respectively, but the differences were not statistically significant. The ability of mixtures challenged with M. graminicola to suppress disease appears to be inconsistent. In this system, host genotype mixtures evidently do not consistently confer either fitness benefits or liabilities on pathogen populations.
Evaluation of Relationships Between Weather Patterns and Prevalence of Sorghum Ergot in the Texas Panhandle
Authors: Workneh F. , Rush C. M.
Published: May 2007

Sorghum ergot caused by Claviceps africana was observed for the first time in the United States in Southern Texas in 1997. That year there was a widespread ergot epidemic in hybrid seed production fields in the Texas Panhandle. However, occurrence of the disease has been sparse during the past 3 years, easing fears that the hybrid seed industry in the region might be endangered. To determine whether climatic factors were associated with observed variations in prevalence of ergot, weather data (temperature, precipitation, and relative humidity) were collected from seven weather stations in the Texas Panhandle. Sorghum ergot prevalence data for the period 1997 to 2000 were collected from records of seed companies in the Panhandle and related to weather variables. Results showed that, in the southern section of the Panhandle, maximum temperature and precipitation between 1 and 15 August were associated (r2= 0.98, P = 0.001 and r2= 0.81, P = 0.0193, respectively) with variations in the prevalence of ergot during the 4-year period. In the northern section, only maximum temperature during 16 to 31 July was significantly associated (r2= 0.91, P = 0.0111) with disease prevalence. Over all, 1997 was wetter and cooler, during the 1 to 15 August period, than each of the subsequent 3 years. In addition to creating humid conditions for ergot development, precipitation was associated with suppression of maximum temperature, enhancing ergot-favorable temperature conditions. Examination of historic weather data for the region showed that there were many instances in the past where temperature depression was associated with a rise in cumulative precipitation, creating ergot-favorable conditions similar to those in 1997. Cross-spectral analysis was used to determine whether such association is periodic. Weather data from five of the seven locations in the region showed peaks of significant coherency ( α< 0.05) at 2 to 4 years and 7 to 10 years or greater, indicating the existence of a periodic cycle in the temperature-precipitation association. The results of the investigation suggested that association of precipitation with temperature depression is a primary factor in development of ergot in the Texas Panhandle, and such association has a periodic cycle.
Emergence of a New Cucurbit-Infecting Begomovirus Species Capable of Forming Viable Reassortants with Related Viruses in the Squash leaf curl virus Cluster
Authors: Brown J. K. , Idris A. M. , Alteri C. , Stenger Drake C.
Published: May 2007

Cucurbit leaf curl virus (CuLCV), a whitefly-transmitted geminivirus previously partially characterized from the southwestern United States and northern Mexico, was identified as a distinct bipartite begomovirus species. This virus has near sequence identity with the previously partially characterized Cucurbit leaf crumple virus from California. Experimental and natural host range studies indicated that CuLCV has a relatively broad host range within the family Cucurbitaceae and also infects bean and tobacco. The genome of an Arizona isolate, designated CuLCV-AZ, was cloned and completely sequenced. Cloned CuLCV-AZ DNA A and B components were infectious by biolistic inoculation to pumpkin and progeny virus was transmissible by the whitefly vector, Bemisia tabaci, thereby completing Koch's postulates. CuLCV-AZ DNA A shared highest nucleotide sequence identity with Squash leaf curl virus-R (SLCV-R), SLCV-E, and Bean calico mosaic virus (BCaMV) at 84, 83, and 80%, respectively. The CuLCV DNA B component shared highest nucleotide sequence identity with BCaMV, SLCV-R, and SLCV-E at 71, 70, and 68%, respectively. The cis-acting begomovirus replication specificity element, GGTGTCCTGGTG, in the CuLCV-AZ origin of replication is identical to that of SLCV-R, SLCV-E, and BCaMV, suggesting that reassortants among components of CuLCV-AZ and these begomoviruses may be possible. Reassortment experiments in pumpkin demonstrated that both reassortants of CuLCV-AZ and SLCV-E A and B components were viable. However, for CuLCV-AZ and SLCV-R, only one reassortant (SLCV-R DNA A/CuLCV-AZ DNA B) was viable on pumpkin, even though the cognate component pairs of both viruses infect pumpkin. These results demonstrate that reassortment among sympatric begomovirus species infecting cucurbits are possible, and that, if generated in nature, could result in begomoviruses bearing distinct biological properties.
Wheat Leaf Rust Uredospore Production and Carbon and Nitrogen Export in Relation to Lesion Size and Density
Authors: Robert Corinne , Bancal Marie-Odile , Lannou Christian
Published: May 2007

To develop mechanistic yield loss models for biotrophic fungi, we need better account for the export of dry matter, carbon, and nitrogen from the leaf into the spores. Three experiments in controlled environment chambers were performed to study the dynamics of uredospores production of Puccinia triticina on seedling leaves of wheat in relation to time, lesion density, and sporulating surface area. The detrimental effect of lesion density on the sporulation capacity of brown rust lesions was confirmed. When lesion density increased, spores production per lesion strongly decreased. However, our results showed that increasing lesion density also greatly reduces lesion size. A model was developed to summarize these relationships. Our main conclusion is that the density effect on spore production per lesion is accounted for by lesion size. When sporulation was related to the sporulating surface area, it became independent of density. As well, carbon and nitrogen contents of the spores were independent of lesion density. Our data suggest that when nitrogen available in the host is limiting, spore production is reduced but nitrogen content of spores tend to remain stable.
Incidence and Density Relationships of Powdery Mildew on Apple
Authors: Xu Xiangming , Madden Laurence V.
Published: May 2007

The relationships between disease incidence and colony density and between leaf and shoot disease incidences for apple powdery mildew were investigated over four seasons in order to derive a simple relationship for predicting density using incidence. The Neyman type A distribution generally provided a good fit to the observed number of colonies per leaf and shoot, and provided a significantly better fit than the Poisson distribution, indicating a degree of aggregation of mildew colonies. In general, Taylor's power-law satisfactorily described the observed variance-mean relationship for colony density; however, Taylor's power-law broke down at very high levels of mean density. Incidence of leaf infection could be determined based on average number of colonies per leaf assuming a fixed variance-mean relationship and a Neyman type A distribution for colony density. Regression models using the complemen- tary log-log transformation of incidence also provided accurate predictions of leaf (or shoot) disease incidence from colonies per leaf (or per shoot). Similar accuracies of these incidence-density models suggested that variance-mean ratio of colony density was more or less constant over time. Unlike the case with colony density, the number of mildewed leaves per shoot generally had a random pattern, as indicated by the good fit of the binomial distribution. Thus, it was possible to estimate the leaf incidence of the youngest unrolled leaves on a shoot using the shoot incidence. It is argued that the leaf incidence-density relationships developed in the present study may be used in making practical disease management decisions.
Abscission Layer Formation as a Resistance Response of Peruvian Apple Cactus Against Glomerella cingulata
Authors: Kim Young Ho , Kim Kwang-Hyung
Published: May 2007

Stem disks from 2-year-old cacti Cereus tetragonus (susceptible) and C. peruvianus (resistant) were inoculated in the center (pith) with Glomerella cingulata isolated from Colletotrichum stem rot in three-angled cacti. The susceptible cactus became extensively colonized, whereas colonization was limited to a small area in the resistant cactus. The resistant cactus formed prominent abscission layers (ALs) in parenchyma internal to the inoculation site. Ethanol extracts of the fungal culture also stimulated AL formation in the resistant cactus. Initial cell division followed at 2 to 4 days after treatment, and layering of multiple cells at 7 days after treatment. After 10 days, the outer layers were sometimes sloughed from the inner layers. No AL formation was induced in susceptible C. tetragonus treated with ethanol extract or in untreated control cacti. Light and electron microscopy revealed that initial cell division occurred by cell wall formation, and that an additional cell wall was layered in pre-existing parenchyma cells without ordinary cell division. Later, separation layers formed in ALs where inner cell walls appeared to be thickened secondarily, and the cell walls and middle lamella within the layer dissolved. These results suggest that AL formation in the resistant cactus is induced by fungal metabolites, and that it serves as a histological barrier against anthracnose pathogens.
Improving Biological Control by Combining Biocontrol Agents Each with Several Mechanisms of Disease Suppression
Authors: Guetsky Ruth , Shtienberg D. , Elad Y. , Fischer E. , Dinoor A.
Published: May 2007

Two biocontrol agents, a yeast (Pichia guilermondii) and a bacterium (Bacillus mycoides), were tested separately and together for suppression of Botrytis cinerea on strawberry leaves and plants. Scanning electron microscopy revealed significant inhibition of Botrytis cinerea conidial germination in the presence of Pichia guilermondii, whereas Bacillus mycoides caused breakage and destruction of conidia. When both biocontrol agents were applied in a mixture, conidial destruction was more severe. The modes of action of each of the biocontrol agents were elucidated and the relative quantitative contribution of each mechanism to suppression of Botrytis cinerea was estimated using multiple regression with dummy variables. The improvement in control efficacy achieved by introducing one or more mechanisms at a time was calculated. Pichia guilermondii competed with Botrytis cinerea for glucose, sucrose, adenine, histidine, and folic acid. Viability of the yeast cells played a crucial role in suppression of Botrytis cinerea and they secreted an inhibitory compound that had an acropetal effect and was not volatile. Bacillus mycoides did not compete for any of the sugars, amino acids, or vitamins examined at a level that would affect Botrytis cinerea development. Viable cells and the compounds secreted by them contributed similarly to Botrytis cinerea suppression. The bacteria secreted volatile and non-volatile inhibitory compounds and activated the defense systems of the host. The nonvolatile compounds had both acropetal and basipetal effects. Mixture of Pichia guilermondii and Bacillus mycoides resulted in additive activity compared with their separate application. The combined activity was due to the summation of biocontrol mechanisms of both agents. This work provides a theoretical explanation for our previous findings of reduced disease control variability with a mixture of Pichia guilermondii and Bacillus mycoides.
Broad-Spectrum Resistance to Different Geographic Strains ofPapaya ringspot virusin Coat Protein Gene Transgenic Papaya
Authors: Bau Huey-Jiunn , Cheng Ying-Huey , Yu Tsong-Ann , Yang Jiu-Sherng , Yeh Shyi-Dong
Published: May 2007

Papaya ringspot virus (PRSV) is a major limiting factor for cultivation of papaya (Carica papaya) in tropical and subtropical areas throughout the world. Although the coat protein (CP) gene of PRSV has been transferred into papaya by particle bombardment and transgenic lines with high resistance to Hawaii strains have been obtained, they are susceptible to PRSV isolates outside of Hawaii. This strain-specific resistance limits the application of the transgenic lines in other areas of the world. In this investigation, the CP gene of a local strain isolated from Taiwan, designated PRSV YK, was transferred into papaya via Agrobacterium-mediated transformation. A total of 45 putative transgenic lines were obtained and the presence of the transgene in papaya was confirmed by polymerase chain reaction amplification. When the plants of transgenic lines were challenged with PRSV YK by mechanical inoculation, they showed different levels of resistance ranging from delay of symptom development to complete immunity. Molecular analysis of nine selected lines that exhibited different levels of resistance revealed that the expression level of the transgene is negatively correlated with the degree of resistance, suggesting that the resistance is manifested by a RNA-mediated mechanism. The segregation analysis showed that the transgene in the immune line 18-0-9 has an inheritance of two dominant loci and the other four highly resistant lines have a single dominant locus. Seven selected lines were tested further for resistance to three PRSV heterologous strains that originated in Hawaii, Thailand, and Mexico. Six of the seven lines showed varying degrees of resistance to the heterologous strains, and one line, 19-0-1, was immune not only to the homologous YK strain but also to the three heterologous strains. Thus, these CP-transgenic papaya lines with broad-spectrum resistance have great potential for use in Taiwan and other geographic areas to control PRSV.