bluuebunny/crossref_metadata_2025 huggingface dataset:
Host Diversity Can Reduce Potato Late Blight Severity for Focal and General Patterns of Primary Inoculum
Authors: Garrett K. A. , Mundt C. C.
Published: May 2007

The use of host diversity as a tool for management of potato late blight has not been viewed as promising in the past. But the increasing importance of late blight internationally has brought new consideration to all potential management tools. We studied the effect of host diversity on epidemics of potato late blight in Oregon, where there was little outside inoculum. The experimental system consisted of susceptible potato cv. Red LaSoda and a highly resistant breeding selection, inoculated with local isolates of US-8 Phytophthora infestans. Potatoes were grown in single-genotype plots and also in a mixture of 10 susceptible and 26 resistant potato plants. Half of the plots received inoculation evenly throughout the plot (general inoculation) and half received an equal quantity of inoculum in only one corner of the plot (focal inoculation). The area under the disease progress curve (AUDPC) was greater in single genotype stands of susceptible cv. Red LaSoda inoculated throughout the plot than with stands inoculated in one focus. The host-diversity effect on foliar late blight was significant in both years of the investigation; the AUDPC was reduced by an average of 37% in 1997 and 36% in 1998, compared with the mean disease level for the potato genotypes grown separately. Though the evidence for influence of inoculum pattern on host-diversity effects was weak (P = 0.15), in both years there was a trend toward greater host-diversity effects for general inoculation. Statistical significance of host-diversity effects on tuber yield and blight were found only in one of the two years. In that year, tuber yield from both the resistant and susceptible cultivar was increased in mixtures compared with single genotype stands and tuber blight was decreased in mixtures for susceptible cv. Red LaSoda.
Transmission, Characterization, and Serology of a Luteovirus Associated with Yellow Leaf Syndrome of Sugarcane
Authors: Scagliusi Sandra Mansur , Lockhart B. E. L.
Published: May 2007

A previously uncharacterized luteovirus was associated with one form of yellow leaf syndrome (YLS), a widespread disease of sugarcane (Saccharum sp.). The virus was named Sugarcane yellow leaf luteovirus (ScYLV), and was identified in major sugarcane-producing areas of the world. Typical disease symptoms were reproduced when ScYLV was transmitted by Melanaphis sacchari or Rhopalosiphum maidis from infected to healthy sugarcane, suggesting that this virus may be the causal agent of one form of YLS. The only known hosts of ScYLV are Saccharum and Erianthus spp. Virions of ScYLV were 24 to 29 nm in diameter in sodium phosphotungstate at pH 5.0, had a buoyant density of 1.30 g/cm3 in Cs2SO4, and contained a 5.8-kb genomic ssRNA. The capsid protein had an estimated relative molecular mass of 27 kDa and was not glycosylated. A polyclonal rabbit antiserum raised against ScYLV did not detect any of eight other luteoviruses by enzyme-linked immunosorbent assay or immunosorbent electron microscopy, but in immunoblot assays, antibodies to ScYLV detected the RPV serotype of Barley yellow dwarf luteovirus. It is concluded that ScYLV is a previously undescribed luteovirus that is biologically and serologically distinct from other members of the group and may be the causal agent of one form of YLS of sugarcane.
Control of Soilborne Plant Pathogens by Incorporating Fresh Organic Amendments Followed by Tarping
Authors: Blok Wim J. , Lamers Jan G. , Termorshuizen Aad J. , Bollen Gerrit J.
Published: May 2007

A new method for the control of soilborne plant pathogens was tested for its efficacy in two field experiments during two years. Plots were amended with fresh broccoli or grass (3.4 to 4.0 kg fresh weight m-2) or left nonamended, and covered with an airtight plastic cover (0.135 mm thick) or left noncovered. In plots amended with broccoli or grass and covered with plastic sheeting, anaerobic and strongly reducing soil conditions developed quickly, as indicated by rapid depletion of oxygen and a decrease in redox potential values to as low as -200 mV. After 15 weeks, survival of Fusarium oxysporum f. sp. asparagi, Rhizoctonia solani, and Verticillium dahliae in inoculum samples buried 15 cm deep was strongly reduced in amended, covered plots in both experiments. The pathogens were not or hardly inactivated in amended, noncovered soil or nonamended, covered soil. The latter indicates that thermal inactivation due to increased soil temperatures under the plastic cover was not involved in pathogen inactivation. The results show the potential for this approach to control various soilborne pathogens and that it may serve as an alternative to chemical soil disinfestation for high-value crops under conditions where other alternatives, such as solarization or soil flooding, are not effective or not feasible.
Molecular Beacons: A New Approach to Plant Virus Detection
Authors: Eun Alvin Jin-Cherng , Wong Sek-Man
Published: May 2007

Molecular beacons are single-stranded nucleic acid molecules with a stem-loop conformation. The stem portion consists of complementary sequences at the 5′ and 3′ terminals of the molecule, while the loop portion consists of probe sequences that are complementary to the target sequences of choice. A fluorescent moiety is attached to one end, while a quenching moiety is attached to the opposite end. Reverse transcription-polymerase chain reactions are carried out with primers that amplify specific genome sequences of interest, yielding targets complementary to their respective molecular beacons for subsequent detection. Here, we have designed four molecular beacons specific to the RNA-dependent RNA polymerase and coat protein genes of two orchid viruses, namely Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV). This technology is successfully applied to detect as little as 0.5 ng of viral RNA of both orchid viruses simultaneously in 100 mg of coinfected Oncidium orchid leaves. This rapid and specific technique is applicable to the orchid industry, which routinely carries out virus indexing and screening for virus-resistant cultivars. We belief that use of this molecular beacon approach can be extended to the detection of multiple plant viruses in various crops.
Biological and Molecular Characterization of a New Carlavirus Isolated from an Aconitum sp.
Authors: Cohen J. , Zeidan M. , Rosner A. , Gera A.
Published: May 2007

A new virus was isolated from symptomless Aconitum napellus plants. The virus, for which the name Aconitum latent virus (AcLV) is proposed, has flexuous particles 640 nm in length. The experimental host range was limited to Nicotiana clevelandii. Electron microscopy studies of ultrathin sections of infected A. napellus tissues revealed the presence of elongated virus particles. No inclusion bodies characteristic of potyvirus infection were observed. AcLV was purified from naturally infected A. napellus by cesium chloride step gradient centrifugation. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis of dissociated purified virus preparations, a major protein component with a molecular mass of 35 kDa was observed. Diagnostic antibodies that could specifically bind to virus particles were produced. The 5′ terminus (620 nucleotides) of the viral RNA was cloned and sequenced. It comprised 71 nucleotides from the untranslated 5′ terminus and 549 nucleotides of an open reading frame encoding 183 amino acids. Comparison of the predicted amino acid sequence with those of other plant viruses revealed 40 to 60% identity with several carlaviruses. Based on particle morphology, absence of inclusion bodies in ultrathin sections, the relative molecular weight of the coat protein, the nucleotide sequence, and predicted amino acid homology, it is suggested that this virus belongs to the carlavirus group.
Genetic Variation Among Natural Populations of Tilletia controversa and T. bromi
Authors: Pimentel Guillermo , Peever Tobin L. , Carris Lori M.
Published: May 2007

Isolates of Tilletia controversa and T. bromi were sampled from wheat and two Bromus species hosts, respectively, in the Pacific Northwest, and genetic variation within and among populations was determined. Fifty-one random amplified polymorphic DNA markers from eleven primers were treated as phenotypic 1 and 0 character state data to estimate similarities and analyze molecular variance (AMOVA) among populations and as putative genetic loci to carry out analyses of gene diversity. Phenotypic analysis of T. controversa and T. bromi isolates revealed two distinct clusters that were 37% similar. The T. bromi cluster was subdivided further into two groups, corresponding to host, with 40% similarity. Cluster analysis based on allele frequencies produced similar results and also supported two T. bromi groups based on host. No evidence of natural hybridization and introgression was detected between the T. controversa and T. bromi populations. Both AMOVA and gene diversity analyses detected moderate levels of differentiation among T. controversa populations, whereas T. bromi populations were highly differentiated. The level of genetic differentiation observed between the T. bromi populations on different Bromus species hosts supports the hypothesis that a high degree of host specificity exists in the wild grass-infecting smuts. We speculate that the higher level of genetic differentiation among the T. bromi populations compared with the T. controversa populations on wheat may be due to selection by a more genetically diverse host population.
The Role of Chitinase Production by Stenotrophomonas maltophilia Strain C3 in Biological Control of Bipolaris sorokiniana
Authors: Zhang Zhongge , Yuen Garry Y.
Published: May 2007

The role of chitinase production by Stenotrophomonas maltophilia strain C3 in biological control of leaf spot on tall fescue (Festuca arundinacea), caused by Bipolaris sorokiniana, was investigated in vitro and in vivo. The filtrate of a broth culture of C3, with chitin as the carbon source, was separated into fractions. A high molecular-weight fraction (>8 kDa) was chitinolytic and more inhibitory than a low-molecular-weight, nonchitinolytic fraction to conidial germination and hyphal growth by B. sorokiniana and to leaf spot development. A protein fraction derived by ammonium sulfate precipitation and a chitinase fraction purified by chitin affinity chromatography also were chitinolytic and highly antifungal. The chitinolytic fractions caused swelling and vacuolation of conidia and discoloration, malformation, and degradation of germ tubes. When boiled, the chitinolytic fractions lost chitinase activity along with most of the antifungal properties. Two chitinase-deficient and two chitinase-reduced mutants of C3 were compared with the wild-type strain for inhibition of germination of B. sorokiniana conidia on tall fescue leaves and for suppression of leaf spot development in vivo. The mutants exhibited reduced antifungal activity and biocontrol efficacy, but did not lose all biocontrol activity. An aqueous extract of leaves colonized by wild-type C3 had higher chitinase activity than that of noncolonized leaves and was inhibitory to conidial germination. The addition of chitin to leaves along with the wild-type strain increased both chitinase and antifungal activity. The chitinase activity level of extracts from leaves colonized by a chitinase-deficient mutant of C3, with and without added chitin, was no higher than the background, and the extracts lacked antifungal activity. Chitinolysis appears to be one mechanism of biological control by strain C3, and it functions in concert with other mechanisms.
Genetic Diversity of Xanthomonas oryzae pv. oryzae Strains from Sri Lanka
Authors: Ochiai H. , Horino O. , Miyajima K. , Kaku H.
Published: May 2007

Sixty strains of Xanthomonas oryzae pv. oryzae, collected from 29 locations in Sri Lanka in 1995, were analyzed by restriction fragment length polymorphism using either polymerase chain reaction-amplified 16S and 23S rDNA or the repetitive DNA element IS1112 from X. oryzae pv. oryzae as hybridization probes. Two different ribogroups were observed in the Sri Lankan strains using rDNA probes, whereas five clusters were identified by the IS1112 probe. Bootstrap analysis revealed that the five clusters defined by IS1112 were relatively robust. Our results suggest that the Sri Lankan strains are phylogenetically composed of five different groups. Each cluster was partially associated with climatic conditions (intermediate zone and wet zone) and was related to groups based on ribotyping. Based on virulence analysis using 12 rice cultivars, each containing a single resistance gene, 14 pathotypes were identified among the Sri Lankan strains. All strains were virulent to resistance genes Xa1, Xa2, Xa4, Xa10, Xa11, and Xa14. Only one strain (pathotype 1) was virulent to all major resistance genes including Xa21, while strains of the other pathotypes were all avirulent to Xa21. A partial relationship was found between the determined phylogenetic groups using the IS1112 probe and pathotypes for all but two clusters. The results of this study will facilitate the further understanding of the population structure of X. oryzae pv. oryzae in Sri Lanka.
A Point Mutation in the FRNK Motif of the Potyvirus Helper Component-Protease Gene Alters Symptom Expression in Cucurbits and Elicits Protection Against the Severe Homologous Virus
Authors: Gal-On Amit
Published: May 2007

Sequence comparison had previously shown three amino acid changes in conserved motifs in the 455-amino acid sequence of the helper component-protease (HC-Pro) between a severe field strain of Zucchini yellow mosaic virus (ZYMV-NAT) and a mild field strain of ZYMV (ZYMV-WK). In this study, exchange of fragments and site-directed mutagenesis within the HC-Pro gene in an infectious clone of ZYMV enabled the effects of the mutations on symptom expression to be mapped. The substitution of Ile for Arg at position 180 in the conserved motif Phe-Arg-Asn-Lys (FRNK) of potyviruses was found to affect symptom expression. Infection of cucurbits with the engineered ZYMV (ZYMV-AG) that contained this mutation caused a dramatic symptom change from severe to mild in squash and to a symptom-free appearance in cucumber, melon, and watermelon. The Ile to Arg mutation was found to be stable, and no revertant virus was found after several passages through plants after long incubation periods. The AG strain was detected 4 days postinoculation and accumulated in cucurbits to a level and with kinetics similar to that of the wild-type ZYMV-AT strain. Cucurbit plants infected with the AG strain were protected against infection by the severe strain.
Molecular Differentiation of Fusarium solani f. sp. glycines from Other F. solani Based on Mitochondrial Small Subunit rDNA Sequences
Authors: Li Shuxian , Tam Yan Kit , Hartman Glen L.
Published: May 2007

Fusarium solani is a soilborne plant pathogen that infects many different hosts. Within the species, there is some specialization, and a number of forma specialis have been described based on host affiliation. One of these, F. solani f. sp. glycines, infects soybean and causes sudden death syndrome. To differentiate between F. solani f. sp. glycines and other F. solani isolates, a partial sequence of the mitochondrial small subunit (mtSSU) rRNA gene was amplified by polymerase chain reaction and sequenced from 14 F. solani f. sp. glycines and 24 F. solani isolates from various plant hosts. All F. solani f. sp. glycines isolates had identical sequences. A single, unique insertion of cytosine occurred in all F. solaniisolates but not in any of the F. solani f. sp. glycines isolates. Two major lineages, distinguished by sequence divergence and the presence or absence of multiple insertions, occurred in F. solani isolates. Cladistic analysis produced a single most-parsimonious tree with three major clades. The first clade contained all F. solani f. sp. glycines isolates. A second clade grouped together all of the F. solani isolates that had only a single nucleotide insertion difference from the first clade. Genetic distance between these two clades was 0.016. A third clade was formed by five F. solaniisolates that had multiple insertions. Isolates in the third clade had a genetic distance of 0.040 from the first and second clades. Based on the sequence data, it is likely that F. solani f. sp. glycineshas a shorter evolutionary history than other F. solaniisolates that have either single or multiple nucleotide insertions. The differences in nucleotide insertions in part of the mtSSU rRNA gene between F. solani f. sp. glycinesand other F. solani isolates provide a direct and reliable way to distinguish isolates of F. solani.
Clonal Spread of Sclerotium cepivorum in Onion Production with Evidence of Past Recombination Events
Authors: Couch B. C. , Kohn L. M.
Published: May 2007

Three field populations of Sclerotium cepivorum from Ontario and Que-bec in Canada, one field sample from the North Island of New Zealand, as well as isolates from other onion- and garlic-growing areas were classified in mycelial compatibility groups (MCGs) and further characterized by DNA fingerprints and DNA sequence polymorphisms at six genomic regions. MCGs in the population samples were genetically distinct clones. Members of a clone belonged to a unique MCG with an associated multi-locus genotype and DNA fingerprint. The structure in each field population was predominantly clonal based on the repeated sampling of genotypes over a wide geographic area. Only three clones were identified within each of the population samples. The hypothesis that these clones were introduced to the three agricultural production areas is supported by their recovery from additional locations. Some evidence for past recombination was detected when genotypes from the three population samples and from other locations were pooled.
Chino del tomate virus:Relationships to Other Begomoviruses and Identification of A-Component Variants that Affect Symptom Expression
Authors: Brown J. K. , Ostrow Kristin M. , Idris Ali M. , Stenger Drake C.
Published: May 2007

Phylogenetic and distance analyses place Chino del tomate virus (CdTV) in the New World clade of begomoviruses and indicate that CdTV and Tomato leaf crumple virus (TLCrV) are closely related strains of the same virus. One cloned CdTV A component (pCdTV-H6), when inoculated to tomato with the B component (pCdTV-B52), produced mild symptoms and low DNA titers. Another cloned CdTV A component (pCdTV-H8), when coinoculated to tomato with the B component, produced moderate leaf curling and veinal chlorosis similar to that of TLCrV. Coinoculation of both CdTV A components and the B component to tomato produced wild-type chino del tomate (CdT) disease symptoms consisting of severe leaf curling, veinal and interveinal chlorosis, and stunting. The two CdTV A components were nearly identical, except at nucleotide positions 1,722 and 2,324. The polymorphism at nucleotide 1,722 resulted in a change at Rep amino acid 261. The second polymorphism at nucleotide 2,324 resulted in changes at Rep amino acid 60 and AC4 amino acid 10. Two chimeric A components constructed by reciprocal exchange of a fragment bearing the polymorphic site at nucleotide 1,722 were evaluated for symptom phenotype. One chimeric A component (pCdTV-H86) produced wild-type CdT symptoms when coinoculated to tomato with the B component. The reciprocal chimeric A component (pCdTV-H68), when coin-oculated to tomato with the B component, also produced severe leaf curling, veinal chlorosis, and stunting. However, pCdTV-H68 induced less obvious interveinal chlorosis than wild-type or pCdTV-H86. Examination of A component genotypes recovered from tomato coinoculated with pCdTV-H6 and pCdTV-H8 indicated that recombination occurred to produce a genotype identical to pCdTV-H86. These results indicate that subtle genotypic variation has significant effects on symptom expression and may explain phenotypic differences observed among isolates and cloned DNAs of CdTV and TLCrV.
Early Events During Quiescent Infection Development by Colletotrichum gloeosporioides in Unripe Avocado Fruits
Authors: Beno-Moualem D. , Prusky D.
Published: May 2007

Inoculation of avocado pericarp tissue with Colletotrichum gloeospori-oides and treatment of avocado cell cultures with the cell wall elicitor of C. gloeosporioidesboth increased the production of reactive oxygen species (ROS). However, whereas the production of ROS could be detected within minutes in avocado cell suspensions, it was detected only after 2 h following inoculation of pericarp tissue. Protein kinase inhibitors such as K-252a and staurosporine and the phosphatase inhibitor microcystin-LR inhibited the release of H2O2 from avocado cell suspensions. When 1 mM H2O2 was exogenously applied to pericarp tissue, it enhanced ROS, phenyl-alanine ammonia lyase (PAL) activity, and epicatechin levels. But, when H2O2 treatment was applied following staurosporine treatment, PAL activity was no longer induced. The uninduced ROS production in pericarp tissue of freshly harvested, unripe, resistant fruit was twice as high as in ripe, susceptible fruit. Challenge inoculation of resistant fruit further increased the ROS level; however, this increase did not occur in susceptible fruits. The current findings are consistent with the hypothesis that production of ROS is induced by fungal infection of unripe fruits and, consequently, may modulate resistance, resulting in the inhibition of fungal development and quiescence.
A Theoretical Assessment of the Effects of Vector-Virus Transmission Mechanism on Plant Virus Disease Epidemics
Authors: Madden L. V. , Jeger M. J. , van den Bosch F.
Published: May 2007

A continuous-time and deterministic model was used to characterize plant virus disease epidemics in relation to virus transmission mechanism and population dynamics of the insect vectors. The model can be written as a set of linked differential equations for healthy (virus-free), latently infected, infectious, and removed (postinfectious) plant categories, and virus-free, latent, and infective insects, with parameters based on the transmission classes, vector population dynamics, immigration/emigration rates, and virus-plant interactions. The rate of change in diseased plants is a function of the density of infective insects, the number of plants visited per time, and the probability of transmitting the virus per plant visit. The rate of change in infective insects is a function of the density of infectious plants, the number of plants visited per time by an insect, and the probability of acquiring the virus per plant visit. Numerical solutions of the differential equations were used to determine transitional and steady-state levels of disease incidence (d*); d* was also determined directly from the model parameters. Clear differences were found in disease development among the four transmission classes: nonpersistently transmitted (stylet-borne [NP]); semipersistently transmitted (foregut-borne [SP]); circulative, persistently transmitted (CP); and propagative, persistently transmitted (PP), with the highest disease incidence (d) for the SP and CP classes relative to the others, especially at low insect density when there was no insect migration or when the vector status of emigrating insects was the same as that of immigrating ones. The PP and CP viruses were most affected by changes in vector longevity, rates of acquisition, and inoculation of the virus by vectors, whereas the PP viruses were least affected by changes in insect mobility. When vector migration was explicitly considered, results depended on the fraction of infective insects in the immigration pool and the fraction of dying and emigrating vectors replaced by immigrants. The PP and CP viruses were most sensitive to changes in these factors. Based on model parameters, the basic reproductive number (R0)—number of new infected plants resulting, from an infected plant introduced into a susceptible plant population—was derived for some circumstances and used to determine the steady-state level of disease incidence and an approximate exponential rate of disease increase early in the epidemic. Results can be used to evaluate disease management strategies.
Regression Analyses for Evaluating the Influence of Bacillus cereus on Alfalfa Yield Under Variable Disease Intensity
Authors: Kazmar Elizabeth R. , Goodman Robert M. , Grau Craig R. , Johnson David W. , Nordheim Erik V. , Undersander Daniel J. , Handelsman Jo
Published: May 2007

We developed and tested regression methods to exploit the variability in disease inherent in field experiments, and applied the methods to evaluate strains of Bacillus cereus for biocontrol efficacy. Four B. cereus strains were tested for their effect on alfalfa (Medicago sativa) performance in 16 field trials planted during 1993 to 1996 at multiple sites in Wisconsin. To evaluate performance of the strains, we used the ratio of (metalaxyl response)/(untreated control response) as a measure of disease intensity within the experiments. The ratio of (Bacillus response)/(untreated control response) was then regressed as a function of disease intensity. The slope of the resulting line provides a statistical test to compare performance of the Bacillus strain with that of the untreated seed (Ho: slope = 0) and metalaxyl controls (Ho: slope = 1). Under conditions in which disease occurred, forage yield of plots planted with seed treated with B. cereus strain AS4–12 exceeded yield from the untreated control plots (P = 0.002) and was similar to yield of plots planted with metalaxyl-treated seed (P = 0.14). Yield gain associated with AS4–12 and metalaxyl seed treatment averaged 6.1 ± 2.8% (±standard error) and 3.0 ± 2.8%, respectively. In contrast to the regression approach, means analysis by analysis of variance did not detect differences among treatments. Three other B. cereus strains either did not increase alfalfa yield or increased yield less than did AS4–12. Metalaxyl and three of the Bacillus strains increased seedling emergence, but the improved stands were not predictive of increased forage yield. In six additional studies conducted for one season in 1997, AS4–12 enhanced yield of two cultivars at diverse locations in Wisconsin, but there was an apparent cultivar-location interaction. A strong correlation between response to AS4–12 and metalaxyl treatment suggests that these treatments controlled similar pathogens, most likely the oomycete pathogens Phytophthora medicaginis and Pythium spp.
Antifungal Activity Toward Fusarium culmorum in Soluble Wheat Extracts
Authors: Doohan F. M. , Mentewab A. , Nicholson P.
Published: May 2007

This study investigated antifungal activity in soluble extracts from seed of a range of wheat cultivars differing in susceptibility to Fusarium head blight. Antifungal activity was assessed in terms of β-D-glucuronidase (GUS) activity of a Fusarium culmorum GUS transformant using a sensitive laboratory assay. Significant antifungal activity was detected in seed extracts from WEK0609, CM 820036, and Arina. Initial characterization of the Arina seed extract indicated that it contained antifungal proteinaceous compounds. The Arina extract yielded two (60 and 80%) ammonium sulfate fractions containing inhibitory compounds. Gel filtration chromatography and subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis of antifungal fractions showed that the antifungal activities detected in the Arina 60 and 80% ammonium sulfate fractions were associated with putative proteinaceous compounds with apparent molecular masses of approximately 60 and 28 kDa, respectively.
Synopsis on the Taxonomy of the Genus Xanthomonas
Authors: Vauterin Luc , Rademaker Jan , Swings Jean
Published: May 2007

The genus Xanthomonas exhibits a high phytopathogenic diversity in contrast to a phenotypic uniformity, which has hampered the genesis of a stable classification for a long time. In past decades, a large number of Xanthomonas strains have been characterized by a variety of phenotypic and genotypic methods in a multitude of studies. Extensive DNA hybridization studies and repetitive sequence-based polymerase chain reaction and amplified fragment length polymorphism genomic fingerprinting have clearly revealed the genomic diversity and relationships within the genus. A review of the current classification of the genus Xanthomonas based on the synopsis of these studies is given here.
Effects of Quadrat Size and Shape, Initial Epidemic Conditions, and Spore Dispersal Gradient on Spatial Statistics of Plant Disease Epidemics
Authors: Xu X.-M. , Ridout M. S.
Published: May 2007

The spatiotemporal spread of plant diseases was simulated using a stochastic model to study the effects of initial conditions (number of plants initially infected and their spatial pattern), spore dispersal gradient, and size and shape of sampling quadrats on statistics describing the spatiotemporal dynamics of epidemics. The spatial spread of disease was simulated using a half-Cauchy distribution with median dispersal distance μ (units of distance). A total of 54 different quadrat types, including 23 distinct sizes ranging from 4 to 144 plants, were used to sample the simulated epidemics. A symmetric form of the binary power law with two parameters (α, β) was fitted to the sampled epidemic data using each of the 54 quadrats for each replicate simulation run. The α and β estimates were highly correlated positively with each other, and their estimates were comparable to those estimated from observed epidemics. Intraclass correlation (κ) was calculated for each quadrat type; κ decreased exponentially with increasing quadrat size. An asymmetric form of the binary power law with three parameters (α1, β1, β2) was used to relate κ to the disease incidence (p); β1 was highly correlated to β: β1 ≈ β - 1. In general, initial conditions and quadrat size affected α, β, α1, β1, and β2greatly. The parameter estimates increased as quadrat size increased, and the relationships were described well by a linear regression model on the logarithm of quadrat size with the slope or intercept parameters dependent on initial conditions and μ. Compared with initial conditions and quadrat size, the overall effects of μ and quadrat shape were generally small, although within each quadrat size and initial condition they could be substantial. Quadrat shape had the greatest effect when the quadrat was long and thin. The relationship of the index of dispersion (D) to p and quadrat size was determined from the α and β estimates. D was greatest when p was 0.5 and decreased when p approached 0 or 1. It increased with quadrat size and the rate of the increase was maximum when p was 0.5 and decreased when p approached 0 or 1.
Combined Soil Treatments and Sequence of Application in Improving the Control of Soilborne Pathogens
Authors: Eshel Dani , Gamliel Abraham , Grinstein Avshalom , Di Primo Pietro , Katan Jaacov
Published: May 2007

The effects of reduced doses of methyl bromide (MB) or metham sodium, heating, short solarization, and soil microbial activity, alone or in combination, on survival of soilborne fungal pathogens were tested in a controlled-environment system and field plots. Sublethal doses of heating or MB delayed germination of Sclerotium rolfsii sclerotia. Combining MB and heating treatments was more effective than either treatment alone in controlling S. rolfsii and Fusarium oxysporum f. sp. basilici. The application heating followed by fumigation with MB, was significantly more effective in delaying and reducing germination of S. rolfsii sclerotia and in controlling F. oxysporum f. sp. basilici than the opposite sequence. Further, incubation in soil and exposure to microbial activity of previously heated or MB-treated sclerotia increased the mortality rate, indicating a weakening effect. Similarly, incubation of chlamydospores of F. oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici in soil in the field after fumigation further reduced their survival, confirming the laboratory results. In field tests, combining MB or metham sodium at reduced doses with short solarization was more effective in controlling fungal pathogens than either treatment alone. Treatment sequence significantly affected pathogen control in the field, similar to its effect under controlled conditions. This study demonstrates a frequent synergistic effect of combining soil treatments and its potential for improving pathogen control and reducing pesticide dose, especially when an appropriate sequence was followed.
Variability of Three Isolates ofBotrytis cinereaAffects the Inhibitory Effects of Calcium on this Fungus
Authors: Chardonnet Catherine O. , Sams Carl E. , Trigiano Robert N. , Conway William S.
Published: May 2007

Botrytis cinerea is an economically important pathogen. Epidemiological studies are difficult because of the genetic variability within this species. The objectives of this work were to study the variability and to compare the inhibitory effects of Ca on three isolates of B. cinerea from decayed apple (B) and grape (C and C77:4). Among these isolates, B had the least radial growth but had a sporulation rate 40% higher than that of both C77:4 and C. In situ, isolate C incited the largest decay area in the fruit of two of four apple cultivars examined and had the highest polygalacturonase activity in vitro. Maximum mycelial growth was reached with CaCl2at 1 g liter-1for isolates B and C77:4 and at 4 g liter-1for isolate C. Calcium (CaCl2) inhibited polygalacturonase activity at 1 g liter-1for C and C77:4 and at 16 g liter-1for B. Calcium infiltration reduced decay caused by all three isolates by three to five times. Mycelial DNA analysis showed that 42% of the character loci scored were polymorphic and the greatest similarities were found between B and C77:4. These results support the evidence that the biological and statistical variability in research can be affected by the B. cinerea isolate selected. Despite this variation, Ca treatment of apples reduced decay caused by all three Botrytis cinerea isolates.
Rational Management of Didymella rabiei in Chickpea by Integration of Genotype Resistance and Postinfection Application of Fungicides
Authors: Shtienberg D. , Vintal H. , Brener S. , Retig B.
Published: May 2007

Various aspects of the integration of genotype resistance and chemical control of Ascochyta blight (caused by Didymella rabiei) in chickpea were examined in field experiments from 1993 to 1999 and in greenhouse experiments. Four commercially available chickpea cultivars representing a range of resistance to D. rabiei were used. The efficacy of chemical control in a highly susceptible cultivar was significantly (P < 0.01) related to the conduciveness of the environment to the pathogen. Adequate disease suppression (>80% control) was achieved when weather supported mild epidemics, but insufficient control (<20%) was achieved when weather supported severe epidemics. The contribution of genotype resistance to disease suppression in a moderately susceptible cultivar varied from <10% when weather supported severe epidemics to ≈60% when weather supported mild epidemics. Spraying a moderately resistant cultivar resulted in 95% control when weather supported mild epidemics, but only 65% control was achieved when weather supported severe epidemics. The existing level of resistance in a moderately resistant cultivar resulted in 70% control when weather supported severe epidemics; fungicides improved control efficacy significantly to >95%. Under mild epidemics, moderate resistance alone provided >95% control. The level of genotype resistance available in a highly resistant cultivar was sufficient to suppress the disease under all weather conditions, even without application of fungicides. The possibility of relying on postinfection rather than prophylactic application of fungicides was tested in the greenhouse and in four field experiments. Activity of the systemic fungicide tebuconazole was detected when the fungicide was applied up to 3 days postinfection, and application of tebuconazole or difenoconazole in the field as a postinfection treatment (i.e., after rain or overhead irrigation) suppressed the disease as effectively as preventive applications and required fewer sprays. In two experiments, the interaction between genotype resistance and chemical control at various amounts of irrigation applied via overhead sprinklers (as a simulation of rain) was tested. The results show that both the level of genotype resistance and the quantity of water should be taken into account in deciding whether to apply a postinfection spray.
Genetic Analysis of Isolates of Botrytis cinerea Sensitive and Resistant to Benzimidazole and Dicarboximide Fungicides
Authors: Yourman L. F. , Jeffers S. N. , Dean R. A.
Published: May 2007

A total of 56 isolates of B. cinerea collected from ornamental crops from commercial greenhouses were examined by random amplified polymorphic DNA (RAPD) fingerprint analyses. Isolates were examined as two independent sets of 35 and 36 isolates, with 15 isolates common to both sets. The isolates had four phenotypes: 17 were sensitive to two commonly used fungicides, thiophanate-methyl (a benzimidazole) and vinclozolin (a dicarboximide) (STSV); 18 were resistant to both fungicides (RTRV); 16 were resistant to thiophanate-methyl but sensitive to vinclozolin (RTSV); and 5 were sensitive to thiophanate-methyl but resistant to vinclozolin (STRV). Relationships among the isolates were determined by cluster analyses of mean character differences using the unweighted pair group method using arithmetic average and cladograms were constructed. Isolates were clustered primarily by fungicide-sensitivity phenotype. In one set of greenhouse isolates, 6 of 10 STSV isolates clustered together with a bootstrap confidence value of 91%. In the other fingerprint set of greenhouse isolates, 9 of 11 STSV isolates clustered together and had a bootstrap confidence value of 98%. Isolates resistant to thiophanate-methyl, vinclozolin, or both fungicides usually were not clustered with other isolates or were clustered with isolates of the same phenotype. To further elucidate these relationships, variant isolates resistant to one or both fungicides were produced on fungicide-amended agar medium from 14 STSV greenhouse isolates. These 14 STSV parent isolates, 57 resistant variant isolates, and 11 resistant greenhouse isolates were analyzed as three independent RAPD fingerprint sets. Variants selected from eight STSV parent isolates were resistant to both thiophanate-methyl and vinclozolin even though parent isolates were exposed to only one of the fungicides. Isolates resistant only to vinclozolin (STRV) had fingerprint patterns similar to and clustered with those of parent isolates, while fingerprint patterns of isolates resistant to thiophanate-methyl (i.e., RTRV or RTSV), regardless of sensitivity to vinclozolin, clustered differently from both those of STSV parent isolates and those of STRV isolates derived from the same parent. RTRV and RTSV variant isolates derived from the same fungicide-sensitive parents only clustered with other variants having the same phenotype.
Molecular Characterization of Natural Hybrids of Phytophthora nicotianae and P. cactorum
Authors: Bonants Peter J. M. , Hagenaar-de Weerdt Marjanne , Man in 't Veld Willem A. , Baayen Robert P.
Published: May 2007

Hybrid isolates of Phytophthora nicotianae × P. cactorum from five different hosts (Cyclamen, Lavandula, Lewisia, Primula, and Spathiphyllum spp.) were identified by their atypical morphology and their well-defined heterozygous isozyme patterns. The hybrid nature of these isolates was tested by restriction fragment length polymorphism analysis of the internal transcribed spacer (ITS) region of rDNA, generating fragments typical for both P. nicotianae and P. cactorum. In hybrid isolates, polymerase chain reactions (PCR) with primers derived from unique parts of the ITS region (ITS-PCR) of both species yielded a combination of unique amplicons typical of both parental species. Eleven hybrid isolates, three isolates of each parental species and two atypical isolates from Rhododendron and Idesia spp. close to P. cactorum, were analyzed for amplified fragment length polymorphisms (AFLP). Consistent differences in AFLP patterns existed among the hybrid isolates, strongly indicating that these hybrids have arisen from independent hybridization events between P. nicotianae and P. cactorum. The two atypical isolates morphologically resembling P. cactorum were identical to the latter species in ITS-restriction fragment length polymorphism and response to the specific PCR primers but were intermediate between P. nicotianae × P. cactorum and P. cactorum in isozyme profiles and AFLP patterns. Since the introduction of hydroponic systems in greenhouses in the Netherlands, outbreaks of Phytophthora diseases are occurring in previously unaffected host species. This may be due to interspecific hybridization events resulting in novel pathogenic behavior.
Quantitative Trait Loci Associated with Resistance to Gray Leaf Spot of Corn
Authors: Clements M. J. , Dudley J. W. , White D. G.
Published: May 2007

The susceptible parent FR1141, the resistant parent 061, the F1 cross, and 301 families selfed once from backcrosses to the susceptible parent were evaluated for gray leaf spot (GLS) severity for two years in Urbana, IL, and one year in Andrews, NC. Linkage between ear height and GLS severity was suspected. Therefore, plant height characteristics were noted for two years in Urbana, IL. Eighty-six polymorphic probes were used to construct a random fragment length polymorphism linkage map, and the presence, locations, effects, and interactions of quantitative trait loci (QTL) associated with GLS, plant and ear height were determined. Five QTL were significantly associated with GLS resistance across all environments and rating periods. These five regions are associated with additive effects on phenotype and account for between 51.0 and 58.7% of the phenotypic variation associated with GLS severity. Additionally, six QTL were identified with maturity-dependent associations to GLS severity. Heritability of GLS resistance was estimated to be approximately 0.73. Four QTL were identified with associations to ear height relative to total plant height. One of the four was associated with higher ear height and GLS resistance.
The Helper Component-Proteinase ofSweet potato feathery mottle virusFacilitates Systemic Spread ofPotato virus XinIpomoea nil
Authors: Sonoda S. , Koiwa H. , Kanda K. , Kato H. , Shimono M. , Nishiguchi M.
Published: May 2007

When Ipomoea nil was coinfected with Sweet potato feathery mottle virus (SPFMV), a member of the genus Potyvirus, and Potato virus X (PVX) typical symptoms caused by PVX were observed on those by SPFMV on the first upper true leaves at 14 days postinoculation (dpi). On the other hand, no PVX-induced symptoms were observed on the first upper true leaves at 14 dpi when plants were infected with PVX alone. In the case of coinfection with PVX and SPFMV, PVX RNA was detected not only in the inoculated cotyledonary leaves but also in the first upper true leaves at 14 dpi. In the case of single infection with PVX, PVX RNA was detected in the inoculated cotyledonary leaves but not in the first upper true leaves at 14 dpi. The accumulation of SPFMV remained unchanged, regardless of whether the inoculum consisted of SPFMV alone or a mixture of SPFMV and PVX. Although recombinant PVX engineered to express the helper component-proteinase (HC-Pro) of SPFMV (PVX.HC) enhanced symptoms severity in Nicotiana benthamiana, PVX.HC induced the synergism characterized by an enhanced viral movement in Ipomoea nil. Immunofluorescence microscopic examination revealed that the HC-Pro was present in phloem of SPFMV-infected I. nil. These results suggest that SPFMV HC-Pro acts as an enhancer of long distance movement for PVX in I. nil.
A Polymerase Chain Reaction-Based Method to Specifically DetectAlternaria alternataApple Pathotype (A. mali), the Causal Agent of Alternaria Blotch of Apple
Authors: Johnson R. D. , Johnson L. , Kohmoto K. , Otani H. , Lane C. R. , Kodama M.
Published: May 2007

Alternaria alternata apple pathotype (previously A. mali) causes Alternaria blotch on susceptible apple cultivars through the production of a host-specific toxin, AM-toxin. Identification of some Alternaria species, especially those that produce host-specific toxins, has been extremely difficult due to a high level of variability which extends even to nonpathogenic isolates. We have recently cloned and characterized a gene (AMT) that plays a crucial role in AM-toxin biosynthesis and demonstrated that it is only present in isolates of A. alternata apple pathotype. Using primers designed for the AMT gene, we developed a polymerase chainreaction-based method to specifically detect AM-toxin producing isolates of A. alternata apple pathotype.
A Temperature-Based Model for Latent-Period Duration in Stem Rust of Perennial Ryegrass and Tall Fescue
Authors: Pfender W. F.
Published: May 2007

A temperature-response curve for latent-period duration in stem rust (caused by Puccinia graminis subsp. graminicola) on perennial ryegrass and tall fescue was developed from constant-temperature experiments with inoculated plants and evaluated in field experiments. Under constant-temperature conditions, time from infection to 50% of pustules erupted for perennial ryegrass ranged from 54 days at 3.5°C to 5.9 days at 26.5°C. The latent period (LP50) duration of tall fescue was 69 and 8.5 days at these respective temperatures. The dependence of latent-period completion rate on temperature was best described as a linear increase in rate with temperature up to approximately 26°C, then an exponential decline with temperature up to the maximum (lethal) temperature of approximately 35.5°C. LP20, the time required for 20% of open pustules to appear, was used as an estimator of latent-period duration for field observations. Percentage of one latent period completed per half hour (half-hourly rate), for perennial ryegrass was modeled as (0.0156T - 0.0206) {1 - exp[0.497(T - 35.5)]}, where T = average temperature (°C) during the half-hour period. For tall fescue the modeled rate was (0.0109T - 0.00214) {1 - exp[0.417(T - 35.5)]}. Latent periods modeled by these equations were compared with observed latent periods in field experiments with potted plants, where half-hourly temperatures were measured. Linear regressions of modeled versus observed latent periods had adjusted r2 values of 0.96 for perennial ryegrass and 0.93 for tall fescue. The latent-period equations could be used as components of a weather-based disease advisory model to optimize fungicide use in stem-rust management on these crops.