bluuebunny/crossref_metadata_2025 huggingface dataset:
Biological and Molecular Characterization of Fusarium oxysporum f. sp. lycopersici Divides Race 1 Isolates into Separate Virulence Groups
Authors: Mes Jurriaan J. , Weststeijn Emma A. , Herlaar Frits , Lambalk Joep J. M. , Wijbrandi Jelle , Haring Michel A. , Cornelissen Ben J. C.
Published: May 2007

A collection of race 1 and race 2 isolates of Fusarium oxysporum f. sp. lycopersici was screened for vegetative compatibility and characterized by random amplified polymorphic DNA (RAPD) analysis to establish the identity and genetic diversity of the isolates. Comparison of RAPD profiles revealed two main groups that coincide with vegetative compatibility groups (VCGs). In addition, several single-member VCGs were identified that could not be grouped in one of the two main RAPD clusters. This suggests that F. oxysporum f. sp. lycopersici is a polyphyletic taxon. To assign avirulence genotypes to race 1 isolates, they were tested for their virulence on a small set of tomato lines (Lycopersicon esculentum), including line OT364. This line was selected because it shows resistance to race 2 isolates but, unlike most other race 2-resistant lines, susceptibility to race 1 isolates. To exclude the influence of other components than those related to the race-specific resistance response, we tested the virulence of race 1 isolates on a susceptible tomato that has become race 2 resistant by introduction of an I-2 transgene. The results show that both line OT364 and the transgenic line were significantly affected by four race 1 isolates, but not by seven other race 1 isolates nor by any race 2 isolates. This allowed a subdivision of race 1 isolates based on the presence or absence of an avirulence gene corresponding to the I-2 resistance gene. The data presented here support a gene-for-gene relationship for the interaction between F. oxysporum f. sp. lycopersici and its host tomato.
Identification of Molecular Genetic Markers inPyrenophora teresf.teresAssociated with Low Virulence on ‘Harbin’ Barley
Authors: Weiland John J. , Steffenson Brian J. , Cartwright Richard D. , Webster Robert K.
Published: May 2007

Two isolates of the barley net blotch pathogen (Pyrenophora teres f. teres), one possessing high virulence (0-1) and the other possessing low virulence (15A) on the barley cultivar Harbin, were crossed and the progeny of the mating were isolated. Conidia from cultures of the parent and progeny isolates were used as inoculum to determine the inheritance of virulence in the pathogen. Of the 82 progeny tested, 42 exhibited high virulence and 40 exhibited low virulence on ‘Harbin’ barley. The data support a model in which a single, major gene controls virulence in P. teres f. teres on this barley cultivar (1:1 ratio; χ2= 0.05, P = 0.83). Preparations of DNA were made from parental and progeny isolates, and the DNA was subjected to the random amplified polymorphic DNA (RAPD) technique in a search for molecular genetic markers associated with the virulence phenotype. Five RAPD markers were obtained that were associated in coupling with low virulence. The data indicate that the RAPD technique can be used to tag genetic determinants for virulence in P. teres f. teres.
Regional Assessment of Soybean Brown Stem Rot, Phytophthora sojae, and Heterodera glycines Using Area-Frame Sampling: Prevalence and Effects of Tillage
Authors: Workneh F. , Tylka G. L. , Yang X. B. , Faghihi J. , Ferris J. M.
Published: May 2007

The prevalence of brown stem rot (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae in the north central United States was investigated during the fall of 1995 and 1996. Soybean fields were randomly selected using an area-frame sampling design in collaboration with the National Agricultural Statistics Service. Soil and soybean stem samples, along with tillage information, were collected from 1,462 fields in Illinois, Iowa, Minnesota, Missouri, and Ohio. An additional 275 soil samples collected from Indiana were assessed for H. glycines. For each field, the incidence and prevalence of brown stem rot was assessed in 20 soybean stem pieces. The prevalence and recovery (expressed as the percentage of leaf disks colonized) of P. sojae and the prevalence and population densities of H. glycines were determined from the soil samples. The prevalence of brown stem rot ranged from 28% in Missouri to 73% in Illinois; 68 and 72% of the fields in Minnesota and Iowa, respectively, showed symptomatic samples. The incidence of brown stem rot was greater in conservation-till than in conventional-till fields in all states except Minnesota, which had few no-till fields. P. sojae was detected in two-thirds of the soybean fields in Ohio and Minnesota, whereas 63, 55, and 41% of the fields in Iowa, Missouri, and Illinois, respectively, were infested with the pathogen. The recovery rates of P. sojae were significantly greater (P ≤ 0.05) in conservation-till than in conventional-till fields in all states except Iowa. H. glycines was detected in 83% of the soybean fields in Illinois, 74% in Iowa, 71% in Missouri, 60% in Ohio, 54% in Minnesota, and 47% in Indiana. Both the prevalence and population densities of H. glycines were consistently greater in tilled than in no-till fields in all states for which tillage information was available.
Characterization of a Pterostilbene Dehydrodimer Produced by Laccase of Botrytis cinerea
Authors: Breuil Anne-Céline , Jeandet Philippe , Adrian Marielle , Chopin Florence , Pirio Nadine , Meunier Philippe , Bessis Roger
Published: May 2007

In the interaction between grapevines and Botrytis cinerea, one of the main aspects of pathogenicity is fungal ability to degrade phytoalexins synthesized by the plant in response to infection. Laccase-like stilbene oxidase activity in liquid cultures of B. cinerea has been shown to be related to the decrease of phytoalexin concentrations. Recent research and results presented in this paper determined the chemical structure of a pterostilbene metabolite produced by B. cinerea. Study of degradation of pterostilbene that has just one free hydroxy phenyl group function allowed us to determine the oxidative dimerization process undergone by grapevine phytoalexins after B. cinerea infection. The phytopathological significance of this degradation process in the B. cinerea interaction has also been discussed.
Surface Disinfestation of Resting Spores of Plasmodiophora brassicae Used to Infect Hairy Roots of Brassica spp.
Authors: Asano T. , Kageyama K. , Hyakumachi M.
Published: May 2007

Resting spores of Plasmodiophora brassicae were surface-disinfested by treatment with 2% chloramine-T for 20 min and then with an antibiotic solution (1,000 ppm of colistin sulfate, 1,000 ppm of vancomycin hydrochloride, and 6,000 ppm of cefotaxime sodium) for 1 day. The disinfested resting spores were used to inoculate hairy roots of cabbage (Brassica oleracea var. capitata cv. Fuji Wase), Chinese cabbage (B. pekinensis cv. Musou Hakusai), turnip (B. rapa var. rapifera cv. Wase Okabu), and rape (B. napus line Dc 119). Differences among hosts in susceptibility to clubroot in hairy roots were evident. Chinese cabbage and turnip hairy roots supported the highest percentages of root hair infection (53.3 to 80%) and the greatest production of zoosporangial groups (8.5 to 32.5 per root). Moreover, gall formation was observed only on Chinese cabbage and turnip hairy roots. The morphology of zoo-sporangia, plasmodia, and resting spores in diseased hairy roots was found to be identical to that in infected intact plants by both light and scanning electron microscopy. Pathogenicity tests confirmed the infectivity of resting spores produced in hairy roots. Thus, the hairy root culture technique should prove useful as a dual culture system for P. brassicae.
Variation in Genotype and Aggressiveness of Ralstonia solanacearum Race 1 Isolated from Tomato in Taiwan
Authors: Jaunet Thierry X. , Wang Jaw-Fen
Published: May 2007

A population of Ralstonia solanacearum race 1 from tomato (Lycopersicon esculentum) was analyzed for genetic polymorphism and aggressiveness on tomato. The 46 strains were collected from main tomato-growing areas in Taiwan. Genetic analysis was achieved by two polymerase chain reaction (PCR)-based methods: REP-, ERIC-, and BOX-PCR (collectively as rep-PCR) and random amplified polymorphic DNA (RAPD) techniques. RAPD (with three 10-mers) and rep-PCR revealed 35 and 30 haplotypes, respectively, that were grouped in 14 clusters and 3 clusters, respectively. Distribution of strains into genetic clusters did not appear related to biovar or geographic origin in considering RAPD, rep-PCR, or composite data. Although strains were more dissimilar based on RAPD data than on rep-PCR data, the two techniques gave complementary results for strain clustering. A set of 40 strains representing the main haplotypes was inoculated on six tomato cultivars differing in their bacterial wilt resistance. Six groups differing in general level of aggressiveness and cultivar specificity were detected. Although populations were highly diverse in both genotype and aggressiveness, no association was found between the two characteristics. Although the sample sizes in this study were not adequate to draw definite conclusions about population structure, these results will be valuable for future population genetic studies on R. solanacearum.
Specific Diagnosis of Two Root-Knot Nematodes, Meloidogyne chitwoodi and M. fallax, with Satellite DNA Probes
Authors: Castagnone-Sereno P. , Leroy F. , Bongiovanni M. , Zijlstra C. , Abad P.
Published: May 2007

Meloidogyne chitwoodi and M. fallax are serious pests of potato, and both species have been recently designated as quarantine organisms in the European Community and in Canada. The sympatric and less damaging species M. hapla is often found associated with both of them under temperate climates. Here, we describe the use of satellite DNA (satDNA) sequences previously isolated from these three root-knot nematode species for the development of specific diagnostic procedures. In dot-blot experiments, it was unambiguously possible to separate M. chitwoodi and M. fallax from M. hapla using satDNA monomers as probes. In squash-blot experiments, satDNAs allowed discrimination between single individuals of M. chitwoodi or M. fallax from M. hapla, even within root tissues, without the need for DNA purification. The same results were obtained with radioactive or digoxigenin-labeled probes with no loss of sensitivity in detection. M. fallax and M. chitwoodi could not be distinguished. From this study, it is concluded that such cloned satDNA sequences may constitute a powerful tool for the identification and management of Meloidogyne spp. populations in the field and for the implementation of quarantine regulations against these pests.
Phylogenetic Analysis of 16S rRNA Genes and PCR Analysis of the nec1 Gene from Streptomyces spp. Causing Common Scab, Pitted Scab, and Netted Scab in Finland
Authors: Kreuze Jan F. , Suomalainen Sini , Paulin Lars , Valkonen Jari P. T.
Published: May 2007

The sequences of the 16S rRNA genes (nucleotides 29 to 1,521) from various Streptomyces strains pathogenic to potato were compared. These included 10 pathogenic Streptomyces strains isolated from potato scab lesions in Finland, the type strains of S. aureofaciens NRRL 2209T and S. lydicus ATCC 25470T, ‘S. griseus subsp. scabies’ ATCC 10246, and two S. griseus strains that were originally deposited to the collection as pathogens. The nucleotide sequence (>94.5% sequence identity [SI]) and length (1,469 to 1,481 nucleotides) of the analyzed region varied. Phylogenetic analysis of 16S rRNA genes placed Finnish strains into three species, supported by previously characterized morphological and physiological traits. Six Finnish strains, including two strains that deviated from the others in one trait (no spiral sporophores or D-xylose utilization), had identical 16S rRNA genes and were identified as S. scabies (99.9% SI to S. scabies ATCC 49173). Three Finnish strains were identified as S. turgidiscabies, a species previously described only in Japan (99.9% SI to S. turgidiscabies ATCC 700248). Finnish strain 317 and S. aureofaciens NRRL 2209 (99.8% SI) were placed in a distinct phylogenetic cluster together with Kitosatospora spp., which suggests that S. aureofaciens may belong to the recently revived genus Kitosatospora. In pathogenicity tests, S. scabies caused characteristic symptoms of common scab, S. turgidiscabies caused mainly pitted scab, and S. aureofaciens caused netted scab and necrotic lesions on stolons of potato cultivars Bintje and Matilda in the greenhouse. The nec1 gene and the intergenic region between nec1 and the 5′ transposase pseudogene ORFtnp were successfully amplified by polymerase chain reaction from S. scabies ATCC 49173 and the pathogenic Finnish strains of S. scabies, but not from a nonpathogenic strain of S. scabies, three pathogenic and two nonpathogenic strains of S. turgidiscabies, and S. aureofaciens.
Survey Methods for Assessment of Citrus Tristeza Virus Incidence When Toxoptera citricida Is the Predominant Vector
Authors: Hughes G. , Gottwald T. R.
Published: May 2007

Citrus tristeza virus (CTV) incidence may be assessed by sampling groups of citrus trees, recording the groups as CTV positive (one or more infected trees) or CTV negative (no infected trees), and then calculating disease incidence at the scale of the individual tree by means of a formula involving incidence at the group scale and the number of trees per group. This procedure works well when the CTV status of a tree can be regarded as independent of the CTV status of other trees in the same group. This is the case when the main vector species is Aphis gossypii and the groups comprise four adjacent trees, because the spatial pattern of CTV incidence at the within-group scale can be regarded as random. However, when the main vector species is Toxoptera citricida, this simple procedure is not appropriate, because the spatial pattern of CTV incidence at the within-group scale cannot be regarded as random. Using field data and computer simulation, an alternative procedure for assessment of CTV incidence when the main vector species is T. citricida was devised and tested. In the alternative procedure, the sampling scheme is operationally identical to that used when the main vector species is A. gossypii, but the calculation of CTV incidence at the scale of the individual tree is based on incidence at the group scale and an effective sample size. The analysis of CTV-incidence data collected from a number of citrus blocks in reasonable geographical and temporal proximity and the use of CTV-detection methods more sensitive than the enzyme-linked immunosorbent assay used here are also discussed.
Pandemics of Focal Plant Disease, a Model
Authors: van den Bosch F. , Metz J. A. J. , Zadoks J. C.
Published: May 2007

An analytical model of a pandemic, initiated by a single focus and spreading over a continent, is developed using foci as the smallest units of disease and fields as the smallest units of host. A few generalizing assumptions lead to a parameter-sparse model that may answer general questions on pandemics in a qualitative manner. For pandemic spread of disease during one season, a ‘within-season velocity of pandemic spread,’ C, is expressed as a set of integral equations. Reduction of inoculum during the off-season is expressed by a ‘survival ratio’ of inoculum, ε. The effect of the off-season is a ‘push-back’ of the pandemic front over a distance Δh. It will be shown how Δh is related to C and ε. The mean pandemic spread over successive years is calculated as the ‘polyetic velocity of pandemic spread,’ V, which depends on C and the push-back distance. The concept of ‘pandemic effectiveness’ is parameterized. Relations between the two velocities of pandemic spread and several model parameters are studied. Somewhat unexpectedly, velocities of pandemic spread depend only in a very limited way on field density represented by the ‘cropping ratio’ ζ. This implies that our model and methods will also apply to situations with inhomogeneous field distributions. The effect of parameter values on rates of severity increase are analyzed. Finally, generalizations of the model are developed and their applications discussed.
Genetic Structure of Heterobasidion annosum in White Fir Mortality Centers in California
Authors: Garbelotto Matteo , Cobb Fields W. , Bruns T. D. , Otrosina William J. , Popenuck Tina , Slaughter Garey
Published: May 2007

The structure of Heterobasidion annosum populations was studied in 15 mixed-conifer sites in central and northern California. Study sites displayed mortality of white fir trees in enlarging discrete patches (mortality centers). At each site, fungal genotypes were defined by somatic compatibility tests. In two sites, further genetic and molecular analyses were performed on field genotypes and on homokaryons obtained by dedikaryotization of field heterokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of white fir trees, and no significant infections of other tree species were observed. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S intersterility group. Both homokaryotic and heterokaryotic strains were present in all sites. Multiple genotypes were retrieved in individual trees or stumps. Out of 228 fungal genotypes, 86% were found only within a single tree or stump, while 14% had spread to adjacent trees. The two largest genotypes had diameters of 9 and 10 m, and had colonized five and nine trees, stumps, or both, respectively. The maximum distance between two adjacent trees colonized by the same genotype was 6 m, and a highly significant correlation was found between tree diameter and distance of fungal “vegetative” spread. The largest clones were found in areas characterized by high tree and stump densities, and secondary spread of the fungus was more significant in denser stands. In most cases, original infection courts of existing genotypes could be traced to standing trees and not to stumps. The genetic analysis performed in two mortality centers revealed that most local genotypes had different mating alleles, and thus originated from unrelated basidiospores. In a few cases, the same mating allele was shared by two heterokaryons (n+n genome) or by a homokaryon (n genome) and a heterokaryon. Molecular analysis showed that nuclei bearing the same mating allele were identical, providing evidence that the two nuclei forming heterokaryons can act independently in the field and can be shared among isolates, presumably via di-mon mating or by separate matings of different portions of widespread homokaryons.
Suppression of Sclerotinia Stem Rot of Soybean by Lactofen Herbicide Treatment
Authors: Dann E. K. , Diers B. W. , Hammerschmidt R.
Published: May 2007

Severity of Sclerotinia stem rot of soybean after treatment with lactofen (Cobra) and other herbicides was assessed in field experiments conducted in Michigan from 1995 to 1997. At sites where disease pressure was high, disease severity was reduced 40 to 60% compared with controls when lactofen was applied at the V3 (1995 and 1996) or R1 (1997) growth stages. Corresponding seed yields were unchanged or up to 20% greater when lactofen was applied at the R1 stage in 1997. Disease severity was not reduced by lactofen treatments in years and at sites where disease pressure was low to medium, and corresponding yields often were reduced by ≈10%. High levels of glyceollin accumulated in lactofen-injured leaves collected from field plots in 1996 and 1997. High glyceollin content in lactofen-treated leaves was associated with significant reductions in lesion size when leaves were challenge-inoculated with Sclerotinia sclerotiorum.
Examination of the Effect of Aphid Vector Population Composition on the Spatial Dynamics of Citrus Tristeza Virus Spread by Stochastic Modeling
Authors: Gottwald T. R. , Gibson G. J. , Garnsey S. M. , Irey M.
Published: May 2007

Aphid vector species population composition is known to affect the spatial patterns of citrus tristeza virus (CTV) and the changes in these patterns over time. However, the biological processes that are associated with virus spread have not been well defined. The spatiotemporal dynamics of CTV were examined using data collected from research plots in the Dominican Republic and Costa Rica, where the brown citrus aphid (BCA), Toxoptera citricida, was the predominant species, and in Florida, where the BCA was absent and the melon aphid, Aphis gossypii, was the predominant vector. Data were analyzed using a spatiotemporal stochastic model for disease spread, and parameter values were evaluated using Markov chain Monte Carlo stochastic integration methods. Where the melon aphid was the dominant species, the model parameter likelihood values supported the hypothesis that the disease was spread through a combination of random background transmission (transmission originating from inoculum sources outside the plot) and a local interaction (transmission from inoculum sources within the plot) operating over short distances. Conversely, when BCA was present, results often suggested a local short-range transmission interaction that was not restricted to nearest-neighbor interactions and that the presence of background infection was not necessary to explain the observations.
Geographic Distribution and Molecular Variation of Isolates of Three Whitefly-Borne Closteroviruses of Cucurbits: Lettuce Infectious Yellows Virus, Cucurbit Yellow Stunting Disorder Virus, and Beet Pseudo-Yellows Virus
Authors: Rubio Luis , Soong Joyce , Kao John , Falk Bryce W.
Published: May 2007

The geographic incidence and molecular variation of three whitefly-borne closteroviruses (lettuce infectious yellows virus [LIYV], cucurbit yellow stunting disorder virus [CYSDV], and beet pseudo-yellows virus [BPYV]) were studied in cucurbits collected from several distinct geographic locations. Of 498 samples analyzed, none were found to be infected by LIYV. Sixty-nine samples collected in the Middle East and Mediterranean Europe were found infected by CYSDV, and twelve samples from Crete and Italy were infected by BPYV. Reverse-transcription poly-merase chain reaction of a portion of the heat shock protein 70 homolog coding region, followed by single-strand conformation polymorphism and nucleotide sequence analysis, was used to estimate the intra- and inter-isolate molecular variability. These analyses showed that each BPYV and CYSDV isolate was composed of a population of sequence variants with a nucleotide identity greater than 98%. CYSDV isolates could be divided into two divergent groups. Group I was only composed of isolates from Spain, Jordan, and Turkey, and group II isolates were predominantly found in Saudi Arabia. Nucleotide identity between isolates of the same group was greater than 99%, whereas identity between both groups was less than 92%. All BPYV isolates showed a nucleotide identity greater than 98%.
Sensitivity of Penicillium digitatum and P. italicum to Postharvest Citrus Fungicides in California
Authors: Holmes Gerald J. , Eckert Joseph W.
Published: May 2007

Penicillium digitatum isolates (326), collected in California citrus groves and packinghouses, were assayed qualitatively for their sensitivity to imazalil, thiabendazole, and o-phenylphenol. Eighteen typical triple-resistant isolates, acquired in each of 3 years (1988, 1990, and 1994), were assayed quantitatively for their sensitivity to each of the three fungicides. No significant differences were found in the mean sensitivity of the isolates collected in different years. However, the proportion of isolates that were resistant to all three fungicides increased from 43% in 1988 to 77% in 1990 and 74% in 1994. Imazalil-resistant biotypes of P. digitatum were isolated frequently in California packinghouses, while resistant P. italicum was rare. No fungicide-resistant biotypes of either species were collected from citrus groves. Wild-type P. italicum was slightly less sensitive than wild-type P. digitatum to all three fungicides. The concentration of imazalil producing 50% growth inhibition (EC50) was three times greater when the age of the P. digitatum assay inoculum was increased from 12 to 24 h. Activity of imazalil increased with pH of the assay medium in the range pH 5.1 to 5.9, reflecting the greater concentration of dissociated imazalil at the higher pH value.
Identification of Microsatellite Markers and Their Application to Population Genetics of Venturia inaequalis
Authors: Tenzer Isabel , degli Ivanissevich Stefania , Morgante Michele , Gessler Cesare
Published: May 2007

Microsatellite markers of Venturia inaequalis were developed using genomic libraries of V. inaequalis enriched for the simple sequence repeats (TC)n and (AAC)n. Seven markers, three with (TC)n repeats and four with (AAC)n repeats, were selected for the analyses of 350 isolates of V. inaequalis collected from 11 sites in Europe. Polymorphism in the (TC)n repeats was higher than in the (AAC)n repeats. Nei's expected genetic diversity (HE) varied between 0.52 and 0.96 for the microsatellites containing (TC)n stretches and between 0.09 and 0.36 for the microsatellites containing (AAC)n stretches. Within-population diversity (HS) was very high with values ranging from 0.28 to 0.49, whereas differentiation among all European populations (GST) was low with an average of 0.07. In the population from Ahrensburg (northern Germany) where isolates were mainly collected from apple varieties carrying the Vf gene, usually resistant to V. inaequalis, we showed a bottleneck effect with reduced diversity and loss of alleles. The great advantages of microsatellite markers over random amplified polymorphic DNA and polymerase chain reaction-restriction fragment length polymorphism markers are their high specificity, high polymorphism, good reproducibility, and unambiguous scorability.
Mathematical Models of Host Plant Infection by Helper-Dependent Virus Complexes: Why Are Helper Viruses Always Avirulent?
Authors: Zhang X.-S. , Holt J. , Colvin J.
Published: May 2007

Interactions between viruses in plants are common, and some viruses depend on such interactions for their survival. Frequently, a virus lacks some essential molecular function that another provides. In “helper-dependent” virus complexes, the helper virus is transmitted independently by a vector, whereas the dependent virus depends on molecular agents associated with the helper virus for transmission by a vector. A general mathematical model was developed of the dynamics of host plant infection by a helper-dependent virus complex. Four categories of host plants were considered: healthy, infected with helper virus alone, infected with dependent virus alone, and infected with both viruses. New planting of the host crop was constrained by a maximum abundance due to limitation of the cropping area. The ratio of infection rate to host loss rate due to infection is proposed as an important epidemiological quantity, A, that can be used as a measure of the mutual adaptation of the virus and host. A number of alternative equilibria of host infection could occur and were determined exclusively by parameter values; it was informative to display their distribution in the parameter plane: (1/A)helper versus (1/A)dependent. A simple analysis of the distribution of the final equilibria illustrated that the dependent virus could affect the survival of the helper virus, so facilitation between the two can be reciprocal. The distribution of the final equilibria also indicated that a well-adapted helper virus increases the opportunity for a dependent virus to evolve and survive, and the model, therefore, explains why infection with a helper virus usually causes no or little damage to plants, whereas infection with a dependent virus or mixed infection with both often causes very severe damage.
Transmissibility of Cucumber mosaic virus by Aphis gossypii Correlates with Viral Accumulation and Is Affected by the Presence of Its Satellite RNA
Authors: Escriu Fernando , Perry Keith L. , García-Arenal Fernando
Published: May 2007

Satellite RNAs (satRNAs) are associated with Cucumber mosaic virus (CMV) in tomato, most often causing severe epidemics of necrotic plants, and not associated with specific host symptoms. Laboratory studies on virus transmission by the aphid vector Aphis gossypii were performed to better understand the dynamics of field populations of CMV. The presence of satRNAs correlated with lower concentrations of virus in infected plants and with a decrease in the efficiency of transmission from satRNA-infected plants. Both the concentration of virus in CMV-infected tomato and the efficiency of transmission varied more extensively with nonnecrogenic satRNAs than with necrogenic satRNAs. A negative effect of satRNAs on virus accumulation can account, in part, for a decrease in the field transmission and recovery of CMV + satRNAs. Aphids behaved differently and probed less readily on plants infected with CMV + necrogenic satRNAs compared with plants containing non-necrogenic satRNAs. Aphid-mediated satRNA-free CMV infections were observed in test plants when aphids were fed on source plants containing CMV + nonnecrogenic satRNA; no comparable satRNA-free test plants occurred when aphids were fed on source plants containing necrogenic satRNAs. These results indicate that factors associated with transmission can be a determinant in the evolution of natural populations of CMV and its satRNA.
Chayote mosaic virus, a New Tymovirus Infecting Cucurbitaceae
Authors: Bernal Juan J. , Jiménez Ignacio , Moreno Manuel , Hord Melanie , Rivera Carmen , Koenig Renate , Rodríguez-Cerezo Emilio
Published: May 2007

Chayote mosaic virus (ChMV) is a putative tymovirus isolated from chayote crops in Costa Rica. ChMV was characterized at the host range, serological, and molecular levels. ChMV was transmitted mechanically and induced disease symptoms mainly in Cucurbitaceae hosts. Asymptomatic infections were detected in other host families. Serologically, ChMV is related to the Andean potato latent virus (APLV) and the Eggplant mosaic virus (EMV), both members of the genus Tymovirus infecting solanaceous hosts in the Caribbean Basin and South America. The sequence of the genomic RNA of ChMV was determined and its genetic organization was typical of tymoviruses. Comparisons with other tymoviral sequences showed that ChMV was a new member of the genus Tymovirus. The phylogenetic analyses of the coat protein gene were consistent with serological comparisons and positioned ChMV within a cluster of tymoviruses infecting mainly cucurbit or solanaceous hosts, including APLV and EMV. Phylogenetic analyses of the replicase protein gene confirmed the close relationship of ChMV and EMV. Our results suggest that ChMV is related to two tymoviruses (APLV and EMV) of proximal geographical provenance but with different natural host ranges. ChMV is the first cucurbit-infecting tymovirus to be fully characterized at the genomic level.
Resistance to Striga hermonthica in Wild Accessions of the Primary Gene Pool of Pennisetum glaucum
Authors: Wilson J. P. , Hess D. E. , Hanna W. W.
Published: May 2007

Resistance to Striga hermonthica in 274 wild Pennisetum glaucum subsp. monodii and stenostachyum accessions was evaluated at Samanko, Mali in 1997 and 1998, and at Cinzana, Mali and Sadoré, Niger in 1998. Data recorded included number of striga plants per plot at least three times during the season, date of striga emergence, number of Pennisetum plants, Pennisetum anthesis date, and downy mildew incidence (caused by Sclerospora graminicola). Across trials, the average maximum number of striga per host plant ranged from 0.9 to 8.3. Average days to striga emergence ranged from 54 to 68 days across trials, and was negatively correlated (P < 0.01) with maximum striga within trials. Days to Pennisetum flowering ranged from 54 to 74 days across trials. Host flowering was correlated (P ≤ 0.05) positively with maximum striga at Samanko in 1997 and 1998, but negatively at Cinzana. Downy mildew incidence ranged from 10 to 32% across trials, and was negatively correlated with maximum striga in three trials. Days to striga emergence and Pennisetum flowering were significant covariates affecting maximum striga values, but downy mildew incidence was not. Least squares means of maximum striga across trials averaged 5.7, and ranged from -0.2 to 19.1 striga per host plant. Broad-sense heritability of observed maximum striga was estimated to be 0.55. Agar-gel assays conducted in the laboratory were ineffective in identifying differences in stimulating striga seed germination among selected accessions. Variables not directly related to genetic resistance can affect maximum striga emergence in the field, and their identification may allow more effective resistance screening.
Panama Disease: Cell Wall Reinforcement in Banana Roots in Response to Elicitors from Fusarium oxysporum f. sp. cubense Race Four
Authors: De Ascensao Ana R. D. C. F. , Dubery Ian A.
Published: May 2007

The biochemical basis of tolerance in banana to Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. cubense race four, was investigated. Tissue culture banana plants from tolerant cv. Goldfinger and susceptible cv. Williams were maintained in a hydroponic system and inoculated with conidial suspensions to evaluate the degree of tolerance to susceptibility between the two clones and to investigate the effectiveness of this technique as a potential tool for early screening for resistance in breeding programs. Similarly, defense responses were induced by treatment of the plants with an elicitor preparation from the mycelial cell walls of the pathogen. Differences in the induction of lignin and callose deposition, phenolics, and the enzymes involved in cell wall strengthening; phenylalanine ammonia lyase, cinnamyl alcohol dehydrogenase, peroxidase, and polyphenol oxidase were determined. Root tissue of the tolerant cv. Goldfinger responded to the fungal elicitor through the strong deposition of lignin, preceded by the induction or activation of the enzyme activities involved in the synthesis and polymerization thereof, whereas only slight increases were observed for the susceptible cv. Williams. No increase in callose content was observed for either clone. These results indicate an important role for cell wall strengthening due to the deposition of lignin as an inducible defense mechanism of banana roots against F. oxysporum f. sp. cubense race four.
Commercial Fungicide Formulations Induce In Vitro Oospore Formation and Phenotypic Change in Mating Type in Phytophthora infestans
Authors: Groves Carol Trout , Ristaino Jean Beagle
Published: May 2007

A wide range of commercially formulated fungicides cause in vitro effects on mating behavior in specific isolates of Phytophthora infestans, the causal agent of late blight of potato and tomato. Four isolates of P. infestans representing each of the four common US genotypes, US-1, US-6, US-7, and US-8 and varying in their sensitivity to metalaxyl, were exposed to a variety of fungicides used to control late blight in petri dish assays at concentrations ranging from 1 to 100 μg a.i./ml. Exposure of each of these normally heterothallic single mating type isolates of P. infestans to 9 of the 11 commercial fungicide formulations tested resulted in the formation of oospores after 2 to 4 weeks. The highest numbers of oospores were formed on media amended with Ridomil 2E (metalaxyl) and Ridomil Gold EC (mefenoxam) at 0.1 to 10 μg a.i./ml, averaging as many as 471 and 450 oospores per petri dish, respectively. Several other fungicides including Maneb, Manzate (Mancozeb), Curzate (cymoxanil + mancozeb), and Acrobat MZ (dimethomorph + mancozeb) also induced oospore formation, producing from 0 to 200 oospores per plate at fungicide concentrations from 0.1 to 10 μg a.i./ml. The metalaxyl resistant isolates formed oospores in response to the fungicides more often than the metalaxyl sensitive isolates. No oospores were formed on media amended with Bravo (chlorothalonil) or Tattoo C (chlorothalonil + propamocarb HCl) and these compounds completely suppressed growth of the isolates at 0.1 and 1 μg a.i./ml. Three metalaxyl resistant A2 isolates mated with both A1 and A2 isolates after exposure to the fungicides Ridomil 2E and Ridomil Gold EC. Alterations in mating type expression were also observed in a metalaxyl sensitive A1 isolate after exposure to Benlate (benomyl). Copious amounts of chemicals are applied annually to potato and tomato production areas to control late blight. Our results indicate that a wide range of chemically diverse fungicides can induce normally heterothallic metalaxyl resistant isolates of P. infestans to form oospores in vitro after short exposures to the fungicides.
Bean golden yellow mosaic virus from Chiapas, Mexico: Characterization, Pseudorecombination with Other Bean-Infecting Geminiviruses and Germ Plasm Screening
Authors: Garrido-Ramirez E. R. , Sudarshana M. R. , Gilbertson R. L.
Published: May 2007

The complete nucleotide (nt) sequences of the cloned DNA-A (2644 nts) and DNA-B (2609 nts) components of Bean golden yellow mosaic virus (BGYMV-MX) from Chiapas, Mexico were determined. The genome organization of BGYMV-MX is similar to that of other Western Hemisphere bipartite geminiviruses (genus Begomovirus). Infectivity of the cloned BGYMV-MX DNA components in common bean (Phaseolus vulgaris) plants was demonstrated by particle bombardment and agroinoculation. BGYMV-MX was identified as a BGYMV (previously type II BGMV) isolate based on sequence analyses, sap-transmissibility, and pseudorecombination experiments with other bean-infecting begomoviruses. On the basis of differences in the DNA-B hypervariable region, symptom phenotype, and properties of infectious pseudorecombinants, BGYMV-MX may represent a distinct strain of BGYMV. Pseudorecombination experiments further established that BGYMV symptom determinants mapped to DNA-B, and that BGYMV-MX was most closely related to BGYMV from Guatemala. A Tomato leaf crumple virus (TLCrV) DNA-A/BGYMV-MX DNA-B pseudorecombinant was infectious in bean, establishing that a viable reassortant can be formed between begomovirus species from different phylogenetic clusters. Bean germ plasm representing the two major gene pools (Andean and Mesoamerican) was screened for response to BGYMV-MX with three methods of inoculation: sap-inoculation, particle bombardment, and agroinoculation. Andean germ plasm was very susceptible and similar results were obtained with all three methods, whereas Mesoamerican germ plasm showed resistance to BGYMV-MX, particularly with agroinoculation.
Fitness of Botrytis cinerea Associated with Dicarboximide Resistance
Authors: Raposo R. , Gomez V. , Urrutia T. , Melgarejo P.
Published: May 2007

Fitness costs in Botrytis cinerea associated with dicarboximide resistance were studied. Spearman rank correlation coefficients were calculated between resistance to iprodione and survival ability both outside and inside the greenhouse, measured on isolates randomly chosen from a collection done in a survey of commercial greenhouses in Southeastern Spain in 1992. Survival was measured at 47, 83, and 110 days as percentage of surviving mycelia in a sample of artificially inoculated tomato stem pieces and as percentage of viable sclerotia from a sample of sclerotia produced on potato dextrose agar. Resistance to iprodione was measured by the fungicide concentration that reduces fungal growth by 50% (EC50 values). Significant (P < 0.05) negative correlation coefficients between survival of sclerotia and resistance to iprodione were found for some samplings dates, which indicates that sclerotia of resistant isolates survive less well than sclerotia from sensitive isolates. For mycelia, no relationship between survival and resistance was found.
Disease Development Following Infection of Tomato and Basil Foliage by Airborne Conidia of the Soilborne Pathogens Fusarium oxysporum f. sp. radicis-lycopersici and F. oxysporum f. sp. basilici
Authors: Rekah Y. , Shtienberg D. , Katan J.
Published: May 2007

Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of Fusarium crown and root rot of tomato, and F. oxysporum f. sp. basilici, the causal agent of Fusarium wilt in basil, are soilborne pathogens capable of producing conspicuous masses of macroconidia along the stem. The role of the airborne propagules in the epidemics of the disease in tomato plants was studied. In the field, airborne propagules of F. oxysporum f. sp. radicis-lycopersici were trapped with a selective medium and their prevalence was determined. Plants grown in both covered and uncovered pots, detached from the field soil, and exposed to natural aerial inoculum developed typical symptoms (82 to 87% diseased plants). The distribution of inoculum in the growth medium in the pots also indicated the occurrence of foliage infection. In greenhouse, foliage and root inoculations were carried out with both tomato and basil and their respective pathogens. Temperature and duration of high relative humidity affected rate of colonization of tomato, but not of basil, by the respective pathogens. Disease incidence in foliage-inoculated plants reached 75 to 100%. In these plants, downward movement of the pathogens from the foliage to the crown and roots was observed. Wounding enhanced pathogen invasion and establishment in the foliage-inoculated plants. The sporulation of the two pathogens on stems, aerial dissemination, and foliage infection raise the need for foliage protection in addition to soil disinfestation, in the framework of an integrated disease management program.
Fully “Recombinant Enzyme-Linked Immunosorbent Assays” Using Genetically Engineered Single-Chain Antibody Fusion Proteins for Detection ofCitrus tristeza virus
Authors: Terrada Estela , Kerschbaumer Randolf J. , Giunta Giuseppe , Galeffi Patrizia , Himmler Gottfried , Cambra Mariano
Published: May 2007

Recombinant single-chain variable fragment antibodies (scFv) that bind specifically to Citrus tristeza virus (CTV), which cause the most detrimental viral disease in the citrus industry worldwide, were obtained from the hybridoma cell lines 3DF1 and 3CA5. These scFv were genetically fused with dimerization domains as well as with alkaline phosphatase, respectively, and diagnostic reagents were produced by expressing these fusion proteins in bacterial cultures. The engineered antibodies were successfully used for CTV diagnosis in plants by tissue print enzyme-linked immunosorbent assay (ELISA) and double antibody sandwich-ELISA. The fully recombinant ELISAs were as specific and sensitive as conventional ELISAs performed with the parental monoclonal antibodies, showing the usefulness of recombinant antibodies for routine detection of a virus in woody plants for the first time.
Modeling Infection of Strawberry Flowers byBotrytis cinereaUsing Field Data
Authors: Xu Xiangming , Harris David C. , Berrie Angela M.
Published: May 2007

The incidence of strawberry flower infection by Botrytis cinerea was monitored in unsprayed field plots in three successive years together with meteorological data and numbers of conidia in the air. There were large differences in conidia numbers and weather conditions in the 3 years. Three sets of models were derived to relate inoculum and weather conditions to the incidence of flower infection; by inoculum only, by weather variables only, and by both inoculum and weather variables. All the models fitted the observed incidence satisfactorily. High inoculum led to more infection. Models using weather variables only gave more accurate predictions than models using inoculum only. Models using both weather variables and inoculum gave the best predictions, but the improvement over the models based on weather variables only was small. The relationship between incidence of flower infection and inoculum and weather variables was generally consistent between years. Of the weather variables examined, daytime vapor pressure deficit and nighttime temperature had the greatest effect in determining daily incidence of flower infection. Infection was favored by low day vapor pressure deficit and high night temperature. The accuracy and consistency of the weather-based models suggest they could be explored to assist in management of gray mold.
Nucleocapsid Gene-Mediated Transgenic Resistance Provides Protection Against Tomato spotted wilt virus Epidemics in the Field
Authors: Herrero Sonia , Culbreath Albert K. , Csinos Alex S. , Pappu Hanu R. , Rufty Rebeca C. , Daub Margaret E.
Published: May 2007

Transformation of plants with the nucleocapsid (N) gene of Tomato spotted wilt tospovirus (TSWV) provides resistance to disease development; however, information is lacking on the response of plants to natural inoculum in the field. Three tobacco cultivars were transformed with the N gene of a dahlia isolate of TSWV (TSWV-D), and plants were evaluated over several generations in the greenhouse. The resistant phenotype was more frequently observed in ‘Burley 21’ than in ‘KY-14’ or ‘K-326’, but highly resistant ‘Burley 21’ transgenic lines were resistant to only 44% of the heterologous TSWV isolates tested. Advanced generation (R3 and R4) transgenic resistant lines of ‘Burley 21’ and a ‘K-326’ F1 hybrid containing the N genes of two TSWV isolates were evaluated in the field near Tifton, GA, where TSWV is endemic. Disease development was monitored by symptom expression and enzyme-linked immunosorbent assay (ELISA) analysis. Whereas incidence of TSWV infection in ‘Burley 21’ susceptible controls was 20% in 1996 and 62% in 1997, the mean incidence in transgenic lines was reduced to 4 and 31%, respectively. Three transgenic ‘Burley 21’ lines were identified that had significantly lower incidence of disease than susceptible controls over the two years of the study. In addition, the rate of disease increase at the onset of the 1997 epidemic was reduced for all the ‘Burley 21’ transgenic lines compared with the susceptible controls. The ‘K-326’ F1 hybrid was as susceptible as the ‘K-326’ nontransformed control. ELISA analysis demonstrated that symptomless plants from the most resistant ‘Burley 21’ transgenic lines accumulated detectable nucleocapsid protein, whereas symptomless plants from more susceptible lines did not. We conclude that transgenic resistance to TSWV is effective in reducing incidence of the disease in the field, and that accumulation of transgene protein may be important in broad-spectrum resistance.