bluuebunny/crossref_metadata_2025 huggingface dataset:
A Conjugative Plasmid Carrying the efe Gene for the Ethylene-Forming Enzyme Isolated from Pseudomonas syringae pv. glycinea
Authors: Watanabe K. , Nagahama K. , Sato M.
Published: May 2007

Previous work suggested that the efe gene encoding the ethylene-forming enzyme was present in the plasmids of three pathovars of Pseudomonas syringae including glycinea, phaseolicola (kudzu strains), and cannabina. However, no direct evidence to support this assumption had been presented. In the current study, we isolated the conjugative plasmid harboring the efe gene (ethylene plasmid) designated pETH2 from P. syringae pv. glycinea MAFF301683. pETH2 was detected by Southern blot hybridization using the efe probe, marked with the transposon mini-Tn5-Km1, and transferred into P. syringae Ni27n, which does not produce ethylene. The transconjugant Ni27n (pETH2) produced ethylene at a level similar to pv. glycinea MAFF301683. In addition, the plasmid designated pCOR2, which encodes coronatine biosynthesis genes, was detected in the same strain. Although the molecular size of the plasmid pCOR2 was not easily distinguishable from pETH2, pCOR2 transferred independently into Ni27n and the transconjugants produced coronatine. These findings suggested that the efe gene has been horizontally dispersed among pathovars of P. syringae by plasmid-mediated conjugation in nature.
Transmission of Pineapple Mealybug Wilt-Associated Virus by Two Species of Mealybug (Dysmicoccus spp.)
Authors: Sether D. M. , Ullman D. E. , Hu J. S.
Published: May 2007

Closterovirus-like particles associated with mealybug wilt of pineapple were acquired and transmitted by the pink pineapple mealybug, Dysmicoccus brevipes, and the gray pineapple mealybug, D. neobrevipes. Mealybugs acquired pineapple mealybug wilt-associated virus (PMWaV) from infected pineapple plants or detached leaves. The virus was detected in plants by tissue blot immunoassay and confirmed by immunosorbent electron microscopy. Plants exposed to mealybugs reared on PMWaV-free pineapple tissue remained uninfected. The presence of ants was correlated with an increased rate of virus spread when caged with D. brevipes. All stages of D. neobrevipes acquired PMWaV, although vector efficiency decreased significantly in older adult females. The probability of a single third-instar immature transmitting the virus was 0.04. Both species of mealybug acquired and transmitted PMWaV from infected pineapple material that had been clonally propagated for decades, and both species acquired PMWaV from sources previously infected with the virus by the other mealybug species.
Comparative Quantitative Trait Loci Mapping of Partial Resistance toPuccinia sorghiAcross Four Populations of European Flint Maize
Authors: Lübberstedt Thomas , Klein Dietrich , Melchinger Albrecht E.
Published: May 2007

We mapped and characterized quantitative trait loci (QTL) for partial resistance to Puccinia sorghi and investigated consistency across different European flint maize populations. Four independent populations, containing 280 F3lines (A×BI), 120 F5lines (A×BII), 131 F4lines (A×C), and 133 F4lines (C×D) were produced from four European elite flint inbreds (A, B, C, and D) and genotyped at 89, 151, 104, and 122 restriction fragment length polymorphism marker loci, respectively. All Fnlines were evaluated in field trials with two replications in three or five (A×BI) environments. Genotypic variance was highly significant for rust ratings in all populations, and heritabilities exceeded 0.64. Between 4 and 13 QTL were detected in individual populations using composite interval mapping, explaining between 33 and 71% of the phenotypic variance. Twenty QTL were distributed over all ten chromosomes, without preference to chromosomes 3, 4, 6, and 10, which harbor qualitatively acting Rp loci. In most cases, gene action was additive or partially dominant. Four pairs of QTL displayed significant digenic epistatic interactions, and QTL-environment interactions were observed frequently. Approximately half of the QTL were consistent between A×BIand A×BIIor A×C and C×D; fewer were consistent between A×BIand A×C or C×D. In European flint maize germ plasm, conventional selection for partial rust resistance seems to be more promising than marker-assisted selection.
Measuring Immigration and Sexual Reproduction in Field Populations of Mycosphaerella graminicola
Authors: Zhan J. , Mundt C. C. , McDonald B. A.
Published: May 2007

A field experiment was conducted to determine the relative contributions of immigration and sexual reproduction to the genetic structure of Mycosphaerella graminicola populations during the course of an epidemic. The genetic structure of M. graminicola populations sampled from wheat plots inoculated artificially with 10 isolates was compared with control plots infected naturally by airborne ascospores. Restriction fragment length polymorphisms (RFLPs) were used to test the randomness of associations among loci, and DNA fingerprints were used to identify clones. All isolates in the control plots had unique genotypes and RFLP loci were at gametic equilibrium, findings consistent with random mating. The proportion of isolates in the inoculated plots with DNA fingerprints that differed from the 10 inoculated isolates increased from 3% in the early to 39 and 34% in the mid- and late season, respectively. The degree of gametic disequilibrium was higher in the mid-season than in the late-season population. By the end of the growing season, we estimate that 66% of the isolates in the inoculated plots were asexual progeny of the 10 inoculated isolates, 10% were immigrants, and 24% were sexual recombinants. The proportion of infections caused by ascospores increased over the growing season.
Phytoplasma: Ecology and Genomic Diversity
Authors: Lee Ing-Ming , Gundersen-Rindal Dawn E. , Bertaccini Assunta
Published: May 2007

The recent development of molecular-based probes such as mono- and polyclonal antibodies, cloned phytoplasma DNA fragments, and phytoplasma-specific primers for polymerase chain reaction (PCR) has allowed for advances in detection and identification of uncultured phytoplasmas (formerly called mycoplasma-like organisms). Comprehensive phylogenetic studies based on analysis of 16S ribosomal RNA (rRNA) or both 16S rRNA and ribosomal protein gene operon sequences established the phylogenetic position of phytoplasmas as members of the class Mollicutes, and the revealed phylogenetic interrelationships among phytoplasmas formed a basis for their classification. Based on restriction fragment length polymorphism (RFLP) analysis of PCR-amplified 16S rRNA gene sequences, phytoplasmas are currently classified into 14 groups and 38 subgroups that are consistent with groups delineated based on phylogenetic analysis using parsimony of 16S rRNA gene sequences. In the past decades, numerous phyto-plasma strains associated with plants and insect vectors have been identified using molecular-based tools. Genomic diversity of phytoplasma groups appears to be correlated with their sharing common insect vectors, host plants, or both in nature. The level of exchange of genetic information among phytoplasma strains in a given group is determined by three-way, vector-phytoplasma-plant interactions. A putative mechanism for the creation of new ecological niches and the evolution of new ecospecies is proposed.
Expression of Engineered Antibodies in Plants: A Possible Tool for Spiroplasma and Phytoplasma Disease Control
Authors: Chen Y. D. , Chen T. A.
Published: May 2007

In an attempt to develop maize plants with resistance to corn stunt spi-roplasma (CSS), a single-chain Fv fragment (scFv) gene that was constructed from antibodies with strong inhibitory activity against CSS, was expressed in a cell line of maize. However, plants regenerated from this transgenic cell line showed no distinct resistance to CSS infection under the greenhouse conditions. The affinity and functionality of scFv in vivo and the locations of CSS and expressed scFv in maize could be a part of the contributing factors affecting this result. Progress of expressing antibodies in plants for plant pathogen resistance is also discussed.
Carbon Loss and Germinability, Viability, and Virulence of Chlamydospores of Fusarium solani f. sp. phaseoli After Exposure to Soil at Different pH Levels, Temperatures, and Matric Potentials
Authors: Mondal S. N. , Hyakumachi M.
Published: May 2007

14C-labeled chlamydospores of Fusarium solani f. sp. phaseoli were exposed to soil at 5, 15, 25, or 30°C at pH 5 or 8 and water potential of -1 kPa or to soil at 0, -1, or -10 kPa at 25°C at pH 6.9. Total carbon loss was greatest at 25 or 30°C at pH 8 and -1 kPa. 14CO2 from respiration of chlamydospores and from soil microbes utilizing chlamydospore exudates accounted for the largest share of total carbon loss under all conditions. 14(CO)2 from soil microbial metabolism of 14CO2 exudates of chlamydospores was greatest in soil at 15, 25, and 30°C, pH 8, and at either -1 or -10 kPa. Chlamydospore germinability in the absence of a C source (nutrient independence), viability in potato-dextrose broth, and virulence to kidney bean declined rapidly after exposure to soil at high temperatures (25 and 30°C), pH 8, and the higher matric potentials (0 to-1 kPa). By contrast, germinability remained high (>50%), as did virulence, in soil at 5°C and -10 kPa even after 70 days of incubation. Carbon loss was inversely correlated with germinability, viability, and virulence after exposure to soil at different pH levels, temperatures, and matric potentials.
A Single Gene Cluster Controls Incompatibility and Partial Resistance to VariousMelampsora larici-populinaRaces in Hybrid Poplars
Authors: Lefèvre F. , Goué-Mourier M. C. , Faivre-Rampant P. , Villar M.
Published: May 2007

Complete cosegregation for race-specific incompatibility with three Melampsora larici-populina rust races was observed in five F1hybrid progenies of Populus, with different patterns among the various progenies. A single gene cluster could explain these segregations: one locus with multiple alleles or two tightly linked loci controlling complete resistance to E1 and E3, and two tightly linked loci for E2. The random amplified polymorphic DNA marker OPM03/04_480 was linked to that cluster in all families (<1 cM). This marker accounted for more than 70% of the genetic variation for field resistance in each family (heritability ≈ 0.40). The same marker accounted for up to 64% of the clonal variation for growth in the nursery under natural inoculum pressure; the weak tolerance to rust of F1interspecific hybrids was attributed to a genetic background effect. Partial resistance was split into epidemiological components (heritability ranged from 0.35 to 0.87). Genotypic correlations among resistance traits for the different races were high (0.73 to 0.90). However, correlations among different resistance components for a single race were not all significant. A major quantitative trait locus for all components of partial resistance to E2 was associated to the cluster controlling incompatibility to E1 and E3 and marked by OPM03/04_480 (R2from 48 to 68%).
Ultrastructural and Cytochemical Investigation of Asymptomatic Infection by Pythium spp.
Authors: Rey Patrice , Benhamou Nicole , Tirilly Yves
Published: May 2007

The influence exerted by Pythium group F (a minor pathogen ubiquitous in soilless cultures) and P. uncinulatum (a nonpathogenic species) colonization on tomato roots was investigated. In both interactions, infected roots did not exhibit obvious symptoms; however, major physiological changes occurred within the host tissues colonized by Pythium group F compared to P. uncinulatum. According to our cytological observations, Pythium group F colonization involved a series of events: first, development and growth of the fungus in the epidermis and outer cortex tissues, which was associated with marked host cell disorganization and even breakdown. In colonized roots, symptoms were not easily discernible because alterations were restricted to the epidermis and outer cortex tissues. Second, pathogen ingress in the inner cortex and stele tissues was associated with massive induction of host defense reactions and alteration of invading hyphae. In a complex interaction that involved major metabolic changes in root cells, an array of defense-related reactions was produced, as exemplified by the formation of wall appositions and plugging of host cells with osmiophilic, electron-dense, granular, or fibrillar materials. P. uncinulatum growth was restricted to the epidermis and outer cortex tissues and associated with relatively minor damage to host cells. P. uncinulatum colonization of root tissues did not result in defense events similar to those observed in Pythium group F-infected inner cortex and stele tissues. Moreover, most of the invading P. uncinulatum hyphae were moribund. The results of the current study indicated that symptomless infections can be insignificant in terms of pathology for the plant (i.e., P. uncinulatum) or can be potentially dangerous (i.e., Pythium group F). Our results clearly showed a striking difference between nonpathogenic and minor pathogenic Pythium spp. attacks. The description of the Pythium group F infection process suggests that this interaction is unique, because the fungus behaves as a necrotrophic fungus in the epidermis and outer cortex tissues, whereas it is a potential inducer of plant defense reactions in the inner cortex and stele tissues. To our knowledge, cytological events similar to those observed with Pythium group F have not been described until now, even though descriptions of these events could help elucidate several aspects of the relationships between plants and minor pathogens.
Characterization of Mexican Isolates of Colletotrichum lindemuthianum by Using Differential Cultivars and Molecular Markers
Authors: González Mario , Rodríguez Raul , Zavala Maria Elena , Jacobo Juan L. , Hernández Fernando , Acosta Jorge , Martínez Octavio , Simpson June
Published: May 2007

Differential cultivars and molecular markers were used to analyze 59 isolates of the bean anthracnose pathogen, Colletotrichum lindemuthianum, from different regions of Mexico. Ten distinct races were determined, three of which had not been reported previously in Mexico. Isolates were found to infect only a narrow range of the differential cultivars used and were restricted to cultivars of Middle American origin. A comparison of random amplified polymorphic DNA and amplified fragment length polymorphism (AFLP) analyses was carried out on a subset of the fungal isolates. Determination of genetic distances based on AFLP data and production of a dendrogram demonstrated two levels of association: i) isolates classified into two major groups according to the type of cultivar or system of cultivation from which they originated, and ii) isolates could be classified into smaller subgroups generally associated with the geographic location from which they were obtained. Bootstrap analysis and determination of confidence intervals showed these geographic groupings to be extremely robust.
Genetic Structure of Setosphaeria turcica Populations in Tropical and Temperate Climates
Authors: Borchardt Dorothea S. , Welz H. Günter , Geiger Hartwig H.
Published: May 2007

Northern leaf blight, caused by Setosphaeria turcica, is a serious disease of maize in temperate and tropical environments. To examine the pathogen's population structure, we analyzed 264 isolates from four different continents with 70 random amplified polymorphic DNA markers and determined their mating types. Tropical populations (from Kenya, Mexico, and southern China) had an extremely high genotypic diversity, no or only weak gametic phase disequilibrium, and an even distribution of the two mating types, indicating frequent sexual recombination. Temperate populations (from Europe and northern China) had a much lower genotypic diversity, strong gametic phase disequilibrium, and an uneven distribution of mating types, indicating that sexual recombination has been rare. Populations in different continents were genetically isolated. They shared no haplotypes and carried several “private” alleles. The number of migrants between continents and between regions (between northern and southern China, western and central Kenya, and Europe west and east of the Alps) was estimated to be less than one per generation. Multivariate statistics suggested a greater relatedness of populations from the same continents than from different continents. Within agroecological zones, migration must be extensive. The potential within populations of S. turcica for adaptation should be regarded as very high, especially in tropical climates.
Silicon-Mediated Accumulation of Flavonoid Phytoalexins in Cucumber
Authors: Fawe A. , Abou-Zaid M. , Menzies J. G. , Bélanger R. R.
Published: May 2007

The controversial role of silicon in plant disease resistance, described mostly as a passive mechanical protection, has been addressed. Conclusive evidence is presented that silicon is involved in the increased resistance of cucumber to powdery mildew by enhancing the antifungal activity of infected leaves. This antifungal activity was attributable to the presence of low-molecular-weight metabolites. One of these metabolites, described here as a phytoalexin, was identified as a flavonol aglycone rhamnetin (3,5,3′,4′-tetrahydroxy-7-O-methoxyflavone). This is the first report of a phytoalexin for this chemical group in the plant kingdom and of a flavonol phytoalexin in cucumber, a chemical defense long believed to be nonexistent in the family Cucurbitaceae. The antifungal activity of leaf extracts was better expressed after acid hydrolysis, extending to another plant species the concept that some phytoalexins are synthesized as glycosylated phytoalexins or their precursors.
Identification of Quantitative Loci for Tolerance to Barley Yellow Dwarf Virus in Oat
Authors: Jin Hua , Domier Leslie L. , Kolb Frederic L. , Brown Charles M.
Published: May 2007

Molecular markers linked to quantitative trait loci conditioning tolerance to barley yellow dwarf virus (BYDV) were identified in oat (Avena sativa) using amplified fragment length polymorphism (AFLP) analysis. Near-isogenic and recombinant inbred lines (NILs and RILs, respectively) derived from a cross of Clintland64 (BYDV-sensitive) and IL86-5698 (BYDV-tolerant) were evaluated for their responses to an Illinois isolate of the PAV strain of BYDV. Individual markers identified in the analysis of the NILs explained up to 35% of the variability seen in the tolerance response. Single-point analysis of the marker data from the RIL population identified 24 markers in three linkage groups that were associated with tolerance to BYDV infection at P ≤ 0.001. These markers defined three major loci, A, C, and E, that were contributed by the tolerant parent (IL86-5698) and explained 35.0, 20.6, and 17.0% of the variability, respectively. Three minor loci G, H1, and R) were identified at P ≤ 0.01. These loci were contributed by the sensitive parent (Clintland64) and explained 5.8, 5.6, and 5.6% of the variability, respectively. Interval analysis showed that only the loci A, C, and E are associated significantly with BYDV tolerance at log of the likelihood ratio ≥ 3.0. These loci explained about 50% total of the variation in BYDV tolerance in multimarker regression analysis in both years. The BYDV tolerance loci A, C, E, and R were mapped to hexaploid oat restriction fragment length polymorphism linkage groups 2, 8, 36, and 5, respectively, by analyzing the segregation of the AFLP markers in the Kanota × Ogle RIL population.
Pathogenicity of Streptomyces scabies Mutants Altered in Thaxtomin A Production
Authors: Goyer Claudia , Vachon Joanne , Beaulieu Carole
Published: May 2007

To investigate the role of thaxtomin A in the pathogenicity of Streptomyces scabies, mutants altered in thaxtomin A production were obtained by N-methyl-N′-nitro-N-nitrosoguanidine mutagenesis. Mutants of S. scabies EF-35 could be differentiated according to levels of thaxtomin production. Mutants M1, M8, and M19 produced 2 to 20 times less thaxtomin A in oat bran medium than did EF-35. M1 and M19 were deficient in tryptophan catabolism. Thaxtomin production was reduced by about 300 times in mutant M16, which was a glutamic acid auxotroph. No thaxtomin A was detected in M13 culture supernatant. This mutant had a normal growth rate, was prototrophic, and catabolized tryptophan. Pathogenicity of mutants was tested on radish and potato. Mutants M1, M8, and M19 were pathogenic but, in most cases, less virulent than EF-35. M13 and M16 were nonpathogenic. These results suggest that thaxtomin A is an important pathogenicity determinant in S. scabies.
Effect of Relative Humidity on Sporulation of Fusarium oxysporum in Various Formulations and Effect of Water on Spore Movement Through Soil
Authors: Gracia-Garza J. A. , Fravel D. R.
Published: May 2007

Fusarium oxysporum f. sp. erythroxyli is being investigated as a mycoherbicide for the narcotic plant coca. Sporulation of the fungus in seven formulations containing different organic substrates and movement of its propagules through soil were studied. The formulations were a granular wheat flour/kaolin (pesta); an extruded wheat and rice flour (C-6); and five alginate pellet products containing corn cobs, soybean hull fiber, canola meal, rice flour, or rice flour plus canola oil. Formulations were incubated at 25°C for 6 weeks in desiccators with various salt solutions to provide nine relative humidities (RH), ranging from 100% (pure deionized water) to 0% (anhydrous (CaSO4). Hyphae of F. oxysporum f. sp. erythroxyli grew out of alginate pellets with canola meal, rice, and rice plus canola oil as early as 24 h at 100% constant RH. Alginate pellets of rice plus canola oil and granular C-6 and pesta formulations consistently produced more microconidia, macroconidia, and CFU than the other four formulations at all RH tested. The C-6 formulation produced more propagules than the other formulations at low RH (<53%). Canola meal pellets produced more spores than three other formulations when exposed to fluctuating RH (100 to 75%). The effect of percolating water on spore movement through soil was studied using a plant-pathogenic isolate of F. oxysporum f. sp. niveum. To determine the effect of water percolation on propagule movement, formulations were placed on soil columns and artificial rain was applied. In general, 10-fold fewer CFU were recovered at a 8- to 10-cm depth compared with a 0- to 2-cm depth.
Mechanisms of Induced Resistance in Barley Against Drechslera teres
Authors: Lyngs Jørgensen H. J. , Lübeck P. Stephensen , Thordal-Christensen H. , de Neergaard E. , Smedegaard-Petersen V.
Published: May 2007

Quantitative and qualitative histopathological methods and molecular analyses were used to study the mechanisms by which preinoculation with either of the nonbarley pathogens, Bipolaris maydis and Septoria nodorum, inhibited growth of Drechslera teres. Collectively, our data suggest that induced resistance is the principal mechanism responsible for impeding the pathogen. The enhancement of resistance in the host was primarily manifested during penetration by D. teres, and after penetration, where growth of D. teres ceased soon after development of infection vesicles. Thus, 24 h after pretreatment with B. maydis or S. nodorum, the penetration frequency from D. teres appressoria was reduced from 42.7% in the controls to 9.5 and 14.8%, respectively. The reductions were associated with increased formation of fluorescent papillae in induced cells (early defense reaction). The postpenetrational inhibition of D. teres completely stopped fungal growth and was apparently linked to an enhancement of multicellular hypersensitive responses in inducer-treated leaves (late defense reaction). Papillae formation and multicellular hypersensitive reactions were also observed in fully susceptible, noninduced control leaves, but they were inadequate to stop fungal progress. Northern blots from leaves treated with either inducer alone support the conclusion that induced resistance is involved in suppression of D. teres by increased formation of papillae and hypersensitive reactions. Thus, the blots showed strong expression of several defense response genes that are involved in these reactions in barley attacked by Erysiphe graminis f. sp. hordei.
Pathogenic and Genetic Variation in the Japanese Strains of Fusarium oxysporum f. sp. melonis
Authors: Namiki Fumio , Shiomi Toshiki , Nishi Kazufumi , Kayamura Tsuruo , Tsuge Takashi
Published: May 2007

Pathogenic variation among 41 Japanese strains of Fusarium oxysporum f. sp. melonis was analyzed by pathogenicity tests with muskmelon, oriental melon, and oriental pickling melon cultivars. Based on pathogenicity to muskmelon cvs. Amus and Ohi and oriental melon cv. Ogon 9, 41 strains were divided into 3 groups that corresponded completely to Risser's races 0, 2, and 1,2y. To further characterize pathogenic variation within the forma specialis and races, strains were assayed for pathogenicity to 42 additional muskmelon, oriental melon, and oriental pickling melon cultivars. All strains of race 1,2y were pathogenic to all cultivars tested. Strains of race 0 were divided into six variants based on differences in pathogenicity to three muskmelon cultivars; strains of race 2 also were classified into six variants based on differences in pathogenicity to two muskmelon cultivars and one oriental melon cultivar. Genetic variation among strains was analyzed by DNA fingerprinting with four repetitive DNA sequences: FOLR1 to FOLR4. Thirty-six fingerprint types were detected among forty-one strains by pooling results of fingerprinting with four probes. Cluster analysis showed distinct genetic groups correlated with races: the fingerprint types detected in each of races 2 and 1,2y were grouped into a single cluster, and two distinct genetic groups were found in race 0. However, pathogenic variation detected within races 0 and 2 could not be differentiated based on the nuclear markers examined.
Interactions of Pratylenchus thornei and Fusarium oxysporum f. sp. ciceris on Chickpea
Authors: Castillo Pablo , Mora-Rodríguez María P. , Navas-Cortés Juan A. , Jiménez-Díaz Rafael M.
Published: May 2007

Fusarium oxysporum f. sp. ciceris and the root-lesion nematode Pratylenchus thornei coinfect chickpeas in southern Spain. The influence of root infection by P. thornei on the reaction of Fusarium wilt-susceptible (CPS 1 and PV 61) and wilt-resistant (UC 27) chickpea cultivars to F. oxysporum f. sp. ciceris race 5 was investigated under controlled and field conditions. Severity of Fusarium wilt was not modified by coinfection of chickpeas by P. thornei and F. oxysporum f. sp. ciceris, in simultaneous or sequential inoculations with the pathogens. Root infection with five nematodes per cm3 of soil and 5,000 chlamydospores per g of soil of the fungus resulted in significantly higher numbers of propagules of F. oxysporum f. sp. ciceris with the wilt-susceptible cultivar CPS 1, but not with the wilt-resistant one. However, infection with 10 nematodes per cm3 of soil significantly increased root infection by F. oxysporum f. sp. ciceris in both cultivars, irrespective of fungal inoculum densities (250 to 2,000 chlamydospores per g of soil). Plant growth was significantly reduced by P. thornei infection on wilt-susceptible and wilt-resistant chickpeas in controlled and field conditions, except when shorter periods of incubation (45 days after inoculation) were used under controlled conditions. Severity of root necrosis was greater in wilt-susceptible and wilt-resistant cultivars when nematodes were present in the root, irrespective of length of incubation time (45 to 90 days), densities of nematodes (5 and 10 nematodes per cm3 of soil), fungal inocula, and experimental conditions. Nematode reproduction on the wilt-susceptible cultivars, but not on the wilt-resistant one, was significantly increased by F. oxysporum f. sp. ciceris infections under controlled and field conditions.
Lr46: A Gene Conferring Slow-Rusting Resistance to Leaf Rust in Wheat
Authors: Singh R. P. , Mujeeb-Kazi A. , Huerta-Espino J.
Published: May 2007

Wheat (Triticum aestivum) cultivar Pavon 76 carries slow-rusting resistance to leaf rust that has remained effective in Mexico since its release in 1976. ‘Pavon 76’ was crossed with two leaf rust-susceptible wheat cultivars, Jupateco 73S and Avocet S, and between 118 and 148 individual F2 plant-derived F3 and F5 lines were evaluated for adult-plant leaf rust resistance at two field sites in Mexico during different seasons. Evaluation of F1 plants and parents indicated that the slow-rusting resistance was partially dominant. Segregation in the F3 and F5 indicated that the resistance was based on two genes with additive effects. Monosomic analysis was carried out to determine the chromosomal locations of the resistance genes. For this purpose, two or three backcross-derived cytogenetic populations were developed by crossing ‘Pavon 76’ with a monosomic series of adult-plant leaf rust-susceptible cultivar Lal-bahadur. Evaluation of such BC2F3 and BC3F3 lines from 16 confirmed ‘Lalbahadur’ monosomics indicated that one slow-rusting gene was located in chromosome 1B of ‘Pavon 76’. This gene, designated as Lr46, is the second named gene involved in slow-rusting resistance to leaf rust in wheat.
Influence of Light, Relative Humidity, and Maturity of Populations on Discharge of Ascospores of Venturia inaequalis
Authors: Gadoury David M. , Stensvand Arne , Seem Robert C.
Published: May 2007

Ascospore release in 20 populations of Venturia inaequalis was generally suppressed in wind tunnel tests during darkness and simulated rain, but the following relieved this suppression: (i) exposure to low relative humidity during simulated rain and (ii) protracted incubation of leaf samples and the consequent senescence of the pathogen population. No counterpart to (i) was observed under orchard conditions. Although V. inaequalis also released a high percentage of ascospores during darkness in field studies under simulated rain late in the season of ascospore release, this phenomenon has not been reported for natural rain events. A threshold value of 0.5 μW/cm2 at 725 nm was identified as the minimum stimulatory light intensity. Ascospore release increased with increasing light intensity from 0.5 to 5.2 μW/cm2 at 725 nm. There was also an intrinsic increase in ascospore release as duration of rain increased. In orchards, the combined impact of both processes is probably responsible for a delay in reaching peak ascospore release at several hours after sunrise. Ascospore release during darkness will generally constitute a small proportion of the total available supply of primary inoculum. Significant ascospore release, and therefore infection periods, can be assumed to begin shortly after sunrise, when rain begins at night in orchards with low potential ascospore dose (PAD). A PAD level of 1,000 ascospores per m2 of orchard floor per season is suggested as a threshold, above which the night-released ascospores should not be ignored.
Effect of Nectar on Microbial Antagonists Evaluated for Use in Control of Fire Blight of Pome Fruits
Authors: Pusey P. L.
Published: May 2007

Under warm, dry conditions, Erwinia amylovora can become established in relatively high populations on apple (Malus domestica) or pear (Pyrus communis) flower stigmas, and subsequent wet conditions facilitate its movement to the flower hypanthium where infection generally is initiated through the nectarthodes. Research on biological control of fire blight has focused mainly on the flower stigma, and knowledge is lacking regarding the effect of nectar on microbial antagonists in the flower hypanthium. The biocontrol agents Pseudomonas fluorescens strain A506 and Pantoea agglomerans strain C9-1 were cultured in a basal liquid medium with various concentrations (0 to 50% total sugar) of sucrose or synthetic nectar (sucrose/glucose/fructose, 2:1:1). Strain A506 showed less growth and lower survival than strain C9-1 at high sugar levels, and A506 was less effective than C9-1 as a preemptive antagonist of E. amylovora in high-sugar media. Both antagonist strains were less tolerant to high sugar levels than E. amylovora (strain Ea153). The same bacteria were cultured in a medium with 25% total sugar consisting of various proportions of sucrose, glucose, and fructose, and growth response correlated strongly with solute potential. When 28 microbial strains were cultured in synthetic nectar (25% total sugar) and ranked based on growth, strains clustered according to taxonomic group. Yeasts were most osmotolerant, followed by strains of E. amylovora, Pantoea agglomerans, Bacillus spp., and Pseudomonas spp. Further studies done in planta are necessary to determine whether osmotolerance of antagonists is advantageous in the biological control of fire blight.
Biological Control of Fusarium Wilt of Cucumber by Chitinolytic Bacteria
Authors: Singh Pushpinder Paul , Shin Yong Chul , Park Chang Seuk , Chung Young Ryun
Published: May 2007

Two chitinolytic bacterial strains, Paenibacillus sp. 300 and Streptomyces sp. 385, suppressed Fusarium wilt of cucumber (Cucumis sativus) caused by Fusarium oxysporum f. sp. cucumerinum in nonsterile, soilless potting medium. A mixture of the two strains in a ratio of 1:1 or 4:1 gave significantly (P < 0.05) better control of the disease than each of the strains used individually or than mixtures in other ratios. Several formulations were tested, and a zeolite-based, chitosan-amended formulation (ZAC) provided the best protection against the disease. Dose-response studies indicated that the threshold dose of 6 g of formulation per kilogram of potting medium was required for significant (P < 0.001) suppression of the disease. This dose was optimum for maintaining high rhizosphere population densities of chitinolytic bacteria (log 8.1 to log 9.3 CFU/g dry weight of potting medium), which were required for the control of Fusarium wilt. The ZAC formulation was suppressive when added to pathogen-infested medium 15 days before planting cucumber seeds. The formulation also provided good control when stored for 6 months at room temperature or at 4°C. Chitinase and β-1,3-glucanase enzymes were produced when the strains were grown in the presence of colloidal chitin as the sole carbon source. Partial purification of the chitinases, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and activity staining, revealed the presence of five bands with molecular masses of 65, 62, 59, 55, and 52 kDa in the case of Paenibacillus sp. 300; and three bands with molecular masses of 52, 38, and 33 kDa in the case of Streptomyces sp. 385. Incubation of cell walls of F. oxysporum f. sp. cucumerinum with partially purified enzyme fractions led to the release of N-acetyl-D-glucosamine (NAGA). NAGA content was considerably greater when pooled enzyme fractions (64 to 67) from Paenibacillus sp. were used, because they contained high β-1,3-glucanase activity in addition to chitinase activity. Suppression of Fusarium wilt of cucumber by a combination of these two bacteria may involve the action of these hydrolytic enzymes.
Population Genetic Structure and Host Specificity of Alternaria spp. Causing Brown Spot of Minneola Tangelo and Rough Lemon in Florida
Authors: Peever T. L. , Canihos Y. , Olsen L. , Ibañez A. , Liu Y.-C. , Timmer L. W.
Published: May 2007

Alternaria spp. were sampled from two rough lemon (RL) and two Minneola tangelo (MIN) groves in a limited geographic area in central Florida to test for host-specialized forms of the pathogen. Isolates of Alternaria spp. were scored for variation at 16 putative random amplified polymorphic DNA (RAPD) loci and for pathogenicity on both hosts. Subpopulations on each host were differentiated genetically and pathogenically, which was consistent with the hypothesis of host specialization. Highly significant genetic differentiation was detected among all four subpopulations (Nei's coefficient of gene differentiation [GST] = 0.292, P = 0.000); most of the differentiation occurred between hosts (GST = 0.278, P = 0.000). Phenograms of qualitative similarities among isolates within subpopulations revealed two or three distinct clusters of isolates within each subpopulation. The majority of isolates sampled from RL were pathogenic on RL and not on MIN, although a few RL isolates were able to induce disease on MIN, and 44% were nonpathogenic on either host. In contrast, isolates from MIN were pathogenic only on MIN, never on RL, and only 3% of the isolates were nonpathogenic. Overall, three genetically distinct clusters of isolates were detected on both hosts. One of the clusters (cluster A) sampled from RL was pathogenic on RL and not on MIN and consisted almost entirely of one RAPD genotype. This cluster also contained two isolates that were 93% similar to the majority genotype but were pathogenic on MIN and not RL. In isolates from MIN, two distinct clusters of isolates were found in one subpopulation (clusters B and C), and three distinct clusters were found in another subpopulation (clusters A, B, and C). Clusters A and B were found on both hosts, while cluster C was limited to MIN. Populations of Alternaria spp. sampled from RL and MIN showed a high degree of host specificity; however, the specificity obscured a high level of genetic variation within subpopulations.
Identification of Sugarcane Striate Mosaic-Associated Virus by Partial Characterization of Its Double-Stranded RNA
Authors: Choi Yoon Gi , Croft Barry J. , Randles John W.
Published: May 2007

Sugarcane striate mosaic (ScSM)-affected sugarcane leaves contain a disease-associated 9-kilobase (kb) double-stranded RNA (dsRNA), usually together with 6- and 2.6-kb dsRNAs. The purified 9-kb dsRNA was amplified by the randomly primed polymerase chain reaction (PCR) and cloned. The nucleotide sequences of three separate regions, representing about 2.55 kb (28%) of the dsRNA sequence, were found to have significant similarities to viruses in the genera Capillo-, Carla-, Fovea-, Potex-, Poty-, Tricho-, and Tymovirus. Greatest overall similarity was found to apple stem pitting virus, with less similarity to blueberry scorch virus and potato virus M. A standard virus purification procedure was used to identify slightly flexuous filamentous particles that copurified with the disease-associated RNA. Particle modal lengths were approximately 950 and 1,900 nm with a diameter of 15 nm. Preparations contained a 51-kDa putative capsid protein and a 9-kb single-stranded RNA with a probable 3′ polyadenylate tract. These ScSM-associated virus particles differ physically from viruses in existing genera because of their relative rigidity, length, and putative coat protein size. Reverse-transcription PCR with a primer pair designed from the sequenced segments amplified a 820-base pair fragment from ScSM-affected but not healthy sugarcane plants.
Genetic Analysis ofEutypaStrains from California Supports the Presence of Two Pathogenic Species
Authors: DeScenzo Richard A. , Engel Stacia R. , Gomez German , Jackson Erik L. , Munkvold Gary P. , Weller Jennifer , Irelan Nancy A.
Published: May 2007

Eutypa dieback is a perennial canker disease that adversely affects grape (Vitis vinifera) production throughout the world. The causal agent has been known as either Eutypa armeniacae or E. lata, and it has been unclear whether the two taxa are separate species. We analyzed 115 isolates of Eutypa and conspecific strains, including 106 from California, using amplified fragment length polymorphism (AFLP) and sequence analysis of the ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequence. Strains from cultivated plant species exhibited an average genetic distance of 0.34, as calculated by the DICE coefficient (NTSYS-pc software). An unweighted pair-group method with arithmetic averages dendrogram revealed a genetically distinct (distance of 0.73) group of Eutypa strains from valley oak (Quercus lobata) and madrone (Arbutus menziesii) and a strain from grape. Analysis of rDNA ITS sequences strongly supported the genetically distinct cluster detected in the AFLP data. Combined data indicated the presence of two species of Eutypa (E. armeniacae and E. lata) in our sample population. However, both Eutypa species were capable of infecting native and cultivated hosts, suggesting the potential for native tree species to serve as inoculum sources for grape infection in California. Further investigations of E. armeniacae and E. lata would contribute to the development of a successful disease management strategy.
Spatiotemporal Separation of Groundnut Rosette Disease Agents
Authors: Naidu R. A. , Kimmins F. M. , Holt J. , Robinson D. J. , Deom C. M. , Subrahmanyam P.
Published: May 2007

Analysis by triple-antibody sandwich enzyme-linked immunosorbent assay of groundnut samples from fields in two seasons from different regions of Malawi showed the absence of groundnut rosette assistor virus (GRAV) from some plants showing groundnut rosette disease symptoms and the presence of GRAV in some symptomless plants. Viruliferous Aphis craccivora collected from fields transmitted either GRAV alone, groundnut rosette virus (GRV) with its satellite RNA (sat RNA), or all three agents together, in different proportions. More plants became infected with all three agents when increasing numbers of potentially viruliferous aphids were used per plant, suggesting a dosage response. Electrical pentration graph studies of aphid stylet activities indicated successful transmission of GRV and its sat RNA during both the “stylet pathway phase” and salivation into sieve elements, whereas GRAV was transmitted only during the latter phase. Aphids transmitted all three agents together only during the salivation phase. Reverse-transcriptase polymerase chain reaction testing of viruliferous aphids and of inoculated plants revealed no correlation between the presence of all three agents in prospective aphid vectors and their simultaneous transmission to groundnut plants. These results show that separation of the groundnut rosette disease agents occurs over time and space.
RNA 3 Deletion Mutants of Beet Necrotic Yellow Vein Virus Do Not Cause Rhizomania Disease in Sugar Beets
Authors: Tamada Tetsuo , Uchino Hirokatsu , Kusume Toshimi , Saito Minako
Published: May 2007

Two mutant strains of beet necrotic yellow vein virus (BNYVV) containing deletions in RNA 3 were obtained by single lesion transfers in Tetragonia expansa. The deleted regions encode either 94 or 121 amino acids toward the C-terminal part of the 25-kDa protein (P25). Wild-type and mutant virus strains were inoculated by Polymyxa betae to sugar beet seedlings of susceptible and partially resistant cultivars. No differences were found in virus content in rootlets between mutant and wild-type viruses or between susceptible and resistant cultivars after culture for 4 weeks in a growth cabinet. However, when virus-inoculated seedlings were grown in the field for 5 months, the wild-type virus caused typical rhizomania root symptoms (69 to 96% yield loss) in susceptible cultivars, but no symptoms (23% loss) developed in most plants of the resistant cultivar, and BNYVV concentrations in the roots were 10 to 20× lower in these plants than in susceptible plants. In contrast, the mutant strains caused no symptoms in susceptible or resistant cultivars, and the virus content of roots was much lower in both cultivars than in wild-type virus infections. Wild-type RNA 3 was not detectable in most of the taproots of a resistant cultivar without any symptoms, suggesting that replication of undeleted RNA 3 was inhibited. These results indicate that the P25 of BNYVV RNA 3 is essential for the development of rhizomania symptoms in susceptible cultivars and suggest that it may fail to facilitate virus translocation from rootlets to taproots in the partially resistant cultivar.
Chromosomal Gene Transfer by Conjugation in the Plant Pathogen Xanthomonas axonopodis pv. vesicatoria
Authors: Basim Huseyin , Stall Robert E. , Minsavage Gerald V. , Jones Jeffrey B.
Published: May 2007

Genes for copper resistance, located on the chromosome of strain XvP26 of Xanthomonas axonopodis pv. vesicatoria, were transferred by conjugation to a recipient strain of the bacterium. The chromosomal gene transfer was verified by analyses of the genomes of donor, recipient, and putative transconjugants for plasmid profiles, by polymorphism of DNA bands obtained by digesting total genomic DNA by a rare-cutting endonuclease and pulsed-field gel electrophoresis, and by Southern hybridization with a probe containing the copper genes. Transfer of kanamycin resistance to a recipient strain, associated with Tn5 insertion into the chromosome of another strain of the bacterial spot pathogen, was also verified. The frequency of kanamycin resistance transfer to recipient was more than 75 times greater in pepper leaves than in vitro. The transfer of chromosomal sequences containing the hypersensitive reaction and pathogenicity (hrp) genes and pigmentation (pig) genes was linked with transfer of kanamycin resistance (Tn5). Horizontal transfer in planta of the chromosomal genes (i.e., cop, pig, hrp, and Tn5 sequences) among strains of X. axonopodis pv. vesicatoria means that horizontal chromosomal gene transfer is possible in nature. This type of gene transfer may explain the presence of great diversity among strains of the bacterial spot pathogen in terms of DNA polymorphism and may also explain the apparent horizontal transfer of hrp sequences among pathovars of Xanthomonas.
Mechanisms of Action and Dose-Response Relationships Governing Biological Control of Fusarium Wilt of Tomato by NonpathogenicFusariumspp.
Authors: Larkin Robert P. , Fravel Deborah R.
Published: May 2007

Three isolates of nonpathogenic Fusarium spp. (CS-1, CS-20, and Fo47), previously shown to reduce the incidence of Fusarium wilt diseases of multiple crops, were evaluated to determine their mechanisms of action and antagonist-pathogen inoculum density relationships. Competition for nutrients, as represented by a reduction in pathogen saprophytic growth in the presence of the biocontrol isolates, was observed to be an important mechanism of action for isolate Fo47, but not for isolates CS-1 and CS-20. All three biocontrol isolates demonstrated some degree of induced systemic resistance in tomato (Lycopersicon esculentum) and watermelon (Citrullus lanatus) plants, as determined by split-root tests, but varied in their relative abilities to reduce disease. Isolate CS-20 provided the most effective control (39 to 53% disease reduction), while Fo47 provided the least effective control (23 to 25% reduction) in split-root tests. Dose-response relationships also differed considerably among the three biocon-trol isolates, with CS-20 significantly reducing disease incidence at antagonist doses as low as 100 chlamydospores per g of soil (cgs) and at pathogen densities up to 105cgs. Isolate CS-1 also was generally effective at antagonist densities of 100 to 5,000 cgs, but only when pathogen densities were below 104cgs. Isolate Fo47 was effective only at antagonist densities of 104to 105cgs, regardless of pathogen density. Epidemiological dose-response models (described by linear, negative exponential, hyperbolic saturation [HS], and logistic [LG] functions) fit to the observed data were used to quantify differences among the biocontrol isolates and establish biocontrol characteristics. Each isolate required a different model to best describe its dose-response characteristics, with the HS/HS, LG/HS, and LG/LG models (pathogen/biocontrol components) providing the best fit for isolates CS-1, CS-20, and Fo47, respectively. Model parameters (defining effective biocontrol dose (ED50) indicated an ED50of 2.6, 36.3, and 2.1 × 106cgs and estimates of biocontrol efficiency of 0.229, 0.539, and 0.774 for isolates CS-1, CS-20, and Fo47, respectively. Differences in dose-response relationships among the biocontrol isolates were attributed to differences in their mechanisms of action, with CS-20 and CS-1 functioning primarily by induced resistance and Fo47 functioning primarily by competition for nutrients.