bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Modeling of the Effects of Temperature and Inoculum Density of Fusarium oxysporum f. sp. ciceris Races 0 and 5 on Development of Fusarium Wilt in Chickpea Cultivars
Authors: Navas-Cortés Juan A. , Landa Blanca B. , Méndez-Rodríguez Miguel A. , Jiménez-Díaz Rafael M.
Published: May 2007

Races 0 (Foc-0) and 5 (Foc-5) of Fusarium oxysporum f. sp. ciceris differ in virulence and induce yellowing or wilting syndrome, respectively, in chickpea. We modeled the combined effects of soil temperature and inoculum density of Foc-0 and Foc-5 on disease developed in chickpea cvs. P-2245 and PV-61 differing in susceptibility to those races, using quantitative nonlinear models. Disease development over time in the temperature range of 10 to 30°C and inoculum densities between 6 and 8,000 chlamydospores g−1 of soil was described by the Weibull function. Four response variables (the reciprocal incubation period, the final disease intensity, the standardized area under the disease progress curve, and the intrinsic rate of disease development) characterized the disease development. Response surface models that expressed the combined effect of inoculum density and temperature were developed by substituting the intrinsic rate of disease development in the Weibull or exponential functions with a beta function describing the relationship of response variables to temperature. The models estimated 22 to 26°C as the most favorable soil temperature for infection of cvs. P-2245 and PV-61 by Foc-5, and 24 to 28°C for infection of cv. P-2245 by Foc-0. At 10°C, no disease developed except in cv. P-2245 inoculated with Foc-5. At optimum soil temperature, maximum disease intensity developed with Foc-5 and Foc-0 at 6 and 50 chlamydospores g−1 of soil respectively, in cv. P-2245, and with Foc-5 at 1,000 chlamydospores g−1 of soil in cv. PV-61. The models were used to construct risk threshold charts that can be used to estimate the potential risk of Fusarium wilt epidemics in a geographical area based on soil temperature, the race and inoculum density in soil, and the level of susceptibility of the chickpea cultivar.
Simple Sequence Repeat Diversity of a Worldwide Collection of Puccinia triticina from Durum Wheat
Authors: Ordoñez M. E. , Kolmer J. A.
Published: May 2007

Isolates of Puccinia triticina collected from durum wheat from Argentina, Chile, Ethiopia, France, Mexico, Spain, and the United States were analyzed with 11 simple sequence repeat (SSR) markers in order to determine the genetic relationship among isolates. These isolates also were compared with P. triticina isolates from common wheat from North America, and an isolate collected from Aegilops speltoides from Israel, to determine genetic relationships among groups of P. triticina found on different telial hosts. The large majority of isolates from durum wheat were identical for SSR markers or had <8% genetic dissimilarity, except for isolates from Ethiopia, which had 55% dissimilarity with respect to the other durum isolates. Isolates from common wheat had >70% genetic dissimilarity from isolates from durum wheat, and the isolate from A. speltoides was >90% dissimilar from all isolates tested. Analysis of molecular variance tests showed significant levels (P = 0.001) of genetic differentiation among regions and among isolates within countries. Isolates of P. triticina from durum wheat from South America, North America, and Europe were closely related based on SSR genotypes, suggesting a recent common ancestor, whereas P. triticina from Ethiopia, common wheat, and A. speltoides each had distinct SSR genotypes, which suggested different origins.
Flumorph Is a Novel Fungicide That Disrupts Microfilament Organization in Phytophthora melonis
Authors: Sheng Zhu Shu , Li Liu Xi , Fei Liu Peng , Li Yong , Qiang Li Jian , Min Wang Hui , Kui Yuan Shan , Guo Si Nai
Published: May 2007

The mechanism of the effects of flumorph (a novel fungicide) was investigated by analyzing alterations of hyphal morphology, cell wall deposition patterns, F-actin organization, and other organelles in Phytophthora melonis. Calcofluor white staining suggested that flumorph did not inhibit the synthesis of cell wall materials, but disturbed the polar deposition of newly synthesized cell wall materials during cystospore germination and hyphal growth. After exposure to flumorph, zoospores were able to switch into cystospores accompanied with the formation of a cell wall, whereas cystospores failed to induce the isotropic-polar switch and did not produce germ tubes but continued the isotropic growth phase. In flumorph-treated hyphae, the most characteristic change was the development of periodic swelling (“beaded” morphology) and the disruption of tip growth. Newly synthesized cell wall materials were deposited uniformly throughout the diffuse expanded region of hyphae, in contrast to their normal polarized patterns of deposition. These alterations were the result of F-actin disruption, identified with the fluorescein isothiocynate (FITC)-phalloidin staining. The disruption of F-actin also was accompanied by disorganized organelles: each swelling of subapical hyphae was associated with a nucleus. Vesicles did not undergo polarized secretion to the apical hyphae, but diffused around nuclei for the subapical growth; thus, the cell wall was thickened with periodic expansion along the hyphae. Upon removing flumorph, normal tip growth and organized F-actin were observed again. These data, as well as data published earlier, suggest that flumorph may be involved in the impairment of cell polar growth through directly or indirectly disrupting the organization of F-actin. The primary site of action by flumorph in the disruption of the F-actin organization is under investigation.
Characterization of Components of Partial Resistance, Rps2, and Root Resistance to Phytophthora sojae in Soybean
Authors: Mideros Santiago , Nita Mizuho , Dorrance Anne E.
Published: May 2007

Phytophthora root and stem rot of soybeans caused by Phytophthora sojae is a serious limitation to soybean production in the United States. Partial resistance to P. sojae in soybeans is effective against all the races of the pathogen and is a form of incomplete resistance in which the level of colonization of the root is reduced following inoculation. Other forms of incomplete resistance include the single dominant gene Rps2 and Ripley's root resistance, which are both race-specific. To differentiate partial resistance from the other types of incomplete resistance, the components lesion length, numbers of oospores, and infection frequency were measured in eight soybean genotypes inoculated with two P. sojae isolates. The Rps2 and root-resistant genotypes had significantly lower oospore production and infection frequency compared with the partially resistant genotype Conrad, while the root-resistant genotype also had significantly smaller lesion lengths. However, the high levels of partial resistance in Jack were indistinguishable from Rps2 in L76-1988, based on the evaluation of these components. Root resistance in Ripley and Rps2 in L76-1988 had similar responses for all components measured in this study. Partial resistance expressed in Conrad, Williams, Jack, and General was comprised of various components that interact for defense against P. sojae in the roots, and different levels of each component were found in each of the genotypes. However, forms of incomplete resistance expressed via single genes in Ripley and Rps2 in L76-1988, could not be distinguished from high levels of partial resistance based on lesion length, oospore production, and infection frequency.
Influence of Incubation-Period Humidity on the Development of Brown Rot Blossom Blight of Sour Cherry
Authors: Koball D. C. , Wilcox W. F. , Seem R. C.
Published: May 2007

When detached sour cherry (Prunus cerasus) blossoms were inoculated with conidia of Monilinia fructicola and subjected to a standard 8-h wetting treatment at 20°C, blossom blight incidence was proportional to relative humidity (RH) when RH was held constant during the subsequent 6-day incubation period (frequency = 1.0 at the maximum RH of 92%; frequency = 0.38 at the minimum RH of 57%). Similarly, when a primary incubation period at 87% RH was followed by a secondary incubation period at 54% RH, blossom blight incidence was proportional to the number of hours at the higher level (frequencies of 0.94, 0.80, and 0.38 with primary incubation periods of 6 days, 36 h, and 12 h, respectively). When intact blossoms on potted trees were exposed to common inoculation and wetting treatments, disease incidence was consistently high on trees that subsequently were incubated in a controlled environment chamber (20°C, 90 to 95% RH) but was extremely variable when trees were incubated under variable ambient conditions. Ambient incubation temperature had little effect on disease incidence 9 days after inoculation, whereas ambient RH had a pronounced effect: the frequency of blighted blossoms was 0.53 to 0.61 when the number of hours at RH >90% was approximately two to six times that at RH <60%, whereas this frequency was only 0.02 to 0.07 when the number of hours at RH >90% was approximately one-third the number at RH <60%. After 48 h at a constant RH of 89 or 57%, the water potential of excised uninoculated blossoms was -1.15 and -1.93 MPa, respectively; however, growth of M. fructicola on osmotically adjusted potato dextrose agar was unaffected by changes in water potential within this range. Thus, although RH during incubation has an important influence on blossom blight development, the causal mechanism remains uncertain.
Genetic Analysis of Avirulence/Virulence of an Isolate of Magnaporthe grisea from a Rice Field in Texas
Authors: Chao C. T. , Ellingboe A. H.
Published: May 2007

An isolate of Magnaporthe grisea, Tm4, from a rice field in Texas was crossed with a fertile laboratory strain, 70-6. The progenies showed segregation of avirulence/virulence on rice cvs. Newbonnet, Lemont, Lebonnet, Leah, and Katy. The avirulent/virulent segregation ratios were 29:6 on Newbonnet, Lemont, and Lebonnet; 28:7 on Leah; and 33:2 on Katy. There was cosegregation on the first three cultivars. Several avirulent progenies were backcrossed to virulent parent 70-6. Three generations of backcrossing avirulent progenies to 70-6 led to segregation ratios that suggested certain strains had only one avirulence gene. Strains avirulent only on cv. Katy or only on cvs. Newbonnet, Lemont, and Lebonnet were test crossed with virulent siblings. Strains that gave progeny ratios approximating 1 avirulent:1 virulent when crossed with virulent siblings were selected for further test crossing. Intercrosses between strains with possible single avirulence genes were made to determine whether these strains had the same or different avirulence genes. Many lines still segregated two genes for avirulence after three generations of backcrossing. This is based on the recovery of virulent progenies from crossing two avirulent siblings.
Etiology of Apple Sooty Blotch Disease in North Carolina
Authors: Johnson E. M. , Sutton T. B. , Hodges C. S.
Published: May 2007

Sooty blotch disease of apples (Malus × domestica), previously attributed to the fungus Gloeodes pomigena, was shown to be a disease complex caused by fungi previously considered mycelial types of G. pomigena. Peltaster fructicola and Geastrumia polystigmatis were associated with the ramose mycelial type. A fungus similar to P. fructicola, but with smaller pycnothyria and conidia, was associated with the punctate mycelial type. The diffuse fuliginous mycelial type was caused by Leptodontium elatius. Fungi that fit the classical morphological description of G. pomigena were not observed. Pycnothyria of P. fructicola developed on overwintered colonies on blackberries, and conidia were a source for primary infection during the entire growing season. Secondary spread was through conidia produced in colonies on fruit. L. elatius was observed infrequently producing conidia on fruit during periods of extended high moisture. Histological examination did not reveal penetration of the cuticle of apple fruit for any of the fungi of the apple sooty blotch complex.
Genetic Analysis of Metalaxyl Insensitivity Loci in Phytophthora infestans Using Linked DNA Markers
Authors: Fabritius Anna-Liisa , Shattock Richard C. , Judelson Howard S.
Published: May 2007

Previous studies indicated that incompletely dominant loci determine insensitivity by oomycetes to phenylamide fungicides such as metalaxyl. To compare the bases of insensitivity in different strains of the late blight pathogen, Phytophthora infestans, crosses were performed between sensitive isolates and isolates from Mexico, the Netherlands, and the United Kingdom that displayed varying levels of insensitivity. Segregation analyses indicated that metalaxyl insensitivity was determined primarily by one locus in each isolate, and that two of the isolates were heterozygous and the other homozygous for the insensitive allele. Metalaxyl insensitivity was also affected by the segregation of additional loci of minor effect. DNA markers linked to insensitivity were obtained by bulked segregant analysis using random amplified polymorphic DNA (RAPD) markers and the Dutch and Mexican crosses. By studying the linkage relationships between these markers and the insensitivity in each cross by RAPD or restriction fragment length polymorphism analysis, it appeared that the same chromosomal locus conferred insensitivity in the Mexican and Dutch isolates. However, a gene at a different chromosomal position was responsible for insensitivity in the British isolate.
Serological Detection of Grapevine Virus A Using Antiserum to a Nonstructural Protein, the Putative Movement Protein
Authors: Rubinson E. , Galiakparov N. , Radian S. , Sela I. , Tanne E. , Gafny R.
Published: May 2007

Grapevine virus A (GVA) is implicated in the etiology of the rugose wood disease. The coat protein (CP) and the putative movement protein (MP) genes of GVA were cloned and expressed in Escherichia coli and used to produce antisera. Both the CP and the MP were detected with their corresponding antisera in GVA-infected Nicotiana benthamiana. The MP was first detected at an early stage of the infection, 6 to 12 h after inoculation, and the CP was detected 2 to 3 days after inoculation. The CP and MP were detected by immunoblot analysis in rugose wood-affected grapevines. The MP could be detected in GVA-infected grapevines that tested negative for CP, both with CP antiserum and with a commercially available enzyme-linked immunosorbent assay kit. The study shows that detection of the nonstructural MP may be an effective means for serological detection of GVA infection in grapevines.
High Genetic Similarity Among Populations of Phaeosphaeria nodorum Across Wheat Cultivars and Regions in Switzerland
Authors: Keller S. M. , Wolfe M. S. , McDermott J. M. , McDonald B. A.
Published: May 2007

Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-km transect representing three geographical regions in Switzerland to determine the scale of genetic differentiation among subpopulations. Three different wheat cultivars were sampled three times to determine whether differences in host genotype correlated with differences among corresponding pathogen populations. Seven restriction fragment length polymorphism (RFLP) loci and one DNA fingerprint were assayed for each of the 432 isolates in the collection. DNA fingerprints differentiated 426 unique genotypes. Though absolute differences were small, five RFLP loci exhibited significant differences in allele frequencies across the nine sub-populations. Gene diversity within all subpopulations was high (HT = 0.51), but only 3% of the total genetic variation was distributed among the nine subpopulations. When subpopulations were grouped according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespread gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and across the entire Swiss population supported the hypothesis of random mating.
Cloning and Characterization of a Member of theXanthomonas avr/pthGene Family That Evades All Commercially Utilized CottonRGenes in the United States
Authors: Chakrabarty P. K. , Duan Y. P. , Gabriel D. W.
Published: May 2007

The highly virulent African strains of Xanthomonas campestris pv. malvacearum are quarantined pathogens in the United States and can evade or overcome all commercially utilized resistance (R) genes in cotton grown in the United States including the entire set of host differential lines used to distinguish 19 races of the pathogen. Nevertheless, the African strains carry multiple DNA fragments that strongly hybridize with members of the Xanthomonas avirulence (avr)/pathogenicity (pth) gene family. Since all previously tested members of the gene family confer avirulence against one or more R genes in cotton, strains carrying multiple members might not be expected to evade so many different R genes. The hybridizing DNA fragments were cloned from African strain XcmN and found to confer water-soaking ability to a nearly asymptomatic mutant strain of the pathogen. Restriction mapping, Southern hybridization, and DNA sequencing of the cloned fragments from XcmN were used to identify two water-soaking genes, pthN and pthN2, as new members of the Xanthomonas avr/pth gene family. The complete DNA sequence of pthN was obtained, and it is >94% identical with all other sequenced members of the gene family. Gene fusions of pthN with avrb6 (another family member) and other experiments revealed that the ability of African strain XcmN to water-soak cotton and avoid recognition by commercially used cotton R genes is determined by the specific repeats of multiple functional members of the Xanthomonas avr/pth gene family.
Effects of Temperature and Host on the Generation of Tomato Spotted Wilt Virus Defective Interfering RNAs
Authors: Inoue-Nagata Alice K. , Kormelink Richard , Nagata Tatsuya , Kitajima Elliot W. , Goldbach Rob , Peters Dick
Published: May 2007

The generation of defective interfering (DI) RNA molecules of tomato spotted wilt tospovirus (TSWV) was studied by serially passaging in-ocula from plant to plant under different controlled conditions. DI RNAs were generated at higher rates in plants at 16°C than in plants incubated at higher temperatures. Another factor promoting the TSWV DI RNA generation was the use of high virus concentrations in the inocula. The solanaceous species Capsicum annuum, Datura stramonium, Lycopersicon esculentum, Nicotiana benthamiana, and N. rustica supported the generation of DI RNAs, whereas the virus recovered from the inoculated composite species, Emilia sonchifolia, remained free of any DI RNA under all conditions tested. This study resulted in a strategy to maintain DI RNA-free TSWV isolates, as well as in an efficient way to produce a large population of different DI RNA species. A single DI RNA species usually became dominant in an isolate after a few rounds of serial inoculations. The possible mechanisms involved in TSWV DI RNA generation under different inoculation circumstances are discussed.
Germination Induces Accumulation of Specific Proteins and Antifungal Activities in Corn Kernels
Authors: Guo B. Z. , Chen Z.-Y. , Brown R. L. , Lax A. R. , Cleveland T. E. , Russin J. S. , Mehta A. D. , Selitrennikoff C. P. , Widstrom N. W.
Published: May 2007

This study examined protein induction and accumulation during imbibition and germination of corn kernels, as well as antifungal activities of extracts from germinating kernels against Aspergillus flavus and Fusarium moniliforme. Genotypes studied included GT-MAS:gk and Mp420, which are resistant to A. flavus infection and aflatoxin accumulation, and Pioneer 3154 and Deltapine G-4666, which are susceptible to A. flavus infection and aflatoxin accumulation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis resolved five protein bands that were present at higher concentrations in germinated kernels than in nongerminated kernels. Western blot analyses revealed that one of these proteins reacted with the 22-kDa zeamatin antiserum, and a zeamatin-like protein accumulated to a higher concentration in germinated kernels. Two protein bands from dry kernels that reacted with ribosome-inactivating protein (RIP) antiserum were identified as the 32-kDa proRIP-like form and an 18-kDa peptide of the two peptides that form active RIP. However, in germinated kernels, two protein bands that reacted with RIP antiserum were identified as two RIP-like peptides with a molecular mass of ≈18 and 9 kDa. Purified RIP and zeamatin from corn inhibited growth of A. flavus. Bioassays of germinated kernel extracts from all four genotypes exhibited antifungal activity against A. flavus and F. moniliforme, with extracts from the susceptible genotypes showing greater inhibition zones. This study provides evidence of protein induction in corn kernels during imbibition or the early stages of germination, and the induced proteins may be related to our previous findings of germination-associated resistance in the corn kernel, especially in the susceptible kernels.
Detection of New Ethylene-Producing Bacteria, Pseudomonas syringae pvs. cannabina and sesami, by PCR Amplification of Genes for the Ethylene-Forming Enzyme
Authors: Sato M. , Watanabe K. , Yazawa M. , Takikawa Y. , Nishiyama K.
Published: May 2007

Strains of Pseudomonas syringae (78 strains and 43 pathovars) and other strains (79) of plant and insect origin were examined for the presence of the ethylene-forming enzyme gene (efe) by polymerase chain reaction (PCR) assay. The sequence of the efe gene of P. syringae pv. phaseolicola PK2 was used to design two primer sets for amplification of the gene. In addition to P.syringae pv. phaseolicola (the “kudzu strain”) and P.syringae pv. glycinea, which were efficient ethylene producers, several strains of P.syringae pvs. sesami and cannabina generated PCR products of the predicted size. A DNA probe of the efe gene, isolated from strain PK2, hybridized to these PCR products, indicating homology to the P.syringae pv. phaseolicola efe gene. PCR restriction fragment length polymorphism analyses suggested that these four pathovars harbor a similar efe gene. Furthermore, the probe hybridized to an indigenous plasmid of P.syringae pv. cannabina, suggesting that the efe gene could be located on a plasmid in this pathovar, but did not hybridize to plas-mids of P.syringae pv. sesami strains. P.syringae pvs. sesami and cannabina strains produced ethylene in King's medium B at levels similar to those of P.syringae pvs. phaseolicola and glycinea. Thus, two new ethylene-producing bacteria were detected by the PCR assay.
Reassessment of the Role of Saprophytic Activity in the Ecology of Thielaviopsis basicola
Authors: Hood M. E. , Shew H. D.
Published: May 2007

The ability of Thielaviopsis basicola to survive saprophytically in soil was investigated using root tissue from susceptible hosts as organic substrates. Inoculum densities were lower in soils amended with root tissue than in nonamended controls after 2 and 4 weeks of incubation. The greatest decrease occurred in soils containing the highest concentration of root tissue or in soils in which root tissue included the soluble components of the living root. Reproduction by T. basicola also was examined in axenic media containing either killed root pieces or various carbohydrates as the sole carbohydrate source. T. basicola utilized killed root tissue as a carbohydrate source in axenic media, particularly in cultures in which root tissue included the soluble components. Enzymatic activities of T. basicola, however, did not result in maceration of the root tissue. T. basicola utilized sucrose and cellobiose, but did not utilize structural carbohydrates such as cellulose, hemicellulose, or pectin. Based on the absence of significant saprophytic ability, T. basicola should be classified ecologically as an obligate parasite.
Border Cells and Arbuscular Mycorrhizae in Four Amaranthaceae Species
Authors: Arriola Laura , Niemira Brendan A. , Safir Gene R.
Published: May 2007

Four species from the family Amaranthaceae were studied to determine border cell production and arbuscular mycorrhizal colonization. It was found that border cells, also known as sloughed root cap cells, are produced by all plant species studied and increase with increasing root length until a maximum number is reached at a root length of 25 mm. However, the increase in border cells with increasing root length is not uniform between species. Arbuscular mycorrhizal root colonization was found in all the Amaranthaceae species, and arbuscular mycorrhizal colonization was positively correlated with maximum border cell production.
Zinc Improves Biocontrol of Fusarium Crown and Root Rot of Tomato by Pseudomonas fluorescens and Represses the Production of Pathogen Metabolites Inhibitory to Bacterial Antibiotic Biosynthesis
Authors: Duffy Brion K. , Défago Geneviève
Published: May 2007

Crown and root rot of tomato caused by Fusarium oxysporum f. sp. radicis-lycopersici is an increasing problem in Europe, Israel, Japan, and North America. The biocontrol agent Pseudomonas fluorescens strain CHA0 provides only moderate control of this disease. A one-time amendment of zinc EDTA at 33 μg of Zn2+/ml to hydroponic nutrient solution in soilless rockwool culture did not reduce disease when used alone, but did reduce disease by 25% in the presence of CHA0. In in vitro studies with the pathogen, zinc at concentrations as low as 10 μg/ml abolished production of the phytotoxin fusaric acid, a Fusarium pathogenicity factor, and increased production of microconidia over 100-fold, but reduced total biomass. Copper EDTA at 33 μg of Cu2+/ml had a similar effect as zinc on the pathogen in vitro; it reduced disease when used alone, and increased the biocontrol activity of CHA0 in soilless culture. Ammonium-molybdate neither improved the biocontrol activity of CHA0 nor affected production of fusaric acid or microconidia. Strain CHA0 did not degrade fusaric acid. Fusaric acid at concentrations as low as 0.12 μg/ml repressed production by CHA0 of the antibiotic 2,4-diacetylphloroglucinol, a key factor in the biocontrol activity of this strain. Production of pyoluteorin by CHA0 was also reduced, but production of hydrogen cyanide and protease was not affected, suggesting that fusaric acid affects biosynthesis at a regulatory level downstream of gacA and apdA genes. Fusaric acid did not affect the recovery of preformed antibiotics nor did it affect bacterial growth even at concentrations as high as 200 μg/ml. When microbial meta-bolite production was measured in the rockwool bioassay, zinc amendments reduced fusaric acid production and enhanced 2,4-diacetylphloro-glucinol production. We suggest that zinc, which did not alleviate the repression of antibiotic biosynthesis by fusaric acid, improved biocontrol activity by reducing fusaric acid production by the pathogen, which resulted in increased antibiotic production by the biocontrol agent. This demonstrates that pathogens can have a direct negative impact on the mechanism(s) of biocontrol agents.
Importance of Different Pathways for Maize Kernel Infection by Fusarium moniliforme
Authors: Munkvold G. P. , McGee D. C. , Carlton W. M.
Published: May 2007

The relative importance of several infection pathways (silks, stalks, and seed) leading to kernel infection of maize hybrids by Fusarium moniliforme was investigated in field experiments in 1993 and 1994. Systemic movement of specific fungal strains within plants was detected by using vegetative compatibility as a marker. Transmission of F. moniliforme from inoculated seed to stalks and developing kernels was detected in two of three field experiments; the seed-inoculated strain was detected in kernels on approximately 10% of ears. The percentage of kernels infected with the seed-inoculated strain ranged from 0 to 70%, with a mean of 0 to 2.5% (0 to 8.3% of F. moniliforme-infected kernels). Other pathways to kernel infection were more effective than seed transmission and systemic infection. F. moniliforme strains inoculated into the crowns and stalks of plants were found throughout the stalks and in up to 95% of the kernels in individual plants. Infection through the silks was clearly the most effective pathway to kernel infection. This was the only inoculation method that significantly increased overall incidence of F. moniliforme infection in kernels; the silk-inoculated strain infected up to 100% of the kernels in individual ears, with a treatment mean as high as 83.7% of kernels. When plants were silk-inoculated, the percentage of kernels infected by other F. moniliforme strains from the seed or stalk was reduced, apparently due to competition among strains. This study provides evidence that systemic development of F. moniliforme from maize seed and stalk infections can contribute to kernel infection, but silk infection is a more important pathway for this fungus to reach the kernels.
Initial Cellular Interactions BetweenThielaviopsis basicolaand Tobacco Root Hairs
Authors: Hood M. E. , Shew H. D.
Published: May 2007

Cellular events that occur during the initial interactions between Thielaviopsis basicola and root hairs of tobacco (Nicotiana tabacum) were examined microscopically. Time-course documentation of the infection process indicated a dynamic interaction between T. basicola and the living host cell. Upon root hair contact and recognition, the vegetative apex of T. basicola rapidly differentiated to form infection structures, and the host cell responded cytologically. Penetration was achieved by threadlike hyphae that subsequently developed distal swellings, and intracellular hyphae of sickle-shaped morphology advanced from the distal swelling and colonized the cell. Streaming of the host cytoplasm became aggregated near the infection site prior to penetration and accumulated around the infecting hyphae as long as the host cell was viable. Substantial callose deposition, in the form of a bell-shaped collar around infection structures, resulted from the cytological activity at the infection site. Penetration of dead root hairs was common, but did not lead to the development of infection structures or to a sustained association with the host tissue; T. basicola exited dead root hairs and resumed vegetative growth. The establishment of the parasitic relationship by T. basicola was characteristic of hemibiotrophic fungi in that, initially, biotrophic infection led to tissue colonization, and host cell survival was limited under parasitism.
Suppression of Phytophthora Blight in Bell Pepper by a No-Till Wheat Cover Crop
Authors: Ristaino Jean B. , Parra Gregory , Campbell C. Lee
Published: May 2007

Four mechanisms of dispersal of propagules of Phytophthora capsici were investigated through modifications in cultural practices and fungicide applications in field plots of bell pepper (Capsicum annuum). Dispersal of soil inoculum was suppressed, and final incidence of disease was 2.5 to 43% when stubble from a fall-sown, no-till, wheat cover crop was present. Final disease incidence was 71 to 72% and pathogen spread occurred within and across rows when all dispersal mechanisms were operative in plots of pepper planted into bare soil. Final disease incidence was 42 to 78% with black plastic mulch when a sporulating pepper fruit placed on the surface served as the source of initial inoculum. The fungicide metalaxyl applied in the irrigation system did not suppress within-row spread of surface inoculum from a sporulating fruit on plastic, but did limit across-row spread; final disease incidence in metalaxyl-treated plots was 11.5 to 14%. Pathogen dispersal mechanisms were modified most dramatically by the no-till cropping system. Thus, simple changes in cultural practices can have dramatic effects on the development of Phytophthora epidemics. Ecologically based disease management strategies have the potential to reduce our reliance on agrichemicals in this and similar pathosystems.
Monomorphic Nonpathogenic Mutants of Ustilago maydis
Authors: Martínez-Espinoza Alfredo D. , León Claudia , Elizarraraz Guadalupe , Ruiz-Herrera José
Published: May 2007

We have developed conditions which promote the dimorphic transition of haploid cells of Ustilago maydis in vitro by controlling the pH of the media. At low pH (below 5.0) mycelial growth occurs, whereas at neutral pH yeastlike growth takes place. We screened for mutants unable to form mycelium at low pH and obtained 26 mutants. These mutants have been characterized by their cell and colony morphology in different media. Mutations in 18 strains were found to be recessive when these strains were crossed with the wild type. Other crosses indicated that they were affected in genes other than a and b. Crosses between mutants suggest that the mutations fall in at least two complementation groups. In addition, mutants were characterized by their pathogenicity to corn seedlings. Mutations which were recessive for pathogenicity were also recessive for morphogenesis in vitro.
Movement of Xanthomonas oryzae pv. oryzae in Southeast Asia Detected Using PCR-Based DNA Fingerprinting
Authors: George M. L. C. , Bustamam M. , Cruz W. T. , Leach J. E. , Nelson R. J.
Published: May 2007

Two outwardly directed primers complementary to sequences in IS1112, a repetitive element isolated from Xanthomonas oryzae pv. oryzae, were used to fingerprint DNA from a set of 71 bacterial blight pathogen strains using polymerase chain reaction (PCR), PCR-based restriction analysis, and ligation-mediated PCR. To allow amplification of long DNA fragments, standard amplification conditions were altered to increase the pH, add dimethylsulfoxide, decrease denaturation time, and increase extension time. Bands ranging in size from 100 bp to 7 kb and in number from 13 to 48 bands per strain were amplified. The three methods revealed useful polymorphisms among individual strains and allowed their genetic relationships to be efficiently deduced. Good correlation was found between the major clusters obtained by the three methods. The PCR method gave the most robust clusters and was most efficient in terms of speed, simplicity, and economy. Using PCR and restriction fragment length polymorphism to compare strains of the bacterial blight pathogen from Indonesia and the Philippines, we found that, whereas there is regional differentiation of the pathogen populations, the predominant strains in the pathogen collections from both countries are closely related. This indicates the occurrence of regional movement, perhaps as a consequence of germ plasm exchange.
Intraspecific Genomic Variation Within Xanthomonas albilineans, the Sugarcane Leaf Scald Pathogen
Authors: Davis M. J. , Rott P. , Warmuth C. J. , Chatenet M. , Baudin P.
Published: May 2007

To better understand the nature of recent outbreaks of leaf scald disease of sugarcane in a number of sugarcane production regions of the world including Florida, Guadeloupe, Louisiana, Mauritius, Taiwan, and Texas, a study of the worldwide genetic variation of the pathogen was undertaken. A total of 218 strains from 31 geographic locations were examined. Genomic DNA of each strain was digested with the rare cutting restriction enzyme SpeI, and the fragments were separated by pulsed-field gel electrophoresis (PFGE). A total of 102 bands were identified, and 54 different DNA banding patterns (haplotypes) were observed. Eight groups of banding patterns, designated PFGE groups A through H, were consistently detected by visual, principal component, and cluster analyses. Five groups were comprised of multiple haplotypes representing numerous strains, and three were comprised of single haplotypes representing one strain each. The leaf scald outbreaks in Florida, Louisiana, Texas, and possibly Guadeloupe and Taiwan could be attributed to the introduction of strains belonging to PFGE group B. When infection by two strains each of the newly introduced strains (PFGE group B) and those previously present in Florida (PFGE group A) was analyzed in 22 sugarcane cultivars by reisolation 24 weeks after inoculation, a significantly greater mean frequency was detected for PFGE group B strains and no cultivar by PFGE group interaction was observed. Inadvertent dispersal of the pathogen among plants, possibly by means of aerosols or splashing water, was detected in a subsequent experiment. Strains of PFGE group B were recovered from the internal tissues of some plants inoculated with PFGE group A strains and were also found to be epiphytic colonizers of nonsymptomatic, noninoculated plants adjacent to the inoculated plants; whereas strains of PFGE group A were recovered only from plants that had been inoculated with them. Thus, the possibility became more apparent that strain variation might be associated, at least in part, with factors governing plant-to-plant spread of the pathogen in nature.
Identification and Pathogenicity of Rhizoctonia spp. Isolated from Apple Roots and Orchard Soils
Authors: Mazzola Mark
Published: May 2007

Rhizoctonia spp. were isolated from the roots of apple trees and associated soil collected in orchards located near Moxee, Quincy, East Wenatchee, and Wenatchee, WA. The anastomosis groups (AGs) of Rhizoctonia spp. isolated from apple were determined by hyphal anastomosis with tester strains on 2% water agar and, where warranted, sequence analysis of the rDNA internal transcribed spacer region and restriction analysis of an amplified fragment from the 28S ribosomal RNA gene were used to corroborate these identifications. The dominant AG of R. solani isolated from the Moxee and East Wenatchee orchards were AG 5 and AG 6, respectively. Binucleate Rhizoctonia spp. were recovered from apple roots at three of four orchards surveyed and included isolates of AG-A, -G, -I, -J, and -Q. In artificial inoculations, isolates of R. solani AG 5 and AG 6 caused extensive root rot and death of 2- to 20-week-old apple transplants, providing evidence that isolates of R. solani AG 6 can be highly virulent and do not merely exist as saprophytes. The effect of binucleate Rhizoctonia spp. on growth of apple seedlings was isolate-dependent and ranged from growth enhancement to severe root rot. R. solani AG 5 and AG 6 were isolated from stunted trees, but not healthy trees, in an orchard near Moxee, WA, that exhibited severe symptoms of apple replant disease, suggesting that R. solani may have a role in this disease complex.
Salicylic Acid Produced by the Rhizobacterium Pseudomonas aeruginosa 7NSK2 Induces Resistance to Leaf Infection by Botrytis cinerea on Bean
Authors: De Meyer Geert , Höfte Monica
Published: May 2007

Selected strains of nonpathogenic rhizobacteria can induce a systemic resistance in plants that is effective against various pathogens. In an assay with bean plants, we investigated which determinants of the rhizobacterium Pseudomonas aeruginosa 7NSK2 are important for induction of resistance to Botrytis cinerea. By varying the iron nutritional state of the bacterium at inoculation, it was demonstrated that induced resistance by P. aeruginosa 7NSK2 was iron-regulated. As P. aeruginosa 7NSK2 produces three siderophores under iron limitation, pyoverdin, pyochelin, and salicylic acid, we investigated the involvement of these iron-regulated metabolites in induced resistance by using mutants deficient in one or more siderophores. Results demonstrated that salicylic acid production was essential for induction of resistance to B. cinerea by P. aeruginosa 7NSK2 in bean and did not exclude a role for pyochelin. A role for pyoverdin, however, could not be demonstrated. Transcriptional activity of salicylic acid and pyochelin biosynthetic genes was detected during P. aeruginosa 7NSK2 colonization of bean. Moreover, the iron nutritional state at inoculation influenced the transcriptional activity of salicylic acid and pyochelin biosynthetic genes in the same way as it influenced induction of systemic resistance to B. cinerea.
Inheritance of Resistance to Leptosphaeria maculans in Brassica juncea
Authors: Keri M. , van den Berg C. G. J. , McVetty P. B. E. , Rimmer S. R.
Published: May 2007

The inheritance of resistance to Leptosphaeria maculans, the causal agent of black leg of crucifers, was studied in Brassica juncea. Three resistant accessions (UM3021, UM3043, and UM3323) and one susceptible accession (UM3132) of B. juncea were crossed in a complete diallel. Parents, F1, and F2 progenies were evaluated for all crosses using both cotyledon and stem inoculation. Cotyledon reaction was evaluated with two isolates of L. maculans, but stem reaction was evaluated with one isolate. Disease reactions observed for individual plants were the same for both inoculation methods and for both isolates of the pathogen for cotyledon reaction. No segregation was observed for the crosses between resistant accessions (UM3043 × UM3323 and UM3021 × UM3323), but a few susceptible plants were observed in the F2 progeny of crosses between resistant parents (UM3021 × UM3043). This was probably due to heterozygosity in some parental plants of UM3021. For crosses be tween the susceptible parent and resistant parents, F1 plants for two crosses were all resistant. For cross UM3132 × UM3021, some susceptible plants occurred, which was also suggestive of heterozygosity in UM3021. Although resistance in F1 was dominant, for F2 populations, segregation fit either 13:3, 3:1, or 1:3 ratios, indicating that resistance can be either adominant or recessive trait. F3 families derived from some susceptible F2 plants from crosses UM3021 × UM3132 and UM3043 × UM3132 were evaluated using the cotyledon inoculation method only. Segregation of F2 plants and F3 families in crosses involving resistant and susceptible parents indicated that the resistance to L. maculans in B. juncea is controlled by two nuclear genes with dominant recessive epistatic gene action.
Genetics of Host Specificity in Epichloë typhina
Authors: Chung Kuang-Ren , Hollin Walter , Siegel Malcolm R. , Schardl Christopher L.
Published: May 2007

Epichloë typhina perennially and systemically infects grass plants, causing choke disease in which maturation of host inflorescences is suppressed. In seedling-inoculation tests, isolate E8 from perennial ryegrass established and maintained infection in this host but not in orchardgrass. In contrast, isolates E469, E2466, and E2467 from orchardgrass varied in infection frequency and stability in orchardgrass, but all were unable to establish stable infections in perennial ryegrass. To investigate the genetics of host specificity, isolate E8 was crossed with each of the isolates from orchardgrass. Seedlings of parental host species were inoculated with F1 progeny, and the frequencies of seedling infection and stability in adult plants were assessed. In the E8 × E2466 cross, the F1 progeny exhibited a wide range of infection frequency and stability in each parental host. In crosses E8 × E469 and E8 × E2467, where the orchardgrass-derived parents infected 5 to 13% of inoculated perennial ryegrass seedlings, the distributions of infection frequencies for the F1 progeny wereskewed toward levels comparable to that of the parent from perennial ryegrass. In all crosses, most progeny had low frequencies of infection in orchardgrass. However, transgression was evident in a cross of E8 with E469, an isolate that infected orchardgrass seedlings at a low frequency (2 to 3%). The E8 × E469 cross had a few F1 progeny that infected orchardgrass at high efficiency (up to 81%). A Spearman rank correlation applied to the E8 × E2466 progeny indicated a significant negative correlation between infection frequencies in perennial ryegrass and orchardgrass. Also, there was a significant correlation of infection frequency and stability in perennial ryegrass but not in orchardgrass. To test whether only a few genes governed infection frequency in perennial ryegrass, an E8 × E2466 F1 progeny (designated E386.04), which had intermediate compatibility with this host, was backcrossed to E8. The progeny of this backcross exhibited a distribution of infection frequencies in perennial ryegrass between that of E386.04 and the backcross parent, suggesting that multiple genes may determine compatibility at the seedling infection stage. The results of these experiments indicated multiple genetic determinants of compatibility or incompatibility with each host, with intermediate or high heritability.
Differential Inhibition of Sphaeropsis sapinea Morphotypes by a Phenolic Compound and Several Monoterpenes of Red Pine
Authors: Blodgett J. T. , Stanosz G. R.
Published: May 2007

The in vitro effects of a red pine phenolic compound (pinosylvin), a phenolic compound common to other species (tannic acid), and the major red pine monoterpenes (α-pinene, β-pinene, and δ-3-carene) on spore germination and mycelial growth of Sphaeropsis sapinea were examined. Two A and two B morphotype isolates were used. At 88 μg/mm2, pinosylvin inhibited spore germination of all four isolates (98 to 100%). At 8.8μg/mm2, spore germination of B isolates was inhibited more than that of A isolates (73 versus 30%). Pinosylvin also inhibited mycelial growth of B isolates more than that of A isolates (84 versus 13% at 88 μg/mm2). Tannic acid stimulated or had little affect on spore germination and had little affect on mycelial growth of either morphotype. Spore germination of B isolates was inhibited more than that of A isolates by β-pinene at saturation (79 versus 37%). Spore germination of B isolates was inhibited and germination of A isolates was stimulated by δ-3-carene below saturation (49 versus -7%). Mycelial growth of B isolates was inhibited more than that of A isolates by all monoterpenes at saturation. Differences observed between morphotypes below saturation were significant only for β-pinene. These results demonstrate the biological activity of a phenolic compound and monoterpenes that occur in red pine. The differential responses might provide means of distinguishing morphotypes and offer a potential explanation for ecological specialization.
pH Regulation of Sterigmatocystin and Aflatoxin Biosynthesis inAspergillusspp.
Authors: Keller Nancy P. , Nesbitt Clint , Sarr Bashir , Phillips Timothy D. , Burow Gloria B.
Published: May 2007

Aflatoxin (AF) and sterigmatocystin (ST) are toxic secondary metabolites produced by the same biochemical pathway found in several Aspergillus spp. The expression of the homologous ST/AF structural gene, stcU in A. nidulans and ver-1 in A. parasiticus, was affected by external pH of liquid growth media. Both stcU and ver-1 mRNAs appeared earlier and were expressed at higher levels in cultures grown in acidic media (pH 4 to 6) versus neutral (pH 7) and alkali (pH 8) media. Transcript levels correlated with ST/AF production. Visual and spectrophotometric analysis of production of the orange ST/AF intermediate, norsolorinic acid (NOR), also paralleled transcript patterns and indicated that the pH effects were operative in different nitrogen- and carbon-based solid growth media. Five- to 10-fold increases in ST, AF, and NOR were measured in cultures grown in pH 4 or 5 versus pH 8 media. An A. nidulans strain carrying a mutation resulting in constitutive activity of the pH regulatory factor, PacC, produced 10-fold less ST than did wild type. The stcU transcript was not noticeably affected by pH in this strain. The results suggest a general pattern of pH regulation of ST/AF biosynthesis that may override previously noted carbon and nitrogen effects.