bluuebunny/crossref_metadata_2025 huggingface dataset:
One Amino Acid Change in Cucumber Mosaic Virus RNA Polymerase Determines Virulent/Avirulent Phenotypes on Cowpea
Authors: Karasawa Akira , Okada Itaru , Akashi Kayoko , Chida Yutaka , Hase Shuu , Nakazawa-Nasu Yoshiko , Ito Akiko , Ehara Yoshio
Published: May 2007

The elicitation of the hypersensitive response (HR) is known to depend on the interaction between a resistance gene of a host plant and a corresponding avirulence gene of a pathogen. The cv. Kurodane-Sanjaku of cowpea (Vigna unguiculata) has the Cry locus that confers resistance against cucumber mosaic virus strain Y (CMV-Y). The resistance is overcome by infection with a legume strain of CMV (CMV-L). RNA 2, which codes for the 2a protein, a subunit of the viral replicase components, has been known to control virulent/avirulent phenotypes. We generated chimeric constructs of full-length cDNA clones of RNA 2 of both strains and inoculated infectious transcripts to delimit the domain controlling symptoms. A 243-base pair fragment containing a coding region for the GDD RNA-dependent RNA polymerase core sequence was shown to be responsible for the phenotypic differences. From sequence alignment analysis, two amino acids (Phe631 and Ala641) of the HR-type 2a protein encoded in this fragment were specifically exchanged to Tyr and Ser, respectively, in the 2a proteins of resistance-breaking strains. Point mutations introduced into RNA 2 backgrounds of both strains that were designed to change the amino acid at position 631 resulted in a change of symptoms, indicating that a single nucleotide change determines the reactions elicited by both strains. Analysis for one additional mutant RNA 2 showed that symptom determination may be correlated with the nature of the lateral chain of amino acid 631.
Usefulness of Gene Pg10 as a Source of Stem Rust Resistance in Oat Breeding
Authors: Harder Donald E.
Published: May 2007

Infection types produced by Puccinia graminis f. sp. avenae on plants of Avena sativa with the stem rust resistance gene Pg10 are characterized by moderate-sized uredinia surrounded by an area of chlorosis and a larger variable zone of dark brown necrosis. This study was undertaken to assess the effectiveness of gene Pg10 as a source of resistance to stem rust and to determine the interactions of this gene with other common Pg genes. A derived Pg10 line was tested with 58 distinct pathotypes of P. graminis f. sp. avenae and was crossed to substituted single-gene lines carrying the resistance gene Pg1, Pg2, Pg3, Pg4, Pg8, Pg9, Pg13, Pg15, Pg16, or Pga. The Pg10 line showed moderate resistance to all 58 patho-types, and there was no indication of specificity in virulence by any isolate. Gene Pg10 was inherited independently of the other Pg genes and had a complementary effect on the expression of resistance by these genes. An effective level of resistance conferred by Pg10 was demonstrated in a field nursery artificially inoculated with P. graminis f. sp. avenae. It was concluded that Pg10 is a potentially useful source of stem rust resistance in oat breeding, with its main attributes being an apparent broad base of resistance, ease of combining with other Pg genes, and complementary effects on the expression of other Pg genes
Single-Strand Conformation Polymorphism Analysis of Apple Stem Grooving Capillovirus Sequence Variants
Authors: Magome H. , Yoshikawa N. , Takahashi T.
Published: May 2007

In an earlier study, we demonstrated that isolates of apple stem grooving capillovirus (ASGV) from fruit trees comprise at least two to four sequence variants that differ considerably from each other in nucleotide sequence. In order to characterize the population of sequence variants within a single tree, we applied a combination of an immunocapture reverse transcription-polymerase chain reaction (IC-RT-PCR) and a single-strand conformation polymorphism (SSCP) analysis of a nested asymmetric PCR product. In the SSCP analysis of the PCR products from ASGV-infected apple, Japanese pear, or European pear trees, two to four bands were detected in samples from all trees, indicating that ASGV exists as a mixture of sequence variants. The composition of sequence variants (the number of bands and their relative quantity) differed among leaf samples from different branches, showing that each sequence variant is distributed unevenly within an individual tree. The SSCP analysis of isolates after serial passage in Chenopodium quinoa plants indicated that passages changed the composition of sequence variants originally contained in ASGV isolates; i.e., some sequence variants dominated and others decreased to undetectable levels.
Evidence of Cytoplasmic Inheritance of Virulence in Cronartium ribicola to Major Gene Resistance in Sugar Pine
Authors: Kinloch Bohun B. , Dupper Gayle E.
Published: May 2007

Tests for Mendelian segregation of virulence and avirulence in Cronartium ribicola, causal agent of white pine blister rust, to a major gene (R) for resistance in sugar pine were made using haploid basidiospore progenies from single diploid telia as inoculum on resistant genotypes. The telia were sampled from a small deme in the Siskyou Mountains of northern California, where a few mature sugar pines known to be Rr genotypes had become infected after withstanding the chronic blister rust epidemic for several decades and where intermediate frequencies of virulence in the ambient basidiospore population were subsequently measured. Infection type on inoculated seedlings with R was qualitative: all progenies of 81 single telia tested over 3 different years were either virulent (compatible) or avirulent (inducing hypersensitive necrosis), never a mixture of both reactions. The complete absence of heterozygotes in the telia population is strong evidence that virulence is not controlled by a nuclear gene. The data are consistent with earlier tests showing that basidiospore inoculum derived from aeciospores isolated from infected Rr trees produced mostly (>90%) virulent reactions on R— seedlings. The evidence indicates that transmission of virulence is uniparental via the cytoplasm of aeciospores. Exchange of spermatia between haploid thalli does not appear to be involved.
Recessive Genes for Resistance to Puccinia striiformis f. sp. hordei in Barley
Authors: Chen Xianming , Line Roland F.
Published: May 2007

Barley genotypes Abyssinian 14, BBA 2890, Grannelose Zweizeilige, PI 548708, PI 548734, PI 548747, and Stauffers Obersulzer are resistant to all races of Puccinia striiformis f. sp. hordei identified thus far in North America. Astrix, BBA 809, Bigo, Cambrinus, Emir, Heils Franken, Hiproly, I5, Mazurka, Trumpf, and Varunda have specific resistance to certain races, whereas Topper and Steptoe are susceptible to all races. Seedlings of parents and F1, F2, and F3 progeny from crosses of resistant genotypes with Topper and Steptoe were tested for resistance to North American P. striiformis f. sp. hordei races PSH-1, PSH-4, PSH-10, PSH-13, and PSH-20. When tested with PSH-1, one recessive gene was detected in BBA 809, BBA 2890, Bigo, Hiproly, and Grannelose Zweizeilige; two recessive genes were detected in Emir, I5, PI 548708, PI 548734, PI 548747, Varunda, and Astrix; and one dominant gene and one recessive gene were detected in Abyssinian 14 and Stauffers Obersulzer. In tests with PSH-4, one recessive gene was detected in BBA 809, two recessive genes were detected in Trumpf, and two partially recessive genes were detected in Astrix. In tests with PSH-13, one recessive gene was detected in BBA 2890, Grannelose Zweizeilige, I5, and PI 548708 and two recessive genes were detected in Abyssinian 14, Hiproly, PI 548734, and PI 548747. In tests with PSH-20, one recessive gene was detected in Bigo and a recessive gene and a dominant gene were detected in Heils Franken. A gene in Cambrinus for resistance to PSH-10 and a gene in Mazurka for resistance to PSH-20 were dominant on the basis of the response of the first seedling leaf but recessive on the basis of the response of the second leaf. Four different types of epistasis were detected. Information on the number of genes, mode of inheritance, and nonallelic gene interactions should be useful in understanding the host-pathogen interaction and in breeding barley for resistance to stripe rust.
Increased Antifungal Activity of Trichoderma harzianum Transformants That Overexpress a 33-kDa Chitinase
Authors: Limón M. Carmen , Pintor-Toro José A. , Benítez Tahía
Published: May 2007

Transformants of the biocontrol agent Trichoderma harzianum strain CECT 2413 that overexpressed a 33-kDa chitinase (Chit33) were obtained and characterized. Strain CECT 2413 was cotransformed with the amdS gene and its own chit33 gene under the control of the pki constitutive promoter from T. reesei. Southern blotting indicated that the chit33 gene was integrated ectopically, mostly in tandem. Some transformants showed the same restriction pattern, indicating preferable sites of integration. There was no correlation between the number of integrated copies and the level of expression of the chit33 gene in the transformants. When grown in glucose, the extracellular chitinase activity of the transformants was up to 200-fold greater than that of the wild type, whereas in chitin, the activity of both the transformants and the wild type was similar. Under both conditions, the transformants were more effective in inhibiting the growth of Rhizoctonia solani as compared with the wild type. Similar results were obtained when culture supernatants from the transformants and the wild type were tested against R. solani.
Characterization of Populations of Rhizoctonia solani in Paddy Rice Fields in Côte d'Ivoire
Authors: Banniza S. , Sy A. A. , Bridge P. D. , Simons S. A. , Holderness M.
Published: May 2007

Isolates of Rhizoctonia solani were obtained from plant and soil samples that had been systematically collected in a field experiment in Côte d'Ivoire to study the diversity of the pathogen and the influence of three different rice rotations on the pathogen population. Characterization by morphology, anastomosis testing, pathogenicity testing, and restriction fragment length polymorphisms (RFLPs) of AT-rich DNA (AT-DNA) showed that there were no differences in isolates from different experimental plots, suggesting that the soil as well as the plant population of the fungus was indistinguishable throughout the experiment and was not influenced by crop rotation. Analysis of AT-DNA showed that the isolates obtained from plant material and one from soil shared a distinct banding pattern, identical with the AT-DNA RFLP obtained for the reference strain of anastomosis group 1 (AG-1). The remaining soil isolates produced a consistent RFLP pattern that was distinct from that of the plant isolates. Morphological characterization of isolates produced two major clusters consisting of the same groups of isolates as found by AT-DNA RFLP. Diversity in morphological characters was much higher in plant than in soil isolates and indicated that the population might consist of several clones. Anastomosis testing revealed that soil as well as plant isolates were able to fuse with the tester strain of AG-1. Significant differences in disease severity were observed between the two groups of isolates in pathogenicity tests on rice plants, with plant isolates being distinctively more virulent.
Advances in the Characterization of the Pyrenophora tritici-repentis—Wheat Interaction
Authors: Ciuffetti Lynda M. , Tuori Robert P.
Published: May 2007

Tan spot of wheat, caused by the fungus Pyrenophora tritici-repentis, is a destructive disease found in wheat-growing regions worldwide that can lead to serious yield losses. Changes in cultural practices have led to an increase in the severity and incidence of tan spot. Following infection, compatible races of the fungus elicit two distinct symptoms in differential wheat lines: tan necrosis and (extensive) chlorosis. Tan necrosis has been clearly demonstrated by several groups to result from the action of a protein toxin, Ptr ToxA. Wheat sensitivity to this toxin is conditioned by a single dominant gene. The chlorosis response may be more complex and appears to involve at least two other toxins, Ptr ToxB and Ptr ToxC, produced by different races of the fungus. Distinct genes apparently condition the reaction of wheat lines to each of these chlorosis-inducing toxins. This review concentrates on significant advances that have occurred during the past decade in the characterization of this disease interaction, ranging from the epidemiology and management of tan spot to molecular host-parasite interactions. Particular emphasis is placed on work describing fungal race differentiation, production of toxins and their importance in pathogenicity, and the genetics and physiology of host response to infection.
Ultrastructural and Cytochemical Aspects of the Interaction Between the Mycoparasite Pythium oligandrum and Soilborne Plant Pathogens
Authors: Benhamou Nicole , Rey Patrice , Picard Karine , Tirilly Yves
Published: May 2007

The interaction between the oomycete Pythium oligandrum and various soilborne oomycete and fungal plant pathogens (P. ultimum, P. aphanidermatum, Fusarium oxysporum f. sp. radicis-lycopersici, Verticillium albo-atrum, Rhizoctonia solani, and Phytophthora megasperma) was studied by light and electron microscopy in order to assess the relative contribution of mycoparasitism and antibiosis in the antagonistic process. Scanning electron microscope investigations of the interaction regions showed that structural alterations of all pathogenic fungi and oomycetes (except for Phytophthora megasperma) occurred soon after contact with the antagonist. Light and transmission electron microscope studies of the interaction region between the antagonist and P. ultimum revealed that intimate contact between both partners preceded a sequence of degradation events including aggregation of host cytoplasm and penetration of altered host hyphae. Localization of the host wall cellulose component showed that cellulose was altered at potential penetration sites. A similar scheme of events was observed during the interaction between P. oligandrum and F. oxysporum f. sp. radicis-lycopersici, with the exception that complete loss of host protoplasm was associated with antagonist invasion. The interaction between P. oligandrum and R. solani resulted in an abnormal deposition of a wall-like material at potential penetration sites for the antagonist. However, the antagonist displayed the ability to circumvent this barrier and penetrate host hyphae by locally altering the chitin component of the host hyphal wall. Interestingly, antagonist cells also showed extensive alteration as evidenced by the frequent occurrence of empty hyphal shells. In the case of Phytophthora megasperma, hyphal interactions did not occur, but hyphae of the plant pathogen were damaged severely. At least two distinct mechanisms appear to be involved in the process of oomycete and fungal attack by P. oligandrum: (i) mycoparasitism, mediated by intimate hyphal interactions, and (ii) antibiosis, with alteration of the host hyphae prior to contact with the antagonist. However, the possibility that the antagonistic process may rely on the dual action of antibiotics and hydrolytic enzymes is discussed.
Detection of Cymbidium Mosaic Potexvirus and Odontoglossum Ringspot Tobamovirus Using Immuno-Capillary Zone Electrophoresis
Authors: Eun Alvin Jin-Cherng , Wong Sek-Man
Published: May 2007

Immuno-capillary zone electrophoresis (I-CZE) is a technique that combines the specificity afforded by serological assays with the sensitivity, rapidity, and automation in detection provided by capillary zone electrophoresis. Cymbidium mosaic potexvirus (CymMV) and odontoglossum ringspot tobamovirus (ORSV) were detected in their purified forms as well as in the crude saps of infected Nicotiana benthamiana leaves and Oncidium orchid flowers. The two orchid virus-antibody complexes were resolved via the combined actions of electrophoretic migration and electro-osmotic flow along a buffer-filled, uncoated fused-silica capillary. The I-CZE fractions collected from both CymMV- and ORSV-antibody complex peaks, as well as the RNA purified from them, retained their infectivity upon inoculation onto Chenopodium quinoa. I-CZE assays were able to detect as little as 10 fg each of both CymMV and ORSV in their purified forms as well as in the crude saps of infected N. benthamiana and Oncidium orchid. As multiple samples can be analyzed rapidly, I-CZE offers an ideal diagnostic technique for routine mass-indexing programs such as virus-free certification, breeding for virus-resistant cultivars, plant quarantine, and germ plasm screening. This is the first report of the application of I-CZE for the detection of plant viruses.
The Race-Specific Resistance Gene to Powdery Mildew, MlRE, Has a Residual Effect on Adult Plant Resistance of Winter Wheat Line RE714
Authors: Chantret N. , Pavoine M. T. , Doussinault G.
Published: May 2007

Race-specific resistance genes to powdery mildew have been extensively used in wheat breeding programs, but the complete resistance they provide breaks down when confronted by pathogen isolates with matching virulence. However, when overcome, some race-specific genes have a residual action leading to a reduction of the symptoms. Our objective was to determine if the resistance genes MlRE and Pm4b have a residual effect on adult plant resistance (APR) and on vernalized seedling plant resistance (VPR) in the line RE714. Individuals from two populations (double haploid [DH] and F3 families) were genotyped for the race-specific genes MlRE and Pm4b and assessed for their resistance under field conditions at the adult plant stage (in 1996 and 1997 for the DH lines and in 1997 for the F3 families). Vernalized seedlings of the DH population were tested with four powdery mildew isolates. Only the MlRE gene had a significant effect (dominant type) on APR. Neither MlRE nor Pm4b had a significant effect on VPR. The dominant residual effect of the defeated race-specific gene MlRE was a component of APR in the line RE714.
Correlation of Hop Stunt Viroid Variants to Cachexia and Xyloporosis Diseases of Citrus
Authors: Reanwarakorn K. , Semancik J. S.
Published: May 2007

Citrus viroid (CVd) group II is comprised of hop stunt viroid (HSVd)-related variants of 295 to 302 nucleotides. Included in this group are the cachexia-inducing agents citrus cachexia viroid (or CVd-IIb), CVd-IIc, Ca-903, and Ca-909 as well as the non-cachexia-inducing variant CVd-IIa. The cachexia indexing hosts ‘Parson's Special’ mandarin and ‘Orlando’ tangelo as well as Citrus macrophylla responded with symptoms of gumming, discoloration, and stem pitting when infected by CVd-IIb, CVd-IIc, or Ca-903. However, ‘Palestine’ sweet lime, the indicator host used to describe the xyloporosis disease, displayed a distinctly different fine-pitting reaction and no discoloration or gumming when infected by the same viroids. Cachexia-inducing variants contain a number of nucleotide changes more similar to hop-type HSVd sequences than to the citrus-type HSVd sequences, as typified by CVd-IIa. The nucleotide sequence of CVd-IIc was identical to CVd group II isolates common to trees expressing xyloporosis. Experimental evidence indicates that either CVd-IIb or CVd-IIc can cause citrus diseases known as cachexia and xyloporosis and that the two disease designations reflect the distinct responses of different indexing hosts to the same viroids.
A Model for Infestation Foci of Potato Cyst Nematodes Globodera rostochiensis and G. pallida
Authors: Schomaker C. H. , Been T. H.
Published: May 2007

Infestation foci of potato cyst nematodes (Globodera rostochiensis and G. pallida) in the provinces of Zeeland, Friesland, Groningen, and Drente of the Netherlands were sampled to validate a model describing infestation foci based on data from Flevoland. Eighty-two fields were presampled to locate infestation foci using a coarse sampling grid (8 × 3 m). Parts of 37 fields containing one or more foci were sampled intensively by extracting at least 1.5 kg of soil per square meter (1.33 × 0.75 m). Forty foci were analyzed for spatial distribution characteristics of cysts using multiple regression with generalized linear models and classical linear models. The results showed that the data from all investigated cropping areas fit well to an exponential model with two parameters, the length and width gradient parameters. Significant differences in these parameter values between cropping areas could not be demonstrated. As both parameters follow a normal distribution, the probability of any combination can be described by a bivariate normal distribution. Gradient parameters were correlated, but significant correlations between these parameters and certain variables such as the nematode species involved, the time interval between sampling and the last potato crop, soil type, cropping frequency, and cyst density in the focus center could not be demonstrated.
Development of a New Semiselective Medium for Isolating Xanthomonas campestris pv. manihotis from Plant Material and Soil
Authors: Fessehaie Anania , Wydra Kerstin , Rudolph Klaus
Published: May 2007

An effective control for bacterial blight of cassava (Manihot esculenta), caused by Xanthomonas campestris pv. manihotis, requires the use of non-contaminated cuttings and seeds. Using classical agar plating techniques for screening planting material for contamination has not been very successful because of the lack of a reliable semiselective agar medium. The pathogen grows slowly on general plating media and is easily overgrown by saprophytic bacteria during isolation from diseased plants. In an effort to develop a semiselective medium, the utilization of several carbon and nitrogen sources was studied. Results of these tests provided information used to design a basal medium allowing good growth of the target organism while suppressing growth of several common saprophytes. Additional selectivity was achieved by incorporating three antibiotics into the basal medium. The new semiselective agar medium, designated cefazolin trehalose agar (CTA) medium, contained (per liter) 3.0 g of K2HPO4, 1.0 g of NaH2PO4, 0.3 g of MgSO4·7H2O, 1.0 g of NH4Cl, 9.0 g of D(+)-trehalose, 1.0 D(+)-glucose, 1.0 g of yeast extract, 0.025 g of cefazolin, 0.0012 g of lincomycin, 0.0025 g of phosphomycin, 0.25 g of cycloheximide, and 14.0 g of agar. In comparison to a starch-based semiselective medium (SXM), plating efficiencies using pure cultures of 10 strains of X. campestris pv. manihotis were significantly higher on CTA, with an average of 85 and 50%, respectively. Likewise, isolation and recovery of X. campestris pv. manihotis from infected cassava leaves and contaminated soil were much higher on CTA than on SXM agar. When X. campestris pv. manihotis occurs in high concentrations in diseased tissue, the standard yeast trehalose glucose agar medium supplemented with 250 μg of cycloheximide per ml appears to be satisfactory. The newly developed CTA medium should prove useful for control strategies to identify and remove infected planting material of cassava, as well as for basic ecological studies of the pathogen.
Heterogeneity of Xanthomonas campestris pv. hederae Strains from Araliaceous Hosts
Authors: Norman D. J. , Chase A. R. , Stall R. E. , Jones J. B.
Published: May 2007

Xanthomonas campestris pv. hederae (synonym X. hortorum pv. hederae) strains (59 total) were collected from plants in the araliaceae family. Strains were isolated from Hedera helix, Schefflera arboricola, Brassaia actinophylla, and Polyscias spp. from Florida, California, Hawaii, and New Zealand. All strains produced yellow mucoid growth; hydrolyzed esculin, starch, casein and gelatin; were pectolytic; produced urease; and grew on minimal media containing asparagine. All bacterial strains were pathogenic on H. helix (English ivy), B. actinophylla (dwarf schefflera), and Polyscias fruticosa (ming aralia). No differences in symptomatology were detected among strains; however, severity of symptoms usually was greatest on the host of origin. In planta growth rates of representative strains isolated from H. helix, B. actinophylla, and Polyscias spp. also were compared among these three hosts. In all cases, populations grew more rapidly when strains were inoculated to their original host species. All 59 bacterial strains were compared by 95-carbon source GN microplate, fatty acid methyl ester (FAME), and restriction fragment-length polymorphisms (RFLP), with the pulse-field gel electrophoresis method, analyses. All three analyses grouped strains into two distinct groups that correlated with the host of origin. Using metabolic profiles, 75% of the H. helix strains were separated from strains isolated from Brassaia and Schefflera and 95% of the Polyscias strains. FAME analysis separated strains into two distinct groups, with 96% of the H. helix strains placed in one group. RFLP analysis placed all of the H. helix and Schefflera strains in one group, as well as 33% of the Brassaia strains, whereas the other group contained all of the Polyscias strains and the remainder of the Brassaia strains. It is apparent that the pathovar hederae is made up of heterogeneous populations that can be separated by biochemical, pathological, genetic, and physiological analyses into two groups that are closely associated with the host of origin.
Aggressiveness and Competitive Fitness of Phytophthora infestans Isolates Collected from Potato and Tomato in France
Authors: Lebreton Lionel , Lucas Jean-Marie , Andrivon Didier
Published: May 2007

To test the hypothesis that host-related differences in the genotypic composition of populations of the late blight pathogen Phytophthora infestans can be explained by differential pathogenicity, the aggressiveness of isolates of the pathogen collected in France from potato and tomato was measured on detached leaflets of potato (cv. Bintje) and tomato (cv. Marmande). A preliminary trial with four isolates (two each from potato and tomato) showed that lesion appearance and development were similar for each isolate in detached leaflets and in whole plant tests in growth cabinets. Isolates collected from tomato were more pathogenic to tomato than isolates collected from potato. This was particularly the case for isolates belonging to the A2 mating type. Isolates originating from potato had a higher infection efficiency and a higher sporulation capacity on this host, but they induced lesions that generally spread more slowly than those caused by isolates from tomato. Extensive variation for components of aggressiveness on potato, and to a lesser extent on tomato, was observed in collections of isolates from each of the two hosts. Competition experiments between one potato isolate and one tomato isolate in field plots of the susceptible potato cv. Bintje clearly demonstrated the higher competitive fitness of the potato isolate on its host of origin. Therefore, differential pathogenicity to potato and tomato certainly contributes to the differentiation between P. infestans populations present on potato and tomato in France; however, additional factors, possibly related to survival ability or random genetic drift, are probably also involved and may explain the persistence of weakly pathogenic isolates in these populations.
Genotypic and Pathotypic Diversity in Xanthomonas oryzae pv. oryzae in Nepal
Authors: Adhikari Tika B. , Mew T. W. , Leach Jan E.
Published: May 2007

Among the 171 strains of Xanthomonas oryzae pv. oryzae (the bacterial blight pathogen of rice) collected from eight rice-producing zones in Nepal, 31 molecular haplotypes were distinguished using two polymerase chain reaction-based assays. Six common haplotypes represented nearly 63% of the strains, and some haplotypes were geographically dispersed. Multiple correspondence analysis divided the collection into five putative genetic lineages. Lineages 1, 2, and 4 were the most frequently detected and occurred in diverse geographic populations. Twenty-six pathotypes (virulence phenotypes) of X. oryzae pv. oryzae were identified using 11 near-iso-genic rice lines, each containing a single gene for resistance. The 26 pathotypes grouped into five clusters, and cluster 1 contained wide virulence spectrum strains from all geographic populations. Although molecular variation was greatest between strains of different virulence phenotypes, some variation was observed among strains with identical virulence. There was a weak correlation (r = 0.52) between molecular haplotypes and virulence phenotypes. There are two major groups of X. oryzae pv. oryzae in Nepal. One group consists of strains with high molecular polymorphism and many pathotypes that are either virulent to the 11 major resistance genes or avirulent only to Xa21. Strains in the second group have low molecular polymorphism and are avirulent to Xa4, xa5, Xa7, and Xa21.
Tobacco Plants Transformed with a Modified Coat Protein of Tomato Mottle Begomovirus Show Resistance to Virus Infection
Authors: Sinisterra X. H. , Polston J. E. , Abouzid A. M. , Hiebert E.
Published: May 2007

Tobacco plants (Nicotiana tabacum ‘Xanthi’) were transformed with a binary vector containing the coat protein gene of tomato mottle begomo-virus (ToMoV) modified by the deletion of 30 nucleotides in the 5′ end. The R1 generation was screened for resistance to ToMoV by inoculation with viruliferous whiteflies. Fifteen days after inoculation, symptom development was recorded weekly for up to 120 days using a visual scale, and ToMoV infection was confirmed by polymerase chain reaction and enzyme-linked immunosorbent assay. The response to high inoculation levels of ToMoV varied and ranged from susceptibility to immunity. The transgene transcript was detected by northern blot analysis; however, the transgene product could not be detected by protein blot analysis using antisera reactive with ToMoV coat protein. The lack of detection of the transgene product in resistant plants suggests that it is not involved in eliciting the resistance response and that resistance may be mediated by the transgene transcript.
Salicylic Acid Induces Resistance to Alternaria solani in Hydroponically Grown Tomato
Authors: Spletzer Matthew E. , Enyedi Alexander J.
Published: May 2007

Alternaria solani is the causal agent of early blight disease in tomato and is responsible for significant economic losses sustained by tomato producers each year. Because salicylic acid (SA) is an important signal molecule that plays a critical role in plant defense against pathogen invasion, we investigated if the exogenous application of SA would activate systemic acquired resistance (SAR) against A. solani in tomato leaves. The addition of 200 μM SA to the root system significantly increased the endogenous SA content of leaves. Free SA levels increased 65-fold over basal levels to 5.85 μg g-1 fresh weight (FW) after 48 h. This level of SA had no visible phytotoxic effects. Total SA content (free SA + SA-glucose conjugate) increased to 108 μg g-1 FW after 48 h. Concomitant with elevated SA levels, expression of the tomato pathogenesis-related (PR)-1B gene was strongly induced within 24 h of the addition of 200 μM SA. PR-1B expression was still evident after 48 h; however, PR-1B induction was not observed in plants not receiving SA treatment. Challenge inoculation of SA-treated tomato plants using conidia of A. solani resulted in 83% fewer lesions per leaf and a 77% reduction in blighted leaf area as compared with control plants not receiving SA. Our data indicate that root feeding 200 μM SA to tomato plants can (i) significantly elevate foliar SA levels, (ii) induce PR-1B gene expression, and (iii) activate SAR that is effective against A. solani.
Effects of Milsana and Benzothiadiazole on the Ultrastructure of Powdery Mildew Haustoria on Cucumber
Authors: Wurms Kirstin , Labbé Caroline , Benhamou Nicole , Bélanger Richard R.
Published: May 2007

Disease assessments and cytological investigations provided valuable information on the modes of action and efficacies of two prophylactic compounds, Milsana and benzothiadiazole (BTH), against powdery mildew development on long English cucumber. Milsana application significantly reduced disease incidence relative to inoculated controls through induction of localized resistance. Microscopic observations showed most haustoria had collapsed in the localized Milsana treatment and were encapsulated by an amorphous material impregnated by electron-opaque substances. The rapidity of haustorial collapse (within 4 days of treatment application) together with the encasement by electron-dense substances stained blue by toluidine blue O suggest that phenolics are possibly involved in the Milsana defense response. Cytochemical labeling of chitin with a wheat germ agglutinin/ovomucoid-gold complex showed that complete cellular disorganization of the fungus had occurred without disturbance to chitin in the walls of mycelia and haustoria. This may indicate that chitinolytic activity is not important in the Milsana-activated defense response. Application of high doses of BTH induced occasional cell wall thickening and accumulation of a compound that stained purple by toluidine blue O, but the defense response was weak, sporadic, and insufficient to reduce powdery mildew infection on cucumber. Responses to BTH could not be differentiated in terms of timing of the initial application or systemicity.
Virtual Lesion Extension: A Measure to Quantify the Effects of Bacterial Blight on Rice Leaf CO2 Exchange
Authors: Elings Anne , Rossing Walter A. H. , van der Werf Wopke
Published: May 2007

Virtual lesion extension was proposed as a measure to summarize the effects of foliar diseases with single spreading lesions on CO2-exchange parameters at the whole-leaf level. Visible lesion plus virtual lesion extension constitute a virtual lesion, in which CO2 exchange was postulated to be nil. Virtual lesion extension can be derived for each photosynthesis parameter from gas-exchange measurements. Using a leaf-shape function, one-dimensional lesion length was translated into two-dimensional lesion area, and a relationship between visible and virtual severity can be established. The model was applied to measurements of leaf CO2 exchange in rice leaves infected with Xanthomonas campestris pv. oryzae, the causal organism of rice bacterial blight. The model resulted in a virtual lesion extension of 1.1 cm for the gross CO2-exchange rate at light saturation, -3.9 cm for dark respiration rate, and 0 for initial light use efficiency. Reduced light interception due to a visible lesion caused reductions in net CO2 assimilation, and small virtual lesion extensions only marginally reduced net CO2 assimilation further. The additional reduction was smaller in case of longer leaves. Measurement of net photosynthesis rate along a transect from the base to the tip of infected leaves indicated that the location on the leaf blade where net photosynthesis decreased from normal to nil was centered around the lesion tip.
Molecular Detection of Diaporthe phaseolorum and Phomopsis longicolla from Soybean Seeds
Authors: Zhang A. W. , Hartman G. L. , Curio-Penny B. , Pedersen W. L. , Becker K. B.
Published: May 2007

Species-specific detection of Diaporthe phaseolorum and Phomopsis longicolla from soybean seeds was accomplished using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and TaqMan chemistry. To use these detection systems, fungal DNA was released from soybean seed coats using an ultrasonic processor to break the cells. DNA fragment lengths ranged from 200 to 1,200 base pairs (bp), with the majority of fragments <500 bp. Based on DNA sequences of the internal transcribed spacer (ITS) regions of ribosomal DNA, three TaqMan primer/probe sets were designed. Primer/probe set PL-5 amplified a 96-bp fragment within the ITS1 region of P. longicolla, D. phaseolorum var. caulivora, D. phaseolorum var. meridionalis, and D. phaseolorum var. sojae. Set PL-3 amplified a 86-bp DNA fragment within the ITS2 region of P. longicolla. Set DPC-3 amplified a 151-bp DNA fragment within the ITS2 region of D. phaseolorum var. caulivora. TaqMan primer/probe sets were able to detect as little as 0.15 fg (four copies) of plasmid DNA. When using PCR-RFLP for Diaporthe and Phomopsis detection, the sensitivity was as low as 100 pg of pure DNA. Among 13 soybean seed lots from Italy and the United States, the total Diaporthe and Phomopsis detected using a traditional seed-plating technique ranged from 0 to 32%. P. longicolla was most prevalent, followed by D. phaseolorum var. sojae. D. phaseolorum var. caulivora, which only occurred in 0.5% of the Italian seed lots, was not detected in the U.S. seed lots. D. phaseolorum var. meridionalis was not detected in either the U.S. or Italian seed lots. Using TaqMan primer/probe set PL-3, the frequency of P. longicolla was 18% in seed lot I3, similar to the frequency obtained from PCR-RFLP and potato dextrose agar plating detection. The frequencies of D. phaseolorum and P. longicolla in each seed lot obtained by the different detection methods were comparable with respect to total infection and individual species detection. However, TaqMan detection provided the fastest results of all the methods tested.
Two Properties of a Gene-for-Gene Coevolution System under Human Perturbations
Authors: Sun P. , Yang X. B.
Published: May 2007

Recently, the gene-for-gene host-parasite coevolution model of Leonard was extended by incorporating two kinds of perturbations. The first kind was the natural perturbations that include those caused by pathogen migration between the two subpopulations of the host, forward and backward mutations in the host or pathogen populations, and some others. The second kind was human perturbations, such as constantly increasing the percentage of the resistant genotype within the host population each season. In this study, we quantitatively compared the two kinds of perturbations and extended the constantly changing human perturbation to include non-constant perturbations that are more likely to occur in the real world. Two properties of the modified Leonard model were revealed from this study. First, when both human perturbations and natural perturbations are involved, the effects of natural perturbations are very small compared with those of human perturbations. This finding ensures that, in the study of human perturbations, we can simplify the study by ignoring the effects of natural perturbations. Second, through the simulation of nonconstant perturbations, which assumes that the proportion of the resistant genotype of the host population increases over time, we found that the model reproduces the “boom and bust” epidemic cycles that are often found in agroecosystems.
Bacterial-Mediated Induced Resistance in Cucumber: Beneficial Effect of the Endophytic Bacterium Serratia plymuthica on the Protection Against Infection by Pythium ultimum
Authors: Benhamou Nicole , Gagné Serge , Le Quéré Dominique , Dehbi Leila
Published: May 2007

The potential of the endophytic bacterium Serratia plymuthica strain R1GC4 in stimulating defense reactions in cucumber (Cucumis sativus) seedlings inoculated with the soilborne pathogen Pythium ultimum was explored at the cellular level. Bacterial treatment prior to Pythium inoculation resulted in less seedling disease development as compared with that in nontreated control plants, in which typical root symptoms were visible by 3 days after inoculation with the pathogen. Histological investigations of root samples revealed striking differences in the extent of plant defense reactions between bacterized and nonbacterized plants. These observations were further confirmed at the ultrastructural level with the demonstration that restriction of fungal colonization to the outermost root tissues of bacterized seedlings correlated with the deposition of enlarged callose-enriched wall appositions at sites of potential pathogen penetration and the accumulation of an osmiophilic material in the colonized areas. Hyphae of the pathogen, surrounded by this electron-opaque material, exhibited considerable changes including cytoplasm disorganization and, in many cases, loss of the protoplasm. However, labeling with the β-1,4-exoglucanase resulted in a regular labeling of Pythium cell walls, even at a time when these walls were entirely coated by the osmiophilic material. This material was also found to infiltrate into the invading hyphae to form either an internal coating of the cell wall or a network of polymorphic droplets in the area previously occupied by the cytoplasm. Cytochemical investigations revealed that callose, pectin, and cellulose appeared in the wall appositions. In addition, glucosides, lipids, and phenolics were detected in the electron-dense aggregates forming the core of most wall appositions. Finally, galactose residues were among the minor polysaccharidic compounds detected in the wall appositions. Evidence is provided in this study showing that treatment with S. plymuthica sensitizes susceptible cucumber plants to react more rapidly and more efficiently to Pythium attack through the formation of physical and chemical barriers at sites of potential fungal entry.
Spore Release of Bremia lactucae on Lettuce Is Affected by Timing of Light Initiation and Decrease in Relative Humidity
Authors: Su H. , van Bruggen A. H. C. , Subbarao K. V.
Published: May 2007

A suction-impaction mini—spore trap was developed to study the effect of light initiation and decreasing relative humidity (RH) on spore release of Bremia lactucae in a controlled environment. Three light periods (from 0400 to 1600, 0600 to 1800, and 0800 to 2000 h, circadian time) at a constant RH of 99 to 100% were used for studying the effect of light initiation on spore release. Few spores were released during the dark periods. Spore release increased sharply after the initiation of the three light periods, reached a maximum 1 to 2 h after light initiation, and then declined until only a few spores could be detected. The effect of reduction in RH on spore release was studied by comparing decreases in RH 2 h before and 2 h after light initiation at 0800 h. When RH decreased from 100 to 94% 2 h before light initiation, spore release increased within 1 h, followed by a second increase after light initiation. When RH decreased 2 h after light initiation, spore release continued to increase after the initial increase after light initiation, reached a maximum 1 h after the reduction in RH, and then declined. The results suggest that both light initiation and reduction in RH can trigger spore release and that these factors have separate effects on spore release of Bremia lactucae.
Mapping of Quantitative Trait Loci for Fusarium Head Blight Resistance in Barley
Authors: Ma Zhengqiang , Steffenson Brian J. , Prom Louis K. , Lapitan Nora L. V.
Published: May 2007

Fusarium head blight (FHB) is a devastating disease that causes significant reductions in yield and quality in wheat and barley. Barley grains infected with deoxynivalenol (DON), a vomitoxin produced by Fusarium graminearum, are rejected for malting and brewing. Among six-rowed barley cultivars tested thus far, only cv. Chevron exhibited resistance. This study was conducted to map genes and to identify DNA markers for marker-assisted breeding for FHB resistance in cv. Chevron with restriction fragment length polymorphism (RFLP) markers. A doubled haploid (DH) population was created from a cross between cv. Chevron and susceptible cv. Stander. Seven field experiments were conducted in four different locations in 2 years. A RFLP map containing 211 loci and covering over 1,000 centimorgans (cM) of the genome was used to map quantitative trait loci (QTL) associated with relatively low FHB severity and DON concentration. Morphological traits differing between the parents were also measured: heading date, plant height, spike angle, number of nodes per cm of rachis in the spike, and kernel plumpness. Many of the QTL for FHB and DON coincided with QTLs for these morphological traits. The “fix-QTL” algorithm in Mapmaker QTL was used to remove the part of the variance for FHB resistance that may be explained by heading date or plant height. Results from this study suggest that QTLs with major effects for FHB resistance probably do not exist in cv. Chevron. Three QTL intervals, Xcmwg706-Xbcd441 on chromosome 1H, Xbcd307b-Xcdo684b on chromosome 2H, and Xcdo959b-Xabg472 on chromosome 4H, that are not associated with late heading or height may be useful for marker-assisted selection.
Characterizing the Mechanism of Biological Control of Postharvest Diseases on Fruits with a Simple Method to Study Competition for Nutrients
Authors: Janisiewicz W. J. , Tworkoski T. J. , Sharer C.
Published: May 2007

Biocontrol agents may compete with pathogens for nutrients and space to delay or prevent decay of fruits after harvest. These mechanisms of biological control have been difficult to study because no method has been available to determine the significance of each of the components of competition. We developed a nondestructive method using tissue culture plates with cylinder inserts containing defusing membrane at one end to study competition for nutrients without competition for space. Other biocontrol mechanisms in which direct contact between an antagonist and a pathogen is not required also can be studied. The method was used to determine the competition between the yeastlike biocontrol agent, Aureobasidium pullulans, and Penicillium expansum for limited nutrients in apple juice during 24 h incubation, simulating a fruit wound. The antagonist depleted amino acids and inhibited germination of P. expansum conidia. Exposing these conidia to fresh apple juice increased conidial germination to the level comparable to that exhibited by conidia which were not exposed to the antagonist. Because the culture plate method was nondestructive, follow-up experiments in an agar diffusion test were conducted. Juice in which the antagonist grew did not inhibit germination of P. expansum conidia that were seeded on the plates. This corroborates findings from the culture plate method that inhibition of the conidia germination resulted from competition for nutrients. The new method can be coupled with existing techniques to improve understanding of antagonist-pathogen interaction for biological control of postharvest diseases.
Two Serotypes ofSweetpotato feathery mottle virusin Uganda and Their Interaction with Resistant Sweetpotato Cultivars
Authors: Karyeija Robert F. , Kreuze Jan F. , Gibson Richard W. , Valkonen Jari P. T.
Published: May 2007

Isolates of Sweetpotato feathery mottle virus (SPFMV, genus Potyvirus, family Potyviridae) were obtained in several districts of Uganda from sweetpotato plants infected with the sweetpotato virus disease (SPVD), the most important disease of this crop in Africa. A monoclonal antibody (MAb 7H8) raised against the coat proteins (CP) of a mixture of the SPFMV strain C (United States) and the isolate SPV-I (West Africa) distinguished Ugandan SPFMV isolates into those detectable and not detectable by the MAb. These two serotypes differed in prevalence in different districts of Uganda and in two common sweetpotato cultivars. Both serotypes could be transmitted simultaneously by single aphids. The serotypes differed in their ability to systemically coinfect sweetpotatoes that were infected with Sweetpotato chlorotic stunt virus (SPCSV, genus Crinivirus), the virus required to induce SPVD in SPFMV-infected plants. One sweetpotato breeding line, resistant to SPFMV from the New World, was infected by graft-inoculation with all SPFMV isolates from Uganda. Another SPFMV-resistant sweetpotato line became infected with SPFMV and developed SPVD only following coinoculation with SPCSV.