bluuebunny/crossref_metadata_2025 huggingface dataset:
Ecology of Bacillus and Paenibacillus spp. in Agricultural Systems
Authors: McSpadden Gardener Brian B.
Published: May 2007

Diverse populations of aerobic endospore-forming bacteria occur in agricultural fields and may directly and indirectly contribute to crop productivity. This paper describes recent advances in our understanding of the ecology of Bacillus and Paenibacillus spp. and how different subpopulations of these two genera can promote crop health. The abundance, diversity, and distribution of native populations and inoculant strains in agricultural fields have been characterized using a variety of methods. While native populations of these two genera occur abundantly in most agricultural soils, plant tissues are differentially colonized by distinct subpopulations. Multiple Bacillus and Paenibacillus spp. can promote crop health in a variety of ways. Some populations suppress plant pathogens and pests by producing antibiotic metabolites, while others may directly stimulate plant host defenses prior to infection. Some strains can also stimulate nutrient uptake by plants, either by promoting rhizobial and mycorrhizal symbioses or by fixing atmospheric nitrogen directly. Despite a wealth of new information on the genetics and physiology of Bacillus and related species, a better understanding of the microbial ecology of these two genera must be developed. To this end, several important, but unanswered, questions related to the ecological significance and potential for managing the beneficial activities of these bacteria are discussed.
Induced Systemic Resistance and Promotion of Plant Growth by Bacillus spp.
Authors: Kloepper Joseph W. , Ryu Choong-Min , Zhang Shouan
Published: May 2007

Elicitation of induced systemic resistance (ISR) by plant-associated bacteria was initially demonstrated using Pseudomonas spp. and other gram-negative bacteria. Several reviews have summarized various aspects of the large volume of literature on Pseudomonas spp. as elicitors of ISR. Fewer published accounts of ISR by Bacillus spp. are available, and we review this literature for the first time. Published results are summarized showing that specific strains of the species B. amyloliquefaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B. sphaericus elicit significant reductions in the incidence or severity of various diseases on a diversity of hosts. Elicitation of ISR by these strains has been demonstrated in greenhouse or field trials on tomato, bell pepper, muskmelon, watermelon, sugar beet, tobacco, Arabidopsis sp., cucumber, loblolly pine, and two tropical crops (long cayenne pepper and green kuang futsoi). Protection resulting from ISR elicited by Bacillus spp. has been reported against leaf-spotting fungal and bacterial pathogens, systemic viruses, a crown-rotting fungal pathogen, root-knot nematodes, and a stem-blight fungal pathogen as well as damping-off, blue mold, and late blight diseases. Reductions in populations of three insect vectors have also been noted in the field: striped and spotted cucumber beetles that transmit cucurbit wilt disease and the silver leaf whitefly that transmits Tomato mottle virus. In most cases, Bacillus spp. that elicit ISR also elicit plant growth promotion. Studies on mechanisms indicate that elicitation of ISR by Bacillus spp. is associated with ultrastructural changes in plants during pathogen attack and with cytochemical alterations. Investigations into the signal transduction pathways of elicited plants suggest that Bacillus spp. activate some of the same pathways as Pseudomonas spp. and some additional pathways. For example, ISR elicited by several strains of Bacillus spp. is independent of salicylic acid but dependent on jasmonic acid, ethylene, and the regulatory gene NPR1—results that are in agreement with the model for ISR elicited by Pseudomonas spp. However, in other cases, ISR elicited by Bacillus spp. is dependent on salicylic acid and independent of jasmonic acid and NPR1. In addition, while ISR by Pseudomonas spp. does not lead to accumulation of the defense gene PR1 in plants, in some cases, ISR by Bacillus spp. does. Based on the strains and results summarized in this review, two products for commercial agriculture have been developed, one aimed mainly at plant growth promotion for transplanted vegetables and one, which has received registration from the U.S. Environmental Protection Agency, for disease protection on soybean.
Genetic Variability and Structure of Canadian Populations of the Sapstain Fungus Ceratocystis resinifera
Authors: Morin Chantal , Breuil Colette , Bernier Louis
Published: May 2007

Genomic DNA was extracted from 129 isolates of Ceratocystis resinifera, a species belonging to the C. coerulescens complex, and 19 polymorphic random amplified polymorphic DNA markers were used to study the population genetic structure of this fungus. The analysis suggested a moderate value for genetic diversity (HS = 0.209). However, when monomorphic markers and rare alleles, representing 89 markers, also were included in the calculation, the genetic diversity of Canadian populations of C. resinifera appeared to be much lower (HS = 0.045). This could be explained by two hypotheses: (i) recent introduction of this species into North America and (ii) clonal reproduction (by selfing). No specialization by C. resinifera for coniferous tree species was observed based on genetic differentiation index between isolates sampled from Pinus and Picea spp. and on phylogenetic analysis using Dice coefficient of association. In spite of a low genetic diversity, a very high genetic differentiation was observed among the nine geographical populations studied (FST = 20.8%). The genetic differences were especially striking when populations from Eastern Canada were compared with populations from Western Canada (φST = 0.27%; P < 0.001), suggesting that a geographic reproductive barrier occurs in Central Canada. This barrier may be the consequence of a weak migration of insect vectors of C. resinifera due to reduced presence of hosts in the Canadian Great Plains, where extensive agriculture occurs. However, results from pairwise FST matrix and phylogeny of haplotypes suggest that the barrier is not totally impenetrable because some gene flow occurred from the west and from the east in the Big River (Saskatchewan) population located in the middle of the Great Plains.
Natural Incidence of Mixed Infections and Experimental Cross Protection Between Two Genotypes of Tobacco mild green mosaic virus
Authors: Bodaghi Sohrab , Mathews Deborah M. , Dodds J. Allan
Published: May 2007

Isolates of Tobacco mild green mosaic virus (TMGMV), a member of the genus Tobamovirus, from Nicotiana glauca in southern California fall into two major genotypes, large (TMGMV-L) and small (TMGMV-S), distinguishable by the size of the coat protein (CP) subgenomic RNA. Mixed infections in the field were rare (1.6%), even at sites where both genotypes were common in single infections (62% for TMGMV-S; 37% for TMGMV-L). When plants experimentally protected by TMGMV-L were challenged by TMGMV-S, almost complete cross protection (90% of total plants challenged) was observed regardless of the protective time period (minimum 12 h and maximum 14 days). When plants protected by TMGMV-S were challenged with TMGMV-L, complete cross protection was observed when the protective time was 5 to 14 days. However, when the protective time was 3 days or less, protection by TMGMV-S was greatly reduced (11%), with mixed infections of TMGMV-S and -L predominating (69%), and single infections of the challenge virus TMGMVL were frequently observed (20%). When TMGMV-S and -L virions were co-inoculated, the virus progeny from individual plants most often contained only the TMGMV-L genome (61%) or, less frequently (39%), both genotypes. Therefore, TMGMV-L was more competitive than TMGMV-S and was able to displace TMGMV-S in experimental situations. The results obtained from cross-protection experiments in the greenhouse would explain the low frequency of natural mixed infections. It is possible that the experimental superior competitiveness of the novel L genotype has already or will play a role in its abundance in southern California.
A Model-Based Approach to Preplanting Risk Assessment for Gray Leaf Spot of Maize
Authors: Paul P. A. , Munkvold G. P.
Published: May 2007

Risk assessment models for gray leaf spot of maize, caused by Cercospora zeae-maydis, were developed using preplanting site and maize genotype data as predictors. Disease severity at the dough/dent plant growth stage was categorized into classes and used as the response variable. Logistic regression and classification and regression tree (CART) modeling approaches were used to predict severity classes as a function of planting date (PD), amount of maize soil surface residue (SR), cropping sequence, genotype maturity and gray leaf spot resistance (GLSR) ratings, and longitude (LON). Models were development using 332 cases collected between 1998 and 2001. Thirty cases collected in 2002 were used to validate the models. Preplanting data showed a strong relationship with late-season gray leaf spot severity classes. The most important predictors were SR, PD, GLSR, and LON. Logistic regression models correctly classified 60 to 70% of the validation cases, whereas the CART models correctly classified 57 to 77% of these cases. Cases misclassified by the CART models were mostly due to overestimation, whereas the logistic regression models tended to misclassify cases by underestimation. Both the CART and logistic regression models have potential as management decision-making tools. Early quantitative assessment of gray leaf spot risk would allow for more sound management decisions being made when warranted.
Diversity Among Potato virus Y Isolates Obtained from Potatoes Grown in the United States
Authors: Piche L. M. , Singh R. P. , Nie X. , Gudmestad N. C.
Published: May 2007

Potato field isolates (Solanum tuberosum) of Potato virus Y (PVY) collected from the midwestern and western United States were characterized using serological, molecular, and biological assays. PVY field isolates were grouped into the previously defined categories: PVYO, European PVYNTN, North American PVYNTN, and PVYN:O recombinant and four previously undefined groups. Studies reported here agree with published reports from Europe and elsewhere in North America as PVY isolates capable of causing veinal necrosis in tobacco indicator plants appear in high frequency. In contrast to European experiences, PVY tuber necrosis isolates have a PVYO coat protein rather than that of PVYN. Several PVYN:O recombinant isolates induced potato tuber necrotic ringspot disease (PTNRD) in the highly susceptible potato cv. Yukon Gold. The PTNRD symptoms produced by these PVYN:O recombinants were atypical compared with lesions found on the same cultivar infected with either the European or North American PVYNTN isolates. These PVYN:O isolates produced a roughly circular, sunken necrotic lesion on the surface of the tuber instead of the typical external sunken ring pattern displayed by PVYNTN isolates. This study establishes the complex nature of PVY populations within the U.S. potato industry and clearly demonstrates the diverse nature of PVY in the United States.
Systemic Acquired Resistance Delays Race Shifts to Major Resistance Genes in Bell Pepper
Authors: Romero A. M. , Ritchie D. F.
Published: May 2007

The lack of durability of host plant disease resistance is a major problem in disease control. Genotype-specific resistance that involves major resistance (R) genes is especially prone to failure. The compatible (i.e., disease) host-pathogen interaction with systemic acquired resistance (SAR) has been studied extensively, but the incompatible (i.e., resistant) interaction less so. Using the pepper-bacterial spot (causal agent, Xanthomonas axonopodis pv. vesicatoria) pathosystem, we examined the effect of SAR in reducing the occurrence of race-change mutants that defeat R genes in laboratory, greenhouse, and field experiments. Pepper plants carrying one or more R genes were sprayed with the plant defense activator acibenzolar-S-methyl (ASM) and challenged with incompatible strains of the pathogen. In the greenhouse, disease lesions first were observed 3 weeks after inoculation. ASM-treated plants carrying a major R gene had significantly fewer lesions caused by both the incompatible (i.e., hypersensitive) and compatible (i.e., disease) responses than occurred on nonsprayed plants. Bacteria isolated from the disease lesions were confirmed to be race-change mutants. In field experiments, there was a delay in the detection of race-change mutants and a reduction in disease severity. Decreased disease severity was associated with a reduction in the number of race-change mutants and the suppression of disease caused by the race-change mutants. This suggests a possible mechanism related to a decrease in the pathogen population size, which subsequently reduces the number of race-change mutants for the selection pressure of R genes. Thus, inducers of SAR are potentially useful for increasing the durability of genotype-specific resistance conferred by major R genes.
Temporal and Spatial Dynamics of Primary and Secondary Infection by Armillaria ostoyae in a Pinus pinaster Plantation
Authors: Lung-Escarmant Brigitte , Guyon Dominique
Published: May 2007

Epidemiological investigations were performed in a 3-ha maritime pine (Pinus pinaster) plantation established on a site heavily infested by Armillaria ostoyae. Geostatistics were used to examine the density and the distribution of the initial inoculum. Disease dynamics were monitored for 17 years after planting. On the whole site, the cumulative mortality rate reached 35% over this period, plateauing at 12 years. Disease progress curves differed according to the density of the initial inoculum, although in all the cases, the Gompertz model described the epidemics well. The epidemiological contributions of both primary (initially colonized stumps) and secondary inoculum (newly dead pines) were evaluated by analyzing their spatial relation to annual mortality. Newly dead pines acted as secondary inoculum from year 3 and their role increased with time. When the initial inoculum density was low, the contribution of secondary inoculum to epidemic development increased faster and halted sooner than when the density of primary inoculum was high. Regardless of its density, the primary inoculum acted throughout the dynamic phase of the epidemic. When the inoculum density was low, the probability of mortality during the first 6 years of the epidemic depended on the tree distance from the nearest stumps colonized by Armillaria sp. When the inoculum density was high, the probability of mortality was higher and not related to the distance between trees and colonized stumps.
Elicitors of Plant Defense Responses from Biocontrol Strains of Trichoderma viren
Authors: Hanson L. E. , Howell C. R.
Published: May 2007

Effective biocontrol strains of Trichoderma virens can induce the production of defense-related compounds in the roots of cotton. Ineffective strains do not induce these compounds to significant levels. This elicittation was found to be heat stable, insoluble in chloroform, passed through a 5K molecular weight cut-off (MWCO) filter, but not a 3K MWCO filter, and was sensitive to treatment by proteinase K. When the active material was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, several bands were present in the material from biocontrol-active strains that were lacking in inactive strains. When eluted and tested for elicitation activity, with or without renaturation, four bands stimulated cotton terpenoid production. One band showed cross-reaction with an antibody to the ethylene-inducing xylanase from T. viride. Another band of approximately 18 kDa, gave significant stimulation of cotton terpenoid production and increased peroxidase activity in cotton radicles in all tests, with or without renaturation. The 18-kDa protein was subjected to amino-terminal sequence analysis, and the first 19 amino acids at the amino terminus were determined to be DTVSYDTGYDNGSRSLNDV. A database homology search using the BLASTp algorithm showed the highest similarity to a serine proteinase from Fusarium sporotrichioides.
Identification of Lentil Germ Plasm Resistant to Colletotrichum truncatum and Characterization of Two Pathogen Races
Authors: Buchwaldt L. , Anderson K. L. , Morrall R. A. A. , Gossen B. D. , Bernier C. C.
Published: May 2007

A total of 1,771 lentil accessions from the U.S. lentil collection (U.S. Department of Agriculture-Agricultural Research Service, Pullman, WA) and the Institut für Pflanzengenetik und Kulturpflanzenforschung (Gatersleben, Germany) were screened for resistance to Colletotrichum truncatum, the cause of anthracnose. About 95% of the accessions were susceptible when inoculated with a single isolate in the field. Retesting, under controlled conditions, of accessions rated as resistant or moderately resistant in the field resulted in identification of anthracnose resistance in four accessions from the U.S. collection (PI 320937, PI 320952 [cv. Indianhead], PI 345629, and 468901), and 12 accessions from the German collection (Lens 3, 102, 104, 106, 107, 119, 122, 134, 135, 177, 195, and 209). Seven of the accessions were used as host differentials to characterize pathogenic variability of 50 single-spore isolates collected in Manitoba and Saskatchewan, Canada. The presence of two distinct races was demonstrated. Isolates of C. truncatum avirulent on cv. Indianhead, PI 320937, PI 345629, PI 468901, Lens 102, Lens 104, and Lens 195 were designated race Ct1. Isolates that were virulent on these seven entries were designated race Ct0, indicating their lack of avirulence genes. Race Ct0 was isolated more frequently from commercial seed samples than race Ct1, but the two races were isolated with similar frequency from plants in commercial fields planted to susceptible cultivars. Race Ct0, to which no resistance has yet been identified, presents a high risk to lentil production in Canada and potentially worldwide.
Forecasting Sclerotinia Disease on Lettuce: Toward Developing a Prediction Model for Carpogenic Germination of Sclerotia
Authors: Clarkson John P. , Phelps Kath , Whipps John M. , Young Caroline S. , Smith Julie A. , Watling Martyn
Published: May 2007

The feasibility of developing a forecasting system for carpogenic germination of Sclerotinia sclerotiorum sclerotia was investigated in the laboratory by determining key relationships among temperature, soil water potential, and carpogenic germination for sclerotia of two S. sclerotiorum isolates. Germination of multiple burials of sclerotia to produce apothecia also was assessed in the field with concurrent recording of environmental data to examine patterns of germination under different fluctuating conditions. Carpogenic germination of sclerotia occurred between 5 and 25°C but only for soil water potentials of ≥-100 kPa for both S. sclerotiorum isolates. Little or no germination occurred at 26 or 29°C. At optimum temperatures of 15 to 20°C, sclerotia buried in soil and placed in illuminated growth cabinets produced stipes after 20 to 27 days and apothecia after 27 to 34 days. Temperature, therefore, had a significant effect on both the rate of germination of sclerotia and the final number germinated. Rate of germination was correlated positively with temperature and final number of sclerotia germinated was related to temperature according to a probit model. Thermal time analysis of field data with constraints for temperature and water potential showed that the mean degree days to 10% germination of sclerotia in 2000 and 2001 was 285 and 279, respecttively, and generally was a good predictor of the observed appearance of apothecia. Neither thermal time nor relationships established in the laboratory could account for a decline in final percentage of germination for sclerotia buried from mid-May compared with earlier burials. Exposure to high temperatures may explain this effect. This, and other factors, require investigation before relationships derived in the laboratory or thermal time can be incorporated into a forecasting system for carpogenic germination.
Isolation and Characterization of a Novel Arabidopsis thaliana Mutant Unable to Develop Wilt Symptoms After Inoculation with a Virulent Strain of Ralstonia solanacearum
Authors: Feng Dong-Xin , Deslandes Laurent , Keller Harald , Revers Frédéric , Favery Bruno , Lecomte Philippe , Hirsch Judith , Olivier Jocelyne , Marco Yves
Published: May 2007

To characterize host genes required for a compatible interaction, we identified a novel recessive Arabidopsis thaliana mutant, nws1 (no wilt symptoms), that failed to develop wilt symptoms in response to virulent strains of the phytopathogenic bacterium, Ralstonia solanacearum. The absence of wilting in nws1 plants was not correlated with a cell death phenotype or a constitutive expression of salicylic acid-, jasmonic acid- or ethylene-associated genes. In addition, this mutation, which conferred a symptomless phenotype in response to all the R. solanacearum strains tested, was highly specific to this pathogen, because nws1 responses to other plant pathogens, including oomycetes, nematodes, viruses, and other bacteria, were identical to those of wild-type Col-5 plants. Finally, the lack of disease development was shown to be different than RRS1-R-mediated resistance. The identification of mutants such as nws1, that are unable to develop disease, should lead to the isolation of target host factors required for pathogen growth or fitness, or of factors modified by the invading microorganism to avoid or inactivate plant defense mechanisms, and should bring a better understanding of bacterial wilt diseases.
The Purification, Physical/Chemical Characterization, and cDNA Sequence of Cantaloupe Fruit Polygalacturonase-Inhibiting Protein
Authors: Fish Wayne W. , Davis Angela R.
Published: May 2007

Polygalacturonase-inhibiting proteins (PGIPs) are believed to aid in plant defense against fungal pathogens by inhibiting polygalacturonases (PGs) secreted by the invading organism. In an effort to better understand this type of plant-pathogen interaction in cucurbits, we have isolated a cantaloupe PGIP (CmPGIP) from 5 to 15 day postanthesis cantaloupe fruit. CmPGIP inhibited crude extracts of PG from two of four fungal pathogens of cantaloupe that were tested. Results from assays for PG activity that utilized rate of substrate viscosity reduction or rate of reducing group formation were consistent with CmPGIP inhibition of endo-PG activity. The Mrof CmPGIP by sedimentation equilibrium or MALDITOF MS was 38,500. The pI of CmPGIP was ≈8.2, and its absorptivity at 280 nm was 0.93 ml/mg. The circular dichroism spectrum of native CmPGIP exhibited strong negative ellipticity in the near UV and possessed a far UV spectrum indicative of β-sheet periodic structure. Amino acid sequences of the N terminus and a cyanogen bromide peptide were used to construct oligonucleotide primers for polymerase chain reaction sequencing. The sequenced open reading frame predicts a mature protein of 307 amino acids with up to 68% identity to other PGIP molecules. Northern blot analysis revealed differential expression during fruit development. The isolation and structural information obtained for CmPGIP by this investigation provide a foundation for the development of molecular strategies for pre- and postharvest crop protection.
Breakdown of the Scab Resistance Gene Vf in Apple Leads to a Founder Effect in Populations of the Fungal Pathogen Venturia inaequalis
Authors: Guérin Fabien , Le Cam Bruno
Published: May 2007

The recent breakdown of Vf, a major resistance gene to apple scab, provided an opportunity to analyze a population genetic process within the matching virulent subpopulation of the fungus Venturia inaequalis. We utilized the amplified fragment length polymorphism technique and allelic variation at four microsatellite loci to assess genetic structure of 133 isolates of V. inaequalis from a single commercial apple orchard sampled from one cultivar carrying the Vf gene (Judeline) and three cultivars devoid of the Vf gene. Both analyses indicated a strong decrease of the genetic diversity among isolates from the Vf cultivar compared with the high level of diversity among isolates from the three other cultivars. This leads to a high genetic differentiation between virVf and avrVf groups (FST > 0.17). Analyses of the genetic distance between AFLP patterns based on the Jaccard index indicate that all virVf isolates could be assigned to a single clonal lineage. These results lead us to conclude that the clonal structure of the population isolated from the Vf cultivar is an example of a founder effect in response to a resistance gene breakdown and it is likely that this event occurred in the orchard during the sampling year.
Rootstock Effects on Pistachio Trees Grown in Verticillium dahliae-Infested Soil
Authors: Epstein L. , Beede R. , Kaur S. , Ferguson L.
Published: May 2007

In a field trial in soil infested with Verticillium dahliae, we compared the yield, growth, incidence of symptoms of Verticillium wilt, and mortality of two interspecific hybrid pistachio tree rootstocks (UCBI and PGII) with the standard rootstocks: the V. dahliae-resistant and susceptible Pistacia integerrima and P. atlantica, respectively. After 10 years, the trees were destructively sampled for V. dahliae in the xylem at the graft union. The results indicate that trees on the (P. atlantica ‘KAC’ × P. integerrima) hybrid UCBI rootstock grew and yielded as well as those on P. integerrima. Trees on the hybrid PGII yielded the least. Analysis of variance and log-linear models indicate that in soil infested with V. dahliae, three associations significantly affect pistachio nut yield. Rootstock affects scion vigor and extent of infection. Third, the extent of infection and scion vigor are inversely associated. Although trees on the P. integerrima rootstock had the highest ratings in a visual assessment of vigor, 65% were infected with V. dahliae in the trunk in the graft region compared with 73% in P. atlantica and 25% in UCBI. Thus, P. integerrima and UCBI have at least one different mechanism for resistance to V. dahliae.
Sporulation of Bremia lactucae Affected by Temperature, Relative Humidity, and Wind in Controlled Conditions
Authors: Su H. , van Bruggen A. H. C. , Subbarao K. V. , Scherm H.
Published: May 2007

The effects of temperature (5 to 25°C), relative humidity (81 to 100%), wind speed (0 to 1.0 m s-1), and their interactions on sporulation of Bremia lactucae on lettuce cotyledons were investigated in controlled conditions. Sporulation was affected significantly (P < 0.0001) by temperature, with an optimum at 15°C, and by relative humidity (RH), with sporulation increasing markedly at RH ≥ 90%. There was a significant effect of exposure time in relation to temperature (P = 0.0007) but not to RH. In separate experiments, both RH and wind speed significantly (P < 0.0001) affected the number of cotyledons with sporulation and the number of sporangia produced per cotyledon. No sporulation was observed at wind speeds of >0.5 m s-1, regardless of RH. In still air, the number of sporangiophores produced per cotyledon increased linearly with RH from 81 to 100% (P = 0.0001, r = 0.98). Histological observations indicated that sporulation may be affected by stomatal aperture in response to RH, as more closed stomata and correspondingly fewer sporangiophores were present at lower RH. These results are important for understanding the mechanism of RH effects on sporulation and for predicting conditions conducive to downy mildew development.
Development of Conductive Polymer Analysis for the Rapid Detection and Identification of Phytopathogenic Microbes
Authors: Wilson A. D. , Lester D. G. , Oberle C. S.
Published: May 2007

Conductive polymer analysis, a type of electronic aroma detection technology, was evaluated for its efficacy in the detection, identification, and discrimination of plant-pathogenic microorganisms on standardized media and in diseased plant tissues. The method is based on the acquisition of a diagnostic electronic fingerprint derived from multisensor responses to distinct mixtures of volatile metabolites released into sampled headspace. Protocols were established to apply this technology specifically to plant disease diagnosis. This involved development of standardized cultural methods, new instrument architecture for sampling, sample preparation, prerun procedures, run parameters and schedules, recognition files and libraries, data manipulations, and validation protocols for interpretations of results. The collective output from a 32-sensor array produced unique electronic aroma signature patterns diagnostic of individual microbial species in culture and specific pathogen-host combinations associated with diseased plants. The level of discrimination applied in identifications of unknowns was regulated by confidence level and sensitivity settings during construction of application-specific reference libraries for each category of microbe or microbe-host combination identified. Applications of this technology were demonstrated for the diagnosis of specific disease systems, including bacterial and fungal diseases and decays of trees; for host identifications; and for determinations of levels of infection and relatedness between microbial species. Other potential applications to plant pathology are discussed with some advantages and limitations for each type of diagnostic application.
Host Barriers and Responses toUncinula necatorin Developing Grape Berries
Authors: Ficke Andrea , Gadoury David M. , Seem Robert C. , Godfrey Dale , Dry Ian B.
Published: May 2007

Grape berries are highly susceptible to powdery mildew 1 week after bloom but acquire ontogenic resistance 2 to 3 weeks later. We recently demonstrated that germinating conidia of the grape powdery mildew pathogen (Uncinula necator) cease development before penetration of the cuticle on older resistant berries. The mechanism that halts U. necator at that particular stage was not known. Several previous studies investigated potential host barriers or cell responses to powdery mildew in berries and leaves, but none included observation of the direct effect of these factors on pathogen development. We found that cuticle thickness increased with berry age, but that ingress by the pathogen halted before formation of a visible penetration pore. Cell wall thickness remained unchanged over the first 4 weeks after bloom, the time during which berries progressed from highly susceptible to nearly immune. Autofluorescent polyphenolic compounds accumulated at a higher frequency beneath appressoria on highly susceptible berries than on highly resistant berries; and oxidation of the above phenolics, indicated by cell discoloration, developed at a significantly higher frequency on susceptible berries. Beneath the first-formed appressoria of all germinated conidia, papillae occurred at a significantly higher frequency on 2- to 5-day-old berries than on 30- to 31-day-old fruit. The relatively few papillae observed on older berries were, in most cases (82.8 to 97.3%), found beneath appressoria of conidia that had failed to produce secondary hyphae. This contrasted with the more abundantly produced papillae on younger berries, where only 35.4 to 41.0% were located beneath appressoria of conidia that had failed to produce secondary hyphae. A pathogenesis-related gene (VvPR-1) was much more highly induced in susceptible berries than in resistant berries after inoculation with U. necator. In contrast, a germin-like protein (VvGLP3) was expressed within 16 h of inoculation in resistant, but not in susceptible berries. Our results suggest that several putative barriers to infection, i.e., cuticle and cell wall thickness, antimicrobial phenolics, and two previously described pathogenesis-related proteins, are not principal causes in halting pathogen ingress on ontogenically resistant berries, but rather that infection is halted by one or more of the following: (i) a preformed physical or biochemical barrier near the cuticle surface, or (ii) the rapid synthesis of an antifungal compound in older berries during the first few hours of the infection process.
Involvement of Phosphoglucose Isomerase in Pathogenicity of Xanthomonas oryzae pv. oryzae
Authors: Tsuge Seiji , Ochiai Hirokazu , Inoue Yasuhiro , Oku Takashi , Tsuno Kazunori , Kaku Hisatoshi , Kubo Yasuyuki
Published: May 2007

Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight of rice, was subjected to transposon mutagenesis to generate mutants defective in pathogenicity. A novel mutant 74M913 was attenuated in virulence but retained its ability to cause the hypersensitive response in leaf blight-resistant rice and tomato. Cloning and sequence analysis revealed that the transposon in 74M913 was inserted in a gene homologous to the phosphoglucose isomerase (pgi) gene of X. axonopodis pv. citri. Growth of the mutant in a synthetic medium containing fructose or xylose as a sole carbohydrate source was much reduced, indicating the transposon disrupted pgi function. The interaction between expression of pgi and hypersensitive response and pathogenicity (hrp) genes was investigated because we had demonstrated previously that expression of hrp genes of X. oryzae pv. oryzae is induced in a synthetic medium containing xylose. However, pgi and the hrp gene (hrcU) were expressed independently. This study suggests that PGI is involved in pathogenicity of X. oryzae pv. oryzae.
A Reovirus Causes Hypovirulence of Rosellinia necatrix
Authors: Kanematsu S. , Arakawa M. , Oikawa Y. , Onoue M. , Osaki H. , Nakamura H. , Ikeda K. , Kuga-Uetake Y. , Nitta H. , Sasaki A. , Suzaki K. , Yoshida K. , Matsumoto N.
Published: May 2007

White root rot, caused by Rosellinia necatrix, is a serious soilborne disease of fruit trees and other woody plants. R. necatrix isolate W370 contains 12 segments of double-stranded RNA (dsRNA) that is believed to represent a possible member of the family Reoviridae. W370 was weakly virulent and its hyphal-tip strains became dsRNA free and strongly virulent. The 12 segments of W370dsRNA were transmitted to hygromycin B-resistant strain RT37-1, derived from a dsRNA-free strain of W370 in all or none fashion through hyphal contact with W370. The W370dsRNA-transmitted strains were less virulent than their parent strain RT37-1 on apple seedlings, with mortality ranging between 0 to 16.7% in apple seedlings that were inoculated with the W370dsRNA-containing strains and 50 to 100% for seedlings inoculated with the dsRNA-free strains. Some W370dsRNA-containing strains killed greater than 16.7% of seedlings, but these were found to have lost the dsRNA in planta. These results indicate that W370dsRNA is a hypovirulence factor in R. necatrix. In addition, a strain lost one segment (S8) of W370dsRNA during subculture, and the S8-deficient mutant strain also exhibits hypovirulence in R. necatrix.
Integrated Control of Grape Black Rot: Influence of Host Phenology, Inoculum Availability, Sanitation, and Spray Timing
Authors: Hoffman Lisa Emele , Wilcox Wayne F. , Gadoury David M. , Seem Robert C. , Riegel Duane G.
Published: May 2007

The epidemiology and control of black rot (Guignardia bidwellii) was studied from 1995 to 1999 in vineyards in Dresden and Naples, NY, where disease pressure was moderate and extreme, respectively. The efficacy of serial applications of myclobutanil, provided at 2-week intervals and varied with respect to their number and time of initiation, was examined within the context of host phenology, inoculum availability, and sanitation. At Dresden, sprays applied over 4 weeks through the immediate prebloom stage provided only 13 to 91% control of diseased clusters, despite the release of 95% of the season's ascosporic inoculum during the period of fungicidal protection. However, applications immediately prior to bloom plus 2 and 4 weeks later, which afforded protection while fruit are highly susceptible to infection, provided virtually complete control. At Naples, where mummified berries were retained in the canopy after mechanical pruning, this same regime provided only approximately 80% disease control, but applying a fourth spray 2 weeks prebloom generally improved control. Hand-pruning mummies to the ground in selected plots significantly (P ≤ 0.05) improved control in some spray regimes. Although this sanitation practice did not affect inoculum dynamics through bloom, very few spores were recovered thereafter from mummies collected from the ground, whereas abundant ascospores and conidia were recovered from mummies in the trellis for an additional 6 to 8 weeks.
A Promoter from Pea Gene DRR206 Is Suitable to Regulate an Elicitor-Coding Gene and Develop Disease Resistance
Authors: Choi Jane Junghae , Klosterman Steven Joseph , Hadwiger Lee A.
Published: May 2007

Plant nonhost disease resistance is characterized by the induction of multiple defense genes. The pea DRR206 gene is induced following inoculation with pathogens and treatment with abiotic agents, and moderately induced by wounding. A deletion series of DRR206 promoter segments was fused with the β-glucuronidase (GUS) reporter gene and transiently transferred to tobacco, potato, and pea. GUS activity revealed that two upstream regions of the DRR206 promoter were particularly important for activation in the three plant species. Putative cis regulatory elements within the DRR206 promoter included a wound/pathogen- inducible box (W/P-box) and a WRKY box (W-box). Gel shift assays with nuclear extracts from treated and untreated tissue with the W/P-box revealed both similar and unique protein-DNA complexes from pea, potato, and tobacco. Tobacco was stably transformed with gene constructs of the DRR206 promoter fused with a DNase elicitor gene from Fusarium solani f. sp. phaseoli, FsphDNase. Pathogenicity tests indicated that the FsphDNase elicitor conferred resistance against Pseudomonas syringae pv. tabaci and Alternaria alternata in tobacco. Transgenic potatoes showed some sensitivity to the FsphDNase gene providing less protection against Phytophthora infestans. Thus, the elicitor-coding gene, FsphDNase, is capable of generating resistance in a heterologous plant system (tobacco) when fused with defined regions of the pea DRR206 promoter.
Genetic and Phenotypic Analysis of Soybean mosaic virus Resistance in PI 88788 Soybean
Authors: Gunduz Irfan , Buss Glenn R. , Chen Pengyin , Tolin Sue A.
Published: May 2007

Resistance to Soybean mosaic virus (SMV) was identified in PI 88788 soybean, a germ plasm accession from China that is used widely as a source of resistance to soybean cyst nematode. Strains SMV-G1 through -G7 infected the inoculated leaves of PI 88788 but were not detected in upper, noninoculated trifoliolate leaves. Inheritance of resistance was determined by inoculating progenies of crosses of PI 88788 with susceptible cvs. Essex and Lee 68 with SMV strains G1 and G7. Allelomorphic relationships with known genes for resistance to SMV were tested in crosses with the resistant genotypes PI 96983, L29, and V94-5152, possessing Rsv1, Rsv3, and Rsv4 genes, respectively. Data analyses showed that resistance in PI 88788 to SMV-G1 is controlled by a single, partially dominant gene; however, to SMV-G7, the same gene was completely dominant. The PI 88788 gene was independent of the Rsv1 and Rsv3 loci, but allelic to Rsv4 in V94-5152. Expression of the Rsv4 gene in PI 88788 resulted in a reduced number of infection sites and restricted short- and long-distance movement of virus, rather than hypersensitivity. A unique late susceptible phenotype was strongly associated with heterozygosity. This gene has potential value for use in gene pyramiding to achieve resistance to several SMV strains, as well as for rate-reducing resistance.
Genetic Characterization of Cercospora sorghi from Cultivated and Wild Sorghum and Its Relationship to Other Cercospora Fungi
Authors: Okori P. , Rubaihayo P. R. , Ekwamu A. , Fahleson J. , Dixelius C.
Published: May 2007

Genetic variability and population structure of Cercospora sorghi from wild and cultivated sorghum were investigated to gain insight into their potential impact on epidemics of gray leaf spot of sorghum in Africa. Population structure was examined using data derived from amplified fragment length polymorphism (AFLP) of C. sorghi by Nei's test for population differentiation, GST, and analysis of molecular variation (AMOVA). Two ecological populations of C. sorghi in Uganda were devoid of population structure (GST = 0.03, фFST = 0.01, P = 0.291). AMOVA revealed that genetic variability was due mainly to variations within (99%) rather than between (0.35%) populations, and Nei's genetic distance between the two populations was 0.014. Phenetic analysis based on AFLP data and polymerase chain reaction-restriction fragment length polymorphism analyses of the internal transcribed spacer regions of rDNA and mitochondrial small subunit rDNA separated Cercospora cereal pathogens from dicot pathogens but did not differentiate among C. sorghi isolates from wild and cultivated sorghum. Our results indicate that Ugandan populations of C. sorghi compose one epidemiological unit and suggest that wild sorghum, while not affecting genetic variability of the pathogen population, provides an alternative host for generating additional inoculum.
Abiotic and Biological Suppression of Phytophthora parasitica in a Horticultural Medium Containing Composted Swine Waste
Authors: Fichtner E. J. , Benson D. M. , Diab H. G. , Shew H. D.
Published: May 2007

Horticultural potting media have been amended with compost to enhance biological suppression and with Al2(SO4)3 to enhance abiotic suppression of plant pathogens, but these factors have not been simultaneously incorporated into the same medium. In this study, the efficacy of aluminum (Al)-amended potting medium containing 20% composted swine waste (CSW) was assessed for control of Phytophthora parasitica (syn. P. nicotianae), a soilborne pathogen causing damping-off of many horticultural bedding plants. Steamed and unsteamed media were amended with no Al or Al at 0.0079 g of Al g-1 of medium with an Al2(SO4)3 solution at either pH 4 or pH 6. Infested leaf disks were buried for 2-day durations beginning 0, 6, 13, and 21 days after Al amendment. The number of sporangia produced on infested leaf disks was assessed. A similar experiment was conducted to determine the effect of steaming and Al amendments on pathogen populations. Medium treated with the pH 4 solution consistently reduced sporangia production between 38 and 65% on day 0, but no Al effect was noted at subsequent time points. The pH 6 amendment did not consistently affect sporangia production. Exchangeable Al levels decreased over time, and abiotic suppression was only observed at >2 μM Al g-1 of medium. Pathogen populations were occasionally affected by steaming and Al. Sporangia production in unsteamed medium was reduced by 50% on leaf disks buried on days 6, 13, and 21, but not on day 0. Al amendment of a 20% CSW potting medium enhanced suppression of P. parasitica and abiotic suppression occurred before biological suppression developed.
Minimum Phylogenetic Coverage: An Additional Criterion to Guide the Selection of Microbial Pathogens for Initial Genomic Sequencing Efforts
Authors: Goodwin Stephen B.
Published: May 2007

The scientific community has made a persuasive case to increase funding for genomic sequencing of microbial plant pathogens, and a number of objective criteria were developed to guide the selection of sequencing targets. However, this task still is complicated for fungi and Oomycetes, which are extremely diverse evolutionarily. Lack of a definite target number of organisms to be sequenced and inclusion of only a small segment of fungal diversity weaken the effort, which may reduce support and hinder efforts to increase the level of funding. These problems can be minimized by inclusion of phylogenetic relationship as an additional criterion to prioritize organisms for genomic sequencing. A phylogenetic analysis of 18S ribosomal DNA sequences revealed that many of the species proposed for genomic sequencing are very closely related, while other evolutionarily important groups, such as the Taphrinales, are underrepresented or ignored. By choosing one representative from each of the major clusters on the 18S tree, a minimum phylogenetic coverage (MPC) of the fungi and Oomycetes that have been proposed for genomic sequencing can be achieved by including seven Ascomycetes, four Basidiomycetes, and one Oomycete. A second round of 14 species could be sequenced to cover the major sub-branches within each of the 12 major clusters. This approach defines a tangible, achievable goal for genomic sequencing that could be used to lobby for additional funding. MPC also would assure support and participation by the largest number of plant pathologists because most would have access to a sequenced genome from a close relative of their organism of interest. Allocating funds for this effort might be achieved best through a competitive bidding process among sequencing centers rather than the traditional grants programs. MPC provides a logical criterion to help prioritize fungi, Oomycetes, and other microbial pathogens for genomic sequencing that generates a defined, tractable number of sequencing targets and could serve as a rallying point to unify plant pathologists toward achieving a common goal.
The Potential Geographic Range of Pyrenophora semeniperda
Authors: Yonow Tania , Kriticos Darren J. , Medd Richard W.
Published: May 2007

There is no evidence that Pyrenophora semeniperda, the causal agent of leaf spotting in many annual and perennial grasses, currently occurs in Europe or Asia. However, there is potential phytosanitary concern that the importation of infected commodities could result in the introduction of this fungus into Eurasia, putting crops at risk and possibly resulting in economic losses. To assist in assessing the risk of geographic range extension of P. semeniperda, an analysis was undertaken to estimate the potential global distribution of this species, based on climatic suitability. Geographic distribution data for P. semeniperda in part of its current range were used to fit parameter values in a CLIMEX pest risk assessment model, and the remaining distribution data were used to validate the model. The CLIMEX model correctly predicts that virtually all locations where P. semeniperda has been found are climatically suitable. Only five locations worldwide where the fungus was recorded present are predicted as being unsuitable. These “outliers” may have been transient populations occurring during a favorable season and then dying out. Exploratory adjustments of the model to accommodate these records created unsatisfactory distortions in the projected climatic suitability surfaces, extending the suitable climatic zone beyond well-established traditional range boundaries. We are therefore confident that the model is credibly predicting the potential distribution of P. semeniperda worldwide. The CLIMEX model suggests that P. semeniperda could potentially extend its range throughout Europe and temperate regions of Asia, Africa, and South America. Our heavy reliance upon geographic data to build this CLIMEX model departs from most previous published examples in plant pathology, which have depended primarily upon experimentally derived physiological data to estimate model parameters. The use of geographic data to infer climate parameters is popular in CLIMEX models of weeds and arthropod pests and can provide decision-makers with early risk assessments of potential pathogen invasions, particularly where the pathogens have long, or difficult-to-study, lifecycles.