bluuebunny/crossref_metadata_2025 huggingface dataset:
Environmental Sensitivity of Soybean Cultivar Response to Sclerotinia sclerotiorum
Authors: Pennypacker B. W. , Risius M. L.
Published: May 2007

The sporadic occurrence of Sclerotinia stem rot in soybeans often is attributed to the sensitivity of Sclerotinia sclerotiorum to environmental factors. Environmental sensitivity in soybean response to the pathogen also could contribute to the unpredictable nature of this disease. We used stability analysis to determine whether soybean cultivar response to S. sclerotiorum was sensitive to light and temperature. Five greenhouse experiments examined the response of seven cultivars to limited-term inoculation with S. sclerotiorum. The cultivars, selected at random from Pennsylvania variety trials, represented maturity groups grown in Pennsylvania and other states sharing that latitude. Photon flux density of photosynthetically active radiation (PAR) and temperature were recorded hourly and varied among experiments. Environmental sensitivity was detected in the response of five cultivars to S. sclerotiorum when individual cultivar disease ratings (assessed 6 days after inoculation) were regressed against the mean disease rating of each experiment. Stability analysis with temperature during the 48-h inoculation period as the environmental index found that all cultivars responded similarly to the number of hours that temperatures were <19°C, 19 to 22°C, or >22°C. In contrast, cultivars separated into PAR-sensitive and PAR-insensitive groups when the environmental index was moles of PAR at a photon flux density ≥475 μmol m-2 s-1 during the inoculation period. The photon flux density of PAR on a cloudy day in the field is ≤200 μmol m-2 s-1. The two cultivar groups were identical to those identified as environmentally sensitive and insensitive in the initial analysis. The PAR-sensitive cultivars had homogeneous negative regression coefficients different (P = 0.001) from zero. As the accumulated moles of PAR during the inoculation period increased, the disease ratings of the PAR-sensitive cultivars decreased; the disease ratings of the PAR-insensitive cultivars remained stable. The photon flux density of PAR during the 48-h inoculation period explained the environmental sensitivity of soybean cultivar response to S. sclerotiorum in these experiments.
Population Genetic Analysis Corroborates Dispersal of Fusarium oxysporum f. sp. radicis-lycopersici from Florida to Europe
Authors: Rosewich U. Liane , Pettway R. E. , Katan Talma , Kistler H. C.
Published: May 2007

Fusarium oxysporum isolates from tomato plants displaying crown and root rot symptoms were collected in central and southern Florida and analyzed using vegetative compatibility grouping (VCG) and nuclear restriction fragment length polymorphism (RFLP) data. VCG 0094 of F. oxysporum f. sp. radicis-lycopersici, previously known only from northwestern Europe, was predominant among 387 isolates assessed. In addition, two newly described VCGs (0098 and 0099) were detected at low frequencies. Floridian VCG 0094 isolates displayed a continuum of compatibilities, which is in contrast to the three distinct subgroups previously identified among European VCG 0094 isolates. RFLP haplotypes were constructed using one repetitive and three low-copy probes. Population subdivision of VCG 0094 from various Floridian counties and from northwestern Europe (Belgium, the Netherlands, and the United Kingdom) was evaluated by analysis of molecular variance. A “natural” population structure was revealed, differentiating populations from the east and west coasts of Florida. In addition, isolates from Europe were statistically indistinguishable from the Palm Beach County, FL, population. Furthermore, gene diversity among Palm Beach County VCG 0094 isolates was more than five times greater than among European isolates. Results from both VCG and RFLP analyses strongly support the inference that the European VCG 0094 constitutes a founder population that resulted from intercontinental migration of a few isolates from Palm Beach County, FL.
Quantitative Trait Loci Mapping of Resistance to Sugarcane Mosaic Virus in Maize
Authors: Xia Xianchun , Melchinger Albrecht E. , Kuntze Lissy , Lübberstedt Thomas
Published: May 2007

Sugarcane mosaic virus (SCMV) is an important virus disease of maize (Zea mays) in Europe. In this study, we mapped and characterized quantitative trait loci (QTL) affecting resistance to SCMV in a maize population consisting of 219 F3 or immortalized F2 families from the cross of two European maize inbreds, D32 (resistant) × D145 (susceptible). Resistance was evaluated in replicated field trials across two environments under artificial inoculation. The method of composite interval mapping was employed for QTL detection with a linkage map based on 87 restriction fragment length polymorphism and 7 mapped microsatellite markers. Genotypic and genotype × environment interaction variances for SCMV resistance were highly significant in the population. Heritabilities ranged from 0.77 to 0.94 for disease scores recorded on seven consecutive dates. Five QTL for SCMV resistance were identified on chromosomes 1, 3, 5, 6, and 10 in the joint analyses. Two major QTL on chromosomes 3 and 6 were detected consistently in both environments. Significant epistatic effects were found among some of these QTL. A simultaneous fit with all QTL in the joint analyses explained between 70 and 77% of the phenotypic variance observed at various stages of plant development. Resistance to SCMV was correlated with plant height and days to anthesis.
Multiple Loci Determining Insensitivity to Phenylamide Fungicides in Phytophthora infestans
Authors: Judelson Howard S. , Roberts Samuel
Published: May 2007

The diversity of mechanisms causing insensitivity to phenylamide fungicides in Phytophthora infestans was addressed by comparative genetic analyses of isolates from North America, Europe, and Mexico. Both semidominant major loci (MEX loci) and genes of minor effect were previously shown to determine insensitivity based on studies of isolates from Europe and Mexico. In this investigation, genetic analyses of three highly insensitive isolates from the United States and Canada revealed a similar pattern involving major and minor loci. However, MEX alleles in two Canadian isolates conferred higher levels of insensitivity than those examined previously, particularly in a heterozygous state. This suggested that not all MEX alleles in P. infestans were functionally equivalent. The chromosomal locations of the major insensitivity loci were also shown to vary in different isolates based on linkage analyses performed with the aid of DNA markers. The major determinant of insensitivity in the North American, Dutch, and Mexican isolates mapped to the same locus, which was named MEX1. In a British isolate, a different locus, dubbed MEX2, was implicated that mapped to the same linkage group as MEX1 but to a distinct site.
Using Predictions Based on Geostatistics to Monitor Trends in Aspergillus flavus Strain Composition
Authors: Orum Thomas V. , Bigelow Donna M. , Cotty Peter J. , Nelson Merritt R.
Published: May 2007

Aspergillus flavus is a soil-inhabiting fungus that frequently produces aflatoxins, potent carcinogens, in cottonseed and other seed crops. A. flavus S strain isolates, characterized on the basis of sclerotial morphology, are highly toxigenic. Spatial and temporal characteristics of the percentage of the A. flavus isolates that are S strain (S strain incidence) were used to predict patterns across areas of more than 30 km2. Spatial autocorrelation in S strain incidence in Yuma County, AZ, was shown to extend beyond field boundaries to adjacent fields. Variograms revealed both short-range (2 to 6 km) and long-range (20 to 30 km) spatial structure in S strain incidence. S strain incidence at 36 locations sampled in July 1997 was predicted with a high correlation between expected and observed values (R = 0.85, P = 0.0001) by kriging data from July 1995 and July 1996. S strain incidence at locations sampled in October 1997 and March 1998 was markedly less than predicted by kriging data from the same months in prior years. Temporal analysis of four locations repeatedly sampled from April 1995 through July 1998 also indicated a major reduction in S strain incidence in the Texas Hill area after July 1997. Surface maps generated by kriging point data indicated a similarity in the spatial pattern of S strain incidence among all sampling dates despite temporal changes in the overall S strain incidence. Geostatistics provided useful descriptions of variability in S strain incidence over space and time.
Spatial Distribution and Temporal Development of Fusarium Crown and Root Rot of Tomato and Pathogen Dissemination in Field Soil
Authors: Rekah Yael , Shtienberg D. , Katan J.
Published: May 2007

The spatial distribution and temporal development of tomato crown and root rot, caused by Fusarium oxysporum f. sp. radicis-lycopersici, were studied in naturally infested fields in 1996 and 1997. Disease progression fit a logistic model better than a monomolecular one. Geostatistical analyses and semivariogram calculations revealed that the disease spreads from infected plants to a distance of 1.1 to 4.4 m during the growing season. By using a chlorate-resistant nitrate nonutilizing (nit) mutant of F. oxysporum f. sp. radicis-lycopersici as a “tagged” inoculum, the pathogen was found to spread from one plant to the next via infection of the roots. The pathogen spread to up to four plants (2.0 m) on either side of the inoculated focus plant. Root colonization by the nit mutant showed a decreasing gradient from the site of inoculation to both sides of the inoculated plant. Simulation experiments in the greenhouse further established that this soilborne pathogen can spread from root to root during the growing season. These findings suggest a polycyclic nature of F. oxysporum f. sp. radicis-lycopersici, a deviation from the monocyclic nature of many nonzoosporic soilborne pathogens.
Effect of Fall Application of Fungal Antagonists on Spring Ascospore Production of the Apple Scab Pathogen,Venturia inaequalis
Authors: Carisse Odile , Philion Vincent , Rolland Daniel , Bernier Julie
Published: May 2007

The influences of Microsphaeropsis sp., M. arundinis, Ophiostoma sp., Diplodia sp., and Trichoderma sp., all antagonists of Venturia inaequalis, on ascospore production were evaluated under natural conditions and compared with urea and Athelia bombacina, a known antagonist. In the autumn, the fungi were applied to leaf disks artificially inoculated with V. inaequalis and to scabbed apple (Malus domestica) leaves incubated under controlled and natural conditions. In addition, large-scale trials were conducted with Microsphaeropsis sp. applied either as a foliar postharvest spray or as a ground application at 90% leaf fall. All fungal isolates, except Ophiostoma sp., were recovered from the leaf material that overwintered in the orchard. All treatments, except those with Ophiostoma sp., resulted in a significant reduction in V. inaequalis ascospore production on the leaf disks incubated under controlled conditions or in the orchard. In 1997, leaves with apple scab lesions treated with urea or Microsphaeropsis sp. produced significantly fewer ascospores of V. inaequalis than did nontreated leaves, with a reduction of 73.0 and 76.3%, respectively. In 1998, leaves treated with Microsphaeropsis sp., urea, Trichoderma sp., A. bombacina, and M. arundinis reduced ascospore production by 84.3, 96.6, 75.2, 96.6, and 52.2%, respectively. Based on all tests combined, the most efficient isolate was Microsphaeropsis sp. Postharvest applications of Microsphaeropsis sp. reduced the total amount of airborne ascospores trapped by 70.7 and 79.8% as compared with the nontreated plots in 1997 and 1998, respectively. Microsphaeropsis sp. provided a significant and consistent reduction in ascospore production in all tests.
Effect of Bactericides on Population Sizes and Spread of Clavibacter michiganensis subsp. michiganensis on Tomatoes in the Greenhouse and on Disease Development and Crop Yield in the Field
Authors: Hausbeck M. K. , Bell J. , Medina-Mora C. , Podolsky R. , Fulbright D. W.
Published: May 2007

Chemical applications, with the exception of mancozeb, reduced population sizes and spread of Clavibacter michiganensis subsp. michiganensis among tomato seedlings in the greenhouse and impacted subsequent plant development and yield in the field. While applications of copper hydroxide, copper hydroxide/mancozeb, copper hydroxide/mancozeb (premixed 12 h before spraying), streptomycin, and streptomycin/copper hydroxide to seedlings in the greenhouse did not differ significantly from the inoculated control, the trend was for these treatments to increase the survival of inoculated transplants in the field in comparison to the inoculated control. In the field, inoculated controls produced yields that were 63% (1995) and 51% (1996) of those produced by uninoculated controls. In both years, with the exception of mancozeb in 1995, all treatments resulted in yields similar to those obtained with the uninoculated control. Plant survival and yield in the field were severely affected when transplants had a pathogen population of ≥ × 108 CFU/g of tissue. All treatments, with the exception of mancozeb, limited C. michiganensis subsp. michiganensis populations to <5.0 × 105. None of the treatments significantly reduced the incidence of fruit spotting compared with that of the inoculated control.
Expression of Endochitinase fromTrichoderma harzianumin Transgenic Apple Increases Resistance to Apple Scab and Reduces Vigor
Authors: Bolar Jyothi Prakash , Norelli John L. , Wong Kwai-Weng , Hayes Christopher K. , Harman Gary E. , Aldwinckle Herb S.
Published: May 2007

The goal of this research was to improve scab resistance of apple by transformation with genes encoding chitinolytic enzymes from the bio-control organism Trichoderma harzianum. The endochitinase gene, as cDNA and genomic clones, was transferred into apple cv. Marshall McIntosh by Agrobacterium-transformation. A total of 15 lines were identified as transgenic by NPTII enzyme-linked immunosorbent assay and polymerase chain reaction and confirmed by Southern analysis. Substantial differences in endochitinase activity were detected among different lines by enzymatic assay and western analysis. Eight lines propagated as grafted and own-rooted plants were inoculated with Venturia inaequalis. Six of these transgenic lines expressing endochitinase were more resistant than nontransformed cv. Marshall McIntosh. Disease severity compared with cv. Marshall McIntosh was reduced by 0 to 99.7% (number of lesions), 0 to 90% (percentage of leaf area infected), and 1 to 56% (conidia recovered) in the transgenic lines tested. Endochitinase also had negative effects on the growth of both inoculated and uninoculated plants. There was a significant negative correlation between the level of endochitinase production and both the amount of disease and plant growth.
Barrier to Gene Flow Between Eastern and Western Populations of Cronartium ribicola in North America
Authors: Hamelin R. C. , Hunt R. S. , Geils B. W. , Jensen G. D. , Jacobi V. , Lecours N.
Published: May 2007

The population structure of Cronartium ribicola from eastern and western North America was studied to test the null hypothesis that populations are panmictic across the continent. Random amplified polymorphic DNA markers previously characterized in eastern populations were mostly fixed in western populations, yielding high levels of genetic differentiation between eastern and western populations (φst = 0.55; θ = 0.36; P < 0.001). An unweighted pair-group method, arithmetic mean dendro-gram based on genetic distances separated the four eastern and four western populations into two distinct clusters along geographic lines. Similarly, a principal component analysis using marker frequency yielded one cluster of eastern populations and a second cluster of western populations. The population from New Mexico was clearly within the western cluster in both analyses, confirming the western origin of this recent introduction. This population was completely fixed (Hj = 0.000; n = 45) at all loci suggesting a severe recent population bottleneck. Genetic distances were low among populations of western North America (0.00 to 0.02) and among eastern populations (0.00 to 0.02), indicating a very similar genetic composition. In contrast, genetic distances between eastern and western populations were large, and all were significantly different from 0 (0.07 to 0.19; P < 0.001). Indirect estimates of migration were high among western populations, including the number of migrants among pairs of populations (Nm > 1) between New Mexico and British Columbia populations, but were smaller than one migrant per generation between eastern and western populations. These results suggest the presence of a barrier to gene flow between C. ribicola populations from eastern and western North America.
Influence of Temperature and Leaf Wetness Duration on Infection of Strawberry Leaves by Mycosphaerella fragariae
Authors: Carisse O. , Bourgeois G. , Duthie J. A.
Published: May 2007

In controlled environment studies, the influence of temperature and wetness duration on infection of strawberry leaves by Mycosphaerella fragariae was quantified by inoculating plants with a conidial suspension and incubating them at various combinations of temperature (5 to 35°C) and leaf wetness duration (0 to 96 h). Infection was expressed as the number of lesions per square centimeter of leaf surface and relative infection was used to develop an infection model. Younger leaves were more susceptible to infection. Regardless of temperature and duration of leaf wetness, only few lesions developed on the oldest (19 to 21 days old) and intermediate leaves (12 to 15 days old), respectively (maximum of 1.7 and 2.3 lesions per cm2) as compared to the youngest leaves (5 to 7 days old; maximum of 12.6 lesions per cm2). On the youngest leaves, lesions developed at all temperatures except at 35°C, and the number of lesions, for all leaf wetness durations, increased gradually from 5 to 25°C and decreased sharply from 25 to 30°C. For temperatures of 15 and 20°C, the number of lesions increased gradually when leaf wetness duration increased from 12 to 96 h. At 25°C, the number of lesions increased with increasing leaf wetness from 12 to 48 h and then at a higher rate from 48 to 96 h. The optimal temperature for infection was 25°C. For most temperatures, a minimum of 12 h of leaf wetness was necessary for infection (more than 1 lesion per cm2). Relative infection was modeled as a function of both temperature and wetness duration using a modified version of the Weibull equation (R 2 = 0.98). The resulting equations provided a precise description of the response of M. fragariae to temperature. The model was sufficiently flexible to account for most characteristics of the response of M. fragariae to wetness duration. The model was used to construct a risk chart that can be used to estimate the potential risk for infection based on observed or forecasted temperature and leaf wetness duration.
A PCR-Based Assay by Sequence-Characterized DNA Markers for the Identification and Detection of Aphanomyces euteiches
Authors: Vandemark G. J. , Kraft J. M. , Larsen R. C. , Gritsenko M. A. , Boge W. L.
Published: May 2007

Polymerase chain reaction (PCR) products were identified and amplified from isolates of Aphanomyces euteiches and A. cochlioides. The products were cloned and sequenced, and the data were used to design pairs of extended PCR primers to amplify sequence-characterized DNA markers. The primer pair OPC7-FS-30 and OPC7-RS-25 amplified a single 1,332-bp product from all isolates of A. euteiches that were not amplified from any other isolates tested. A single 718-bp product was selectively amplified only from isolates of A. cochlioides with the primer pair OPB10-FS-25 and OPB10-RS-25. A. euteiches was detected in roots of several varieties of field-grown peas collected from a root rot trial site. PCR also detected A. euteiches in the organic fraction of field soil samples. Both pairs of extended primers were used in a multiplex reaction to unambiguously discriminate between A. euteiches and A. cochlioides. Both pairs of primers were used in two-step PCR reactions in which annealing and extension was done in a single step at 72°C. This reduced the time required for amplification of the diagnostic PCR product and its resolution by electrophoresis to less than 3 h.
A Target-Site-Specific Screening System for Antifungal Compounds on Appressorium Formation inMagnaporthe grisea
Authors: Oh Hong-Sik , Lee Yong-Hwan
Published: May 2007

Chemical fungicides are a major method of control for plant diseases in spite of potential negative effects on the environment and the appearance of resistant strains. Development of new chemical fungicides has been largely dependent upon in vivo efficacy tests in the greenhouse or in fields, which is in contrast to target-oriented in vitro screening systems widely used in the pharmaceutical field. To establish a target-site—specific screening system for antifungal compounds, specific inhibition on appressorium formation of the rice blast fungus Magnaporthe grisea was employed. For many plant-pathogenic fungi, including M. grisea, appressorium formation is an essential step to infect host plants. Among 1,000 culture filtrates of members of the class Actinomycetes and fungi, five (A5005, A5008, A5314, A5387, and A5397) from the class Actinomycetes showed differential inhibitory effects on appressorium formation of M. grisea in a dosage-dependent manner. Three (A5005, A5314, and A5387) of these were further fractionated into ethyl acetate and water fractions. The ethyl acetate fraction of A5005 and both the ethyl acetate and water fractions from A5314 and A5387 inhibited appressorium formation, while conidial germination remained little affected. Inhibition of appressorium formation by the ethyl acetate or water fraction was reversed by the exogenous addition of cyclic AMP. Significantly reduced numbers of conidia with appressoria were observed on rice leaves in the presence of culture filtrates. Furthermore, these culture filtrates also exhibited significant disease control of rice blast in the greenhouse. This rapid and target-oriented screening system could be adopted to screen candidate compounds for rice blast control and could be applicable for other appressorium-forming, plant-pathogenic fungi.
Species-Dependent Effects of Border Cell and Root Tip Exudates on Nematode Behavior
Authors: Zhao Xiaowen , Schmitt Mark , Hawes Martha C.
Published: May 2007

Effects of border cell and root tip exudates on root knot nematode (Meloidogyne incognita) behavior were examined. In whole-plant assays using pea, M. incognita second-stage juveniles (J2) accumulated rapidly around the 1- to 2-mm apical region ensheathed by border cells, but not in the region of elongation. Within 15 to 30 min, J2 which had accumulated within detached clumps of border cells lost motility and entered into a quiescent state. When border cells (and associated root tip exudates) were washed from pea roots prior to challenge with nematodes, no such accumulation and quiescence was induced. Attraction of nematodes by roots was species dependent: no attraction or accumulation occurred in snap bean. Using a quantitative assay, three categories of chemotaxis responses occurred: attraction (pea and alfalfa cv. Thor), repulsion (alfalfa cv. Moapa 69), and no response (snap bean and alfalfa cv. Lahonton). In contrast, total root tip exudates from all three plant species acted as a repellent for M. incognita in the sand assay. An in vitro assay was developed to characterize the induced quiescence response. When total root tip exudate from the tested legumes (as well as corn) was incubated with J2 populations, >80% of the nematodes lost motility. A similar response occurred in Caenorhabditis elegans. Border cell exudates did not induce or contribute to the induction of quiescence. Cocultivation of pea border cells with M. incognita resulted in changes in border cell shape similar to those observed in response to exogenous plant hormones. No such changes occurred in snap bean border cells. Understanding the cell- and host-specific extracellular recognition that occurs between roots and pathogenic nematodes in the early stages before infection occurs could lead to new avenues for disease control.
Yield Loss in Chickpeas in Relation to Development of Fusarium Wilt Epidemics
Authors: Navas-Cortés Juan A. , Hau Bernhard , Jiménez-Díaz Rafael M.
Published: May 2007

Development of 108 epidemics of Fusarium wilt of chickpea caused by Fusarium oxysporum f. sp. ciceris were studied on cvs. P-2245 and PV-61 in field microplots artificially infested with races 0 and 5 of F. oxysporum f. sp. ciceris in 1986 to 1989. Disease progression data were fitted to the Richards model using nonlinear regression. The shape parameter was influenced primarily by date of sowing and, to a lesser extent, by chick-pea cultivars and races of F. oxysporum f. sp. ciceris. Fusarium wilt reduced chickpea yield by decreasing both seed yield and seed weight. These effects were related to sowing date, chickpea cultivar, and virulence of the prevalent F. oxysporum f. sp. ciceris race. Regression models were developed to relate chickpea yield to Fusarium wilt disease intensity with the following independent variables: time to initial symptoms (tis), time to inflection point (tip) of the disease intensity index (DII) progress curve, final DII (DIIfinal), standardized area under DII progress curve (SAUDPC), and the Richards weighted mean absolute rate of disease progression (rho). Irrespective of the chickpea cultivar × pathogen race combination, the absolute and relative seed yields decreased primarily by delayed sowing. The relative seed yield increased with the delay in tis and tip and decreased with increasing DIIfinal, SAUDPC, and rho. A response surface as developed in which seed yield loss decreased in a linear relationship with the delay in tis and increased exponentially with the increase of rho.
Recovery of Mutants Impaired in Pathogenicity After Transposition ofImpalainFusarium oxysporumf. sp.melonis
Authors: Migheli Quirico , Steinberg Christian , Davière Jean-Michel , Olivain Chantal , Gerlinger Catherine , Gautheron Nadine , Alabouvette Claude , Daboussi Marie-Josée
Published: May 2007

The ability of transposon impala to inactivate genes involved in pathogenicity was tested in Fusarium oxysporum f. sp. melonis. Somatic excision of an impala copy inserted in the nitrate reductase-encoding niaD gene was positively selected through a phenotypic assay based on the restoration of nitrate reductase activity. Independent excision events were analyzed molecularly and shown to carry reinsertedimpala in more than 70% of the cases. Mapping of reinserted impala elements on large NotI-restriction fragments showed that impala transposes randomly. By screening 746 revertants on plants, a high proportion (3.5%) of mutants impaired in their pathogenic potential was recovered. According to the kinetics of wilt symptom development, the strains that were impaired in pathogenicity were clustered in three classes: class 1 grouped two strains that never induced Fusarium wilt symptoms on the host plant; class 2 and class 3 grouped 15 and 9 revertants which caused symptoms more than 50 and 30 days after inoculation, respectively. The first results demonstrate the efficiency of transposition in generating mutants affected in pathogenicity, which are usually difficult to obtain by classical mutagenesis, and open the possibility to clone the altered genes with impala as a tag.
Effects of Planting Density and the Composition of Wheat Cultivar Mixtures on Stripe Rust: An Analysis Taking into Account Limits to the Replication of Controls
Authors: Garrett K. A. , Mundt C. C.
Published: May 2007

The effect of plant density on disease is not well understood in populations of a single host plant genotype and has been studied even less in mixtures of host genotypes. We performed an experiment to evaluate the effect of wheat planting density on infection by Puccinia striiformis in experimental plots with a single wheat genotype and in plots with two genotypes making up a range of frequencies. Stripe rust severity in single-genotype plots increased with planting density in 1997 but decreased with planting density in 1998. Disease in host mixtures was compared to the weighted mean of disease levels in the corresponding single-genotype plots. The design of the field experiment included limited replication of these reference treatments (that is, there was not a unique pair of single-genotype plots for each mixture plot); therefore, we devised an analysis based on collapsing the data into independent mean observations. Disease reduction due to host diversity was less when one genotype predominated than when both host genotypes were present at nearly equal frequencies. The greatest mean host-diversity effect for reduced disease was at the intermediate planting density of 250 seeds per m2.
Effects of Infection by Mycosphaerella graminicola on Translocation of Fluquinconazole in Wheat Seedlings
Authors: Metcalfe Richard J. , Shaw Michael W. , Russell Philip E.
Published: May 2007

Translocation of 14C-labeled fluquinconazole was measured using combustion analysis and radio thin-layer-chromatographic analysis in seedling wheat leaves uninfected and infected with Mycosphaerella graminicola. Two isolates were used with differing sensitivity to demethylation inhibitor fungicides. Fluquinconazole was translocated acropetally, but not basipetally. Fluquinconazole accumulated around infection sites within 6 days after treatment. Accumulation occurred before M. graminicola hyphae had colonized the host mesophyll further than one host cell around the invasion site. This suggested that the accumulation was caused by a host response to infection. Infrared gas analysis showed that rates of transpiration and stomatal conductance in inoculated leaves were significantly increased very soon after inoculation but net photosynthesis was decreased. The actual mechanism of fungicide accumulation was not determined.
Survival of Ralstonia solanacearum Biovar 2, the Causative Agent of Potato Brown Rot, in Field and Microcosm Soils in Temperate Climates
Authors: van Elsas Jan Dirk , Kastelein Pieter , van Bekkum Petra , van der Wolf Jean M. , de Vries Philippine M. , van Overbeek Leo S.
Published: May 2007

After outbreaks of potato brown rot in three different fields in the Netherlands, the fate of the brown rot pathogen, Ralstonia solanacearum biovar 2, was monitored in soil by immunofluorescence colony staining (IFC) supported by R. solanacearum division-2 specific polymerase chain reaction. In selected areas of all fields, the R. solanacearum population densities were initially on the order 104 to 106 per g of topsoil. These population densities then declined progressively over time. In two fields, however, the pathogen persisted for periods of 10 to 12 months. The survival of a selected R. solanacearum biovar 2 isolate, strain 1609, in three soils, a loamy sand and two different silt loam soils, was further studied in soil microcosm experiments. The effects of temperature and soil moisture content were assessed. At 12 or 15 and 20°C, a gradual decline of the population densities was observed in all three soils, from the established 105 to 106 CFU g-1 of dry soil to significantly reduced levels, occasionally bordering the limit of detection (102 CFU g-1of dry soil), in periods of approximately 90 to 210 days. Soil type affected the rate of population decline at 20°C, with the greatest decline occurring in loamy sand soil. In all three soils, the survival of IFC-detectable R. solanacearum 1609 cells at 4°C was severely impaired, reflected in an accelerated decline of CFU counts, to undetectable numbers. Moreover, indications were found for the occurrence of viable but nonculturable strain 1609 cells in the loamy sand as well as in one silt loam soil under these conditions. In addition, a single freezing-thawing cycle caused a significant additional reduction of the culturable R. solanacearum 1609 populations in the three soils, though detectable populations remained. Moderate soil moisture fluctuations of approximately pF 2 did not affect the survival of R. solanacearum 1609 in soil. Severe drought, however, drastically reduced the populations of strain 1609 CFU in all three soils.
Neural Network Classification of Tan Spot and Stagonospora Blotch Infection Periods in a Wheat Field Environment
Authors: De Wolf E. D. , Francl L. J.
Published: May 2007

Tan spot and Stagonospora blotch of hard red spring wheat served as a model system for evaluating disease forecasts by artificial neural networks. Pathogen infection periods on susceptible wheat plants were measured in the field from 1993 to 1998, and incidence data were merged with 24-h summaries of accumulated growing degree days, temperature, relative humidity, precipitation, and leaf wetness duration. The resulting data set of 202 discrete periods was randomly assigned to 10 modeldevelopment or -validation (n = 50) data sets. Backpropagation neural networks, general regression neural networks, logistic regression, and parametric and nonparametric methods of discriminant analysis were chosen for comparison. Mean validation classification of tan spot incidence was between 71% for logistic regression and 76% for backpropagation models. No significant difference was found between methods of modeling tan spot infection periods. Mean validation prediction accuracy of Stagonospora blotch incidence was 86 and 81% for backpropagation and logistic regression, respectively. Prediction accuracies of other modeling methods were ≤78% and were significantly different (P = 0.01) from backpropagation, but not logistic regression, results. The best backpropagation models of tan spot and Stagonospora blotch incidences correctly classified 82 and 84% of validation cases, respectively. High classification accuracy and consistently good performance demonstrate the applicability of neural network technology to plant disease forecasting.
Evaluation of Proposed Amended Names of Several Pseudomonads and Xanthomonads and Recommendations
Authors: Schaad N. W. , Vidaver A. K. , Lacy G. H. , Rudolph K. , Jones J. B.
Published: May 2007

In 1980, over 90% of all plant-pathogenic pseudomonads and xanthomonads were lumped into Pseudomonas syringae and Xanthomonas campestris, respectively, as pathovars. The term “pathovar” was created to preserve the name of plant pathogens, but has no official standing in nomenclature. Proposals to elevate and rename several pathovars of the genera Pseudomonas and Xanthomonas to the rank of species has caused great confusion in the literature. We believe the following changes have merit and expect to adopt them for publication in a future American Phytopathological Society Laboratory Guide for Identification of Plant Pathogenic Bacteria. Upon review of published data and the Rules of The International Code of Nomenclature of Bacteria, we make the following recommendations. We reject the proposal to change the name of P. syringae pvs. phaseolicola and glycinea to P. savastanoi pvs. phaseolicola and glycinea, respectively, because both pathogens are easily differentiated phenotypically from pv. savastanoi and convincing genetic data to support such a change are lacking. We accept the elevation of P. syringae pv. savastanoi to the rank of species. We accept the reinstatement of X. oryzae to the rank of species with the inclusion of X. oryzicola as a pathovar of X. oryzae and we accept the species X. populi. We agree with the elevation of the pvs. cassavae, cucurbitae, hyacinthi, pisi, and translucens to the rank of species but not pvs. melonis, theicola, and vesicatoria type B. We recommend that all type A X. vesicatoria be retained as X. campestris pv. vesicatoria and all type B X. vesicatoria be named X. exitiosa. We reject the newly proposed epithets arboricola, bromi, codiaei (poinsettiicola type B), hortorum, sacchari, and vasicola and the transfer of many pathovars of X. campestris to X. axonopodis. The proposed pathovars of X. axonopodis should be retained as pathovars of X. campestris.
Competitive Fitness of Phytophthora infestans Isolates Under Semiarid Field Conditions
Authors: Miller J. S. , Johnson D. A.
Published: May 2007

Spread of US-1 and US-8 isolates of Phytophthora infestans were observed in field plots of potato (cv. Russet Burbank) grown in Pullman, WA, in 1996 and 1997. Infected greenhouse-grown potato plants with similar lesion numbers for both strains were transplanted to field plots with four replications. Spread of the pathogen was favored by sprinkler irrigation during evening hours. Diseased leaves and stems were sampled over time to determine the spread of US-1 and US-8 isolates. In 1996, late blight developed in two of the four replications (105 and 87 total isolates recovered). From those two replications, two US-1 isolates were recovered, both from the same replication. Nine isolates from one replication and six isolates from another displayed a phenotype different from the initial isolates, as determined by compatibility type, allozyme genotype, and restriction fragment length polymorphism genotype. These putative recombinant isolates may have arisen from sexual recombination between the US-1 and US-8 isolates. The remaining isolates were of the US-8 strain. In 1997, late blight developed in all four replications (123, 122, 81, and 34 total isolates recovered). One US-1 isolate was recovered (out of 123) from one replication and three (out of 122) from another, and the remaining isolates were of the US-8 strain. Isolates with phenotypes differing from the initial isolates were not recovered in 1997. In both years, oospores were not observed in the plant tissue examined. The low number of putative recombinant isolates in 1996 and their absence in 1997 suggests that sexual reproduction between US-8 and US-1 isolates in a field setting is a rare event. The predominance of US-8 isolates recovered is a measure of the increased fitness and aggressiveness of the US-8 isolates relative to the US-1 isolate used in this study. This further substantiates the increased aggressiveness of the US-8 genotype observed on excised tissues and potted plants in previous laboratory and greenhouse studies.
Microbe-Mediated Germination of Ascospores of Monosporascus cannonballus
Authors: Stanghellini M. E. , Kim D. H. , Waugh M.
Published: May 2007

Ascospores of Monosporascus cannonballus germinated readily in the rhizosphere of cantaloupe plants growing in field soil. However, little or no germination occurred in the rhizosphere of melon plants growing in field soil that was autoclaved prior to infestation with ascospores. The latter data suggested that root exudates alone do not stimulate ascospore germination and that the soil microflora may be involved in the induction of ascospore germination. Amending field soil with streptomycin (which inhibits gram-negative microorganisms) did not suppress ascospore germination in the rhizosphere of cantaloupe plants. However, amending the soil with penicillin (which inhibits gram-positive microorganisms) did suppress ascospore germination. Pentachloronitrobenzene (PCNB), which inhibits the gram-positive actinomycetes but does not inhibit gram-positive or gram-negative bacteria, also suppressed ascospore germination. These results suggest that actinomycetes, either directly or indirectly, are involved in the induction of ascospore germination in field soil in the presence of exudates from cantaloupe roots. Optimum germination occurred at temperatures ranging from 25 to 35°C, and data indicate that a high percentage (≥72%) of the ascospore population within 500 μm of a root are capable of germination and subsequent penetration of cantaloupe roots.
Inheritance of Resistance to Aspergillus Ear Rot and Aflatoxin Production of Corn from Tex6
Authors: Hamblin A. M. , White D. G.
Published: May 2007

The inheritance of resistance to Aspergillus ear rot and aflatoxin production in corn (Zea mays L.) caused by Aspergillus flavus was studied in progeny derived from crosses between the resistant corn inbred cv. Tex6 and susceptible inbred cvs. B73 and Mo17. From 1994 to 1996, plant generations included were the P1(susceptible B73 or Mo17), P2(resistant Tex6), F1, F2, F3, BCP1, BCP1-selfed, and BCP2. The BCP2-selfed generation was added in 1995 and 1996 for the B73 × Tex6 cross. Primary ears were pinboard inoculated and evaluated for Aspergillus ear rot severity. F1means deviated from the midparent value toward resistance for aflatoxin production and toward susceptibility for ear rot in both crosses. Analyses of generation means indicate that additive gene action was most important in the resistance to both ear rot and aflatoxin production in the B73 × Tex6 cross. Mo17 was somewhat resistant to both traits, so resistance from Tex6 was not well defined in this cross. Broad-sense heritabilities for ear rot and aflatoxin production were 58 and 63% for Mo17 × Tex6, and 66 and 65% for B73 × Tex6. Narrow-sense heritabilities for ear rot and aflatoxin production were 39 and 45% for B73 × Tex6. It is estimated that one cycle of selection for resistance within B73 × Tex6 F3families would reduce the percentage of ear rot severity by 8.5% and aflatoxin concentration by 19 ng/g.
A Rapid Bioassay for Screening Rhizosphere Microorganisms for Their Ability to Induce Systemic Resistance
Authors: Han D. Y. , Coplin D. L. , Bauer W. D. , Hoitink H. A. J.
Published: May 2007

We developed a rapid and miniaturized bioassay for screening large numbers of rhizosphere microorganisms for their ability to induce systemic resistance to bacterial leaf spot of radish caused by Xanthomonas campestris pv. armoraciae. In this bioassay, Pantoea agglomerans strain E278Ar controlled symptoms of disease as effectively as 2,6-dichloroisonicotinic acid when applied to the roots of seedlings produced in growth pouches in a soilless system. E278Ar essentially did not migrate from seedling roots to the foliage. This suggests that induction of systemic resistance could best explain the observed reduction in disease severity. Three mini-Tn5Km-induced mutants of strain E278Ar were isolated that had lost the ability to induce resistance. The bioassay also was used to demonstrate that the fungal biocontrol agent Trichoderma hamatum strain 382 induces systemic resistance in radish. The bioassay required only 14 to 18 days from seeding until rating for disease severity, which is 10 to 14 days less than earlier bioassays.
Biological Control of Pseudomonas syringae pv. syringae, the Causal Agent of Basal Kernel Blight of Barley, by Antagonistic Pantoea agglomerans
Authors: Braun-Kiewnick Andrea , Jacobsen Barry J. , Sands David C.
Published: May 2007

Strains of Pantoea agglomerans (synanamorph Erwinia herbicola) suppressed the development of basal kernel blight of barley, caused by Pseudomonas syringae pv. syringae, when applied to heads prior to the Pseudomonas syringae pv. syringae infection window at the soft dough stage of kernel development. Field experiments in 1994 and 1995 revealed 45 to 74% kernel blight disease reduction, whereas glasshouse studies resulted in 50 to 100% disease control depending on the isolate used and barley cultivar screened. The efficacy of biocontrol strains was affected by time and rate of application. Percentage of kernels infected decreased significantly when P. agglomerans was applied before pathogen inoculation, but not when coinoculated. A single P. agglomerans application 3 days prior to the pathogen inoculation was sufficient to provide control since populations of about 107 CFU per kernel were established consistently, while Pseudomonas syringae pv. syringae populations dropped 100-fold to 2.0 × 104 CFU per kernel. An application to the flag leaf at EC 49 (before heading) also reduced kernel infection percentages significantly. Basal blight decreased with increasing concentrations (103 to 107 CFU/ml) of P. agglomerans, with 107 CFU/ml providing the best control. For long-term preservation and marketability, the survival of bacterial antagonists in several wettable powder formulations was tested. Over all formulations tested, the survival declined between 10- to >100-fold over a period of 1.5 years (r = -0.7; P = 0.000). Although not significant, storage of most formulations at 4°C was better for viability (90 to 93% survival) than was storage at 22°C (73 to 79%). However, long-term preservation had no adverse effect on biocontrol efficacy.
Evolution of Virulence in Natural Populations of the Satellite RNA of Cucumber mosaic virus
Authors: Escriu Fernando , Fraile Aurora , García-Arenal Fernando
Published: May 2007

From 1986 to 1992, an epidemic of tomato necrosis caused by Cucumber mosaic virus (CMV) plus CMV satellite RNAs (satRNAs) occurred in eastern Spain. From 1989 onward, the frequency of tomato necrosis di-minshed, and it almost completely disappeared after 1991. Analyses of plants infected with CMV and with CMV satRNA and of the phenotype (necrogenic or nonnecrogenic for tomato) induced by some CMV satRNA variants, showed that the disappearance of tomato necrosis was due to changes in the genetic composition of the satRNA population (i.e., to its evolution toward decreased virulence). Analysis of components of the fitness of satRNA variants, necrogenic or nonnecrogenic for tomato, showed that necrogenic and nonnecrogenic variants did not differ in infectivity or in their accumulation level in tomato and that they represented the same fraction of encapsidated RNA. Other fitness components were positively correlated with the greater virulence of necrogenic variants, in that they were favored in mixed infections with nonnecrogenic variants and were more effectively passed into CMV progeny than were nonnecrogenic variants. On the other hand, necrogenic CMV satRNA variants caused a more pronounced depression in the accumulation of CMV than did nonnecro-genic variants, which could affect the efficiency of aphid transmission. Thus, the evolution of virulence in the CMV satRNA population can be explained by trade-offs between factors that determine virulence and factors that affect transmission, as predicted by theoretical models on the evolution of virulence in parasites.
Relationships Between Several Quadrat-Based Statistical Measures Used to Characterize Spatial Aspects of Disease Incidence Data
Authors: Ridout M. S. , Xu X.-M.
Published: May 2007

This article investigates the relationships between various statistical measures that are used to summarize spatial aspects of disease incidence data. The focus is on quadrat data in which each plant in a quadrat is classified as diseased or healthy. We show that spatial autocorrelation plays a central role via the mean intraclass correlation, ρ, which is defined as the average correlation of the disease status of all pairs of plants within the quadrat. The value of ρ determines the variance of the number of infected plants in the quadrat and, if this variable follows a beta-binomial distribution, the heterogeneity parameter of the beta-binomial distribution is directly related to the mean intraclass correlation. We consider in detail a model in which the spatial autocorrelation depends only on the distance between the plants. For illustration, we consider a specific autocorrelation model that was derived from simulated data. We show that this model leads, approximately, to the binary form of the power law relating the variance of the number of infected plants per quadrat to the mean. Using an approximation technique, we then show how the index of dispersion is related to quadrat size and shape. The index of dispersion increases with quadrat size. The rate of increase is dependent on quadrat shape, but the effect of quadrat shape is small in comparison to the effect of quadrat size. Finally, we note that if the spatial autocorrelation depends on the relative orientation of the plants, as well as the distance between them, there are connections with distance class methods.