Field Resistance to Strobilurin (Q(o)I) Fungicides in Pyricularia grisea Caused by Mutations in the Mitochondrial Cytochrome b Gene

Laboratory Evaluation of Three Rapid, Agar-Based Assays to Assess Fungicide Sensitivity in Monilinia fructicola

Three rapid, agar-based assays were compared with a traditional petri dish method for assessing the sensitivity of Monilinia fructicola to propiconazole (0.3 and 2.0 μg/ml), thiophanate-methyl (1.0 and 50 μg/ml), and azoxystrobin (1.0 and 35 μg/ml) in the laboratory. The three assays were based on mycelial growth inhibition on agar disks sliced from lipbalm tubes filled with fungicide-amended potato dextrose agar (PDA), on PDA-coated cotton swabs, or in PDA-filled microcentrifuge tubes. Mycelial growth inhibition of eight previously characterized isolates (two resistant to propiconazole, two highly resistant to thiophanate-methyl, two with low levels of resistance to thiophanate-methyl, and two sensitive to all three fungicides) was determined visually 24, 48, and 72 h after inoculation. The 48-h time point was the earliest suitable time to collect data for all methods because insufficient growth was recorded in the petri dish and tube assays after 24 h. With the exception of the swab assay, all methods classified the isolates previously determined to be fungicide sensitive correctly (i.e., no fungal growth was observed for these isolates). For propiconazole-resistant isolates, the lipbalm assay resulted in levels of growth inhibition very similar to the petri dish method, whereas the swab assay and the tube assay overestimated and underestimated, respectively, the level of resistance. Both the lipbalm and the swab assays classified isolates correctly as being thiophanate-methyl resistant, and both were able to discriminate the isolates previously classified as having low versus high levels of resistance when treated with this fungicide at 50 μg/ml, as was the petri dish method. None of the eight isolates which previously were determined to be azoxystrobin sensitive grew on azoxystrobin-amended media, regardless of the assay type. Overall, the average percentage of correct isolate classifications (relative to their previously determined resistance status) on propiconazole- and thiophanate-methyl-amended media after 48 h ranged from 87.5 to 100, 85.3 to 100, 63.2 to 94.5, and 50.5 to 81.0% for the petri dish, lipbalm, swab, and tube assays, respectively. The lipbalm assay provided the most accurate assessments (85.3 to 100%) after only 24 h of incubation, supporting its use as a rapid and simple tool to monitor resistance levels in M. fructicola field populations.

Baseline Sensitivity of Ascochyta rabiei to Azoxystrobin, Pyraclostrobin, and Boscalid

Ascochyta rabiei, causal agent of Ascochyta blight on chickpea (Cicer arietinum), can cause severe yield loss in the United States. Growers rely on applications of fungicides with site-specific modes of action such as the quinone outside inhibiting (QoI) fungicides azoxystrobin and pyraclostrobin, and the carboximide fungicide boscalid, to manage disease. In all, 51 isolates collected prior to QoI fungicide registration and 71 isolates collected prior to boscalid registration in the United States were tested in an in vitro assay to determine the effective fungicide concentration at which 50% of conidial germination was inhibited (EC₅₀) for each isolate-fungicide combination. The effect of salicylhydroxamic acid (SHAM) on conidia of A. rabiei in the presence and absence of azoxystrobin also was assessed to determine whether the fungus is capable of using alternative respiration. Five of nine A. rabiei isolates tested had significantly higher (P ≤ 0.05) EC₅₀ values when SHAM was not included in media amended with azoxystrobin, indicating that A. rabiei has the potential to use alternative respiration to overcome fungicide toxicity in vitro. EC₅₀ values of azoxystrobin and pyraclostrobin ranged from 0.0182 to 0.0338 μg/ml and from 0.0012 to 0.0033 μg/ml, with mean values of 0.0272 and 0.0023 μg/ml, respectively. EC₅₀ values of boscalid ranged from 0.0177 to 0.4960 μg/ml, with a mean of 0.1903 μg/ml. Establishment of these baselines is the first step in developing a monitoring program to determine whether shifts in sensitivity to these fungicides are occurring in the A. rabiei pathogen population.

Baseline and Differential Sensitivity to Two QoI Fungicides Among Isolates of Phytophthora cactorum That Cause Leather Rot and Crown Rot on Strawberry

Sensitivities of 89 isolates of Phytophthora cactorum, the causal agent of crown rot and leather rot on strawberry plants, from seven states (Florida, Maine, North Carolina, Ohio, Oregon, South Carolina, and New York) to the QoI fungicide azoxystrobin were determined based on mycelium growth and zoospore germination. Radial growth of mycelia on lima bean agar amended with azoxystrobin at 0.001, 0.01, 0.1, 1.0, 10, and 30 μg/ml and salicylhydroxamic acid (SHAM) at 100 μg/ml was measured after 6 days. Effect on zoospore germination was evaluated in aqueous solutions of azoxystrobin at 0.005, 0.01, 0.05, 0.1, 0.5, and 1.0 μg/ml in 96-well microtiter plates by counting germinated and nongerminated zoospores after 4 h at room temperature. SHAM was not used to evaluate zoospore sensitivity. The effective dose to reduce mycelium growth by 50% (ED₅₀) ranged from 0.16 to 12.52 μg/ml for leather rot isolates and 0.10 to 15 μg/ml for crown rot isolates. The Kolmogorov-Smirnov test showed significant differences (P < 0.001) between the two distributions. Zoospores were much more sensitive to azoxystrobin than were mycelia. Differences between sensitivity distributions for zoospores from leather rot and crown rot isolates were significant at P = 0.05. Estimated ED₅₀ values ranged from 0.01 to 0.24 μg/ml with a median of 0.04 μg/ml. Experiments with pyraclostrobin, another QoI fungicide, demonstrated that both mycelia and zoospores of P. cactorum were more sensitive to pyraclostrobin than to azoxystrobin. Sensitivities to azoxystrobin and pyraclostrobin were moderately but significantly correlated (r = 0.60, P = 0.0001).

Occurrence and Distribution of QoI-Resistant Isolates of Colletotrichum cereale from Annual Bluegrass in California

Turfgrass anthracnose, caused by Colletotrichum cereale (ex. Colletotrichum graminicola), is an important disease of turf used on golf course putting greens. Recent management of the disease has become increasingly difficult, partly due to the possible development of practical resistance to the QoI fungicides. In all, 558 single-conidia isolates of C. cereale were collected from 10 California golf courses, 8 of which had been exposed to QoI fungicides and 2 where no fungicides had been used. Isolates were tested using a mycelial expansion assay on azoxystrobinamended media. For the two nonexposed populations, in vitro 50% effective dose (ED₅₀) values ranged from 0.0060 to 0.089 μg/ml. All isolates from the exposed populations could not be fully inhibited by doses of azoxystrobin as high as 8.0 μg/ml. A subset of these isolates were tested in vitro with the QoI fungicides pyraclostrobin and trifloxystrobin and found to be similar in response, indicating that these isolates were fully cross-resistant to all three fungicides. In greenhouse pot experiments, three isolates nonresponsive to QoI fungicides in vitro were not controlled by label rates of the fungicides. Spore germination assays also were examined; for 10 isolates identified as sensitive by mycelial expansion assays, ED₅₀ values for axoystrobin ranged from 0.0040 to 0.0047 μg/ml; for 25 isolates identified as QoI-resistant, 93 to 100% of the conidia germinated at azoxystrobin concentrations as high as 8.0 μg/ml relative to the nonamended check treatments. Mitochondrial cytochrome b genes from a subset of 15 isolates (12 resistant and 3 sensitive) were partially cloned and sequenced; all resistant isolates had an alanine substitution that corresponded to position 143 of the gene product. These results indicate that QoI resistance is present in California populations of C. cereale and is contributing to the difficulty in controlling this disease.

Molecular Characterization and Diagnosis of QoI Resistance in Cucumber and Eggplant Fungal Pathogens

ABSTRACT The molecular mechanism of QoI fungicide resistance was studied using isolates of cucumber Corynespora leaf spot fungus (Corynespora cassiicola) and the eggplant leaf mold (Mycovellosiella nattrassii). In both pathogens, a mutation at position 143 from glycine to alanine (G143A) was detected in the cytochrome b gene that encodes for the fungicide-targeted protein. Moreover, the nucleotide sequence at amino acid position 143 was converted from GGT or GGA in sensitive (wild-type) to GCT or GCA in resistant (mutant-type) isolates. The methods of polymerase chain reaction restriction fragment length polymorphism commonly used for QoI resistance monitoring were employed successfully, leading to the amplified gene fragment from resistant isolates being cut with the restriction enzyme ItaI. However, heteroplasmy (the coexistence of wild-type and mutated alleles) was found when the resistant isolates of C. cassiicola, M. nattrassii, and Colletotrichum gloeosporioides (strawberry anthracnose fungus) were subcultured in the presence or absence of QoI fungicides. QoI resistance of cucumber powdery and downy mildew isolates persisted for a few years following the removal of the selection pressure imposed by the fungicide under both laboratory and commercial greenhouse conditions. The proportion of mutated sequences in cytochrome b gene decreased over time in the pathogen population. The protective efficacy of the full dose of azoxystrobin decreased when the populations of powdery and downy mildews contained resistant isolates at 10%. Using FMBIO, a fluorescence bio-imaging analyzer, the mutant allele from the QoI-resistant isolates could be detected at the level of 1%, whereas the detection sensitivity of ethidium-bromide-stained gels was approximately 10 times lower.

At least two origins of fungicide resistance in grapevine downy mildew populations

Quinone outside inhibiting (QoI) fungicides represent one of the most widely used groups of fungicides used to control agriculturally important fungal pathogens. They inhibit the cytochrome bc1 complex of mitochondrial respiration. Soon after their introduction onto the market in 1996, QoI fungicide-resistant isolates were detected in field plant pathogen populations of a large range of species. However, there is still little understanding of the processes driving the development of QoI fungicide resistance in plant pathogens. In particular, it is unknown whether fungicide resistance occurs independently in isolated populations or if it appears once and then spreads globally by migration. Here, we provide the first case study of the evolutionary processes that lead to the emergence of QoI fungicide resistance in the plant pathogen Plasmopara viticola. Sequence analysis of the complete cytochrome b gene showed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Phylogenetic analysis of a large mitochondrial DNA fragment including the cytochrome b gene (2,281 bp) across a wide range of European P. viticola isolates allowed the detection of four major haplotypes belonging to two distinct clades, each of which contains a different QoI fungicide resistance allele. This is the first demonstration that a selected substitution conferring resistance to a fungicide has occurred several times in a plant-pathogen system. Finally, a high population structure was found when the frequency of QoI fungicide resistance haplotypes was assessed in 17 French vineyards, indicating that pathogen populations might be under strong directional selection for local adaptation to fungicide pressure.

Cytochrome b gene sequence and structure of Pyrenophora teres and P. tritici-repentis and implications for QoI resistance

Resistance to QoI fungicides in Pyrenophora teres (Dreschsler) and P. tritici-repentis (Died.) Dreschsler was detected in 2003 in France and in Sweden and Denmark respectively. Molecular analysis revealed the presence of the F129L mutation in resistant isolates of both pathogens. In 2004, the frequency of the F129L mutation in populations of both pathogens further increased. The G143A mutation was also detected in a few isolates of P. tritici-repentis from Denmark and Germany. In 2005, the F129L mutation in P. teres increased in frequency and geographical distribution in France and the UK but remained below 2% in Germany, Switzerland, Belgium and Ireland. In P. tritici-repentis, both mutations were found in a significant proportion of the isolates from Sweden, Denmark and Germany. The G143A mutation conferred a significantly higher level of resistance (higher EC50 values) to Qo inhibitors (QoIs) than did the F129L mutation. In greenhouse trials, resistant isolates with G143A were not well controlled on plants sprayed with recommended field rates, whereas satisfactory control of isolates with F129L was achieved. For the F129L mutation, three different single nucleotide polymorphisms (SNPs), TTA, TTG and CTC, can code for L (leucine) in P. teres, whereas only the CTC codon was detected in P. tritici-repentis isolates. In two out of 250 isolates of P. tritici-repentis from 2005, a mutation at position 137 (G137R) was detected at very low frequency. This mutation conferred similar resistance levels to F129L. The structure of the cytochrome b gene of P. tritici-repentis is significantly different from that of P. teres: an intron directly after amino acid position 143 was detected in P. teres which is not present in P. tritici-repentis. This gene structure suggests that resistance based on the G143A mutation may not occur in P. teres because it is lethal. No G143A isolates were found in any P. teres populations. Although different mutations may evolve in P. tritici-repentis, the G143A mutation will have the strongest impact on field performance of QoI fungicides.

Multiple displacement amplification, a powerful tool for molecular genetic analysis of powdery mildew fungi

Powdery mildew fungi (Erysiphales) are probably the largest group of plant pathogens that remain uncharacterized from genetic and molecular points of view, with the only exception of the powdery mildew of cereals, Blumeria graminis. Their nature as obligate biotrophic parasites and consequent inability to grow on culture media has significantly hampered research. A common bottleneck to the molecular genetic analysis of powdery mildew fungi is the availability of genomic DNA of suitable quality and in sufficient quantity. The so-called whole genome amplification technology has the potential to overcome this limitation. Here we present the application of phi29 DNA polymerase-mediated multiple displacement amplification (MDA) to amplify the whole genome of Podosphaera fusca, the main causal agent of powdery mildew in cucurbits, to address this problem. The genome coverage and fidelity of the MDA process was evaluated by PCR amplification and sequencing of two genetics markers: the nuclear rDNA internal transcribed spacer (ITS) regions and the mitochondrial cytochrome b gene (CYTB). Our results show that MDA is a valuable tool for molecular genetic analysis of powdery mildew fungi that can be used for a number of downstream applications in different fields, such as epidemiology and population genetics or systematics.

Cytochrome b gene structure and consequences for resistance to Qo inhibitor fungicides in plant pathogens

The cytochrome b (cyt b) gene structure was characterized for different agronomically important plant pathogens, such as Puccinia recondita f sp tritici (Erikss) CO Johnston, P graminis f sp tritici Erikss and Hennings, P striiformis f sp tritici Erikss, P coronata f sp avenae P Syd & Syd, P hordei GH Otth, P recondita f sp secalis Roberge, P sorghi Schwein, P horiana Henn, Uromyces appendiculatus (Pers) Unger, Phakopsora pachyrhizi Syd & P Syd, Hemileia vastatrix Berk & Broome, Alternaria solani Sorauer, A alternata (Fr) Keissl and Plasmopara viticola (Berk & Curt) Berlese & de Toni. The sequenced fragment included the two hot spot regions in which mutations conferring resistance to QoI fungicides may occur. The cyt b gene structure of these pathogens was compared with that of other species from public databases, including the strobilurin-producing fungus Mycena galopoda (Pers) P Kumm, Saccharomyces cerevisiae Meyer ex Hansen, Venturia inaequalis (Cooke) Winter and Mycosphaerella fijiensis Morelet. In all rust species, as well as in A solani, resistance to QoI fungicides caused by the mutation G143A has never been reported. A type I intron was observed directly after the codon for glycine at position 143 in these species. This intron was absent in pathogens such as A alternata, Blumeria graminis (DC) Speer, Pyricularia grisea Sacc, Mycosphaerella graminicola (Fuckel) J Schröt, M fijiensis, V inaequalis and P viticola, in which resistance to QoI fungicides has occurred and the glycine is replaced by alanine at position 143 in the resistant genotype. The present authors predict that a nucleotide substitution in codon 143 would prevent splicing of the intron, leading to a deficient cytochrome b, which is lethal. As a consequence, the evolution of resistance to QoI fungicides based on G143A is not likely to evolve in pathogens carrying an intron directly after this codon.

Magnaporthe oryzae isolates causing gray leaf spot of perennial ryegrass possess a functional copy of the AVR1-CO39 avirulence gene

SUMMARY Gray leaf spot of perennial ryegrass (Lolium perenne) is a severe foliar disease caused by the ascomycete fungus Magnaporthe oryzae (formerly known as Magnaporthe grisea). Control of gray leaf spot is completely dependent on the use of fungicides because currently available perennial ryegrass cultivars lack genetic resistance to this disease. M. oryzae isolates from perennial ryegrass (prg) were unable to cause disease on rice cultivars CO39 and 51583, and instead triggered a hypersensitive response. Southern hybridization analysis of DNA from over 50 gray leaf spot isolates revealed that all of them contain sequences corresponding to AVR1-CO39, a host specificity gene that confers avirulence to rice cultivar CO39, which carries the corresponding resistance gene Pi-CO39(t). There was also an almost complete lack of restriction site polymorphism at the avirulence locus. Cloning and sequencing of the AVR1-CO39 gene (AVR1-CO39(Lp)) from 16 different gray leaf spot isolates revealed just two point mutations, both of which were located upstream of the predicted open reading frame. When an AVR1-CO39(Lp) gene copy was transferred into ML33, a rice pathogenic isolate that is highly virulent to rice cultivar CO39, the transformants were unable to cause disease on CO39 but retained their virulence to 51583, a rice cultivar that lacks Pi-CO39(t). These data demonstrate that the AVR1-CO39 gene in the gray leaf spot pathogens is functional, and suggest that interaction of AVR1-CO39(Lp) and Pi-CO39(t) is responsible, at least in part, for the host specificity expressed on CO39. This indicates that it may be possible to use the Pi-CO39(t) resistance gene as part of a transgenic strategy to complement the current deficiency of gray leaf spot resistance in prg. Furthermore, our data indicate that, if Pi-CO39(t) can function in prg, the resistance provided should be broadly effective against a large proportion of the gray leaf spot pathogen population.

Molecular characterization and biological response to respiration inhibitors of Pyricularia isolates from ctenanthe and rice plants

The molecular profile and the biological response of isolates of Pyricularia oryzae Cavara obtained from ctenanthe to two strobilurins (azoxystrobin, kresoxim-methyl) and the phenylpyridinamine fungicide fluazinam were characterized, and compared with isolates from rice plants. Five different isozymes (alpha-esterase, lactate, malate, isocitrate and sorbitol dehydrogenases) and five random decamer primers for RAPD-PCR were used to generate molecular markers. Using unweighted pair-group with arithmetic average analysis, ctenanthe isolates were found to form a separate group distinct from that of the rice isolates for both sets of markers. Amplified polymorphic sequences of mitochondrial cytochrome b that were digested with Fnu4HI or StyI revealed no differences among Pyricularia isolates at amino acid positions 143 or 129 which confer resistance to strobilurins in several fungi. In absence of the alternative respiration inhibitor salicylhydroxamic acid (SHAM) the three fungicides showed inferior and variable efficacy, with a trend toward the rice isolate being less sensitive. The addition of SHAM enhanced the effectiveness of all fungicides against isolates regardless of their origin. Appressorium formation was the most vulnerable target of action of the respiration inhibitors and azoxystrobin the most effective. This is the first report of a comparison between the molecular profiles and sensitivities to respiration inhibitors for Pyricularia oryzae isolates from a non-gramineous host and from rice.

Telomere hypervariability in Magnaporthe oryzae

SUMMARY The gray leaf spot disease of perennial ryegrass and tall fescue is caused by the fungus Magnaporthe oryzae (anamorph = Pyricularia oryzae). A collection of single-copy and repetitive DNA markers was used to investigate genetic diversity among 22 isolates of the gray leaf spot pathogen. The single-copy DNA markers revealed only three polymorphisms among 95 restriction fragments spanning approximately 0.6% of the genome. In addition, Southern hybridization analysis and mating tests revealed that all isolates possessed the MAT1-2 mating-type allele. Fingerprinting of repetitive DNA loci using the Pot2 and MGR583 probes also revealed a high degree of genetic similarity (> 85%) among isolates. These data are consistent with the gray leaf spot pathogens having a recent evolutionary origin. In contrast to the results obtained with probes for internal chromosome loci, a telomere probe revealed that the chromosome ends of the very same isolates are highly divergent, with most isolates sharing less than 20% fingerprint similarity with any other isolate. Telomere mutations arise extremely frequently and changes in telomere fingerprint profiles were readily observed during vegetative growth and among cultures derived from single spores isolated from agar medium and from lesions on perennial ryegrass leaves.

Effect of the F129L Mutation in Alternaria solani on Fungicides Affecting Mitochondrial Respiration

Isolates of Alternaria solani previously collected from throughout the Midwestern United States and characterized as being azoxystrobin sensitive or reduced sensitive were tested for sensitivity to the Quinone outside inhibitor (Q_oI) fungicides famoxadone and fenamidone and the carboxamide fungicide boscalid. All three fungicides affect mitochondrial respiration: famoxadone and fenamidone at complex III, and boscalid at complex II. A. solani isolates possessing reducedsensitivity to azoxystrobin also were less sensitive in vitro to famoxadone and fenamidone compared with azoxystrobin-sensitive isolates, but the shift in sensitivity was of lower magnitude, approximately 2- to 3-fold versus approximately 12-fold for azoxystrobin. The in vitro EC₅₀ values, the concentration that effectively reduces germination by 50% relative to the untreated control, for sensitive A. solani isolates were significantly lower for famoxadone and azoxystrobin than for fenamidone and boscalid; whereas, for reduced-sensitive isolates, famoxadone EC₅₀ values were significantly lower than all other fungicides. Isolates of A. solani with reducedsensitivity to azoxystrobin were twofold more sensitive in vitro to boscalid than were azoxystrobin-sensitive wild-type isolates, displaying negative cross-sensitivity. All isolates determined to have reduced-sensitivity to azoxystrobin also were determined to possess the amino acid substitution of phenylalanine with leucine at position 129 (F129L mutation) using real-time polymerase chain reaction. In vivo studies were performed to determine the effects of in vitro sensitivity shifts on early blight disease control provided by each fungicide over a range of concentrations. Reduced-sensitivity to azoxystrobin did not significantly affect disease control provided by famoxadone, regardless of the wide range of in vitro famoxadone EC₅₀ values. Efficacy of fenamidone was affected by some azoxystrobin reduced-sensitive A. solani isolates, but not others. Boscalid controlled azoxystrobin-sensitive and reduced-sensitive isolates with equal effectiveness. These results suggest that the F129L mutation present in A. solani does not convey cross-sensitivity in vivo among all Q_oI or related fungicides, and that two- to threefold shifts in in vitro sensitivity among A. solani isolates does not appreciably affect disease control.

Sensitivity of California Isolates of Uncinula necator to Trifloxystrobin and Spiroxamine, and Update on Triadimefon Sensitivity

Sensitivities of Uncinula necator to spiroxamine and trifloxystrobin were established by assay of 36 and 35 isolates, respectively, recovered from California grape vineyards in 2002 and increased as single-spore lines for laboratory testing. Twenty-nine single-spore isolates also were evaluated for levels of sensitivity to the fungicide triadimefon to determine if there had been a reversion to sensitivity following the development of resistance in 1986. Although triadimefon use was limited after 1992, other demethylation inhibitor (DMI) fungicides (fenarimol and myclobutanil) were used extensively in California vineyards. For spiroxamine, the sample mean value of the median effective concentration (EC50 value) was 365 μg/liter (95% confidence interval [CI] from 251 to 531 μg/liter) and values were distributed log-normally. The corresponding mean for trifloxystrobin was 12.8 μg/liter bounded by 8.9 to 18.5 μg/liter for the 95% CI. State-wide, the triadimefon mean EC₅₀ was 8.8 mg/liter, bounded by a 5.3 to 14.5 mg/liter 95% CI, and those values were significantly higher than those obtained in the last assay 12 years earlier. Significant differences in sensitivity of U. necator to triadimefon were detected at a regional scale by comparison of mean EC₅₀ values of frequency distributions representative of regions within California, although the relations between those regions were different from the prior survey.

Shift in Sensitivity of Alternaria solani in Response to QoI Fungicides

Isolates of Alternaria solani, cause of potato early blight, collected in 1998 through 2001 from various potato growing areas across the midwestern United States, were tested for sensitivity to azoxystrobin. Isolates collected in 1998, prior to the introduction of azoxystrobin, were tested to establish the baseline sensitivity of the fungus to this fungicide. Isolates collected in subsequent years, not necessarily from the same sites as baseline isolates, were tested to determine if populations of A. solani had become less sensitive to azoxystrobin. Azoxystrobin sensitivity was determined utilizing an in vitro spore germination assay. The effective fungicide concentration that inhibited spore germination by 50% (EC₅₀) was determined for each isolate. There was no significant difference in mean EC₅₀ values between baseline isolates and all other isolates collected through 1999. Mean azoxystrobin EC₅₀ values of A. solani isolates collected in 2000 and 2001 were significantly higher compared with means from previous years, and mean azoxystrobin EC₅₀ values from 2001 were significantly higher than means from isolates collected in 2000. A subset of 54 A. solani isolates was evaluated in vitro for cross-sensitivity to pyraclostrobin and trifloxystrobin. A highly significant and strong correlation among the isolates tested for fungicide cross-sensitivity was detected between azoxystrobin and pyraclostrobin; however, the correlation between azoxystrobin and trifloxystrobin, and between trifloxystrobin and pyraclostrobin, was significant but weak. A second subset of five isolates was chosen for in vivo assessment of azoxystrobin, pyraclostrobin, and trifloxystrobin sensitivity. Disease severity on plants treated with azoxystrobin and pyraclostrobin was significantly greater with reduced-sensitive A. solani isolates compared with sensitive isolates. Disease severity was not statistically different between azoxystrobin reduced-sensitive and sensitive A. solani isolates on plants treated with trifloxystrobin. This is the first report of a shift in sensitivity to Q_oI fungicides in a fungus possessing only an anamorphic stage.

Essential role of urease in pathogenesis of gastritis induced by Helicobacter pylori in gnotobiotic piglets

A mutant strain of Helicobacter pylori with weak urease activity was created by using N-methyl-N'-nitro-N-nitrosoguanidine. The urease activity of the mutant (0.036 +/- 0.009 nmol of urea per micrograms of bacterial protein per min) was 0.4% of that of the parental strain (8.20 +/- 2.30 nmol of urea per micrograms of bacterial protein per min). The mutant was otherwise indistinguishable from the parental strain. Both demonstrated prominent catalase and oxidase activities, and both produced vacuolating cytotoxin. Restriction endonuclease and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) patterns and ultrastructure were identical for the two strains. The mutant was fully motile, as evaluated by spreading in soft agar and by direct microscopic examination. Growth rate and colony size and morphology were identical for the mutant and parental strains. Seventeen gnotobiotic piglets were challenged with either the mutant or the parental strain and sacrificed 3 or 21 days after challenge. Gastric tissue was examined histologically and cultured for H. pylori. Of seven piglets challenged with the parental strain, all became infected. H. pylori was not recovered from any of 10 piglets challenged with the urease-negative strain. Lymphofollicular gastritis was present in all seven piglets challenged with the parental strain but in none of the piglets challenged with the urease-negative strain. These results suggest that prominent urease activity is essential for colonization by H. pylori.