Browsing by Author "Baudoin, Antonius B."

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    2021 Horticultural and Forest PMG - Authors
    Askew, Shawn D.; Baudoin, Antonius B.; Bergh, J. Christopher; Chamberlin, Lori; Dary, Eric R.; Del-Pozo, Alejandro; Derr, Jeffrey F.; Frank, Daniel; Hansen, Mary Ann; Hong, Chuan X.; Johnson, Charles S.; Laub, Curtis A.; McCall, David S.; Nita, Mizuho; Parson, Rachel; Peer, Kyle; Pfeiffer, Douglas G.; Richardson, Robert J.; Salom, Scott M.; Schultz, Peter B.; Wilson, James (Virginia Cooperative Extension, 2021-02-12)
    Horticultural and Forest Crops 2021 Author Contact List
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    2021 Horticultural and Forest PMG - Grapes
    Pfeiffer, Douglas G.; Baudoin, Antonius B.; Bergh, J. Christopher; Nita, Mizuho (Virginia Cooperative Extension, 2021-02-12)
    This is a chapter from the 2021 Horticulture and Forest Pest Management Guide. The Virginia Pest Management Guide (PMG) series lists options for management of major pests: diseases, insects, nematodes, and weeds. These guides are produced by Virginia Cooperative Extension and each guide is revised annually. PMG recommendations are based on research conducted by the Research and Extension Division of Virginia Tech, in cooperation with other land-grant universities, the USDA, and the pest management industry.
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    Abiotic stressors in the dogwood anthracnose complex
    Crozier, James Brooks (Virginia Tech, 1994-03-15)
    Acidic precipitation reportedly enhances disease severity of dogwood anthracnose (DA) caused by Discula destructiva, on Cornus florida, the flowering dogwood. Seedlings were subjected to acidic fog episodes at pHs 2.5, 3.5, 4.5, and 5.5, using a simulated acidic rain solution. Leaf discs from these and non-treated plants were examined by scanning electron microscopy (SEM). Damage was noted at all pH levels. Discula destructiva conidia may germinate at trichome bases where damage may cause the leaching of nutrients. Also, the difference in stomatal damage may account, in part, for differences in disease susceptibility. Cardinal growth temperatures and response to thermal stress regimes were determined for isolates of Discula destructiva. This information may lead to an understanding of possible climatic barriers, and the thermal treatment of plant material.
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    Accurate identification and grouping of Rhizoctonia isolates infecting turfgrasses in MD and VA and their sensitivity to selected fungicides in vitro
    Amaradasa, Bimal Sajeewa (Virginia Tech, 2011-07-19)
    Rhizoctonia blight (sensu lato) is a common and serious disease of many turfgrass species. The most widespread causal agent R. solani consists of several genetically different anastomosis groups (AGs) and subgroups. Though anastomosis or hyphal fusion reactions have been used to group Rhizoctonia species, they are time consuming and sometimes difficult to interpret. Anastomosis reactions are incapable of identifying isolates belonging to different AG subgroups within an AG. This study evaluated molecular techniques in comparison with traditional anastomosis grouping (AG) to identify and group isolates of Rhizoctonia. More than 400 Rhizoctonia isolates were collected from diseased turfgrass leaves from eight geographic areas in Virginia and Maryland. A random sample of 86 isolates was selected and initially characterized by colony morphology, nuclei staining and anastomosis grouping. Molecular identification was performed by analysis of rDNA-ITS region and DNA fingerprinting techniques universally primed PCR (UP-PCR) and amplified fragment length polymorphism (AFLP). The cladistic analysis of ITS sequences and UP-PCR fragments supported seven clusters. Isolates of R. solani AG 1-IB (n=18), AG 2-2IIIB (n=30) and AG 5 (n=1) clustered separately. Waitea circinata var. zeae (n=11), and var. circinata (n=4) grouped separately. A cluster of six isolates (UWC) did not fall into any known Waitea group. Most of the binucleate Rhizoctonia-like fungi (BNR) (n=16) grouped separately. AFLP grouping also largely agreed with the above results. However, UWC isolates clustered into two groups. Molecular analyses corresponded well with traditional anastomosis grouping by clustering isolates within an AG or AG subgroup together. UP-PCR cross-hybridization could distinguish closely related Rhizoctonia isolates to their infraspecies level. Genetically related isolates belonging to the same AG subgroups cross-hybridized strongly, while isolates of different AGs did not cross-hybridize or did so weakly. Sequence-characterized amplified region (SCAR) markers were generated from UP-PCR products to identify isolates of major pathogenic groups AG 1-IB and AG 2-2IIIB. Specific primer pairs successfully distinguished isolates of AG 1-IB and AG 2-2IIIB from isolates of other AGs. Sensitivity of Rhizoctonia species and AGs was tested in vitro to commercial formulations of iprodione, triticonazole and pyraclostrobin. W. circinata isolates were moderately sensitive to iprodione while isolates of R. solani and BNR were extremely sensitive. Isolates of AG 2-2IIIB showed less sensitivity to triticonazole than other Rhizoctonia isolates. W. circinata var. zeae isolates were moderately sensitive to pyraclostrobin while most of the other isolates were extremely sensitive.
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    Biological Control Agent Rhizobium vitis, ARK-1 Reduces Incidence and Severity of Grapevine Crown Gall in Virginia
    Wong, Alexander Thomas (Virginia Tech, 2018-08-06)
    Crown gall of grapevine (Vitis spp.) is a serious and economically important disease caused by the bacterial pathogen Rhizobium vitis, which transforms healthy plant cell genomes leading to hypertrophic and hyperplastic growth of affected plant cells. Recent studies have documented a strong inhibitory effect against Japanese tumorigenic R. vitis isolates by a newly identified non-tumorigenic strain of R. vitis, ARK-1. We conducted co-inoculation assays in tomato (Solanum lycopersicum) and wine grape cultivars (Vitis vinifera) with four tumorigenic isolates of R. vitis from Virginia. These tumorigenic isolates were co-inoculated with ARK-1 in various ratios and resulting gall incidence and gall size were measured. Analysis was conducted with the generalized linear mixed model (GLIMMIX) in SAS (ver. 9.4). ARK-1 significantly reduced both the mean probability of gall formation and the mean gall size (P < 0.05). ARK-1 efficacy against combinations of two or four tumorigenic isolates and up to twice as many cells of tumorigenic isolates was also significant. However, there was an indication of a loss of efficacy when ARK-1 was challenged with four isolates at four times the cell number of ARK-1. Also, the efficacy of ARK-1 was influenced by both the specific isolate and host plant used in the study. Our results suggest that ARK-1 has promising potential as an effective biological control agent for grapevine crown gall in the United States.
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    Biological control of the invasive Ailanthus altissima (tree-of-heaven) in Virginia using naturally occurring Verticillium wilt fungi
    Brooks, Rachel Keys (Virginia Tech, 2020-06-08)
    The invasive tree-of-heaven, Ailanthus altissima (Miller) Swingle, is widespread and damaging throughout North America. Verticillium wilt disease is emerging as a potentially exciting biological control option for this difficult to control tree. In Virginia, Verticillium nonalfalfae has been confirmed causing significant mortality to A. altissima, while V. dahliae is suspected to be present and causing lower levels of disease. Little else is known regarding these two fungal species in this state. The purpose of this research was to gain a better understanding of how Verticillium wilt impacts A. altissima and its potential as a biological control agent. We first confirmed V. dahliae's presence in Virginia and its pathogenicity to A. altissima using Koch's postulates. We then completed a regional field-inoculation experiment to show that V. nonalfalfae effectively kills and spreads to adjacent A. altissima, regardless of V. dahliae presence or other climate and stand variables. Additionally, we showed that V. dahliae causes lower levels of disease than V. nonalfalfae, and does not spread rapidly. Next, we surveyed all Virginia A. altissima stands known to be naturally infected with V. nonalfalfae to determine whether V. nonalfalfae persists long-term, that it considerably reduces A. altissima numbers, and that its local prevalence may be higher than initially suspected. However, we were unable to infect A. altissima seedlings using soil collected at these infested sites, suggesting that V. nonalfalfae's survival within field soil may be limited. Lastly, using paired A. altissima invaded-uninvaded sites, we found that A. altissima presence is associated with a decreased proportion of native plants and species in the woody and herbaceous understory, but not the germinable seedbank. Furthermore, we found that this impact on the woody understory appears to increase over time, supporting early management actions and helping us predict post-management restoration needs. We conclude that V. nonalfalfae has a high potential of successfully limiting A. altissima throughout Virginia, supporting its registration as a biopesticide.
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    Biological watermelon (Citrullus lanatus L.) seed treatments for control of Acidovorax citrulli
    Klein, Rachel (Virginia Tech, 2020-06-03)
    Acidovorax citrulli is a seedborne pathogen responsible for bacterial fruit blotch (BFB), an economically important disease in melon and watermelon throughout the world. BFB is highly virulent and in affected fields can cause yield reduction of up to 95%, which has resulted in over $100,000 in losses to melon growers in some cases. The efficacy of green tea as an antimicrobial seed treatment against A. citrulli was tested. Watermelon seeds were treated with green tea after inoculation with transgenic A. citrulli expressing green fluorescent protein (GFP). Forty five percent of watermelon seedlings inoculated with a high level (OD600:1.0, ~8 x 108 cells/ml) of A. citrulli displayed GFP in their cotyledons. When these seeds were treated with green tea, only 11.2% displayed GFP in their cotyledons. None of the treated watermelon seedlings inoculated with a low level (OD600:0.001, ~8 x 105 cells/ml) of A. citrulli displayed GFP in their cotyledons. Green tea treatments effectively controlled the disease when administered as a liquid to infected watermelon seeds. Green tea has potential as an effective commercial treatment for pericarp infected seeds that could also be used by growers participating in the National Organic Program.
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    Causal factors of Macrophoma rot observed on Petit Manseng grapes
    Encardes, Nicole A. (Virginia Tech, 2020-06-22)
    Macrophoma rot is a general term for fruit rots of Vitis spp. caused by the fungus Neofusicoccum ribis (syn. Botryosphaeria ribis) or closely related or renamed taxa, including Botryosphaeria dothidea. While mainly observed as a fruit pathogen of muscadine grape, the disease has recently been observed on bunch grapes in Virginia. Isolates (N = 835) were collected from Petit Manseng fruit clusters from seven Virginia vineyards in 2018 and 2019. A subset of these isolates was sequenced using three primer sets (ITS, RPB2, and EF). The preliminary result showed a single taxonomic strain of N. ribis. A controlled inoculation study of Petit Manseng clusters verified that infection could occur anytime between bloom and 2 weeks post-veraison; however, both the mean cluster incidence and the severity of Macrophoma rot did not differ from each other at any growth stage during the season. A season-long cluster exposure experiment showed that any amount of sun exposure significantly increased Macrophoma rot severity compared to shaded clusters, and that full sun exposure was associated with greatest rot severity. This finding contravenes current management recommendations for Macrophoma rot, and it raises yet unanswered questions as to why exposed clusters are more susceptible to Macrophoma rot than are shaded clusters. An in vitro fungicide assay study using nine fungicides identified captan, thiophanate-methyl, and tetraconazole as potential candidates for management of Macrophoma rot which need to be investigated further.
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    Characterization of Phytophthora Species in Recycled Irrigation Water at a Container Nursery in Southwestern Virginia
    Bush, Elizabeth A. (Virginia Tech, 2002-05-03)
    The potential of increasing disease problems through the use of recycled irrigation water in horticultural operations is a serious concern, yet basic research on waterborne plant pathogens in Virginia is lacking. In this work seasonal fluctuations and locations of Pythiaceae in a recycled water irrigation system at a container nursery were determined. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Species of Phytophthora recovered in filtering assays were identified as P. capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae. P. cryptogea and P. drechsleri were the only Phytophthora spp. recovered from baits placed on the surface of the irrigation reservoir, whereas a greater diversity of species was recovered from baits placed at depths. Hymexazol-amended medium was found to have limitations in recovery of Phytophthora spp. In pathogenicity tests, P. cactorum, P. capsici, P. citrophthora, and P. nicotianae caused significant mortality of Salvia officinalis and P. cactorum showed limited pathogenicity on Gerbera jamesonii. Asymptomatic (aboveground) plants were found to harbor inoculum long after Phytophthora-inoculation. Fresh weight analyses of roots and shoots of asymptomatic plants demonstrated that Phytophthora inoculation may either reduce or stimulate plant shoot growth, but little effect is apparent on roots. Irrigation with naturally infested irrigation water reduced plant growth. This research provides data for prioritizing development of detection technology and management practices for plant pathogens in irrigation water. The results may also lead to improvements in conventional water assay protocols for plant pathogens.
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    Characterization of Chitinase Activity and Gene Expression in Muskmelon Seeds
    Zou, Xiaohong (Virginia Tech, 2000-11-16)
    Chitinase has been suggested to play a role in defense mechanisms. In this study, the activity and expression of chitinase in muskmelon seeds were investigated. Multiple chitinase isoforms were detected in muskmelon seeds from early development through radicle emergence. One acidic and three basic chitinase isoforms were detected in developing seeds at 40 days after anthesis (DAA). Both acidic and basic chitinase isoforms were detected in endosperm tissue during seed imbibition and after radicle emergence. Basic chitinase isoforms, but not acidic isoforms, were detected in embryo tissue. Basic chitinase isoforms were also detected in the embryonic axis or radicle tissue. Taken together, these observations indicate that chitinases are regulated developmentally and in a tissue-specific manner in muskmelon seeds. Therefore the potential function of chitinases in muskmelon seeds is discussed. Two complete cDNAs, Cmchi1 and Cmchi2, and a partial genomic clone of Cmchi2 have been isolated from muskmelon seeds. Cmchi2 gene has two introns in the coding region while Cmchi1 is intronless. Cmchi1 cDNA encodes a class III chitinase while Cmchi2 cDNA encodes a class II chitinase. Cmchi1 and Cmchi2 proteins might be targeted to secretory pathways because they possess signal peptides. Southern blotting suggested that there is at least one additional gene similar to Cmchi1 in the muskmelon seed genome, while there is only one copy of Cmchi2. Northern blotting analysis showed that both Cmchi1 and Cmchi2 are expressed in the radicle tissue at the time of radicle emergence. This indicates that the expression is regulated developmentally and in a tissue-specific manner. Salicylic acid (SA) and benzothiadiazole (BTH) stimulated the expression of Cmchi1 but not Cmchi2 in seeds after radicle emergence, indicating that SA might be involved in inducing the expression of Cmchi1, while a different signal might be involved in triggering the expression of Cmchi2. The protein encoded by Cmchi1cDNA was expressed in E.coli. It did not show any enzymatic activity. Western blotting using an antibody raised against the class III chitinase protein in cucumber was inconclusive, as this antibody recognized the purified Cmchi1 fusion protein and other unknown proteins isolated from the embryonic axis or the radicle tissue.
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    Characterization of fungicide resistance in grape powdery and downy mildew using field trials, bioassays, genomic, and transcriptomic approaches: quinoxyfen, phosphite, and mandipropamid
    Feng, Xuewen (Virginia Tech, 2018-02-06)
    Development of fungicide resistance in fungal and oomycete pathogens is a serious problem in grape production. Quinoxyfen is a fungicide widely used against grape powdery mildew (Erysiphe necator). In 2013, E. necator isolates with reduced quinoxyfen sensitivity (designated as quinoxyfen lab resistance or QLR) were detected in Virginia. Field trials were conducted in 2014, 2015, and 2016 at the affected vineyard to determine to what extent quinoxyfen might still contribute to disease control. Powdery mildew control by quinoxyfen was good, similar to, or only slightly less, than that provided by myclobutanil and boscalid in all three years. The frequency of QLR in vines not treated with quinoxyfen declined only slowly over the three years, from 65% to 46%. Information about the mode of action of quinoxyfen is limited; previous research suggests that quinoxyfen interferes with the signal transduction process. We profiled the transcriptomes of QLR and sensitive isolates in response to quinoxyfen treatment, providing support for this hypothesis. Additional transcriptional targets of quinoxyfen were revealed to be involved in the positive regulation of the MAPK signaling cascade, pathogenesis, and sporulation activity. Grape downy mildew (Plasmopara viticola), another important grape pathogen, is commonly controlled by phosphite fungicides. A field trial and laboratory bioassays were conducted to determine whether P. viticola isolates from vineyards with suspected control failures showed reduced sensitivity against phosphite fungicides. Prophyt applied at 14-day intervals under high disease pressure provided poor downy mildew control in the field. Next-generation sequencing technologies were utilized to identify 391,930 single nucleotide polymorphisms (SNPs) and generated a draft P. viticola genome assembly at ~130 megabase (Mb). Finally, field isolates of P. viticola collected from a Virginia vineyard with suspected mandipropamid control failure were bioassayed. The EC50 values of the isolates were >240 μg.ml-1 for mandipropamid, well above the field rate. The PvCesA3 gene of two resistant isolates was sequenced revealing that these isolates had a GGC-to-AGC substitution at codon 1105, the same mutation that has been found associated with CAA resistance elsewhere.
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    Characterization of Fungicide Resistance in Venturia inaequalis Populations in Virginia
    Marine, Sasha Cahn (Virginia Tech, 2012-03-28)
    Apple scab (causal organism: Venturia inaequalis) is an economically devastating disease of apples that is predominantly controlled with fungicides. Of the chemical classes currently available, the sterol-inhibiting (SI) and strobilurin (QoI) fungicides are the most commonly used. Recent observations indicate that V. inaequalis populations in Virginia have developed resistance to myclobutanil and other SIs. However, little is known about the frequency and distribution of SI and QoI resistance in Virginia's scab populations. The first objective of this research was to evaluate V. inaequalis populations in Virginia for SI and QoI resistance. Fungal isolates were collected from experimental orchards at the Alson H. Smith Jr., Agricultural Research and Extension Center (AHS AREC) and from commercial orchards in Virginia and Maryland. Sensitivities were determined by assessing colony growth at 19°C on potato dextrose agar (PDA) amended with 0 or 1.0 µg ml-1 of myclobutanil (SI) (N=87) or trifloxystrobin (QoI) (N=25) at 28 days. A range of fungicide sensitivity was observed for both chemical classes. The second objective of this research was to monitor the temporal dynamics of SI resistance over five sequential field seasons. To monitor shoot growth, neon rubber bands were placed over actively growing shoot tips following myclobutanil application or sample collection. Fungal isolates were collected from the same trees from 2007 through 2010 (N=176) and compared with isolates collected from wild apple seedlings (N=3). A continuum of SI resistance was observed for each year, and the V. inaequalis population exhibited a baseline shifted toward reduced sensitivity. The third objective of this research was to examine the spatial distribution of SI fungicide resistance within the tree canopy in a lower-density orchard (less than 150 trees A-1). Leaves collected from larger trees (>8m) in a lower-density orchard at the AHS AREC were analyzed for manganese deposition, pre- and post-mancozeb application. Fungal isolates (N=105) were collected from several locations within the canopy in replicated trees in the same orchard. Weather sensors also monitored the microclimates within those tree canopies. Spray deposition, microclimate and SI resistance were influenced by canopy location. The fourth objective of this research was to investigate potential SI resistance mechanisms. Previously classified isolates were screened for point mutations within the CYP51A1 gene (Appendix C), differences in polymorphic bands (alleles) (Appendix D), and differences in metabolism of myclobutanil (Appendix E). The consensus sequences for the CYP51A1 gene were identical for all isolates tested (N=9), and results from amplified fragment length polymorphism experiment (N=82) were inconclusive. There were, however, significant differences among incubation time and myclobutanil concentration in the bioassay (N=11). Our results indicate that myclobutanil is still an effective compound for control of apple scab in many areas of Virginia.
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    Characterizing resistance of Erysiphe necator to fungicides belonging to the quinone outside inhibitors and demethylation inhibitors
    Rallos, Lynn Esther E. (Virginia Tech, 2013-01-21)
    Practical resistance of Erysiphe necator to quinone outside inhibitors (QoIs) is now widespread, and resistance to demethylation inhibitors (DMIs) has also developed.  The goal of this research was to characterize fungicide resistance by elucidating resistance mechanisms and determining its stability.  QoI resistance persisted for several years in a field population after QoI application ended.  Resistant isolates were highly competitive in mixed populations in competition assays under laboratory conditions, indicating a lack of fitness cost.  In one competition trial under field conditions, resistance frequency declined, possibly due to spore migration and influx of background inoculum, but in a second trial, it did not decline.  Double resistance to QoI and DMI was detected and DMI application may have been partially responsible for maintaining QoI resistance in the field.  One isolate with QoI resistance but an undetectable level of the major QoI mutation was shown to be heteroplasmic -- resistant strains could be selected from this isolate. DMI resistance mechanisms in E. necator included the Y136F mutation in CYP51 and cyp51 over-expression.  The first mechanism was present in almost all isolates with substantial levels of resistance, and cyp51 expression level was correlated with resistance level.  Three cyp51 genotypes were detected.  Wildtype isolates with the TAT genotype were sensitive to DMIs, while isolates with increased resistance had either a TTT or TWT genotype; TWT indicated the presence of both wildtype and mutant alleles.  Cyp51 was expressed 1.4 to 19 times more in mutants than in wildtype.  It is not known whether the significant differences in cyp51 expression level among isolates and among genotype groups are due to gene copy number variation.  DMI resistance was found to decline after years of subculturing, and the decline appeared to occur after a few culture transfers of field samples on fungicide-free host leaves.  The observed decline, together with the finding that isolates could be "trained" to increase resistance, and may be slightly induced in cyp51 expression when successively challenged to grow in increasing fungicide concentration, indicate instability of DMI resistance.
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    Chemical and biological control of silvery threadmoss on creeping bentgrass putting greens
    Post, Angela R. (Virginia Tech, 2013-07-31)
    Silvery threadmoss is a problematic weed of golf putting greens, growing interspersed with turf, decreasing aesthetic quality and playability.  Moss is typically controlled postemergence and currently only one herbicide, carfentrazone, is registered for silvery threadmoss control on greens.  Carfentrazone controls moss up to 75% applied at a three week interval throughout the growing season.  Alternatives providing longer residual or more effective control are desirable.  Studies were conducted to examine the growth of moss gametophytes from spores and bulbils and to evaluate turf protection products for pre and postemergence moss control.  Moss gametophytes develop best from spores at 30"aC and from bulbils at 23"aC.  Products which control moss equivalent to carfentrazone (>70%) both pre and postemergent include sulfentrazone, saflufenacil, flumioxazin, oxadiazon, and oxyfluorfen.  Fosamine and fosetyl-Al alone controlled moss equivalent to carfentrazone post-, but not preemergent.  14C glyphosate absorption and translocation through moss colonies was examined from 12 to 192 hours after treatment (HAT) to understand how herbicides are absorbed by silvery threadmoss.  It appears that 14C reaches equilibrium by 24 HAT in capillary water of the moss colony and inside moss tissues.  Subsequently, 14C is lost to the system presumably through microorganism degradation of 14C glyphosate in capillary water.  The final objective of this work was to identify and evaluate two fungal organisms observed to cause disease of silvery threadmoss on putting greens in efforts to develop a biological control.  The organisms were identified by morphology and ITS sequence as Alternaria sp. and Sclerotium rolfsii.  Alternaria sp. causes a leaf disease of silvery threadmoss and Sclerotium rolfsii causes Southern blight of silvery threadmoss.  Host specificity testing demonstrated moderate pathogenicity of S. rolfsii to annual bluegrass but not to "¥Penn A4"" creeping bentgrass.  Both organisms have potential to be effective biological controls for silvery threadmoss; however, host specificity indicates Alternaria sp. may be a better choice.  Data from these experiments suggest herbicides in two chemical classes control mosses both pre and postemergence, and sulfentrazone, fosetyl-Al, and Alternaria sp. may be new alternatives to carfentrazone for use on golf putting greens.  
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    Co-Occurrence of Two Allelic Variants of CYP51 in Erysiphe necator and Their Correlation with Over-Expression for DMI Resistance
    Rallos, Lynn Esther E.; Baudoin, Antonius B. (PLOS, 2016-02-03)
    Demethylation inhibitors (DMIs) have been an important tool in the management of grapevine powdery mildew caused by Erysiphe necator. Long-term, intensive use of DMIs has resulted in reduced sensitivity in field populations. To further characterize DMI resistance and understand resistance mechanisms in this pathogen, we investigated the cyp51 sequence of 24 single-spored isolates from Virginia and surrounding states and analyzed gene expression in isolates representing a wide range of sensitivity. Two cyp51 alleles were found with respect to the 136th codon of the predicted EnCYP51 sequence: the wildtype (TAT) and the mutant (TTT), which results in the known Y136F amino acid change. Some isolates possessed both alleles, demonstrating gene duplication or increased gene copy number and possibly a requirement for at least one mutant copy of CYP51 for resistance. Cyp51 was over-expressed 1.4- to 19-fold in Y136F-mutant isolates. However, the Y136F mutation was absent in one isolate with moderate to high resistance factor. Two additional synonymous mutations were detected as well, one of which, A1119C was present only in isolates with high cyp51 expression. Overall, our results indicate that at least two mechanisms, cyp51 over-expression and the known target-site mutation in CYP51, contribute to resistance in E. necator, and may be working in conjunction with each other.
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    Comparing Digital and Visual Evaluations for Accuracy and Precision in Estimating Tall Fescue Brown Patch Severity
    Sykes, Virginia R.; Horvath, Brandon J.; Warnke, Scott E.; Askew, Shawn D.; Baudoin, Antonius B.; Goatley, James M. (2017-11)
    Brown patch (Rhizoctonia solani Kuhn), a destructive disease of tall fescue (Festuca arundinacea Schreb.), is typically evaluated visually. The subjectivity of visual evaluations may be reduced using technology like digital image analysis (DIA). This study compared DIA and visual evaluations for accuracy and precision of brown patch ratings of glasshouse grown tall fescue plants. Across four experiments, 112 plants were inoculated with R. solani. Disease was rated visually and using DIA-WP (digital image analysis whole plant canopy). In two experiments, disease evaluations were replicated using three images and three visual evaluations per pot. Absolute error was calculated as the difference between actual disease severity [calculated using an individual leaf DIA method previously quantified as highly predictive of actual brown patch disease severity on tall fescue (r(2) = 0.99)] and DIA-WP and visual evaluations, respectively. Standard deviations within repeated measures were also calculated. A mixed-model ANOVA was used to determine differences (P < 0.05) in mean absolute error and mean standard deviation by method, disease range, and method by disease range. Disease ranged from 0 to 100%. Mean absolute error did not differ between methods but did by disease range, exhibiting a bell-shaped curve from 0% to 100% disease severity. Mean standard deviation exhibited significant method by disease range interaction. Mean standard deviation did not differ across the disease range within DIA-WP evaluations but did across the disease range within visual evaluations. The more consistent precision of DIA across the disease range could reduce variability in brown patch evaluations of tall fescue.
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    Conidial production and viability of Calonectria pseudonaviculata on infected boxwood leaves as affected by temperature, wetness, and dryness periods
    Avenot, Herve F.; Baudoin, Antonius B.; Hong, Chuanxue (Wiley, 2021-10-31)
    Calonectria pseudonaviculata causes lesions on boxwood leaves and twigs. Controlled-environment experiments were conducted to determine the effects of temperature and leaf wetness period on C. pseudonaviculata sporulation on diseased (cv. Suffruticosa) leaves and of dryness periods and high temperature on conidial survival. Infected leaves were incubated in moist chambers and subjected to six temperatures (9, 13, 17, 21, 25, and 29°C) and six leaf wetness periods (0, 12, 24, 40, 48, and 72 h). Spore production was influenced significantly by wetness period, temperature, and their interaction. Increasing duration of leaf wetness and increasing temperature generally increased sporulation, with no sporulation occurring at 29°C or 9 and 13°C, except at 72 h of wetness exposure, while it was optimal at 21°C. Detached leaves with profuse conidia were subjected to a range of drying (relative humidity at 65%) times (0, 2, 4, 6, and 8 h) at two temperatures of 21 and 29°C. Conidia were then harvested and plated on water agar. Germinating conidia were counted to measure the spore viability. Spore mortality increased with increasing dryness duration at both temperatures but occurred more quickly and severely at 29 than 21°C. Overall, this study extended biological knowledge of conditions required for crucial stages of the C. pseudonaviculata disease cycle and the obtained results will be vital for developing boxwood blight forecasting and management tools.
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    The Contribution of Within-Field Inoculum Sources of Gibberella zeae to Fusarium Head Blight in Winter Wheat and Barley
    Keller, Melissa Dawn (Virginia Tech, 2011-04-26)
    Fusarium head blight (FHB) is one of the most economically important diseases of small grains and continues to impact crops when environmental conditions are favorable to Gibberella zeae (Fusarium graminearum), the causal agent of the disease. Corn residues are considered to be primary sources of inoculum for epidemics of FHB. Therefore, knowledge of the movement of Gibberella zeae from a local source of infested corn residue is critical to the management of FHB in wheat and barley. Previous research made significant progress in defining the spatial dissemination of inoculum sources of G. zeae within agricultural fields, but was unable to clearly distinguish between within-field and background sources. Using amplified fragment length polymorphism, released clones of G. zeae were tracked within wheat and barley fields. This strategy allowed the distinction between the contributions of released clones to FHB, compared to that of background inocula. Corn residue infested with clones of G. zeae was placed into small replicated plots in winter wheat fields in New York and Virginia in 2007 and 2008 and wheat spikes were collected at 0, 3, 6, and ≥24 m from the inoculum sources. Recovery of released clones decreased an average of 90% between 3 and 6 m from inoculum sources. Various amounts of corn residue infested with a single clone of G. zeae were placed into small replicated plots in winter wheat and barley fields in Virginia from 2008 to 2010. The use of minimal or conventional tillage and a moderately resistant cultivar of wheat or barley may reduce the contribution of within-field inocula to FHB; however, environmental conditions play an important role in the effectiveness of these management strategies. With the increase of corn production due to incentives for ethanol-based fuel, overwintering sites for G. zeae on corn residue are likely to increase. Our work contributes to an increased understanding of the influence of overwintered corn residue to FHB which will also direct future research on how to reduce the inoculum potential from within-field sources.
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    Control of anthracnose (caused by Colletotrichum gloeosporioides s.l.) on mango in Senegal by fungicides and biofungicides
    Diallo, Yaya (Virginia Tech, 2016-06-15)
    Senegal ranks second for mango production among West African countries and has the potential to competitively produce mangoes for the European market. Colletotrichum gloeosporioides Penz. and Sacc. has historically been considered the causal pathogen of anthracnose of mango and other fruits, but is now known to represent a species complex. Field trials to examine the efficacy of fungicides and biofungicides were conducted in southern and northern production regions of Senegal. In three southern trials, Sonata (Bacillus pumilus strain QST 2808), Serenade Optimum (Bacillus subtilis strain QST 713), and sodium molybdate provided 63%, 67% and 76% control of disease severity, respectively, whereas thiophanate methyl and azoxystrobin provided 77% and 78% control, respectively. Disease severities of all treatments were significantly lower than that of the control. Thiophanate methyl and azoxystrobin produced more disease-free mangoes (64 and 62%, respectively) than Serenade (49%), sodium molybdate (44%), and Sonata (38%). Differences within each trial were not statistically significant, but when all trials were combined, all treatments resulted in significantly more disease-free mangoes than the control. Among treatments, thiophanate methyl and azoxystrobin resulted in significantly more disease-free mangoes than Sonata. No results were obtained in the northern orchards due to an absence of disease development. Sequencing of the ITS gene region of 30 Colletotrichum isolates from mangoes from different regions in Senegal indicated that all belonged to the C. gloeosporioides species complex. Sequencing of the ApMat intergenic region identified all of them as most closely resembling C. siamense.
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    Cucurbit Downy Mildew (Pseudoperonospora cubensis): Cucumber Resistance
    Cooper, Jessica G. (Virginia Tech, 2013-01-23)
    Pseudoperonospora cubensis (Bert. et Curt) Rost. is the causal agent of cucurbit downy mildew (CDM). It is the most damaging cucumber pathogen on the Eastern Shore of Virginia and eastern parts of the United States. Pseudoperonospora cubensis is an obligate oomycete pathogen, infecting crops within the Cucurbitaceae family. The disease is characterized by angular chlorotic lesions and a downy or felt-like appearance on the abaxial side of the leaf. Control of this pathogen includes use of resistant cucumber cultivars and costly fungicide programs. Continuous use has led to resistance to commonly used fungicides. This has become a major concern and in response, seed companies have developed cucumber cultivars which claim downy mildew resistance.  This study evaluates different cucumber cultivars and assesses their level of resistance to CDM. The results indicate that an integrated management approach of reduced fungicide application and the use of resistant cultivars can suppress levels of CDM and yield a cucumber crop. Additionally, a molecular study was conducted, comparing the relative expression of genes encoding a basic PR-1 protein, a cytosolic ascorbate peroxidase protein and three resistance (R) gene proteins, in nineteen cultivars. All of the selected genes were analyzed using real-time PCR. The relative expression levels of the R-genes varied between cultivars. The basic PR-1 protein decreased expression in the majority of the cultivars, suggesting no involvement in the first twenty-four hours. Cytosolic ascorbate peroxidase relative expression levels suggest an increase in susceptible cultivars and a decrease in tolerant cultivars.