Browsing by Author "Kong, Ping"

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    The avocado subgroup of Phytophthora citricola constitutes a distinct species, Phytophthora mengei sp. Nov.
    Hong, Chuanxue; Gallegly, Mannon E.; Browne, Greg T.; Bhat, Ravindra G.; Richardson, Patricia A.; Kong, Ping (Mycological Society of America, 2009-07)
    Isolates from avocado tree cankers have been recognized as a distinct subgroup within the P. citricola complex since 1974, both morphologically and molecularly (isozyme and amplified fragment length polymorphism [AFLP] analyses). This sub-group is formally separated from P. citricola after comparative DNA Fingerprinting and sequence analyses of the ITS region, as well as by morphological examinations. This new taxon is homothallic, produces plerotic oospores with paragynous antheridia and noncaducous semipapillate sporangia. Morphologically it differs from Other species of Waterhouse group III by producing many large bizarre-shaped sporangia and smaller oogonia with asymmetric capitate antheridia. It belongs to clade 2 and is phylogenetically closer to A siskiyouensis, P. capsici and P. tropicalis than to P. citricola. P. mengei can be easily differentiated from its relatives in the same clade and other species of this morpho-group by DNA fingerprints and sequence analysis. This new taxon is named Phytophthora mengei sp. nov.
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    Boxwood phyllosphere fungal and bacterial communities and their differential responses to film-forming anti-desiccants
    Li, Xiaoping; Omolehin, Olanike; Hemmings, Ginger; Tseng, Hsien T.; Taylor, Amanda; Taylor, Chad; Kong, Ping; Daughtrey, Margery; Luster, Douglas; Gouker, Fred; Hong, Chuanxue (2023-08-12)
    Background Anti-desiccant is a class of agrochemicals widely used to protect plants from water stresses, rapid temperature variations, heat and sunburn, frost and freeze damages, transplant shock, and pathogen and pest attack. Although anti-desiccants are generally considered non-toxic to organisms, it is unclear whether they may impact the phyllosphere microbial communities. In this study, three film-forming anti-desiccant products, TransFilm, Vapor Gard, and Wilt-Pruf were applied to the canopy of two boxwood cultivars ‘Vardar Valley’ and ‘Justin Brouwers’ on April 13 and August 26, 2021. Shoot samples were collected from boxwood plants treated with each of the three products, as well as nontreated control on June 16, August 26 (before the second treatment), and October 18. Microbial and plant genomic DNA was isolated together and 16S rRNA gene and the extended internal transcribed spacer regions were amplified with PCR and sequenced on a Nanopore MinION platform for bacterial and fungal identification. Results Bacterial communities were more diverse than fungal communities. At the phylum level, the boxwood phyllosphere was dominated by Proteobacteria and Ascomycota; at the genus level, Methylobacterium and Shiraia were the most abundant bacteria and fungi, respectively. Among the three film-forming anti-desiccants, Vapor Gard and Wilt-Pruf had more impact than TransFilm on the microbial communities. Specifically, broader impacts were observed on fungal than bacterial community composition and structure, with most affected fungi being suppressed while bacteria promoted. Conclusion This study addressed several major knowledge gaps regarding boxwood phyllosphere microbiota and the impact of anti-desiccants on plant microbiome. We identified diverse microbial communities of boxwood, a major evergreen woody crop and an iconic landscape plant. We also found differential effects of three film-forming anti-desiccants on the composition and structure of bacterial and fungal communities. These findings advanced our understanding of the associated microbiome of this landmark plant, enabling growers to fully utilize the potentials of microbiome and three anti-desiccants in improving boxwood health and productivity.
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    cDNA Transcriptome of Arabidopsis Reveals Various Defense Priming Induced by a Broad-Spectrum Biocontrol Agent Burkholderia sp. SSG
    Kong, Ping; Li, Xiaoping; Gouker, Fred; Hong, Chuanxue (MDPI, 2022-03-15)
    Burkholderia sp. SSG is a potent biological control agent. Even though its survival on the leaf surface declined rapidly, SSG provided extended, moderate plant protection from a broad spectrum of pathogens. This study used Arabidopsis Col-0 and its mutants, eds16-1, npr1-1, and pad4-1 as model plants and compared treated plants with non-treated controls to elucidate whether SSG triggers plant defense priming. Only eds16-1 leaves with SSG became purplish, suggesting the involvement of salicylic acid (SA) in SSG-induced priming. cDNA sequencing of Col-0 plants and differential gene expression analysis identified 120 and 119 differentially expressed genes (DEGs) at 6- and 24-h post-treatment (hpt) with SSG, respectively. Most of these DEGs encoded responses to biotic and abiotic stimuli or stresses; four DEGs had more than two isoforms. A total of 23 DEGs were shared at 6 and 24 hpt, showing four regulation patterns. Functional categorization of these shared DEGs, and 44 very significantly upregulated DEGs revealed that SSG triggered various defense priming mechanisms, including responses to phosphate or iron deficiency, modulation of defense-linked SA, jasmonic acid, ethylene, and abscisic acid pathways, defense-related gene regulation, and chromatin modification. These data support that SSG is an induced systemic resistance (ISR) trigger conferring plant protection upon pathogen encounter.
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    Characterization of Boxwood Shoot Bacterial Communities and Potential Impact from Fungicide Treatments
    Li, Xiaoping; Tseng, Hsien Tzer; Hemmings, Ginger; Omolehin, Olanike; Taylor, Chad; Taylor, Amanda; Kong, Ping; Daughtrey, Margery; Gouker, Fred; Hong, Chuanxue (American Society for Microbiology, 2023-04)
    Agrochemicals are important tools for safeguarding plants from invasive pathogens, insects, mites, and weeds. How they may affect the plant microbiome, a critical component of crop health and production, was poorly understood. Phyllosphere bacterial communities play important roles in plant fitness and growth. The objective of this study was to characterize the epiphytic and endophytic bacterial communities of boxwood shoots and determine how they may respond to commonly used fungicides. In early summer and early fall, shoot samples were collected immediately before and 1, 7, and 14days after three fungicides containing chlorothalonil and/or propiconazole were applied to the canopy. Total genomic DNA from shoot surface washings and surface-sterilized shoot tissues was used as the template for 16S rRNA metabarcoding, and the amplicons were sequenced on a Nanopore MinION sequencer to characterize the epiphytic and endophytic communities. The bacterial communities were phylogenetically more diverse on the boxwood shoot surface than in the internal tissue, although the two communities shared 12.7% of the total 1,649 identified genera. The most abundant epiphytes were Methylobacterium and Pantoea, while Stenotrophomonas and Brevundimonas were the dominant endophytes. Fungicide treatments had strong impacts on epiphytic bacterial community structure and composition. Analysis of compositions of microbiomes with bias correction (ANCOM-BC) and analysis of variance (ANOVA)-like differential expression (ALDEx2) together identified 312 and 1,362 epiphytes changed in abundance due to fungicide treatments in early summer and early fall, respectively, and over 50% of these epiphytes were negatively impacted by fungicide. The two chlorothalonil-based contact fungicides demonstrated more marked effects than the propiconazole-based systemic fungicide. These results are foundational for exploring and utilizing the full potential of the microbiome and fungicide applications and developing a systems approach to boxwood health and production. IMPORTANCE Agrochemicals are important tools for safeguarding plants from invasive pathogens, insects, mites, and weeds. How they may affect the plant microbiome, a critical component of crop health and production, was poorly understood. Here, we used boxwood, an iconic low-maintenance landscape plant, to characterize shoot epiphytic and endophytic bacterial communities and their responses to contact and systemic fungicides. This study expanded our understanding of the above-ground microbiome in ornamental plants and is foundational for utilizing the full benefits of the microbiome in concert with different fungicide chemistries to improve boxwood health. This study also sets an example for a more thorough evaluation of these and other agrochemicals for their effects on boxwood microbiomes during production and offers an expanded systems approach that could be used with other crops for enhanced integrated pest management.
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    Characterization of the Soil Bacterial Community from Selected Boxwood Gardens across the United States
    Li, Xiaoping; Kong, Ping; Daughtrey, Margery; Kosta, Kathleen; Schirmer, Scott; Howle, Matthew; Likins, Michael; Hong, Chuanxue (MDPI, 2022-07-26)
    In a recent study, we observed a rapid decline of the boxwood blight pathogen Calonectria pseudonaviculata (Cps) soil population in all surveyed gardens across the United States, and we speculated that these garden soils might be suppressive to Cps. This study aimed to characterize the soil bacterial community in these boxwood gardens. Soil samples were taken from one garden in California, Illinois, South Carolina, and Virginia and two in New York in early summer and late fall of 2017 and 2018. Soil DNA was extracted and its 16S rRNA amplicons were sequenced using the Nanopore MinION® platform. These garden soils were consistently dominated by Rhizobiales and Burkholderiales, regardless of garden location and sampling time. These two orders contain many species or strains capable of pathogen suppression and plant fitness improvement. Overall, 66 bacterial taxa were identified in this study that are known to have strains with biological control activity (BCA) against plant pathogens. Among the most abundant were Pseudomonas spp. and Bacillus spp., which may have contributed to the Cps decline in these garden soils. This study highlights the importance of soil microorganisms in plant health and provides a new perspective on garden disease management using the soil microbiome.
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    Complete genome sequence of a boxwood endophyte Burkholderia sp. SSG with broad biotechnological application potential
    Kong, Ping; Hong, Chuanxue (Elsevier, 2020-06-01)
    Burkholderia sp. strain SSG is a boxwood endophyte with potent antagonistic activities against a variety of plant pathogens. Here we present its complete genome sequence that is 8.6 Mb long with a GC content of 66.9%, 10,209 predicted protein-coding sequences, and 866 secondary metabolism gene clusters. Many of these genes and clusters involve antibiosis and other antagonistic activities against plant pathogens and insect pests as well as plant growth promoting traits but none for the Burkholderia cepacia epidemic strain marker. This genome sequence supports SSG as a potent biocontrol agent and source of other biotechnological applications.
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    Differential Adaptation Has Resulted in Aggressiveness Variation of Calonectria pseudonaviculata on Hosts Buxus, Pachysandra, and Sarcococca
    Kong, Ping; Daughtrey, Margery L.; Hong, Chuanxue (MDPI, 2023-01-29)
    Calonectria pseudonaviculata (Cps) infects Buxus (boxwood), Pachysandra (pachysandra), and Sarcococca spp. (sweet box); yet, how it adapts to its hosts has been unclear. Here, we performed serial passage experiments with the three hosts and measured Cps changes in three aggressiveness components: infectibility, lesion size, and conidial production. The detached leaves of individual hosts were inoculated with isolates (P0) from the originating host, followed by nine serial inoculations of new leaves of the same host with conidia from the infected leaves of the previous inoculation. All boxwood isolates maintained their capability of infection and lesion expansion through the 10 passages, whereas most non-boxwood isolates lost these abilities during the passages. Isolates from plants of origin (*-P0) and their descendants isolated from passages 5 (*-P5) and 10 (*-P10) were used to evaluate aggressiveness changes on all three hosts with cross-inoculation. While post-passage boxwood isolates gave enlarged lesions on pachysandra, sweet box P5 and pachysandra P10 isolates showed reduced aggressiveness on all hosts. Cps appears to be most adapted to boxwood and less adapted to sweet box and pachysandra. These results suggest speciation of Cps, with its coevolutionary pace with the hosts the fastest with boxwood, intermediate with sweet box, and the slowest with pachysandra.
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    Differential Tolerance to Calonectria pseudonaviculata of English Boxwood Plants Associated with the Complexity of Culturable Fungal and Bacterial Endophyte Communities
    Kong, Ping; Sharifi, Melissa; Bordas, Adria; Hong, Chuanxue (MDPI, 2021-10-21)
    Isolated boxwood endophytes have been demonstrated to effectively protect boxwood plants from infection by Calonectria pseudonaviculata (Cps). However, the roles of endophytes as communities in plant defense are not clear. Here, we demonstrated differential tolerance to Cps of English boxwood (Buxus sempervirens ‘Suffruticosa’), an iconic landscape plant and generally regarded as highly susceptible, and its link to endophyte complexity. Fifteen boxwood twig samples were collected in triplicates from three historic gardens—Colonial Williamsburg, George Washington’s Mount Vernon and River Farm, and Virginia Tech’s research farm in Virginia Beach in the summer and fall of 2019. A portion of individual samples was inoculated with Cps under controlled conditions. Significant differences in disease severity were observed among samples but not between the two seasons. Examining the endophyte cultures of the summer samples revealed that bacterial and fungal abundance was negatively and positively correlated with the disease severity. Nanopore metagenomics analysis on genomic DNA of the tolerant and susceptible group representatives confirmed the associations. Specifically, tolerant English boxwood plants had an endophyte community dominated by Bacilli and Betaproteobacteria, while susceptible ones had a distinct endophyte community dominated by Gammaproteobacteria, Actinobacteria, and diverse fungi. These findings may lead to boxwood health management innovations—devising and utilizing cultural practices to manipulate and increase the abundance and performance of beneficial endophytes for enhanced boxwood resistance to Cps.
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    Endophytic Burkholderia sp. SSG as a potential biofertilizer promoting boxwood growth
    Kong, Ping; Hong, Chuanxue (2020-07-16)
    Background. Burkholderia sp. SSG is a bacterial endophyte isolated from boxwood leaves showing a resistant response to infection by the boxwood blight pathogen Calonectria pseudonaviculata. SSG acted as a protective and curative biocontrol agent for boxwood blight and as a bio-sanitizer of disease inoculum in the field. Many gene clusters involved in antibiotic production and plant growth promotion (PGP) were found in the genome, giving this endophyte great application potential as a treatment for plant protection. However, the PGP features have not been documented. This study investigated the plant growth promotion activity of SSG in boxwood. Methods. To determine whether SSG is a plant growth promoting bacterium, four PGP traits, auxin and siderophore production, nitrogen fixation and phosphate solubilization, were examined in the laboratory with colorimetric or agar plate assays. The plant growth promoting activity of SSG was tested on three boxwood varieties characterized by slow, intermediate and fast growth rates, namely Justin Brouwers, Buddy and Winter Gem, respectively. These plants were drenched with an SSG cell suspension or water and washed plant weight was compared before and after treatment to determine growth changes after 10 months. Results. The SSG culture was sustainable on nitrogen free media, suggesting that SSG may fix atmospheric nitrogen. It was also a strong phosphate solubilizer and a potent siderophore and indole-3-acetic acid (IAA) producer. Significant growth promotion was observed on boxwood cultivars Justin Brouwers, Buddy and Winter Gem 10 months after plant roots were drenched with SSG cells. The growth rate of treated plants was 76.1, 58.3, and 37.3% higher than that of the control, respectively. The degree of growth promotion was significantly different among plant varieties, notably more pronounced with the slow and intermediate growers. This study demonstrates that the SSG bacterium has multiple PGP traits and is a prospective plant biofertilizer.
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    Evaluation of 1021Bp, a close relative of Pseudomonas eucalypticola, for potential of plant growth promotion, fungal pathogen suppression and boxwood blight control
    Kong, Ping; Hong, Chuanxue (2024-09-14)
    Background: Pseudomonas eucalypticola, a new species of the P. fluorescens group that generates most Pseudomonas-based biocontrol agents, has not been found in any plants other than Eucalyptus dunnii leaves. Except for antagonism to the growth of a few fungi, its features in plant growth promotion and disease control have not been evaluated. Here, we identified a similar species of P. eucalypticola, 1021Bp, from endophyte cultures of healthy leaves of English boxwood (Buxus sempervirens ‘Suffruticosa’) and investigated its antifungal activity, plant growth promotion traits, and potential for boxwood blight control. Results: Colorimetric or plate assays showed the properties of 1021Bp in nitrogen fixation, phosphate solubilization, and production of indole-3-acetic acid (IAA) and siderophores, as well as the growth suppression of all five plant fungal pathogens, including causal agents of widespread plant diseases, gray mold, and anthracnose. Boxwood plant leaves received 87.4% and 65.8% protection from infection when sprayed with cell-free cultural supernatant (CFS) but not the resuspended bacterial cells at 108–9/mL of 1021Bp at one and seven days before inoculation (dbi) with boxwood blight pathogen, Calonectria pseudonaviculata, at 5 × 104 spores/mL. They also received similarly high protection with the 1021Bp cell culture without separation of cells and CFS at 14 dbi (67.5%), suggesting a key role of 1021Bp metabolites in disease control. Conclusions: Given the features of plant growth and health and its similarity to P. eucalypticola with the P. fluorescens lineage, 1021Bp has great potential to be developed as a safe and environmentally friendly biofungicide and biofertilizer. However, its metabolites are the major contributors to 1021Bp activity for plant growth and health. Application with the bacterial cells alone, especially with nonionic surfactants, may result in poor performance unless survival conditions are present.
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    Evaluation of fungicides for management of boxwood blight caused by Calonectria spp. under field conditions in Northern Germany
    Brand, Thomas; Beltz, Heinrich; Adhikari, Urmila; Daughtrey, Margery; Luster, Douglas G.; Kong, Ping; Hong, Chuanxue (Springer, 2022-12-21)
    Fungicide protection is the current approach for management of boxwood blight caused by Calonectria pseudonaviculata (Cps) and C. henricotiae (Che). However, published studies evaluating fungicides under field conditions have been focused on Cps. The objective of this study was to evaluate fungicides in Northern Germany where both Cps and Che were present. Three trials were conducted between 2006 and 2016. In 2006, plants were artificially inoculated with a conidial suspension, while infested soil and plant debris were added to a different field as inoculum for the 2012 trial and this field was used again without further addition of inoculum in 2015. Fungicides were applied one to five times and assessments were done three to six times, depending upon the trial. The highest level of disease severity occurred in 2015 (0.91), while the lowest occurred in 2012 (0.01). Among the fungicides evaluated in 2006, preventive sprays of Cercobin FL, Switch, Harvesan, Pugil 75 WG, Dithane NeoTec and Euparen M WG were most effective, with blight control above 65%. In 2012, all treatments including Askon, Cabrio Top, Malvin WG, Dithane NeoTec and Osiris showed > 75% blight control. In 2015, Bayer Rosen-Pilzfrei Baymat and Switch were the most effective (> 82%). Extended in-season blight control was also observed with some fungicides. Additionally, a few fungicides that were evaluated in more than 1 year showed reduced effectiveness over time. This study filled several major knowledge gaps especially regarding fungicide efficacy against Che under field conditions and thus provides crucial information for developing chemical control strategies.
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    Multi-year Field Plantings Evaluating Boxwood Cultivars for Susceptibility to the Blight Pathogens (Calonectria spp.) in Northern Germany
    Brand, Thomas; Beltz, Heinrich; Ehsen, Björn; Adhikari, Urmila; Daughtrey, Margery; Luster, Douglas G.; Kong, Ping; Hong, Chuanxue (American Phytopathological Society, 2022-07-22)
    Two multi-year field trials were conducted to evaluate boxwood cultivars for their susceptibility to the blight pathogens Calonectria pseudonaviculata and C. henricotiae in northern Germany. Fifteen cultivars were included in the first trial from 2007 to 2012, and 46 cultivars in the second trial from 2014 to 2017. Both trials were done in a naturally infested field, supplemented with infected plant tissue added to the soil before planting. Each cultivar had three replicate hedge sections with ten plants per section and they were assessed annually for blight severity expressed as proportion of leaves blighted and fallen. Blight severity varied significantly among years (P < 0.0001) and cultivars (P < 0.05) within each trial. In the first trial, mean severity ranged from 0.03 to 0.11 for the most resistant cultivars and 0.35 to 0.96 for the most susceptible ones. Similarly, in the second trial, mean severity ranged from 0.06 to 0.27 and 0.71 to 0.97 for the most resistant and susceptible cultivars, respectively. 'Suffruticosa' was consistently the most susceptible cultivar, followed by 'Marianne', 'Myosotidifolia', 'Raket' and 'Morris Midget'. 'Herrenhausen' was the most resistant cultivar, followed by B. microphylla var. japonica, B. microphylla var. koreana, 'Green Mound', 'Faulkner', and 'Winter Beauty'. This study provides field data showing the performance of boxwood cultivars under different levels of disease pressure in an area where C. henricotiae was dominant. This knowledge will help boxwood growers and gardeners to choose less susceptible cultivars and help plant breeders to select for disease resistance.
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    Phytophthora aquimorbida sp nov and Phytophthora taxon 'aquatilis' recovered from irrigation reservoirs and a stream in Virginia, USA
    Hong, Chuanxue; Richardson, Patricia A.; Hao, Wei; Ghimire, Sita R.; Kong, Ping; Moorman, Gary W.; Lea-Cox, John D.; Ross, David S. (Mycological Society of America, 2012-04)
    Two distinct subgroups (L2 and A(-2)) were recovered from irrigation reservoirs and a stream in Virginia, USA. After molecular, morphological and physiological examinations, the L2 subgroup was named Phytophthora aquimorbida and the A(-2) designated as Phytophthora taxon 'aquatilis'. Both taxa are homothallic. P. aquimorbida is characterized by its noncaducous and nonpapillate sporangia, catenulate and radiating hyphal swellings and thick-walled plerotic oospores formed in globose oogonia mostly in the absence of an antheridium. P. taxon 'aquatilis' produces plerotic oospores in globose oogonia mostly with a paragynous antheridium. It has semi-papillate, caducous sporangia with variable pedicels, but it does not have hyphal swelling. Analyses of ITS, CO1, beta-tubulin and NADH1 sequences revealed that P. aquimorbida is closely related to P. hydropathica, P. irrigata and P. parsiana, and P. taxon 'aquatilis' is related to P. multivesiculata. The optimum temperature for culture growth is 30 and 20 C for E aquimorbida and P. taxon 'aquatilis' respectively. Both taxa were pathogenic to rhododendron plants and caused root discoloration, pale leaves, wilting, tip necrosis and dieback. Their plant biosecurity risk also is discussed.
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    Phytophthora pini Leonian resurrected to distinct species status
    Hong, Chuanxue; Gallegly, Mannon E.; Richardson, Patricia A.; Kong, Ping (Mycological Society of America, 2010-10)
    Phytophthora pini was named by Leonian in 1925, but this species was largely ignored until 1956 and then merged with P. citricola by Waterhouse in 1963. This study compared the ex-type and exauthentic cultures of these two species with isolates of P. plurivora and the P. citricola subgroups Cil I and 111 reported previously. Examination of these isolates revealed that the ex-type culture of P. pini is identical to P. citricola I. Phytophthora pini Leonian therefore is resurrected to distinct species status and redescribed here with a Latin description, replacing P. citricola I. Molecular, physiological and morphological descriptions of this species are presented. The molecular description includes DNA sequences of five nuclear and mitochondria] regions as well as PCR-SSCP fingerprints. The relationship among the above species and other species recently segregated from the P. citricola complex also is discussed.
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    A Potent Burkholderia Endophyte against Boxwood Blight Caused by Calonectria pseudonaviculata
    Kong, Ping; Hong, Chuanxue (MDPI, 2020-02-24)
    Calonectria pseudonaviculata (Cps) poses an increasing threat to boxwood, a major nursery crop and iconic landscape plant worldwide. Here, we report on a potent biocontrol agent that produces small sage green (SSG) colonies on potato dextrose agar. SSG is a bacterial strain recovered from Justin Brouwers boxwood leaves with unusual response to Cps inoculation. Water-soaked symptoms developed on leaves 2 days after inoculation then disappeared a few days later. This endophyte affected several major steps of the boxwood blight disease cycle. SSG at 107 cfu/mL lysed all conidia in mixed broth culture. SSG at 108 cfu/mL reduced blight incidence by >98% when applied one day before or 3 h after boxwood were inoculated with Cps. Its control efficacy decreased with decreasing bacterial concentration to 103 cfu/mL and increasing lead time up to 20 days. When applied on diseased leaf litter under boxwood plants, SSG reduced Cps sporulation and consequently mitigated blight incidence by 90%. SSG was identified as a new member of the Burkholderia cepacia complex with distinct characters from known clinical strains. With these protective, curative, and sanitizing properties, this Burkholderia endophyte offers great promise for sustainable blight management at production and in the landscape.
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    Rapid decline of Calonectria pseudonaviculata soil population in selected gardens across the United States
    Hong, Chuanxue; Daughtrey, Margery; Howle, Matthew; Schirmer, Scott; Kosta, Kathleen; Kong, Ping; Likins, Michael; Suslow, Karen (American Phytopathological Society, 2022-11-01)
    Calonectria pseudonaviculata (Cps) poses a serious threat to boxwood, an iconic landscape plant in American and European gardens. Under the mild climatic conditions of the United Kingdom, Cps remained recoverable in infected leaf debris after being left on the soil surface or buried for 5 years. The primary objective of this study was to determine how this fungus may be affected by the warmer summers and colder winters in the United States by sampling and baiting soil with boxwood cuttings and by on-site testing with sentinel plants. Soil sampling started in a Virginia garden in January 2016 and was extended to California, Illinois, New York, and South Carolina in early summer of 2017 through late fall of 2018. The Cps soil population as measured by the percentage of infected bait leaves declined sharply within the first year of blighted boxwood removal and fell to an almost undetectable level at the end of this study. To validate these baiting results, the Virginia garden was tested on site four times with container-grown boxwood plants while the South Carolina garden and three New York gardens were tested once. Each test began with sentinel plants set out for field exposure, followed by evaluation on site and then in the laboratory after plants were retrieved from these gardens and incubated under conducive environments for 2 weeks. Cps was not observed on any sentinel boxwood plant on site or in the laboratory with one exception. These observations indicate that Cps did not survive in the United States garden soil over time as well as it did in the United Kingdom. These results have important practical implications while challenging the notion that fungi producing microsclerotia will always survive in the soil for many years.
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    Seasonal dynamics of cyanobacteria and eukaryotic phytoplankton in a multiple-reservoir recycling irrigation system
    Kong, Ping; Richardson, Patricia A.; Hong, Chuanxue (2019-09-10)
    Background Agricultural runoff recycling systems are manmade aquatic ecosystems of growing significance to global water sustainability, crop health, and production. This study investigated the seasonal dynamics of microbial community in a three-reservoir recycling irrigation system with a stepwise water flow and compared with that of an adjacent runoff-free stream. Runoff water from all production areas was captured in a sedimentation reservoir which overflowed to a transition reservoir then retention reservoir through a culvert. Stream water was pumped to replenish the reservoirs as needed during growing seasons. Results 6S rDNA PCR clone libraries of quarterly water samples from three reservoirs and one stream were sequenced, and 575 operational taxonomic units (OTUs) were identified and assigned to cyanobacteria, eukaryotic phytoplankton, and other bacteria. When compared to the stream, three reservoirs consistently had low microbial diversity. A distinct seasonal pattern of microbial community structure was observed for each reservoir and the stream. Stream was consistently dominated by other bacteria. Retention reservoir was dominated by cyanobacteria during the summer and fall and eukaryotic phytoplankton during the winter and spring. Sedimentation reservoir was dominated by cyanobacteria beginning in the spring but that dominance was altered when water was pumped from the stream from early spring to fall seasons. Transition reservoir had the greatest shift of microbial community structure, being dominated by other bacteria in the summer, cyanobacteria in the fall, and eukaryotic phytoplankton in the winter and spring. Water temperature and ammonium level were the two most important contributing factors to the seasonality of microbial community in these reservoirs. Conclusions The three recycling irrigation reservoirs consistently had lower microbial diversity and distinct community structure when compared to the stream. These reservoirs were typically dominated by cyanobacteria during warm seasons and eukaryotic phytoplankton during cool seasons. This seasonal pattern was altered when water was pumped from the stream. The cyanobacteria dominance was associated with rising water temperature and ammonium level. These results highlight the importance of preventing agricultural runoff from entering natural waterways and water resources and provide a useful framework for further investigations into the ecological processes of this emerging ecosystem.
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    Zoospore exudates from Phytophthora nicotianae affect immune responses in Arabidopsis
    Kong, Ping; McDowell, John M.; Hong, Chuanxue (PLoS, 2017-06-29)
    Zoospore exudates play important roles in promoting zoospore communication, homing and germination during plant infection by Phytophthora. However, it is not clear whether exudates affect plant immunity. Zoospore-free fluid (ZFF) and zoospores of P. nicotianae were investigated comparatively for effects on resistance of Arabidopsis thaliana Col-0 and mutants that affect signaling mediated by salicylic acid (SA) and jasmonic acid (JA): eds16 (enhanced disease susceptibility16), pad4 (phytoalexin deficient4), and npr1 (nonexpressor of pathogenesis-related genes1). Col-0 attracted more zoospores and had severe tissue damage when flooded with a zoospore suspension in ZFF. Mutants treated with ZFF alone developed disease symptoms similar to those inoculated with zoospores and requirements of EDS16 and PAD4 for plant responses to zoospores and the exudates was apparent. Zoospore and ZFFs also induced expression of the PR1 and PDF1.2 marker genes for defense regulated by SA and JA, respectively. However, ZFF affected more JA defense signaling, down regulating PR1 when SA signaling or synthesis is deficient, which may be responsible for Arabidopsis mutant plants more susceptible to infection by high concentration of P. nicotianae zoospores. These results suggest that zoospore exudates can function as virulence factors and inducers of plant immune responses during plant infection by Phytophthora.
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    Zoospore exudates from Phytophthora nicotianae affect immune responses in Arabidopsis
    Kong, Ping; McDowell, John M.; Hong, Chuanxue (PLOS, 2017-06-29)
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    Zoospore interspecific signaling promotes plant infection by Phytophthora
    Kong, Ping; Tyler, Brett M.; Richardson, Patricia A.; Lee, Bobby W. K.; Zhou, Zhaohui S.; Hong, Chuanxue (Biomed Central, 2010-12-07)
    Background Oomycetes attack a huge variety of economically and ecologically important plants. These pathogens release, detect and respond to signal molecules to coordinate their communal behaviors including the infection process. When signal molecules are present at or above threshold level, single zoospores can infect plants. However, at the beginning of a growing season population densities of individual species are likely below those required to reach a quorum and produce threshold levels of signal molecules to trigger infection. It is unclear whether these molecules are shared among related species and what their chemistries are. Results Zoospore-free fluids (ZFF) from Phytophthora capsici, P. hydropathica, P. nicotianae (ZFFnic), P. sojae (ZFFsoj) and Pythium aphanidermatum were cross tested for stimulating plant infection in three pathosystems. All ZFFs tested significantly increased infection of Catharanthus roseus by P. nicotianae. Similar cross activities were observed in infection of Lupinus polyphyllus and Glycine max by P. sojae. Only ZFFnic and ZFFsoj cross induced zoospore aggregation at a density of 2 x 103 ml-1. Pure autoinducer-2 (AI-2), a component in ZFF, caused zoospore lysis of P. nicotianae before encystment and did not stimulate plant infection at concentrations from 0.01 to 1000 μM. P. capsici transformants with a transiently silenced AI-2 synthase gene, ribose phosphate isomerase (RPI), infected Capsicum annuum seedlings at the same inoculum concentration as the wild type. Acyl-homoserine lactones (AHLs) were not detected in any ZFFs. After freeze-thaw treatments, ZFF remained active in promoting plant infection but not zoospore aggregation. Heat treatment by boiling for 5 min also did not affect the infection-stimulating property of ZFFnic. Conclusion Oomycetes produce and use different molecules to regulate zoospore aggregation and plant infection. We found that some of these signal molecules could act in an inter-specific manner, though signals for zoospore aggregation were somewhat restricted. This self-interested cooperation among related species gives individual pathogens of the same group a competitive advantage over pathogens and microbes from other groups for limited resources. These findings help to understand why these pathogens often are individually undetectable until severe disease epidemics have developed. The signal molecules for both zoospore aggregation and plant infection are distinct from AI-2 and AHL.