Field Evaluation of Biological Control of Fire Blight in the Eastern United States

Multisite Field Evaluation of Bacteriophages for Fire Blight Management: Incorporation of Ultraviolet Radiation Protectants and Impact on the Apple Flower Microbiome

Fire blight, a disease of pome fruits caused by the bacterium Erwinia amylovora, has become increasingly difficult to manage after the emergence of streptomycin-resistant strains. Alternative antibiotics and copper are available; however, these chemicals have use restrictions in some countries and also can carry risks of phytotoxicity. Therefore, there is growing interest in biological-based management options, with bacteriophage (phages) showing promise, as these naturally occurring pathogens of bacteria are easy to isolate and grow. However, there are several technical challenges regarding the implementation of phage biocontrol in the field, as the viral molecules suffer from ultraviolet radiation (UVR) degradation and can die off rapidly in the absence of the host bacterium. In this work, we assessed the efficacy of Erwinia phages and a commercial phage product for blossom blight control in the field across multiple locations in the eastern United States. In these tests, disease control ranged from 0.0 to 82.7%, and addition of a UVR protectant only resulted in significantly increased disease control in 2 of 12 tests. We also analyzed microbial community population changes in response to phage application. Changes in bacterial community diversity metrics over time were not detected; however, relative abundances of target taxa were temporarily reduced after phage applications, indicating that these phage applications did not have deleterious effects on the flower microbiome. We have demonstrated that biological control of fire blight with phages is achievable, but a better understanding of phage-pathogen dynamics is required to optimize disease control efficacy.

Inter-species interactions between two bacterial flower commensals and a floral pathogen reduce disease incidence and alter pathogen activity

Flowers are colonized by a diverse community of microorganisms that can alter plant health and interact with floral pathogens. Erwinia amylovora is a flower-inhabiting bacterium and a pathogen that infects different plant species, including Malus × domestica (apple). Previously, we showed that the co-inoculation of two bacterial strains, members of the genera Pseudomonas and Pantoea, isolated from apple flowers, reduced disease incidence caused by this floral pathogen. Here, we decipher the ecological interactions between the two flower-associated bacteria and E. amylovora in field experimentation and in vitro co-cultures. The two flower commensal strains did not competitively exclude E. amylovora from the stigma habitat, as both bacteria and the pathogen co-existed on the stigma of apple flowers and in vitro. This suggests that plant protection might be mediated by other mechanisms than competitive niche exclusion. Using a synthetic stigma exudation medium, ternary co-culture of the bacterial strains led to a substantial alteration of gene expression in both the pathogen and the two microbiota members. Importantly, the gene expression profiles for the ternary co-culture were not just additive from binary co-cultures, suggesting that some functions only emerged in multipartite co-culture. Additionally, the ternary co-culture of the strains resulted in a stronger acidification of the growth milieu than mono- or binary co-cultures, pointing to another emergent property of co-inoculation. Our study emphasizes the critical role of emergent properties mediated by inter-species interactions within the plant holobiont and their potential impact on plant health and pathogen behavior.

IMPORTANCE

Fire blight, caused by Erwinia amylovora, is one of the most important plant diseases of pome fruits. Previous work largely suggested plant microbiota commensals suppressed disease by antagonizing pathogen growth. However, inter-species interactions of multiple flower commensals and their influence on pathogen activity and behavior have not been well studied. Here, we show that co-inoculating two bacterial strains that naturally colonize the apple flowers reduces disease incidence. We further demonstrate that the interactions between these two microbiota commensals and the floral pathogen led to the emergence of new gene expression patterns and a strong alteration of the external pH, factors that may modify the pathogen's behavior. Our findings emphasize the critical role of emergent properties mediated by inter-species interactions between plant microbiota and plant pathogens and their impact on plant health.

The Floral Microbiome and Its Management in Agroecosystems: A Perspective

Disease management is critical to ensuring healthy crop yields and is often targeted at flowers because of their susceptibility to pathogens and direct link to reproduction. Many disease management strategies are unsustainable however because of the potential for pathogens to evolve resistance, or nontarget effects on beneficial insects. Manipulating the floral microbiome holds some promise as a sustainable alternative to chemical means of disease control. In this perspective, we discuss the current state of research concerning floral microbiome assembly and management in agroecosystems as well as future directions aimed at improving the sustainability of disease control and insect-mediated ecosystem services.

Bonactin and Feigrisolide C Inhibit Magnaporthe oryzae Triticum Fungus and Control Wheat Blast Disease

Wheat blast caused by the Magnaporthe oryzaeTriticum (MoT) pathotype is one of the most damaging fungal diseases of wheat. During the screening of novel bioactive secondary metabolites, we observed two marine secondary metabolites, bonactin and feigrisolide C, extracted from the marine bacteria Streptomyces spp. (Act 8970 and ACT 7619), remarkably inhibited the hyphal growth of an MoT isolate BTJP 4 (5) in vitro. In a further study, we found that bonactin and feigrisolide C reduced the mycelial growth of this highly pathogenic isolate in a dose-dependent manner. Bonactin inhibited the mycelial development of BTJP 4 (5) more effectively than feigrisolide C, with minimal concentrations for inhibition being 0.005 and 0.025 µg/disk, respectively. In a potato dextrose agar (PDA) medium, these marine natural products greatly reduced conidia production in the mycelia. Further bioassays demonstrated that these secondary metabolites could inhibit the MoT conidia germination, triggered lysis, or conidia germinated with abnormally long branched germ tubes that formed atypical appressoria (low melanization) of BTJP 4 (5). Application of these natural products in a field experiment significantly protected wheat from blast disease and increased grain yield compared to the untreated control. As far as we are aware, this is the first report of bonactin and feigrisolide C that inhibited mycelial development, conidia production, conidial germination, and morphological modifications in the germinated conidia of an MoT isolate and suppressed wheat blast disease in vivo. To recommend these compounds as lead compounds or biopesticides for managing wheat blast, more research is needed with additional MoT isolates to identify their exact mode of action and efficacy of disease control in diverse field conditions.

Orchard Management and Landscape Context Mediate the Pear Floral Microbiome

Crop-associated microbiota are a key factor affecting host health and productivity. Most crops are grown within heterogeneous landscapes, and interactions between management practices and landscape context often affect plant and animal biodiversity in agroecosystems. However, whether these same factors typically affect crop-associated microbiota is less clear. Here, we assessed whether orchard management strategies and landscape context affected bacterial and fungal communities in pear (Pyrus communis) flowers. We found that bacteria and fungi responded differently to management schemes. Organically certified orchards had higher fungal diversity in flowers than conventional or bio-based integrated pest management (IPM) orchards, but organic orchards had the lowest bacterial diversity. Orchard management scheme also best predicted the distribution of several important bacterial and fungal genera that either cause or suppress disease; organic and bio-based IPM best explained the distributions of bacterial and fungal genera, respectively. Moreover, patterns of bacterial and fungal diversity were affected by interactions between management, landscape context, and climate. When examining the similarity of bacterial and fungal communities across sites, both abundance- and taxon-related turnovers were mediated primarily by orchard management scheme and landscape context and, specifically, the amount of land in cultivation. Our study reveals local- and landscape-level drivers of floral microbiome structure in a major fruit crop, providing insights that can inform microbiome management to promote host health and high-yielding quality fruit. IMPORTANCE Proper crop management during bloom is essential for producing disease-free tree fruit. Tree fruits are often grown in heterogeneous landscapes; however, few studies have assessed whether landscape context and crop management affect the floral microbiome, which plays a critical role in shaping plant health and disease tolerance. Such work is key for identification of tactics and/or contexts where beneficial microbes proliferate and pathogenic microbes are limited. Here, we characterize the floral microbiome of pear crops in Washington State, where major production occurs in intermountain valleys and basins with variable elevation and microclimates. Our results show that both local-level (crop management) and landscape-level (habitat types and climate) factors affect floral microbiota but in disparate ways for each kingdom. More broadly, these findings can potentially inform microbiome management in orchards for promotion of host health and high-quality yields.

An In Vitro Pipeline for Screening and Selection of Citrus-Associated Microbiota with Potential Anti-"Candidatus Liberibacter asiaticus" Properties

Huanglongbing (HLB) is a destructive citrus disease that is lethal to all commercial citrus plants, making it the most serious citrus disease and one of the most serious plant diseases. Because of the severity of HLB and the paucity of effective control measures, we structured this study to encompass the entirety of the citrus microbiome and the chemistries associated with that microbial community. We describe the spatial niche diversity of bacteria and fungi associated with citrus roots, stems, and leaves using traditional microbial culturing integrated with culture-independent methods. Using the culturable sector of the citrus microbiome, we created a microbial repository using a high-throughput bulk culturing and microbial identification pipeline. We integrated an in vitro agar diffusion inhibition bioassay into our culturing pipeline that queried the repository for antimicrobial activity against Liberibacter crescens, a culturable surrogate for the nonculturable "Candidatus Liberibacter asiaticus" bacterium associated with HLB. We identified microbes with robust inhibitory activity against L. crescens that include the fungi Cladosporium cladosporioides and Epicoccum nigrum and bacterial species of Pantoea, Bacillus, and Curtobacterium Purified bioactive natural products with anti-"Ca. Liberibacter asiaticus" activity were identified from the fungus C. cladosporioides Bioassay-guided fractionation of an organic extract of C. cladosporioides yielded the natural products cladosporols A, C, and D as the active agents against L. crescens This work serves as a foundation for unraveling the complex chemistries associated with the citrus microbiome to begin to understand the functional roles of members of the microbiome, with the long-term goal of developing anti-"Ca Liberibacter asiaticus" bioinoculants that thrive in the citrus holosystem.IMPORTANCE Globally, citrus is threatened by huanglongbing (HLB), and the lack of effective control measures is a major concern of farmers, markets, and consumers. There is compelling evidence that plant health is a function of the activities of the plant's associated microbiome. Using Liberibacter crescens, a culturable surrogate for the unculturable HLB-associated bacterium "Candidatus Liberibacter asiaticus," we tested the hypothesis that members of the citrus microbiome produce potential anti-"Ca Liberibacter asiaticus" natural products with potential anti-"Ca Liberibacter asiaticus" activity. A subset of isolates obtained from the microbiome inhibited L. crescens growth in an agar diffusion inhibition assay. Further fractionation experiments linked the inhibitory activity of the fungus Cladosporium cladosporioides to the fungus-produced natural products cladosporols A, C, and D, demonstrating dose-dependent antagonism to L. crescens.

Management of Fire Blight Using Pre-bloom Application of Prohexadione-Calcium

Fire blight, a bacterial disease of rosaceous plants caused by Erwinia amylovora, is one of the most important diseases affecting commercial apple production worldwide. Antibiotics, applied at bloom to protect against blossom infection, are the most effective means of management but raise concern due to the potential for antibiotic resistance in both the pathogen population and nontarget organisms. In addition, most fire blight outbreaks in New York State often emerge in late June to July as shoot blight, calling into question the role of blossom infections and the antibiotic applications made to manage them. Prohexadione-calcium (PhCa) is a gibberellic acid inhibitor used post-bloom to control shoot vigor and to manage shoot blight. However, the magnitude of shoot blight management is directly related to the suppression of shoot growth, which is undesirable, especially in young orchards during establishment years. PhCa is believed to control shoot blight by thickening cell walls in cortical parenchyma, preventing invasion of host tissues by E. amylovora. We hypothesize that PhCa applied pre-bloom could similarly prevent invasion of blossom pedicels following infection, leading to reduced disease incidence. We evaluated novel pre-bloom PhCa programs for their effects on disease management (blossom and shoot blight) as well as their impact on shoot growth for three years in a mature 'Gala' orchard in New York. In all three years of the study, all PhCa programs resulted in less than 27% incidence (71% control) of blossom blight and less than 13% incidence (77% control) of shoot blight with minimal effect on tree growth. Inclusion of a biopesticide during bloom further reduced the incidence of blossom blight in one year of three. Using light microscopy, we found that cell walls in the cortical parenchyma of fruitlet pedicels on trees receiving pre-bloom PhCa applications were significantly thicker than those of untreated trees 40 days after full bloom and inoculation. Overall, we found that pre-bloom applications of PhCa had utility in reducing blossom blight and shoot blight with minimal impacts on tree growth. These pre-bloom programs would fit with standard production practices and may contribute toward the development of fire blight management programs without the use of antibiotics.

Pseudomonas orientalis F9 Pyoverdine, Safracin, and Phenazine Mutants Remain Effective Antagonists against Erwinia amylovora in Apple Flowers

Pseudomonas orientalis F9 is an antagonist of the economically important phytopathogen Erwinia amylovora, the causal agent of fire blight in pomme fruit. On King’s B medium, P. orientalis F9 produces a pyoverdine siderophore and the antibiotic safracin. P. orientalis F9 transposon mutants lacking these factors fail to antagonize E. amylovora, depending on the in vitro assay. On isolated flowers and in soil microcosms, however, pyoverdine, safracin, and phenazine mutants control phytopathogens as clearly as their parental strains.

The recently characterized strain Pseudomonas orientalis F9, an isolate from apple flowers in a Swiss orchard, exhibits antagonistic traits against phytopathogens. At high colonization densities, it exhibits phytotoxicity against apple flowers. P. orientalis F9 harbors biosynthesis genes for the siderophore pyoverdine as well as for the antibiotics safracin and phenazine. To elucidate the role of the three compounds in biocontrol, we screened a large random knockout library of P. orientalis F9 strains for lack of pyoverdine production or in vitro antagonism. Transposon mutants that lacked the ability for fluorescence carried transposons in pyoverdine production genes. Mutants unable to antagonize Erwinia amylovora in an in vitro double-layer assay carried transposon insertions in the safracin gene cluster. As no phenazine transposon mutant could be identified using the chosen selection criteria, we constructed a site-directed deletion mutant. Pyoverdine-, safracin-, and phenazine mutants were tested for their abilities to counteract the fire blight pathogen Erwinia amylovoraex vivo on apple flowers or the soilborne pathogen Pythium ultimumin vivo in a soil microcosm. In contrast to some in vitro assays, ex vivo and in vivo assays did not reveal significant differences between parental and mutant strains in their antagonistic activities. This suggests that, ex vivo and in vivo, other factors, such as competition for resources or space, are more important than the tested antibiotics or pyoverdine for successful antagonism of P. orientalis F9 against phytopathogens in the performed assays.

IMPORTANCEPseudomonas orientalis F9 is an antagonist of the economically important phytopathogen Erwinia amylovora, the causal agent of fire blight in pomme fruit. On King’s B medium, P. orientalis F9 produces a pyoverdine siderophore and the antibiotic safracin. P. orientalis F9 transposon mutants lacking these factors fail to antagonize E. amylovora, depending on the in vitro assay. On isolated flowers and in soil microcosms, however, pyoverdine, safracin, and phenazine mutants control phytopathogens as clearly as their parental strains.

Exploring Rain as Source of Biological Control Agents for Fire Blight on Apple

Poor survival on plants can limit the efficacy of Biological Control Agents (BCAs) in the field. Yet bacteria survive in the atmosphere, despite their exposure to high solar radiation and extreme temperatures. If conditions in the atmosphere are similar to, or more extreme than, the environmental conditions on the plant surface, then precipitation may serve as a reservoir of robust BCAs. To test this hypothesis, two hundred and fifty-four rain-borne isolates were screened for in vitro inhibition of Erwinia amylovora, the causal agent of fire blight, as well as of other plant pathogenic bacteria, fungi and oomycetes. Two isolates showed strong activity against E. amylovora and other plant pathogenic bacteria, while other isolates showed activity against fungal and oomycete pathogens. Survival assays suggested that the two isolates that inhibited E. amylovora were able to survive on apple blossoms and branches similarly to E. amylovora. Pathogen population size and associated fire blight symptoms were significantly reduced when detached apple blossoms were treated with the two isolates before pathogen inoculation, however, disease reduction on attached blossoms within an orchard was inconsistent. Using whole genome sequencing, the isolates were identified as Pantoea agglomerans and P. ananatis, respectively. A UV-mutagenesis screen pointed to a phenazine antibiotic D-alanylgriseoluteic acid synthesis gene cluster as being at the base of the antimicrobial activity of the P. agglomerans isolate. Our work reveals the potential of precipitation as an under-explored source of BCAs, whole genome sequencing as an effective approach to precisely identify BCAs, and UV-mutagenesis as a technically simple screen to investigate the genetic basis of BCAs. More field trials are needed to determine the efficacy of the identified BCAs in fire blight control.

Biological control of bacterial plant diseases with Lactobacillus plantarum strains selected for their broad-spectrum activity

The use of lactic acid bacteria (LAB) to control multiple pathogens that affect different crops was studied, namely, Pseudomonas syringae pv. actinidiae in kiwifruit, Xanthomonas arboricola pv. pruni in Prunus and Xanthomonas fragariae in strawberry. A screening procedure based on in vitro and in planta assays of the three bacterial pathogens was successful in selecting potential LAB strains as biological control agents. The antagonistic activity of 55 strains was first tested in vitro and the strains Lactobacillus plantarum CC100, PM411 and TC92, and Leuconostoc mesenteroides CM160 and CM209 were selected because of their broad-spectrum activity. The biocontrol efficacy of the selected strains was assessed using a multiple-pathosystem approach in greenhouse conditions. L. plantarum PM411 and TC92 prevented all three pathogens from infecting their corresponding plant hosts. In addition, the biocontrol performance of PM411 and TC92 was comparable to the reference products (Bacillus amyloliquefaciens D747, Bacillus subtilis QST713, chitosan, acibenzolar-S-methyl, copper and kasugamycin) in semi-field and field experiments. The in vitro inhibitory mechanism of PM411 and TC92 is based, at least in part, on a pH lowering effect and the production of lactic acid. Moreover, both strains showed similar survival rates on leaf surfaces. PM411 and TC92 can easily be distinguished because of their different multilocus sequence typing and random amplified polymorphic DNA profiles.

New potential bacterial antagonists for the biocontrol of fire blight disease (Erwinia amylovora) in Morocco

The effectiveness of antagonistic bacteria to control Erwinia amylovora was evaluated under in vitro and field conditions. Among 61 bacteria isolated from soil and flowers of fire blight host plants of different Moroccan areas, 20 bacterial isolates showed higher antagonistic activity against the pathogen during agar-diffusion-test, attached blossoms assay and in a bioassay on immature pear fruits. Effective isolates were identified by using biochemical tests and 16 S rRNA genes sequencing. These isolates were grouped into the following genera: Alcaligenes (ACBC1), Pantoea (ACBC2, ACBP1, and ACBP2), Serratia (HC4), Brevibacterium (SF3, SF4, SF7, and SF15), Pseudomonas (SP9), and Bacillus (CPa12, CPa2, HF6, JB2, LMR2, SF14, SF16, SP10, SP13, and SP18). Furthermore, isolates were reported in the NCBI nucleotide sequence database (Genbank) under the accession numbers from KY357285 to KY357304. A 2-year field trials consisted of spray treatments with different bacterial antagonists was conducted on the susceptible apple cultivars 'Gala', 'Golden Parsi' and 'Golden Smoothee'. Their efficacies were evaluated 15 days post-inoculation on detached blossoms and were ranged from 54.6 to 95.0% for 11 strains, most of them were slightly better or better than that obtained with commercial bacterial strains P10c (66%) and QST713 (63%). In field trials, the most effective were P. agglomerans ACBP2, B. amyloliquefaciens LMR2, B. halotolerans (SF3 and SF4), and B. mojarvensis SF16. In addition, effective bacterial isolates did not show the pathogenicity signs towards plant tissue and are, therefore, considered as potential candidates to be integrated in actives ingredients of microbial formulation for the effective control of Fire Blight.

Comparative Programs for Arthropod, Disease and Weed Management in New York Organic Apples

Organic apple production in the eastern US is small and is mostly based on existing varieties, which are susceptible to scab, and rootstocks, which are susceptible to fire blight. This requires numerous sprays per year of various pesticides to produce acceptable fruit. From 2014 to 2016, we tested different arthropod, disease and weed management programs in an advanced tall spindle high-density production system that included disease-resistant cultivars and rootstocks, in an organic research planting of apples in Geneva, New York. Arthropod and disease management regimens were characterized as Advanced Organic, Minimal Organic, or Untreated Control. Results varied by year and variety, but, in general, the Advanced program was more effective than the Minimal program in preventing damage from internal-feeding Lepidoptera, plum curculio, and obliquebanded leafroller, and less effective than the Minimal program against damage by foliar insects. Both organic programs provided comparable control of sooty blotch, cedar apple rust, and fire blight, with some variability across cultivars and years. The advanced selection CC1009 and Modi seemed to possess complete resistance to cedar apple rust, while Pristine had partial resistance. For weed control, bark chip mulch, organic soap sprays, and limonene sprays tended to be most effective, while mechanical tillage and flame weeding had lower success.

Exploration of Using Antisense Peptide Nucleic Acid (PNA)-cell Penetrating Peptide (CPP) as a Novel Bactericide against Fire Blight Pathogen Erwinia amylovora

Erwinia amylovora is a Gram-negative bacterial plant pathogen in the family Enterobacteriaceae and is the causal agent of fire blight, a devastating disease of apple and pear. Fire blight is traditionally managed by the application of the antibiotic streptomycin during bloom, but this strategy has been challenged by the development and spread of streptomycin resistance. Thus, there is an urgent need for effective, specific, and sustainable control alternatives for fire blight. Antisense antimicrobials are oligomers of nucleic acid homologs with antisense sequence of essential genes in bacteria. The binding of these molecules to the mRNA of essential genes can result in translational repression and antimicrobial effect. Here, we explored the possibility of developing antisense antimicrobials against E. amylovora and using these compounds in fire blight control. We determined that a 10-nucleotide oligomer of peptide nucleic acid (PNA) targeting the start codon region of an essential gene acpP is able to cause complete growth inhibition of E. amylovora. We found that conjugation of cell penetrating peptide (CPP) to PNA is essential for the antimicrobial effect, with CPP1 [(KFF)3K] being the most effective against E. amylovora. The minimal inhibitory concentration (MIC) of anti-acpP-CPP1 (2.5 μM) is comparable to the MIC of streptomycin (2 μM). Examination of the antimicrobial mechanisms demonstrated that anti-acpP-CPP1 caused dose-dependent reduction of acpP mRNA in E. amylovora upon treatment and resulted in cell death (bactericidal effect). Anti-acpP-CPP1 (100 μM) is able to effectively limit the pathogen growth on stigmas of apple flowers, although less effective than streptomycin. Finally, unlike streptomycin that does not display any specificity in inhibiting pathogen growth, anti-acpP-CPP1 has more specific antimicrobial effect against E. amylovora. In summary, we demonstrated that PNA-CPP can cause an effective, specific antimicrobial effect against E. amylovora and may provide the basis for a novel approach for fire blight control.

Effects of Consecutive Streptomycin and Kasugamycin Applications on Epiphytic Bacteria in the Apple Phyllosphere

Antibiotic applications are essential for fire blight management in the eastern United States. Recently, streptomycin-resistant Erwinia amylovora strains were found in New York. There are growing concerns that streptomycin resistance may develop from postbloom streptomycin applications in local orchards. Our goal was to investigate the impacts of increasing streptomycin and kasugamycin applications on bacterial epiphyte community composition and antibiotic resistance in the phyllosphere of 'Idared' apple plantings in 2014 and 2015. Rinsate samples from leaves treated with 0, 3, 5, and 10 applications of streptomycin and kasugamycin were collected to isolate, enumerate, and identify epiphytic bacterial species. The majority of isolated epiphytic bacteria were identified as Pantoea agglomerans and fluorescent Pseudomonas spp., whereas E. amylovora was rarely found. Overall, postbloom streptomycin use did not result in an increased recovery of streptomycin-resistant E. amylovora. However, other streptomycin-resistant epiphytes (P. agglomerans and Pseudomonas spp.) did increase with increasing streptomycin applications. Increasing kasugamycin applications reduced the overall number and percentage of streptomycin-resistant epiphytes in the phyllosphere, which has important implications regarding the use of kasugamycin in orchards where streptomycin resistance is a concern.

Bacterial disease management: challenges, experience, innovation and future prospects: Challenges in Bacterial Molecular Plant Pathology

Plant diseases caused by bacterial pathogens place major constraints on crop production and cause significant annual losses on a global scale. The attainment of consistent effective management of these diseases can be extremely difficult, and management potential is often affected by grower reliance on highly disease-susceptible cultivars because of consumer preferences, and by environmental conditions favouring pathogen development. New and emerging bacterial disease problems (e.g. zebra chip of potato) and established problems in new geographical regions (e.g. bacterial canker of kiwifruit in New Zealand) grab the headlines, but the list of bacterial disease problems with few effective management options is long. The ever-increasing global human population requires the continued stable production of a safe food supply with greater yields because of the shrinking areas of arable land. One major facet in the maintenance of the sustainability of crop production systems with predictable yields involves the identification and deployment of sustainable disease management solutions for bacterial diseases. In addition, the identification of novel management tactics has also come to the fore because of the increasing evolution of resistance to existing bactericides. A number of central research foci, involving basic research to identify critical pathogen targets for control, novel methodologies and methods of delivery, are emerging that will provide a strong basis for bacterial disease management into the future. Near-term solutions are desperately needed. Are there replacement materials for existing bactericides that can provide effective disease management under field conditions? Experience should inform the future. With prior knowledge of bactericide resistance issues evolving in pathogens, how will this affect the deployment of newer compounds and biological controls? Knowledge is critical. A comprehensive understanding of bacterial pathosystems is required to not only identify optimal targets in the pathogens, but also optimal seasonal timings for deployment. Host resistance to effectors must be exploited, carefully and correctly. Are there other candidate genes that could be targeted in transgenic approaches? How can new technologies (CRISPR, TALEN, etc.) be most effectively used to add sustainable disease resistance to existing commercially desirable plant cultivars? We need an insider's perspective on the management of systemic pathogens. In addition to host resistance or reduced sensitivity, are there other methods that can be used to target these pathogen groups? Biological systems are variable. Can biological control strategies be improved for bacterial disease management and be made more predictable in function? The answers to the research foci outlined above are not all available, as will become apparent in this article, but we are heading in the right direction. In this article, we summarize the contributions from past experiences in bacterial disease management, and also describe how advances in bacterial genetics, genomics and host-pathogen interactions are informing novel strategies in virulence inhibition and in host resistance. We also outline potential innovations that could be exploited as the pressures to maximize a safe and productive food supply continue to become more numerous and more complex.

Comparison of Methods of Acibenzolar-S-Methyl Application for Post-Infection Fire Blight Suppression in Pear and Apple

Greenhouse-grown, 1-year-old potted 'Bosc' pear and apple rootstock cultivars 'M.9' and 'M.26' were inoculated with the fire blight pathogen, Erwinia amylovora, and subjected to trunk paint, root drench, or foliar spray treatments with acibenzolar-S-methyl (ASM, 4 to 30 mg a.i./tree) to induce systemic acquired resistance. Each method of ASM treatment suppressed fire blight canker expansion by 22 to 25%. Furthermore, ASM application method and ASM treatment timing (at or ±3 weeks relative to inoculation) interacted significantly (P ≤ 0.02) in each experiment. A root drench was most effective when applied 3 weeks before inoculation (36% suppression) whereas trunk paints and foliar sprays were more effective at inoculation (43 and 34%, suppression, respectively). Sizes of fire blight cankers in potted apple rootstocks M.9 and M.26 (under scions 'Gala' or 'Cameo') inoculated directly with the pathogen were reduced by 82 and 87% after two pretreatments of ASM applied as a trunk paint or root drench, respectively. Expression of pathogenesis-related (PR) genes PR-1 and -2 in apple leaves sampled after an ASM trunk paint were elevated significantly (P ≤ 0.05) relative to control trees for at least 9 weeks after treatment. Results of this study are being used to guide field research on postinfection therapy with ASM in 1- to 10-year-old pear and apple trees where fire blight has proven difficult to manage with therapeutic pruning only.

Prevalence of Streptomycin-Resistant Erwinia amylovora in New York Apple Orchards

Resistance to streptomycin in Erwinia amylovora was first observed in the United States in the 1970s but was not found in New York until 2002, when streptomycin-resistant (SmR) E. amylovora was isolated from orchards in Wayne County. From 2011 to 2014, in total, 591 fire blight samples representing shoot blight, blossom blight, and rootstock blight were collected from 80 apple orchards in New York. From these samples, 1,280 isolates of E. amylovora were obtained and assessed for streptomycin resistance. In all, 34 SmR E. amylovora isolates were obtained from 19 individual commercial orchards. The majority of the resistant isolates were collected from orchards in Wayne County, and the remaining were from other counties in western New York. Of the 34 resistant isolates, 32 contained the streptomycin resistance gene pair strA/strB in the transposon Tn5393 on the nonconjugative plasmid pEA29. This determinant of streptomycin resistance has only been found in SmR E. amylovora isolates from Michigan and the SmR E. amylovora isolates discovered in Wayne County, NY in 2002. Currently, our data indicate that SmR E. amylovora is restricted to counties in western New York and is concentrated in the county with the original outbreak. Because the resistance is primarily present on the nonconjugative plasmid, it is possible that SmR has been present in Wayne County since the introduction in 2002, and has spread within and out of Wayne County to additional commercial growers over the past decade. However, research is still needed to provide in-depth understanding of the origin and spread of the newly discovered SmR E. amylovora to reduce the spread of streptomycin resistance into other apple-growing regions, and address the sustainability of streptomycin use for fire blight management in New York.

Antibacterial Activity of Wild Mushroom Extracts on Bacterial Wilt Pathogen Ralstonia solanacearum

In total, 150 protein extracts from 94 different basidiomycete and ascomycete wild mushroom species were tested for antibacterial activity against the quarantine plant-pathogen bacterium Ralstonia solanacearum. In in vitro microtiter plate assays, 15 extracts with moderate to high antibacterial activities were identified: 11 completely inhibited bacterial growth and 4 showed partial inhibition. Of these 15 extracts, 5 were further tested and 3 extracts slowed disease progression and reduced disease severity in artificially inoculated tomato and potato plants. However, the in vitro activities of the extracts did not always correlate with their in vivo activities, which emphasizes the importance of performing early screening tests also in vivo. Testing of selected extracts against 12 R. solanacearum strains identified 6 with potential for broader applicability. Further analysis of extracts from Amanita phalloides and Clitocybe geotropa showed that the active substances are proteins with an approximate size of 180 kDa. To our knowledge, this is the first in vitro and in vivo study that demonstrates that mushroom protein extracts can be promising for treatment of bacterial wilt caused by R. solanacearum.

Fire Blight Control: The Struggle Goes On. A Comparison of Different Fire Blight Control Methods in Switzerland with Respect to Biosafety, Efficacy and Durability

Fire blight (FB), caused by Erwinia amylovora, is one of the most important pome fruit pathogens worldwide. To control this devastating disease, various chemical and biological treatments are commonly applied in Switzerland, but they fail to keep the infection at an acceptable level in years of heavy disease pressure. The Swiss authorities therefore currently allow the controlled use of the antibiotic streptomycin against FB in years that are predicted to have heavy infection periods, but only one treatment per season is permitted. Another strategy for controlling Erwinia is to breed resistant/tolerant apple cultivars. One way of accelerating the breeding process is to obtain resistant cultivars by inserting one or several major resistance genes, using genetic engineering. To date, no study summarizing the impact of different FB control measures on the environment and on human health has been performed. This study consequently aims to compare different disease-control measures (biological control, chemical control, control by antibiotics and by resistant/tolerant apple cultivars obtained through conventional or molecular breeding) applied against E. amylovora, considering different protection goals (protection of human health, environment, agricultural diversity and economic interest), with special emphasis on biosafety aspects. Information on each FB control measure in relation to the specified protection goal was assessed by literature searches and by interviews with experts. Based on our results it can be concluded that the FB control measures currently applied in Switzerland are safe for consumers, workers and the environment. However, there are several gaps in our knowledge of the human health and environmental impacts analyzed: data are missing (1) on long term studies on the efficacy of most of the analyzed FB control measures; (2) on the safety of operators handling streptomycin; (3) on residue analyses of Equisetum plant extract, the copper and aluminum compounds used in apple production; and (4) on the effect of biological and chemical control measures on non-target fauna and flora. These gaps urgently need to be addressed in the near future.

Absence of lysogeny in wild populations of Erwinia amylovora and Pantoea agglomerans

Lytic bacteriophages are in development as biological control agents for the prevention of fire blight disease caused by Erwinia amylovora. Temperate phages should be excluded as biologicals since lysogeny produces the dual risks of host resistance to phage attack and the transduction of virulence determinants between bacteria. The extent of lysogeny was estimated in wild populations of E. amylovora and Pantoea agglomerans with real-time polymerase chain reaction primers developed to detect E. amylovora phages belonging to the Myoviridae and Podoviridae families. Pantoea agglomerans, an orchard epiphyte, is easily infected by Erwinia spp. phages, and it serves as a carrier in the development of the phage-mediated biological control agent. Screening of 161 E. amylovora isolates from 16 distinct geographical areas in North America, Europe, North Africa and New Zealand and 82 P. agglomerans isolates from southern Ontario, Canada showed that none possessed prophage. Unstable phage resistant clones or lysogens were produced under laboratory conditions. Additionally, a stable lysogen was recovered from infection of bacterial isolate Ea110R with Podoviridae phage ΦEa35-20. These laboratory observations suggested that while lysogeny is possible in E. amylovora, it is rare or absent in natural populations, and there is a minimal risk associated with lysogenic conversion and transduction by Erwinia spp. phages.

Control of fire blight (Erwinia amylovora) on apple trees with trunk-injected plant resistance inducers and antibiotics and assessment of induction of pathogenesis-related protein genes

Management of fire blight is complicated by limitations on use of antibiotics in agriculture, antibiotic resistance development, and limited efficacy of alternative control agents. Even though successful in control, preventive antibiotic sprays also affect non-target bacteria, aiding the selection for resistance which could ultimately be transferred to the pathogen Erwinia amylovora. Trunk injection is a target-precise pesticide delivery method that utilizes tree xylem to distribute injected compounds. Trunk injection could decrease antibiotic usage in the open environment and increase the effectiveness of compounds in fire blight control. In field experiments, after 1-2 apple tree injections of either streptomycin, potassium phosphites (PH), or acibenzolar-S-methyl (ASM), significant reduction of blossom and shoot blight symptoms was observed compared to water injected control trees. Overall disease suppression with streptomycin was lower than typically observed following spray applications to flowers. Trunk injection of oxytetracycline resulted in excellent control of shoot blight severity, suggesting that injection is a superior delivery method for this antibiotic. Injection of both ASM and PH resulted in the significant induction of PR-1, PR-2, and PR-8 protein genes in apple leaves indicating induction of systemic acquired resistance (SAR) under field conditions. The time separating SAR induction and fire blight symptom suppression indicated that various defensive compounds within the SAR response were synthesized and accumulated in the canopy. ASM and PH suppressed fire blight even after cessation of induced gene expression. With the development of injectable formulations and optimization of doses and injection schedules, the injection of protective compounds could serve as an effective option for fire blight control.

Improvement of a dry formulation of Pseudomonas fluorescens EPS62e for fire blight disease biocontrol by combination of culture osmoadaptation with a freeze-drying lyoprotectant

AIMS

To study the effect of lyoprotectants and osmoadaptation on viability of Pseudomonas fluorescens EPS62e during freeze-drying and storage and to evaluate the formulation in terms of efficacy in biocontrol and fitness on pear flowers.

METHODS AND RESULTS

A wettable powder formulation of a biocontrol agent of fire blight was optimized by means of lyoprotectants and culture osmoadaptation. Freeze-drying was used to obtain dehydrated cells, and the best viability (70% of survival) was obtained using lactose as lyoprotectant. Survival during lyophilization was additionally improved using physiological adaptation of cells during cultivation under salt-amended medium (osmoadaptation). The procedure increased the survival of cells after freeze-drying attaining viability values close to a 100% in the lactose-formulated product (3 × 10(11) CFU g(-1) ), and through the storage period of 1 year at 4°C. The dry formulation showed also an improved biocontrol efficacy and survival of EPS62e on pear flowers under low relative humidity conditions.

CONCLUSIONS

Cell viability after freeze-drying was improved using lactose as lyoprotectant combined with a procedure of osmoadaptation during cultivation. The powder-formulated product remained active for 12 months and retained biocontrol levels similar to that of fresh cells. The formulation showed an improved survival of EPS62e on flowers and an increase of the efficacy of biocontrol of fire blight at low relative humidity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results have a potential value for commercial application in biocontrol agents not only of fire blight but also of other plant diseases.

Sensitivity of Erwinia amylovora in Illinois Apple Orchards to Streptomycin, Oxytetracyline, Kasugamycin, and Copper

In 2008 and 2009, severe fire blight (Erwinia amylovora) occurred in Illinois apple orchards, leading to speculation that streptomycin-resistant strains of E. amylovora might be present in some orchards. Statewide surveys were conducted in 2010, 2011, and 2012, and 117, 129, and 170, E. amylovora isolates were collected, respectively, from 20 counties. None of the 416 E. amylovora isolates tested were resistant to streptomycin (Agri-Mycin 17WP) at 50 mg/liter. Seven non-E. amylovora bacterial isolates were collected from E. amylovora-infected shoots that contained both a strA-strB streptomycin resistance gene and IS1133 on transposon Tn5393, which could be a potential source of streptomycin resistance for E. amylovora in Illinois in the future. Colony development of all 84 E. amylovora isolates tested was inhibited on Luria-Bertani medium amended with oxytetracycline at 50 mg/liter and kasugamycin (Kasumin 2L) at 100 mg/liter. Similarly, colony development of the 84 E. amylovora isolates was inhibited on casitone-yeast extract medium amended with copper sulfate at 0.16 mM. In 2011 and 2012, field trials were conducted to evaluate the efficacy of oxytetracycline (Mycoshield 17WP), kasugamycin (Kasumin 2L and ARY-4016-06), copper hydroxide (Kocide-3000 41.6DF), Bacillus subtilis (Serenade Max, QST713), and Pseudomonas fluorescens (Blight Ban A506) for management of fire blight in an apple orchard. Only kasugamycin (Kasumin 2L and ARY-4016-06) reduced blossom infection significantly. There was a significant interaction of kasugamycin (Kasumin 2L) with prohexadione calcium (Apogee 27.5DF) in reducing shoot blight incidence in the field in 2012.

Draft Genome Sequence of the Antibiotic-Producing Cystic Fibrosis Isolate Pantoea agglomerans Tx10

Pantoea agglomerans is an enteric bacterium that is capable of causing both plant and human disease. Here, we report the genome sequence of a cystic fibrosis isolate, P. agglomerans Tx10, which produces an antibiotic that is effective against Staphylococcus aureus.

Evaluation of Strategies for Fire Blight Control in Organic Pome Fruit Without Antibiotics

Apple and pear produced organically under the U.S. National Organic Program (NOP) standard can be treated with antibiotics for suppression of fire blight caused by Erwinia amylovora. Recent regulatory actions by the NOP, however, have lessened the likelihood of antibiotic use after the 2014 season. In response, western U.S. organic apple and pear stakeholders identified two immediate-need research objectives related to fire blight control: development of effective non-antibiotic control programs based on combinations of registered biological products; and, in apple, integration of these products with lime sulfur, which is sprayed at early bloom to reduce fruit load. In orchard trials in Oregon, increasing the frequency of treatment with biological products improved suppression of floral infection. In apple, fruit load thinning with 2% lime sulfur plus 2% fish oil (LS+FO) at 30 and 70% bloom significantly (P ≤ 0.05) reduced the proportion of blighted flower clusters in four of five orchard trials. Moreover, lime sulfur significantly (P ≤ 0.05) suppressed epiphytic populations of E. amylovora after their establishment on apple flowers. Over four trials, treatment with Aureobasidium pullulans (Blossom Protect) after LS+FO reduced the incidence of fire blight by an average of 92% compared with water only; this level of control was similar to treatment with streptomycin. In three seasons, a spray of a Pantoea agglomerans product after the 70% bloom treatment of LS+FO established the antagonist on a significantly (P ≤ 0.05) higher proportion of flowers compared with a spray of this bacterium before the thinning treatment. Consequently, in apple, biological treatments for fire blight control are not advised until after lime sulfur treatments for fruit load thinning are completed.

Unexpected diversity during community succession in the apple flower microbiome

Despite its importance to the host, the flower microbiome is poorly understood. We report a culture-independent, community-level assessment of apple flower microbial diversity and dynamics. We collected flowers from six apple trees at five time points, starting before flowers opened and ending at petal fall. We applied streptomycin to half of the trees when flowers opened. Assessment of microbial diversity using tag pyrosequencing of 16S rRNA genes revealed that the apple flower communities were rich and diverse and dominated by members of TM7 and Deinococcus-Thermus, phyla about which relatively little is known. From thousands of taxa, we identified six successional groups with coherent dynamics whose abundances peaked at different times before and after bud opening. We designated the groups Pioneer, Early, Mid, Late, Climax, and Generalist communities. The successional pattern was attributed to a set of prevalent taxa that were persistent and gradually changing in abundance. These taxa had significant associations with other community members, as demonstrated with a cooccurrence network based on local similarity analysis. We also detected a set of less-abundant, transient taxa that contributed to general tree-to-tree variability but not to the successional pattern. Communities on trees sprayed with streptomycin had slightly lower phylogenetic diversity than those on unsprayed trees but did not differ in structure or succession. Our results suggest that changes in apple flower microbial community structure are predictable over the life of the flower, providing a basis for ecological understanding and disease management.

IMPORTANCE

Flowering plants (angiosperms) represent a diverse group of an estimated 400,000 species, and their successful cultivation is essential to agriculture. Yet fundamental knowledge of flower-associated microbiotas remains largely unknown. Even less well understood are the changes that flower microbial communities experience through time. Flowers are particularly conducive to comprehensive temporal studies because they are, by nature, ephemeral organs. Here, we present the first culture-independent time series of bacterial and archaeal communities associated with the flowers of apple, an economically important crop. We found unexpected diversity on apple flowers, including a preponderance of taxa affiliated with Deinococcus-Thermus and TM7, phyla that are understudied but thought to be tolerant to an array of environmental stresses. Our results also suggest that changes in microbial community structure on the apple flower may be predictable over the life of the flower, providing the basis for ecological understanding and disease management.

Advances in bacteriophage-mediated control of plant pathogens

There is continuing pressure to maximise food production given a growing global human population. Bacterial pathogens that infect important agricultural plants (phytopathogens) can reduce plant growth and the subsequent crop yield. Currently, phytopathogens are controlled through management programmes, which can include the application of antibiotics and copper sprays. However, the emergence of resistant bacteria and the desire to reduce usage of toxic products that accumulate in the environment mean there is a need to develop alternative control agents. An attractive option is the use of specific bacteriophages (phages), viruses that specifically kill bacteria, providing a more targeted approach. Typically, phages that target the phytopathogen are isolated and characterised to determine that they have features required for biocontrol. In addition, suitable formulation and delivery to affected plants are necessary to ensure the phages survive in the environment and do not have a deleterious effect on the plant or target beneficial bacteria. Phages have been isolated for different phytopathogens and have been used successfully in a number of trials and commercially. In this paper, we address recent progress in phage-mediated control of plant pathogens and overcoming the challenges, including those posed by CRISPR/Cas and abortive infection resistance systems.

Prospects and limitations of microbial pesticides for control of bacterial and fungal pomefruit tree diseases

The tree constitutes an ecosystem in which microorganisms play an essential role in its functionality. Interactions that microorganisms establish with plants may be beneficial or detrimental and are of extreme importance in the exploitation of trees in agriculture as crop production systems. Fruit trees, especially pomefruit trees including apple, pear and several ornamentals are of great economic importance but its production is affected by several diseases. Fungal and bacterial fruit tree diseases are mainly controlled with chemical fungicides and bactericides, but health and environmental concerns about the use of chemical pesticides have result in strong regulatory actions and have stimulated the development of beneficial microorganisms as microbial pesticides. Up to now, several microorganisms have been registered in different countries and in the EU as biocontrol agents (BCA) covering mainly fire blight, soil-borne fungal diseases and postharvest fruit fungal rot. The key aspects in the success of this technology for disease control are related to biosafety and environmental impact of biocontrol agents, the traceability and fate in the environment and food chain, the improvement by physiological, genetic engineering or the use of mixtures or formulations as well as the industrial production and development of delivery systems for treatment application to trees.

Improvement of fitness and efficacy of a fire blight biocontrol agent via nutritional enhancement combined with osmoadaptation

The efficacy of Pseudomonas fluorescens EPS62e in the biocontrol of Erwinia amylovora was improved by a procedure of physiological adaptation to increase colonization and survival in the phytosphere of rosaceous plants. The procedure consisted of osmoadaptation (OA) and nutritional enhancement (NE). OA was based on saline stress and osmolyte amendment of the growth medium during inoculum preparation. NE consisted of addition of glycine and Tween 80 to the formulation. NE and OA increased the growth rate and carrying capacity of EPS62e under high-relative-humidity (RH) conditions and improved survival at low RH on flowers under controlled environmental conditions. NE did not promote growth or affect infection capacity of E. amylovora. The effect of both methods was tested in the field by following the population of EPS62e using quantitative PCR (Q-PCR) (total population) and CFU counting (culturable population) methods. Following field application, EPS62e colonized blossoms, but it was stressed, as indicated by a sharp decrease in culturable compared to total population levels. However, once established in flowers and at the end of bloom, almost all the total population was culturable. The physiological adaptation treatments increased population levels of EPS62e over those of nonadapted cells during the late stage of the flowering period. Control of fire blight infections in flowers and immature fruits was tested by field application of EPS62e and subsequent inoculation with E. amylovora under controlled-environment conditions. The efficacy of fire blight control increased significantly with the combination of nutritional enhancement and osmoadaptation, in comparison with the absence of physiological adaptation.

Multiple treatment meta-analysis of products evaluated for control of fire blight in the eastern United States

The aim of this analysis was to estimate the effect sizes and consistency of products evaluated for fire blight control in the eastern United States over the last decade. Because only 3% of the 69 studies published from 2000 to 2008 explicitly presented a measure of within-study variability, a method for estimating the least significant difference (LSD) and, hence the sampling variance, for studies with at least two significant mean separations in the presented mean multiple comparisons was developed. Lin's concordance analysis indicated that the estimated LSD was an accurate predictor of the actual LSD based on 35 studies in a calibration evaluation (ρ(c) = 0.997). Separate multi-treatment random-effects meta-analyses were performed for three control categories: antibiotics, biological control, and plant defense-activating products and mean log response ratios relative to the nontreated controls ([Formula: see text]) were computed for each treatment and then back-transformed to obtain the mean percent disease control. None of the products evaluated performed as well as streptomycin, the standard product for fire blight control, for which the mean disease control was 68.6%. As a group, experimental antibiotics provided the best fire blight control with mean effect sizes ranging from 59.7 to 61.7%. Among the biological controls, the best control was noted for treatments combining the antibiotic streptomycin with a product based on Pantoea agglomerans (55.0% mean disease reduction) or Bacillus subtilis (53.9%). Mean disease control was 31.9, 25.7, and 22.6%, respectively, for products based on B. subtilis, Pantoea agglomerans, and Pseudomonas fluorescens without an antibiotic, suggesting that the higher efficacy of the combination treatments was due to the antibiotic. Among the plant defense-activating products, prohexadione calcium had the highest and most consistent effect size (50.7% control), while other products provided modest mean disease control of between 6.1 and 25.8%. Percent control values were significantly moderated by study location and cultivar used in the study, and were smaller, but more variable, when products were tested under high disease intensity compared with low disease intensity. Results indicate that wide-scale use of biological control and plant defense-activating products in the eastern United States is likely to remain low.

Evaluation of kasugamycin for fire blight management, effect on nontarget bacteria, and assessment of kasugamycin resistance potential in Erwinia amylovora

The emergence and spread of streptomycin-resistant strains of Erwinia amylovora in Michigan has necessitated the evaluation of new compounds effective for fire blight control. The aminoglycoside antibiotic kasugamycin (Ks) targets the bacterial ribosome and is particularly active against E. amylovora. The efficacy of Ks formulated as Kasumin 2L for control of fire blight was evaluated in six experiments conducted over four field seasons in our experimental orchards in East Lansing, MI. Blossom blight control was statistically equivalent to the industry standard streptomycin in all experiments. E. amylovora populations remained constant on apple flower stigmas pretreated with Kasumin and were ≈100-fold lower than on stigmas treated with water. Kasumin applied to apple trees in the field also resulted in a 100-fold reduced total culturable bacterial population compared with trees treated with water. We performed a prospective analysis of the potential for kasugamycin resistance (Ks(R)) development in E. amylovora which focused on spontaneous resistance development and acquisition of a transferrable Ks(R) gene. In replicated lab experiments, the development of spontaneous resistance in E. amylovora to Ks at 250 or 500 ppm was not observed when cells were directly plated on medium containing high concentrations of the antibiotic. However, exposure to increasing concentrations of Ks in media (initial concentration 25 μg ml(-1)) resulted in the selection of Ks resistance (at 150 μg ml(-1)) in the E. amylovora strains Ea110, Ea273, and Ea1189. Analysis of mutants indicated that they harbored mutations in the kasugamycin target ksgA gene and that all mutants were impacted in relative fitness observable through a reduced growth rate in vitro and decreased virulence in immature pear fruit. The possible occurrence of a reservoir of Ks(R) genes in orchard environments was also examined. Culturable gram-negative bacteria were surveyed from six experimental apple orchards that had received at least one Kasumin application. In total, 401 Ks(R) isolates (42 different species) were recovered from apple flowers and leaves and orchard soil samples. Although we have not established the presence of a transferrable Ks(R) gene in orchard bacteria, the frequency, number of species, and presence of Ks(R) enterobacterial species in orchard samples suggests the possible role of nontarget bacteria in the future transfer of a Ks(R) gene to E. amylovora. Our data confirm the importance of kasugamycin as an alternate antibiotic for fire blight management and lay the groundwork for the development and incorporation of resistance management strategies.

Mechanistically compatible mixtures of bacterial antagonists improve biological control of fire blight of pear

Mixtures of biological control agents can be superior to individual agents in suppressing plant disease, providing enhanced efficacy and reliability from field to field relative to single biocontrol strains. Nonetheless, the efficacy of combinations of Pseudomonas fluorescens A506, a commercial biological control agent for fire blight of pear, and Pantoea vagans strain C9-1 or Pantoea agglomerans strain Eh252 rarely exceeds that of individual strains. A506 suppresses growth of the pathogen on floral colonization and infection sites through preemptive exclusion. C9-1 and Eh252 produce peptide antibiotics that contribute to disease control. In culture, A506 produces an extracellular protease that degrades the peptide antibiotics of C9-1 and Eh252. We hypothesized that strain A506 diminishes the biological control activity of C9-1 and Eh252, thereby reducing the efficacy of biocontrol mixtures. This hypothesis was tested in five replicated field trials comparing biological control of fire blight using strain A506 and A506 aprX::Tn5, an extracellular protease-deficient mutant, as individuals and combined with C9-1 or Eh252. On average, mixtures containing A506 aprX::Tn5 were superior to those containing the wild-type strain, confirming that the extracellular protease of A506 diminished the biological control activity of C9-1 and Eh252 in situ. Mixtures of A506 aprX::Tn5 and C9-1 or Eh252 were superior to oxytetracycline or single biocontrol strains in suppressing fire blight of pear. These experiments demonstrate that certain biological control agents are mechanistically incompatible, in that one strain interferes with the mechanism by which a second strain suppresses plant disease. Mixtures composed of mechanistically compatible strains of biological control agents can suppress disease more effectively than individual biological control agents.

Meta-analysis to determine the effects of plant disease management measures: review and case studies on soybean and apple

The continuing exponential increase in scientific knowledge, the growing availability of large databases containing raw or partially annotated information, and the increased need to document impacts of large-scale research and funding programs provide a great incentive for integrating and adding value to previously published (or unpublished) research through quantitative synthesis. Meta-analysis has become the standard for quantitative evidence synthesis in many disciplines, offering a broadly accepted and statistically powerful framework for estimating the magnitude, consistency, and homogeneity of the effect of interest across studies. Here, we review previous and current uses of meta-analysis in plant pathology with a focus on applications in epidemiology and disease management. About a dozen formal meta-analyses have been published in the plant pathological literature in the past decade, and several more are currently in progress. Three broad research questions have been addressed, the most common being the comparative efficacy of chemical treatments for managing disease and reducing yield loss across environments. The second most common application has been the quantification of relationships between disease intensity and yield, or between different measures of disease, across studies. Lastly, meta-analysis has been applied to assess factors affecting pathogen-biocontrol agent interactions or the effectiveness of biological control of plant disease or weeds. In recent years, fixed-effects meta-analysis has been largely replaced by random- (or mixed-) effects analysis owing to the statistical benefits associated with the latter and the wider availability of computer software to conduct these analyses. Another recent trend has been the more common use of multivariate meta-analysis or meta-regression to analyze the impacts of study-level independent variables (moderator variables) on the response of interest. The application of meta-analysis to practical problems in epidemiology and disease management is illustrated with case studies from our work on Phakopsora pachyrhizi on soybean and Erwinia amylovora on apple. We show that although meta-analyses are often used to corroborate and validate general conclusions drawn from more traditional, qualitative reviews, they can also reveal new patterns and interpretations not obvious from individual studies.

Control of fire blight by Pseudomonas fluorescens A506 and Pantoea vagans C9-1 applied as single strains and mixed inocula

The biological control agents Pseudomonas fluorescens A506 and Pantoea vagans C9-1 were evaluated individually and in combination for the suppression of fire blight of pear or apple in 10 field trials inoculated with the pathogen Erwinia amylovora. The formulation of pathogen inoculum applied to blossoms influenced establishment of the pathogen and the efficacy of biological control. Pantoea vagans C9-1 suppressed fire blight in all five trials in which the pathogen was applied as lyophilized cells but in none of the trials in which the pathogen was applied as freshly harvested cells. In contrast, Pseudomonas fluorescens A506 reduced disease significantly in only one trial. A mixture of the two strains also suppressed fire blight, but the magnitude of disease suppression over all field trials (averaging 32%) was less than that attained by C9-1 alone (42%). The two biological control agents did not antagonize one another on blossom surfaces, and application of the mixture of A506 and C9-1 to blossoms resulted in a greater proportion of flowers having detectable populations of at least one bacterial antagonist than the application of individual strains. Therefore, the mixture of A506 and C9-1 provided less disease control than expected based upon the epiphytic population sizes of the antagonists on blossom surfaces. We speculate that the biocontrol mixture was less effective than anticipated due to incompatibility between the mechanisms by which A506 and C9-1 suppress disease.

Screening molecules for control of citrus huanglongbing using an optimized regeneration system for 'Candidatus Liberibacter asiaticus'-infected periwinkle (Catharanthus roseus) cuttings

Citrus huanglongbing is one of the most destructive diseases of citrus worldwide. The disease is associated with three different species of 'Candidatus Liberibacter', of which 'Ca. L. asiaticus' is the most widely distributed. An optimized system using 'Ca. L. asiaticus'-infected periwinkle cuttings was developed to screen chemical compounds effective for controlling the bacterial population while simultaneously assessing their phytotoxicity. The optimal regeneration conditions were determined to be the use of vermiculite as a growth medium for the cuttings, and a fertilization routine using half-strength Murashige and Tucker medium supplemented with both naphthalene acetic acid (4 microg/ml) and indole-3-butyric acid (4 microg/ml). This system allowed a plant regeneration rate of 60.6% for 'Ca. L. asiaticus'-infected cuttings in contrast to the <1% regeneration rate with water alone. Two chemical agents, penicillin G sodium and 2,2-dibromo-3-nitrilopropionamide (DBNPA), were found to be effective at eliminating or suppressing the 'Ca. L. asiaticus' bacterium in this periwinkle regeneration system. When treated with penicillin G sodium at 50 microg/ml, all plants regenerated from 'Ca. L. asiaticus'-infected cuttings were 'Ca. L. asiaticus' negative as determined by both nested polymerase chain reaction (PCR) and quantitative real-time PCR. In addition, DBNPA was also able to significantly reduce the percentage of 'Ca. L. asiaticus'-positive plants and the titer of the 'Ca. L. asiaticus' bacterium at 200 microl/liter.

Evidence that prohexadione-calcium induces structural resistance to fire blight infection

Mechanisms of fire blight control by the shoot-growth regulator prohexadione-calcium (ProCa) were investigated by comparing disease development in ProCa-treated potted apple trees (cv. Gala) to paclobutrazol (another shoot-growth regulator)-treated and nontreated trees and in ProCa-treated cv. McIntosh trees in the field. Twenty-eight days after inoculation with Erwinia amylovora Ea110, disease incidence on ProCa- and paclobutrazol-treated shoots was significantly reduced compared with that on nontreated shoots. Disease severity (percent shoot length infected) was also significantly lower on both ProCa- and paclobutrazol-treated shoots than on nontreated shoots. However, bacterial populations within inoculated shoots were high and bacterial growth occurred in all treatments. In addition, the mean cell wall width of the cortical parenchyma midvein tissue of the first and second youngest unfolded leaves of ProCa- and paclobutrazol-treated shoots was significantly wider both 0.5 and 2 cm from the leaf tips compared with the cell walls of the nontreated tissue. Taken together, these results suggest that reduction of fire blight symptoms by ProCa and paclobutrazol is not the result of reduced populations of E. amylovora in shoots. Moreover, because paclobutrazol also reduced disease severity and incidence, changes in flavonoid metabolism induced by ProCa but not paclobutrazol does not appear to be responsible for disease control as suggested in recent literature. Finally, although this study did not directly link disease control to the observed cell wall changes, the possibility that an increase in cell wall width impedes the spread of E. amylovora should be investigated in more depth.

Development of alternative strategies for management of soilborne pathogens currently controlled with methyl bromide

The current standard treatment for management of soilborne pests in some high-value crop production systems is preplant fumigation with mixtures of methyl bromide and chloropicrin. With the impending phase-out of methyl bromide, the agricultural industries that rely on soil fumigation face the need for development of alternative pest management strategies. To maintain farm productivity, immediate term research has focused on evaluation of alternative fumigants, modification of current crop production practices to accommodate their use, and improvement of application technologies to reduce the environmental effects of fumigant applications. Longer-term research goals have focused on developing a more integrated approach for pest management that incorporates the use of cultural practices to reduce pathogen pressure, host resistance to disease, and biological approaches for stimulating plant growth and control of root diseases.